(**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *)

(* This module drives the type checker *)

open Utils_js

(****************** typecheck job helpers *********************)

let clear_errors (files: FilenameSet.t) errors =
  FilenameSet.fold
    (fun file { ServerEnv.local_errors; merge_errors; suppressions; severity_cover_set; } ->
      Hh_logger.debug "clear errors %s" (File_key.to_string file);
      { ServerEnv.
        local_errors = FilenameMap.remove file local_errors;
        merge_errors = FilenameMap.remove file merge_errors;
        suppressions = Error_suppressions.remove file suppressions;
        severity_cover_set = FilenameMap.remove file severity_cover_set;
      }
    ) files errors

let update_errset map file errset =
  if Errors.ErrorSet.is_empty errset then map
  else
    let errset = match FilenameMap.get file map with
    | Some prev_errset ->
      Errors.ErrorSet.union prev_errset errset
    | None -> errset
    in
    FilenameMap.add file errset map

let merge_error_maps = FilenameMap.union ~combine:(fun _ x y -> Some (Errors.ErrorSet.union x y))

(* Filter out duplicate provider error, if any, for the given file. *)
let filter_duplicate_provider map file =
  match FilenameMap.get file map with
  | Some prev_errset ->
    let new_errset = Errors.ErrorSet.filter (fun err ->
      not (Errors.is_duplicate_provider_error err)
    ) prev_errset in
    FilenameMap.add file new_errset map
  | None -> map

let with_timer_lwt ?options timer profiling f =
  let should_print = Option.value_map options ~default:false ~f:(Options.should_profile) in
  Profiling_js.with_timer_lwt ~should_print ~timer ~f profiling

let collate_parse_results ~options parse_results =
  let { Parsing_service_js.
    parse_ok; parse_skips; parse_hash_mismatch_skips; parse_fails; parse_unchanged
  } = parse_results in
  (* No one who is calling collate_parse_results is skipping files with hash mismatches *)
  assert (FilenameSet.is_empty parse_hash_mismatch_skips);
  let local_errors = List.fold_left (fun errors (file, _, fail) ->
    let errset = match fail with
    | Parsing_service_js.Parse_error err ->
      Inference_utils.set_of_parse_error ~source_file:file err
    | Parsing_service_js.Docblock_errors errs ->
      Inference_utils.set_of_docblock_errors ~source_file:file errs
    | Parsing_service_js.File_sig_error err ->
      Inference_utils.set_of_file_sig_error ~source_file:file err
    in
    update_errset errors file errset
  ) FilenameMap.empty parse_fails in

  let local_errors =
    (* In practice, the only `tolerable_errors` are related to well formed exports. If this flag
     * were not temporary in nature, it would be worth adding some complexity to avoid conflating
     * them. *)
    if options.Options.opt_enforce_well_formed_exports then
      FilenameMap.fold (fun file file_sig_errors errors ->
        let errset = Inference_utils.set_of_file_sig_tolerable_errors
          ~source_file:file file_sig_errors in
        update_errset errors file errset
      ) parse_ok local_errors
    else
      local_errors
  in

  let unparsed = List.fold_left (fun unparsed (file, info, _) ->
    (file, info) :: unparsed
  ) parse_skips parse_fails in

  let parse_ok = parse_ok |> FilenameMap.keys |> FilenameSet.of_list in

  parse_ok, unparsed, parse_unchanged, local_errors

let parse ~options ~profiling ~workers parse_next =
  with_timer_lwt ~options "Parsing" profiling (fun () ->
    let%lwt results = Parsing_service_js.parse_with_defaults options workers parse_next in
    Lwt.return (collate_parse_results ~options results)
  )

let reparse ~options ~profiling ~transaction ~workers ~modified ~deleted =
  with_timer_lwt ~options "Parsing" profiling (fun () ->
    let%lwt new_or_changed, results =
      Parsing_service_js.reparse_with_defaults
        ~transaction ~with_progress:true ~workers ~modified ~deleted options
    in
    let parse_ok, unparsed, unchanged, local_errors = collate_parse_results ~options results in
    Lwt.return (new_or_changed, parse_ok, unparsed, unchanged, local_errors)
  )

let parse_contents ~options ~profiling ~check_syntax filename contents =
  with_timer_lwt ~options "Parsing" profiling (fun () ->
    (* always enable types when checking an individual file *)
    let types_mode = Parsing_service_js.TypesAllowed in
    let use_strict = Options.modules_are_use_strict options in
    let max_tokens = Options.max_header_tokens options in

    let docblock_errors, info =
      Parsing_service_js.parse_docblock ~max_tokens filename contents in
    let errors = Inference_utils.set_of_docblock_errors ~source_file:filename docblock_errors in
    let parse_result = Parsing_service_js.do_parse
      ~fail:check_syntax ~types_mode ~use_strict ~info
      contents filename
    in
    Lwt.return (errors, parse_result, info)
  )

(* commit providers for old and new modules, collect errors. *)
let commit_modules, commit_modules_from_saved_state =
  let commit_modules_generic ~introduce_files ~transaction ~all_providers_mutator ~options ~is_init
      ~profiling ~workers ~parsed ~parsed_set ~unparsed ~unparsed_set ~old_modules ~deleted
      ~local_errors ~new_or_changed =
    (* conservatively approximate set of modules whose providers will change *)
    (* register providers for modules, warn on dupes etc. *)
      with_timer_lwt ~options "CommitModules" profiling (fun () ->
        let all_files_set = FilenameSet.union (FilenameSet.union parsed_set unparsed_set) deleted in
        let mutator = Module_heaps.Introduce_files_mutator.create transaction all_files_set in
        let%lwt new_modules =
          introduce_files
            ~mutator ~all_providers_mutator ~workers ~options ~parsed ~unparsed
        in
        let dirty_modules = List.rev_append old_modules new_modules in
        let%lwt providers, changed_modules, errmap =
          Module_js.commit_modules ~transaction ~workers ~options ~is_init new_or_changed dirty_modules in
    (* Providers might be new but not changed. This typically happens when old
      providers are deleted, and previously duplicate providers become new
      providers. In such cases, we must clear the old duplicate provider errors
      for the new providers.

      (Note that this is unncessary when the providers are changed, because in
      that case they are rechecked and *all* their errors are cleared. But we
      don't care about optimizing that case for now.) *)
        let errors = List.fold_left filter_duplicate_provider local_errors providers in
        Lwt.return (
          changed_modules, FilenameMap.fold (fun file errors acc ->
            let errset = List.fold_left (fun acc err ->
              match err with
              | Module_js.ModuleDuplicateProviderError { module_name; provider; conflict; } ->
                let msg = Flow_error.(EDuplicateModuleProvider { module_name; provider; conflict }) in
                let error = Flow_error.error_of_msg ~trace_reasons:[] ~source_file:file msg in
                Errors.ErrorSet.add error acc
            ) Errors.ErrorSet.empty errors in
            update_errset acc file errset
          ) errmap errors
      )
    )
  in
  let commit_modules =
    commit_modules_generic ~introduce_files:Module_js.introduce_files
  in
  let commit_modules_from_saved_state =
    commit_modules_generic ~introduce_files:Module_js.introduce_files_from_saved_state
  in
  commit_modules, commit_modules_from_saved_state

let resolve_requires ~transaction ~options ~profiling ~workers ~parsed ~parsed_set =
  let node_modules_containers = !Files.node_modules_containers in
  let mutator = Module_heaps.Resolved_requires_mutator.create transaction parsed_set in
  with_timer_lwt ~options "ResolveRequires" profiling (fun () ->
    MultiWorkerLwt.call workers
      ~job: (List.fold_left (fun errors_acc filename ->
        let errors = Module_js.add_parsed_resolved_requires filename
          ~mutator ~options ~node_modules_containers in
        if Errors.ErrorSet.is_empty errors
        then errors_acc
        else FilenameMap.add filename errors errors_acc
      )
      )
      ~neutral: FilenameMap.empty
      ~merge: FilenameMap.union
      ~next:(MultiWorkerLwt.next workers parsed)
  )

let commit_modules_and_resolve_requires
  ~transaction
  ~all_providers_mutator
  ~options
  ~profiling
  ~workers
  ~old_modules
  ~parsed_set
  ~unparsed
  ~unparsed_set
  ~new_or_changed
  ~deleted
  ~errors
  ~is_init =
  (* TODO remove after lookup overhaul *)
  Module_js.clear_filename_cache ();

  let { ServerEnv.local_errors; merge_errors; suppressions; severity_cover_set } = errors in

  let parsed = FilenameSet.elements parsed_set in

  let%lwt changed_modules, local_errors = commit_modules
    ~transaction ~all_providers_mutator ~options ~is_init ~profiling ~workers ~parsed ~parsed_set
    ~unparsed ~unparsed_set ~old_modules ~deleted ~local_errors ~new_or_changed
  in

  let%lwt resolve_errors =
    resolve_requires ~transaction ~options ~profiling ~workers ~parsed ~parsed_set
  in
  let local_errors = FilenameMap.union resolve_errors local_errors in

  Lwt.return (
    changed_modules, { ServerEnv.local_errors; merge_errors; suppressions; severity_cover_set }
  )

let error_set_of_merge_error file msg =
  let error = Flow_error.error_of_msg ~trace_reasons:[] ~source_file:file msg in
  Errors.ErrorSet.singleton error

let calc_deps ~options ~profiling ~dependency_graph ~components to_merge =
  with_timer_lwt ~options "CalcDeps" profiling (fun () ->
    let dependency_graph = Dep_service.filter_dependency_graph dependency_graph to_merge in
    let components = List.filter (Nel.exists (fun f -> FilenameSet.mem f to_merge)) components in
    if Options.should_profile options then Sort_js.log components;
    let component_map = List.fold_left (fun component_map component ->
      let file = Nel.hd component in
      FilenameMap.add file component component_map
    ) FilenameMap.empty components in
    Lwt.return (dependency_graph, component_map)
  )

(* The infer_input passed in basically tells us what the caller wants to typecheck.
 * However, due to laziness, it's possible that certain dependents or dependencies have not been
 * checked yet. So we need to calculate all the transitive dependents and transitive dependencies
 * and add them to infer_input, unless they're already checked and in unchanged_checked
 *
 * Note that we do not want to add all_dependent_files to infer_input directly! We only want to
 * pass the dependencies, and later add dependent files as needed. This is important for recheck
 * optimizations. We create the recheck map which indicates whether a given file needs to be
 * rechecked. Dependent files only need to be rechecked if their dependencies change.
 *)
let include_dependencies_and_dependents
    ~options
    ~profiling
    ~unchanged_checked
    ~infer_input
    ~dependency_graph
    ~all_dependent_files
    ~direct_dependent_files =
  let%lwt infer_input, components = with_timer_lwt ~options "PruneDeps" profiling (fun () ->
    (* Don't just look up the dependencies of the focused or dependent modules. Also look up
     * the dependencies of dependencies, since we need to check transitive dependencies *)
    let preliminary_to_merge = CheckedSet.all
      (CheckedSet.add ~dependents:all_dependent_files infer_input) in
    (* So we want to prune our dependencies to only the dependencies which changed. However,
     * two dependencies A and B might be in a cycle. If A changed and B did not, we still need to
     * check both of them. So we need to calculate components before we can prune *)
    (* Grab the subgraph containing all our dependencies and sort it into the strongly connected
     * cycles *)
    let components = Sort_js.topsort ~roots:preliminary_to_merge dependency_graph in
    let dependencies = List.fold_left (fun dependencies component ->
      if Nel.exists (fun fn -> not (CheckedSet.mem fn unchanged_checked)) component
      (* If at least one member of the component is not unchanged, then keep the component *)
      then Nel.fold_left (fun acc fn -> FilenameSet.add fn acc) dependencies component
      (* If every element is unchanged, drop the component *)
      else dependencies
    ) FilenameSet.empty components in
    Lwt.return (CheckedSet.add ~dependencies infer_input, components)
  ) in

  (* NOTE: An important invariant here is that if we recompute Sort_js.topsort with infer_input +
     all_dependent_files (which is = to_merge later) on dependency_graph, we would get exactly the
     same components. Later, we will filter dependency_graph to just to_merge, and correspondingly
     filter components as well. This will work out because every component is either entirely inside
     to_merge or entirely outside. *)

  let to_merge = CheckedSet.add ~dependents:all_dependent_files infer_input in

  let recheck_map =
    let roots = CheckedSet.add ~dependents:direct_dependent_files infer_input in
    (* Definitely recheck inferred and direct_dependent_files. As merging proceeds, other
       files in to_merge may or may not be rechecked. *)
    CheckedSet.fold (fun recheck_map file ->
      FilenameMap.add file (CheckedSet.mem file roots) recheck_map
    ) FilenameMap.empty to_merge
  in

  Lwt.return (to_merge, components, recheck_map)

let run_merge_service
    ~master_mutator
    ~worker_mutator
    ~intermediate_result_callback
    ~options
    ~profiling
    ~workers
    dependency_graph
    component_map
    recheck_map
    acc
    =
  with_timer_lwt ~options "Merge" profiling (fun () ->
    let%lwt merged, skipped_count = Merge_service.merge_strict
      ~master_mutator ~worker_mutator ~intermediate_result_callback ~options ~workers
      dependency_graph component_map recheck_map
    in
    let errs, suppressions, severity_cover_set = List.fold_left (fun acc (file, result) ->
      let component = FilenameMap.find_unsafe file component_map in
      (* remove all errors, suppressions for rechecked component *)
      let errors, suppressions, severity_cover_set =
        Nel.fold_left (fun (errors, suppressions, severity_cover_set) file ->
          FilenameMap.remove file errors,
          Error_suppressions.remove file suppressions,
          FilenameMap.remove file severity_cover_set
        ) acc component
      in
      match result with
      | Ok (new_errors, new_suppressions, new_severity_cover) ->
        update_errset errors file new_errors,
        Error_suppressions.update_suppressions suppressions new_suppressions,
        FilenameMap.union new_severity_cover severity_cover_set
      | Error msg ->
        let new_errors = error_set_of_merge_error file msg in
        update_errset errors file new_errors, suppressions, severity_cover_set
    ) acc merged
    in
    Lwt.return (errs, suppressions, severity_cover_set, skipped_count)
  )

(* This function does some last minute preparation and then calls into the merge service, which
 * typechecks the code. By the time this function is called, we know exactly what we want to merge
 * (though we may later decline to typecheck some files due to recheck optimizations) *)
let merge
  ~transaction
  ~options
  ~profiling
  ~workers
  ~errors
  ~unchanged_checked
  ~to_merge
  ~components
  ~recheck_map
  ~dependency_graph
  ~deleted
  ~persistent_connections
  ~prep_merge =
  let { ServerEnv.local_errors; merge_errors; suppressions; severity_cover_set } = errors in


  let%lwt send_errors_over_connection =
    match persistent_connections with
    | None -> Lwt.return (fun _ -> ())
    | Some clients -> with_timer_lwt ~options "MakeSendErrors" profiling (fun () ->
      (* Each merge step uncovers new errors, warnings, suppressions and lint severity covers.

         While more suppressions and severity covers may come in later steps, the suppressions and
         severity covers we've seen so far are sufficient to filter the errors and warnings we've
         seen so far.

         Intuitively, we will not see an error (or warning) before we've seen all the files involved
         in that error, and thus all the suppressions which could possibly suppress the error. *)
      let open Errors in
      let curr_errors = ref ErrorSet.empty in
      let curr_warnings = ref ErrorSet.empty in
      let curr_suppressions = ref suppressions in
      let curr_severity_cover = ref severity_cover_set in
      let filter = Error_suppressions.filter_suppressed_errors in
      Lwt.return (function lazy results ->
        let new_errors, new_warnings, suppressions, severity_cover =
          List.fold_left (fun (errs_acc, warns_acc, supps_acc, lints_acc) result ->
            let file, errs_and_warns, supps, lints = result in
            let supps_acc = Error_suppressions.union supps_acc supps in
            let lints_acc = FilenameMap.union lints_acc lints in
            (* Filter errors and warnings based on suppressions we've seen so far. *)
            let errs, warns, _, _ = filter supps_acc lints_acc errs_and_warns
              ~unused:Error_suppressions.empty (* TODO: track unused suppressions *)
            in
            (* Only add errors we haven't seen before. *)
            let errs_acc = ErrorSet.fold (fun err acc ->
              if ErrorSet.mem err !curr_errors
              then acc
              else ErrorSet.add err acc
            ) errs errs_acc in
            (* Only add warnings we haven't seen before. Note that new warnings are stored by
               filename, because the clients only receive warnings for files they have open. *)
            let warns_acc =
              let acc = Option.value (FilenameMap.get file warns_acc) ~default:ErrorSet.empty in
              let acc = ErrorSet.fold (fun warn acc ->
                if ErrorSet.mem warn !curr_warnings
                then acc
                else ErrorSet.add warn acc
              ) warns acc in
              if ErrorSet.is_empty acc then warns_acc else FilenameMap.add file acc warns_acc
            in
            errs_acc, warns_acc, supps_acc, lints_acc
          ) (ErrorSet.empty, FilenameMap.empty, !curr_suppressions, !curr_severity_cover) results
        in

        curr_errors := ErrorSet.union new_errors !curr_errors;
        curr_warnings := FilenameMap.fold (fun _ -> ErrorSet.union) new_warnings !curr_warnings;
        curr_suppressions := suppressions;
        curr_severity_cover := severity_cover;

        if not (ErrorSet.is_empty new_errors && FilenameMap.is_empty new_warnings)
        then Persistent_connection.update_clients
          ~clients
          ~calc_errors_and_warnings:(fun () -> new_errors, new_warnings)
      ))
  in

  let%lwt () = match prep_merge with
    | None -> Lwt.return_unit
    | Some callback ->
      (* call supplied function to calculate closure of modules to merge *)
      with_timer_lwt ~options "MakeMergeInput" profiling (fun () ->
        Lwt.return (callback to_merge)
      )
  in

  (* to_merge is the union of inferred (newly inferred files) and the
     transitive closure of all dependents.

     recheck_map maps each file in to_merge to whether it should be rechecked
     initially.
  *)
  Hh_logger.info "to_merge: %s" (CheckedSet.debug_counts_to_string to_merge);
  Hh_logger.info "Calculating dependencies";
  MonitorRPC.status_update ~event:ServerStatus.Calculating_dependencies_progress;
  let files_to_merge = CheckedSet.all to_merge in
  let%lwt dependency_graph, component_map =
    calc_deps ~options ~profiling ~dependency_graph ~components files_to_merge in

  Hh_logger.info "Merging";
  let%lwt merge_errors, suppressions, severity_cover_set, skipped_count =
    let intermediate_result_callback results =
      let errors = lazy (
        List.map (fun (file, result) ->
          match result with
          | Ok (errors, suppressions, severity_cover) ->
            file, errors, suppressions, severity_cover
          | Error msg ->
            let errors = error_set_of_merge_error file msg in
            let suppressions = Error_suppressions.empty in
            let severity_cover =
              Utils_js.FilenameMap.singleton
                file
                (ExactCover.file_cover file (Options.lint_severities options))
            in
            file, errors, suppressions, severity_cover
        ) (Lazy.force results)
      ) in
      send_errors_over_connection errors
    in

    let master_mutator, worker_mutator =
      Context_heaps.Merge_context_mutator.create
        transaction (FilenameSet.union files_to_merge deleted)
    in

    let%lwt merge_errors, suppressions, severity_cover_set, skipped_count =
      run_merge_service
        ~master_mutator
        ~worker_mutator
        ~intermediate_result_callback
        ~options
        ~profiling
        ~workers
        dependency_graph
        component_map
        recheck_map
        (merge_errors, suppressions, severity_cover_set)
    in
    let%lwt () =
      if Options.should_profile options
      then with_timer_lwt ~options "PrintGCStats" profiling (fun () ->
        Lwt.return (Gc.print_stat stderr)
      )
      else Lwt.return_unit
    in
    Hh_logger.info "Done";
    Lwt.return (merge_errors, suppressions, severity_cover_set, skipped_count)
  in

  let checked = CheckedSet.union unchanged_checked to_merge in
  Hh_logger.info "Checked set: %s" (CheckedSet.debug_counts_to_string checked);

  let errors = { ServerEnv.local_errors; merge_errors; suppressions; severity_cover_set } in
  let cycle_leaders = component_map
    |> Utils_js.FilenameMap.elements
    |> List.map (fun (leader, members) -> (leader, Nel.length members))
    |> List.filter (fun (_, member_count) -> member_count > 1) in
  Lwt.return (checked, cycle_leaders, errors, skipped_count)

let ensure_parsed ~options ~profiling ~workers files =
  with_timer_lwt ~options "EnsureParsed" profiling (fun () ->
    let%lwt parse_hash_mismatch_skips =
      Parsing_service_js.ensure_parsed options workers (CheckedSet.all files)
    in

    if FilenameSet.is_empty parse_hash_mismatch_skips
    then Lwt.return_unit
    else begin
      let files_to_recheck = FilenameSet.fold
        (fun f acc -> SSet.add (File_key.to_string f) acc)
        parse_hash_mismatch_skips
        SSet.empty
      in
      ServerMonitorListenerState.push_files_to_recheck files_to_recheck;
      raise Lwt.Canceled
    end
  )

(* When checking contents, ensure that dependencies are checked. Might have more
   general utility.
   TODO(ljw) CARE! This function calls "typecheck" which may emit errors over the
   persistent connection. It's reasonable that anything which checks new files
   should be able to emit errors, even places like propertyFindRefs.get_def_info
   that invoke this function. But it looks like this codepath fails to emit
   StartRecheck and EndRecheck messages. *)
let ensure_checked_dependencies ~options ~profiling ~workers ~env file file_sig =
  let resolved_requires =
    let require_loc_map = File_sig.(require_loc_map file_sig.module_sig) in
    SMap.fold (fun r locs resolved_rs ->
      let resolved_r = Module_js.imported_module
        ~options
        ~node_modules_containers:!Files.node_modules_containers
        file locs r in
      Modulename.Set.add resolved_r resolved_rs
    ) require_loc_map Modulename.Set.empty
  in

  let infer_input = Modulename.Set.fold (fun m acc ->
    match Module_heaps.get_file m ~audit:Expensive.warn with
    | Some f ->
      if FilenameSet.mem f !env.ServerEnv.files && Module_js.checked_file f ~audit:Expensive.warn
      then CheckedSet.add ~dependencies:(FilenameSet.singleton f) acc
      else acc
    | None -> acc (* complain elsewhere about required module not found *)
  ) resolved_requires CheckedSet.empty in
  let unchanged_checked = !env.ServerEnv.checked_files in

  (* Often, all dependencies have already been checked, so infer_input contains no unchecked files.
   * In that case, let's short-circuit typecheck, since a no-op typecheck still takes time on
   * large repos *)
  if CheckedSet.is_empty (CheckedSet.diff infer_input unchanged_checked)
  then Lwt.return_unit
  else begin
    let errors = !env.ServerEnv.errors in
    let all_dependent_files = FilenameSet.empty in
    let direct_dependent_files = FilenameSet.empty in
    let persistent_connections = Some (!env.ServerEnv.connections) in
    let dependency_graph = !env.ServerEnv.dependency_graph in
    let deleted = FilenameSet.empty in

    let%lwt to_merge, components, recheck_map =
      include_dependencies_and_dependents
        ~options ~profiling ~unchanged_checked ~infer_input ~dependency_graph ~all_dependent_files
        ~direct_dependent_files
    in

    let%lwt () = ensure_parsed ~options ~profiling ~workers to_merge in

    let%lwt checked, _cycle_leaders, errors, _skipped_count = Transaction.with_transaction (fun transaction ->
      merge
        ~transaction ~options ~profiling ~workers ~errors
        ~unchanged_checked ~to_merge ~components ~recheck_map
        ~dependency_graph ~deleted
        ~persistent_connections
        ~prep_merge:None
    ) in

    (* During a normal initialization or recheck, we update the env with the errors and
     * calculate the collated errors. However, this code is for when the server is in lazy mode,
     * is trying to using typecheck_contents, and is making sure the dependencies are checked. Since
     * we're messing with env.errors, we also need to set collated_errors to None. This will force
     * us to recompute them the next time someone needs them *)
    !env.ServerEnv.collated_errors := None;
    env := { !env with ServerEnv.
      checked_files = checked;
      errors;
    };
    Lwt.return_unit
  end

(* Another special case, similar assumptions as above. *)
(** TODO: handle case when file+contents don't agree with file system state **)
let typecheck_contents_ ~options ~workers ~env ~check_syntax ~profiling contents filename =
  let%lwt errors, parse_result, info =
    parse_contents ~options ~profiling ~check_syntax filename contents in

  match parse_result with
  | Parsing_service_js.Parse_ok (ast, file_sig) ->
      (* override docblock info *)
      let info = Docblock.set_flow_mode_for_ide_command info in

      (* merge *)
      let%lwt cx, typed_ast = with_timer_lwt ~options "MergeContents" profiling (fun () ->
        let%lwt () =
          ensure_checked_dependencies ~options ~profiling ~workers ~env filename file_sig
        in
        Lwt.return @@ Merge_service.merge_contents_context options filename ast info file_sig
      ) in

      let errors = Context.errors cx in
      let errors =
        if options.Options.opt_enforce_well_formed_exports then
          Inference_utils.set_of_file_sig_tolerable_errors
            ~source_file:filename
            file_sig.File_sig.tolerable_errors
          |> Errors.ErrorSet.union errors
        else
          errors
      in

      (* Suppressions for errors in this file can come from dependencies *)
      let suppressions =
        let open ServerEnv in
        let new_suppressions = Context.error_suppressions cx in
        let { suppressions; _ } = !env.errors in
        Error_suppressions.update_suppressions suppressions new_suppressions
      in

      (* Severity cover info can come from dependencies *)
      let severity_cover =
        let open ServerEnv in
        let file_severity_cover = Context.severity_cover cx in
        let { severity_cover_set; _ } = !env.errors in
        FilenameMap.union file_severity_cover severity_cover_set
      in

      (* Filter out suppressed errors *)
      let errors, warnings, _, _ =
        Error_suppressions.filter_suppressed_errors suppressions severity_cover errors
          ~unused:Error_suppressions.empty (* TODO: track unused suppressions *)
      in

      let warnings = if Options.should_include_warnings options
        then warnings
        else Errors.ErrorSet.empty
      in

      Lwt.return (Some (cx, ast, file_sig, typed_ast), errors, warnings, info)

  | Parsing_service_js.Parse_fail fails ->
      let errors = match fails with
      | Parsing_service_js.Parse_error err ->
          let err = Inference_utils.error_of_parse_error ~source_file:filename err in
          Errors.ErrorSet.add err errors
      | Parsing_service_js.Docblock_errors errs ->
          List.fold_left (fun errors err ->
            let err = Inference_utils.error_of_docblock_error ~source_file:filename err in
            Errors.ErrorSet.add err errors
          ) errors errs
      | Parsing_service_js.File_sig_error err ->
          let err = Inference_utils.error_of_file_sig_error ~source_file:filename err in
          Errors.ErrorSet.add err errors
      in
      Lwt.return (None, errors, Errors.ErrorSet.empty, info)

  | Parsing_service_js.Parse_skip
     (Parsing_service_js.Skip_non_flow_file
    | Parsing_service_js.Skip_resource_file) ->
      (* should never happen *)
      Lwt.return (None, errors, Errors.ErrorSet.empty, info)

let typecheck_contents ~options ~workers ~env ~profiling contents filename =
  let%lwt cx_opt, errors, warnings, _info =
    typecheck_contents_ ~options ~workers ~env ~check_syntax:true ~profiling contents filename in
  Lwt.return (cx_opt, errors, warnings)

let basic_check_contents ~options ~workers ~env ~profiling contents filename =
  try%lwt
    let%lwt cx_opt, _errors, _warnings, info =
      typecheck_contents_
        ~options ~workers ~env ~check_syntax:false ~profiling contents filename in
    let cx, file_sig, typed_ast = match cx_opt with
      | Some (cx, _, file_sig, typed_ast) -> cx, file_sig, typed_ast
      | None -> failwith "Couldn't parse file" in
    Lwt.return (Ok (cx, info, file_sig, typed_ast))
  with
  | Lwt.Canceled as exn -> raise exn
  | exn ->
    Hh_logger.error ~exn "Uncaught exception in basic_check_contents";
    let e = spf "%s\n%s"
      (Printexc.to_string exn)
      (Printexc.get_backtrace ()) in
    Lwt.return (Error e)

let init_package_heap ~options ~profiling parsed =
  with_timer_lwt ~options "PackageHeap" profiling (fun () ->
    FilenameSet.iter (fun filename ->
      match filename with
      | File_key.JsonFile str when Filename.basename str = "package.json" ->
        let ast = Parsing_heaps.get_ast_unsafe filename in
        Module_js.add_package str ast
      | _ -> ()
    ) parsed;
    Lwt.return_unit
  )

let init_libs ~options ~profiling ~local_errors ~suppressions ~severity_cover_set ordered_libs =
  with_timer_lwt ~options "InitLibs" profiling (fun () ->
    let%lwt lib_files =
      let options = match Options.verbose options with
        | Some { Verbose.enabled_during_flowlib = false; _; } ->
          (* Normally we disable verbosity while loading the libs. But if we're running with
           * --verbose-flowlib then we want to leave verbosity on *)
          { options with Options.opt_verbose = None; }
        | _ -> options
      in
      Init_js.init ~options ordered_libs
    in

    Lwt.return @@ List.fold_left (fun acc (lib_file, ok, errs, suppressions, severity_cover) ->
      let all_ok, errors_acc, suppressions_acc, severity_cover_set_acc = acc in
      let all_ok = if ok then all_ok else false in
      let errors_acc = update_errset errors_acc lib_file errs in
      let suppressions_acc =
        Error_suppressions.update_suppressions suppressions_acc suppressions in
      let severity_cover_set_acc = FilenameMap.union severity_cover severity_cover_set_acc in
      all_ok, errors_acc, suppressions_acc, severity_cover_set_acc
    ) (true, local_errors, suppressions, severity_cover_set) lib_files
  )

(* Given a set of focused files and a set of parsed files, calculate all the dependents and
 * dependencies and return them as a CheckedSet
 *
 * This is pretty darn expensive for large repos (on the order of a few seconds). What is taking
 * all that time?
 *
 * - Around 75% of the time is dependent_files looking up the dependents
 * - Around 20% of the time is calc_dependency_graph building the dependency graph
 **)
let focused_files_to_infer ~focused ~dependency_graph =
  let focused = focused |> FilenameSet.filter (fun f ->
    Module_heaps.is_tracked_file f (* otherwise, f is probably a directory *)
    && Module_js.checked_file ~audit:Expensive.warn f)
  in

  let roots = Dep_service.calc_all_reverse_dependencies dependency_graph focused in

  let dependencies = Dep_service.calc_all_dependencies dependency_graph roots in
  let dependents = FilenameSet.diff roots focused in

  Lwt.return (CheckedSet.add ~focused ~dependents ~dependencies CheckedSet.empty)

let filter_out_node_modules ~options =
  let root = Options.root options in
  let file_options = Options.file_options options in
  FilenameSet.filter (fun fn ->
    let filename_str = File_key.to_string fn in
    not (Files.is_within_node_modules ~root ~options:file_options filename_str)
  )

(* Without a set of focused files, we just focus on every parsed file. We won't focus on
 * node_modules. If a node module is a dependency, then we'll just treat it as a dependency.
 * Otherwise, we'll ignore it. *)
let unfocused_files_to_infer ~options ~parsed ~dependency_graph =
  (* All the non-node_modules files *)
  let focused = filter_out_node_modules ~options parsed in

  (* Calculate dependencies to figure out which node_modules stuff we depend on *)
  let dependencies = Dep_service.calc_all_dependencies dependency_graph focused in
  Lwt.return (CheckedSet.add ~focused ~dependencies CheckedSet.empty)

let files_to_infer ~options ~focused ~profiling ~parsed ~dependency_graph =
  with_timer_lwt ~options "FilesToInfer" profiling (fun () ->
    match focused with
    | None ->
      unfocused_files_to_infer ~options ~parsed ~dependency_graph
    | Some focused ->
      focused_files_to_infer ~focused ~dependency_graph
  )

(* We maintain the following invariant across rechecks: The set of
   `files` contains files that parsed successfully in the previous
   phase (which could be the init phase or a previous recheck phase)
*)
let recheck_with_profiling
    ~profiling ~transaction ~options ~workers ~updates env ~files_to_focus =
  let errors = env.ServerEnv.errors in

  (* If foo.js is updated and foo.js.flow exists, then mark foo.js.flow as
   * updated too. This is because sometimes we decide what foo.js.flow
   * provides based on the existence of foo.js *)
  let updates = FilenameSet.fold (fun file updates ->
    if not (File_key.check_suffix file Files.flow_ext) &&
      Parsing_heaps.has_ast (File_key.with_suffix file Files.flow_ext)
    then FilenameSet.add (File_key.with_suffix file Files.flow_ext) updates
    else updates
  ) updates updates in

  (* split updates into deleted files and modified files *)
  (** NOTE: We use the term "modified" in the same sense as the underlying file
      system: a modified file exists, and in relation to an old file system
      state, a modified file could be any of "new," "changed," or "unchanged."
  **)
  let modified, deleted = FilenameSet.partition (fun f ->
    Sys.file_exists (File_key.to_string f)
  ) updates in
  let deleted_count = FilenameSet.cardinal deleted in
  let modified_count = FilenameSet.cardinal modified in

  (* log modified and deleted files *)
  if deleted_count + modified_count > 0 then (
    Hh_logger.info "recheck %d modified, %d deleted files"
      modified_count deleted_count;
    let log_files files msg n =
      Hh_logger.info "%s files:" msg;
      let _ = FilenameSet.fold (fun f i ->
        Hh_logger.info "%d/%d: %s" i n (File_key.to_string f);
        i + 1
      ) files 1
      in ()
    in
    if modified_count > 0 then log_files modified "modified" modified_count;
    if deleted_count > 0 then log_files deleted "deleted" deleted_count
  );

  (* We don't need to delete things from the parsing heaps - they will be automatically oldified.
   * Oldifying something removes it from the heap (but keeps it around in case we need it back) *)

  Hh_logger.info "Parsing";
  (* reparse modified files, updating modified to new_or_changed to reflect
     removal of unchanged files *)
  let%lwt new_or_changed, freshparsed, unparsed, unchanged_parse, new_local_errors =
     reparse ~options ~profiling ~transaction ~workers ~modified ~deleted in

  let%lwt new_or_changed, freshparsed =
    if not (FilenameSet.is_empty files_to_focus) then begin
      (* Normally we can ignore files which are unmodified. However, if someone passed force_focus,
       * then we may need to ressurect some unmodified files into the new_or_changed and freshparsed
       * sets. For example, if someone ran `flow force-recheck --focus a.js` and a.js is not
       * focused, then we need to recheck it even if the file is unchanged
       *
       * We avoid rechecking already-focused unmodified files since they're already focused and
       * haven't changed :P *)
      let unchanged_files_to_focus =
        FilenameSet.diff
          (FilenameSet.inter files_to_focus unchanged_parse) (* unchanged files to focus... *)
          (CheckedSet.focused env.ServerEnv.checked_files)   (* ...which aren't already focused *)
        |> FilenameSet.filter (fun f -> FilenameSet.mem f env.ServerEnv.files) (* Only parsed *)
      in
      let%lwt () = ensure_parsed ~options ~profiling ~workers
        (CheckedSet.add ~focused:unchanged_files_to_focus CheckedSet.empty)
      in
      Lwt.return (
        FilenameSet.union new_or_changed unchanged_files_to_focus,
        FilenameSet.union freshparsed unchanged_files_to_focus
      )
    end else Lwt.return (new_or_changed, freshparsed)
  in

  let unparsed_set =
    List.fold_left (fun set (fn, _) -> FilenameSet.add fn set) FilenameSet.empty unparsed
  in

  (* clear errors for new, changed and deleted files *)
  let errors =
    errors
    |> clear_errors new_or_changed
    |> clear_errors deleted
  in

  (* record reparse errors *)
  let errors =
    let () =
      let error_set: Errors.ErrorSet.t =
        FilenameMap.fold (fun _ -> Errors.ErrorSet.union) new_local_errors Errors.ErrorSet.empty
      in
      if Errors.ErrorSet.cardinal error_set > 0
      then Persistent_connection.update_clients
        ~clients:env.ServerEnv.connections
        ~calc_errors_and_warnings:(fun () -> error_set, FilenameMap.empty)
    in
    let local_errors = merge_error_maps new_local_errors errors.ServerEnv.local_errors in
    { errors with ServerEnv.local_errors }
  in

  (* get old (unchanged, undeleted) files that were parsed successfully *)
  let old_parsed = env.ServerEnv.files in
  let new_or_changed_or_deleted = FilenameSet.union new_or_changed deleted in
  let unchanged = FilenameSet.diff old_parsed new_or_changed_or_deleted in

  Hh_logger.debug
    "recheck: old = %d, del = %d, fresh = %d, unmod = %d"
    (FilenameSet.cardinal old_parsed)
    (FilenameSet.cardinal deleted)
    (FilenameSet.cardinal freshparsed)
    (FilenameSet.cardinal unchanged);

  (** Here's where the interesting part of rechecking begins. Before diving into
      code, let's think through the problem independently.

      Note that changing a file can be conceptually thought of as deleting the
      file and then adding it back as a new file. While such a reduction might
      miss optimization opportunities (so we don't actually implement it), it
      simplifies thinking about correctness.

      We focus on dependency management. Specifically, we discuss how to
      correctly update InfoHeap and NameHeap, and calculate the set of unchanged
      files whose imports might resolve to different files. (With these results,
      the remaining part of rechecking is relatively simple.)

      Recall that InfoHeap maps file names in FS to module names in MS, where
      each file name in FS must exist, different file names may map to the same
      module name, and every module name in MS is mapped to by at least one file
      name; and NameHeap maps module names in MS to file names in FS, where the
      file name mapped to by a module name must map back to the same module name
      in InfoHeap. A file's imports might resolve to different files if the
      corresponding modules map to different files in NameHeap.

      Deleting a file
      ===============

      Suppose that a file D is deleted. Let D |-> m in InfoHeap, and m |-> F in
      NameHeap.

      Remove D |-> m from InfoHeap.

      If F = D, then remove m |-> F from NameHeap and mark m "dirty": any file
      importing m will be affected. If other files map to m in InfoHeap, map m
      to one of those files in NameHeap.

      Adding a file
      =============

      Suppose that a new file N is added.

      Map N to some module name, say m, in InfoHeap. If m is not mapped to any
      file in NameHeap, add m |-> N to NameHeap and mark m "dirty." Otherwise,
      decide whether to replace the existing mapping to m |-> N in NameHeap, and
      pessimistically assuming it might be, mark m "dirty."

      Changing a file
      =============

      What happens when a file C is changed? Suppose that C |-> m in InfoHeap,
      and m |-> F in NameHeap.

      Optimistically, C continues to map to m in InfoHeap and we do nothing.

      However, let's pessimistically assume that C maps to a different m' in
      InfoHeap. Considering C deleted and added back as new, we must remove C
      |-> m from InfoHeap and add C |-> m' to InfoHeap. If F = C, then remove m
      |-> F from NameHeap and mark m "dirty." If other files map to m in
      InfoHeap, map m to one of those files in NameHeap. If m' is not mapped to
      any file in NameHeap, add m' |-> C to NameHeap and mark m' "dirty."
      Otherwise, decide whether to replace the existing mapping to m' |-> C in
      NameHeap, and mark m' "dirty."

      Summary
      =======

      Summarizing, if an existing file F1 is changed or deleted, and F1 |-> m in
      InfoHeap and m |-> F in NameHeap, and F1 = F, then mark m "dirty." And if
      a new file or a changed file F2 now maps to m' in InfoHeap, mark m' "dirty."

      Ideally, any module name that does not map to a different file in NameHeap
      should not be considered "dirty."

      In terms of implementation:

      Deleted file
      ============

      Say it pointed to module OLD_M

      1. need to repick a provider for OLD_M *if OLD_M's current provider is this
      file*
      2. files that depend on OLD_M need to be rechecked if:
        a. the provider for OLD_M is **replaced** or **removed**; or
        b. the provider for OLD_M is **unchanged**, but is a _changed file_

      New file
      ========

      Say it points to module NEW_M

      1. need to repick a provider for NEW_M
      2. files that depend on NEW_M need to be rechecked if:
        a. the provider for NEW_M is **added** or **replaced**; or
        b. the provider for NEW_M is **unchanged**, but is a _changed file_

      Changed file
      ============

      Say it pointed to module OLD_M, now points to module NEW_M

      * Is OLD_M different from NEW_M? *(= delete the file, then add it back)*

      1. need to repick providers for OLD_M *if OLD_M's current provider is this
      file*.
      2. files that depend on OLD_M need to be rechecked if:
        a. the provider for OLD_M is **replaced** or **removed**; or
        b. the provider for OLD_M is **unchanged**, but is a _changed file_
      3. need to repick a provider for NEW_M
      4. files that depend on NEW_M need to be rechecked if:
        a. the provider for NEW_M is **added** or **replaced**; or
        b. the provider for NEW_M is **unchanged**, but is a _changed file_

      * TODO: Is OLD_M the same as NEW_M?

      1. *don't repick a provider!*
      2. files that depend on OLD_M need to be rechecked if: OLD_M's current provider
      is a _changed file_

  **)

  (* remember old modules *)
  let unchanged_checked = CheckedSet.remove new_or_changed_or_deleted env.ServerEnv.checked_files in

  let all_providers_mutator = Module_hashtables.All_providers_mutator.create transaction in

  (* clear out records of files, and names of modules provided by those files *)
  let%lwt old_modules = with_timer_lwt ~options "ModuleClearFiles" profiling (fun () ->
    Module_js.calc_old_modules workers ~all_providers_mutator ~options new_or_changed_or_deleted
  ) in

  MonitorRPC.status_update ServerStatus.Resolving_dependencies_progress;
  let%lwt changed_modules, errors =
    commit_modules_and_resolve_requires
      ~transaction
      ~all_providers_mutator
      ~options
      ~profiling
      ~workers
      ~old_modules
      ~parsed_set:freshparsed
      ~unparsed
      ~unparsed_set
      ~new_or_changed
      ~deleted
      ~errors
      ~is_init:false in

  let parsed = FilenameSet.union freshparsed unchanged in

  (* direct_dependent_files are unchanged files which directly depend on changed modules,
     or are new / changed files that are phantom dependents. dependent_files are
     direct_dependent_files plus their dependents (transitive closure) *)
  let%lwt all_dependent_files, direct_dependent_files =
    with_timer_lwt ~options "DependentFiles" profiling (fun () ->
      Dep_service.dependent_files
        workers
        ~unchanged
        ~new_or_changed
        ~changed_modules
    ) in

  Hh_logger.info "Re-resolving directly dependent files";

  let node_modules_containers = !Files.node_modules_containers in
  (* requires in direct_dependent_files must be re-resolved before merging. *)
  let mutator = Module_heaps.Resolved_requires_mutator.create transaction direct_dependent_files in
  let%lwt () = with_timer_lwt ~options "ReresolveDirectDependents" profiling (fun () ->
    MultiWorkerLwt.call workers
      ~job: (fun () files ->
        List.iter (fun filename ->
          let errors = Module_js.add_parsed_resolved_requires filename
            ~mutator ~options ~node_modules_containers in
          ignore errors (* TODO: why, FFS, why? *)
        ) files
      )
      ~neutral: ()
      ~merge: (fun () () -> ())
      ~next:(MultiWorkerLwt.next workers (FilenameSet.elements direct_dependent_files))
  ) in

  Hh_logger.info "Recalculating dependency graph";
  let%lwt dependency_graph = with_timer_lwt ~options "CalcDepsTypecheck" profiling (fun () ->
    let%lwt updated_dependency_graph = Dep_service.calc_partial_dependency_graph workers
      (FilenameSet.union freshparsed direct_dependent_files) ~parsed in
    let old_dependency_graph = env.ServerEnv.dependency_graph in
    old_dependency_graph
    |> FilenameSet.fold FilenameMap.remove deleted
    |> FilenameMap.union updated_dependency_graph
    |> Lwt.return
  ) in
  let%lwt updated_checked_files, all_dependent_files =
    with_timer_lwt ~options "RecalcDepGraph" profiling (fun () ->
      match Options.lazy_mode options with
      | None (* Non lazy mode treats every file as focused. *)
      | Some Options.LAZY_MODE_FILESYSTEM -> (* FS mode treats every modified file as focused *)
        let old_focus_targets = CheckedSet.focused env.ServerEnv.checked_files in
        let old_focus_targets = FilenameSet.diff old_focus_targets deleted in
        let old_focus_targets = FilenameSet.diff old_focus_targets unparsed_set in
        let parsed = FilenameSet.union old_focus_targets freshparsed in
        let%lwt updated_checked_files = unfocused_files_to_infer
          ~options ~parsed ~dependency_graph in
        Lwt.return (updated_checked_files, all_dependent_files)
      | Some Options.LAZY_MODE_IDE -> (* IDE mode only treats opened files as focused *)
        (* Unfortunately, our checked_files set might be out of date. This update could have added
         * some new dependents or dependencies. So we need to recalculate those.
         *
         * To calculate dependents and dependencies, we need to know what are the focused files. We
         * define the focused files to be the union of
         *
         *   1. The files that were previously focused
         *   2. Modified files that are currently open in the IDE
         *   3. If this is a `flow force-recheck --focus A.js B.js C.js`, then A.js, B.js and C.js
         *
         * Remember that the IDE might open a new file or keep open a deleted file, so the focused
         * set might be missing that file. If that file reappears, we must remember to refocus on
         * it.
         **)
        let old_focused = CheckedSet.focused env.ServerEnv.checked_files in
        let new_focused =
          filter_out_node_modules ~options (FilenameSet.inter files_to_focus freshparsed)
        in

        let open_in_ide =
          let opened_files = Persistent_connection.get_opened_files env.ServerEnv.connections in
          FilenameSet.filter (function
            | File_key.SourceFile fn
            | File_key.LibFile fn
            | File_key.JsonFile fn
            | File_key.ResourceFile fn -> SSet.mem fn opened_files
            | File_key.Builtins -> false
          ) freshparsed
        in
        let focused = old_focused
          |> FilenameSet.union open_in_ide
          |> FilenameSet.union new_focused in
        let%lwt updated_checked_files = focused_files_to_infer ~focused ~dependency_graph in

        (* It's possible that all_dependent_files contains foo.js, which is a dependent of a
         * dependency. That's fine if foo.js is in the checked set. But if it's just some random
         * other dependent then we need to filter it out.
         *)
        let all_dependent_files =
          FilenameSet.inter all_dependent_files (CheckedSet.all updated_checked_files) in
        Lwt.return (updated_checked_files, all_dependent_files)
    )
  in

  let infer_input =
    CheckedSet.filter ~f:(fun fn -> FilenameSet.mem fn freshparsed) updated_checked_files in

  let%lwt to_merge, components, recheck_map =
    include_dependencies_and_dependents
      ~options ~profiling ~unchanged_checked ~infer_input ~dependency_graph ~all_dependent_files
      ~direct_dependent_files
  in

  let%lwt () = ensure_parsed ~options ~profiling ~workers to_merge in

  (* recheck *)
  let%lwt checked, cycle_leaders, errors, skipped_count = merge
    ~transaction
    ~options
    ~profiling
    ~workers
    ~errors
    ~unchanged_checked
    ~to_merge
    ~components
    ~recheck_map
    ~dependency_graph
    ~deleted
    ~persistent_connections:(Some env.ServerEnv.connections)
    ~prep_merge:(Some (fun _to_merge ->
      (* need to merge the closure of inferred files and their deps *)

      let n = FilenameSet.cardinal all_dependent_files in
      if n > 0
      then Hh_logger.info "remerge %d dependent files:" n;

      let _ = FilenameSet.fold (fun f i ->
        Hh_logger.info "%d/%d: %s" i n (File_key.to_string f);
        i + 1
      ) all_dependent_files 1 in
      Hh_logger.info "Merge prep";

      (* merge errors for unchanged dependents will be cleared lazily *)

      (* to_merge is inferred files plus all dependents. prep for re-merge *)
      (* NOTE: Non-@flow files don't have entries in ResolvedRequiresHeap, so
         don't add then to the set of files to merge! Only inferred files (along
         with dependents) should be merged: see below. *)
      (* let _to_merge = CheckedSet.add ~dependents:all_dependent_files inferred in *)
      ()
    ))
  in

  (* Here's how to update unparsed:
   * 1. Remove the parsed files. This removes any file which used to be unparsed but is now parsed
   * 2. Remove the deleted files. This removes any previously unparsed file which was deleted
   * 3. Add the newly unparsed files. This adds new unparsed files or files which became unparsed *)
  let unparsed =
    let to_remove = FilenameSet.union parsed deleted in
    FilenameSet.diff env.ServerEnv.unparsed to_remove
    |> FilenameSet.union unparsed_set
  in

  (* NOTE: unused fields are left in their initial empty state *)
  env.ServerEnv.collated_errors := None;
  Lwt.return (
    ({ env with ServerEnv.
      files = parsed;
      unparsed;
      dependency_graph;
      checked_files = checked;
      errors;
    },
    (new_or_changed, deleted, all_dependent_files, cycle_leaders, skipped_count))
  )

let recheck ~options ~workers ~updates env ~files_to_focus =
  let should_print_summary = Options.should_profile options in
  let%lwt profiling, (env, (modified, deleted, dependent_files, cycle_leaders, skipped_count)) =
    Profiling_js.with_profiling_lwt ~label:"Recheck" ~should_print_summary (fun profiling ->
      SharedMem_js.with_memory_profiling_lwt ~profiling ~collect_at_end:true (fun () ->
        Transaction.with_transaction (fun transaction ->
          recheck_with_profiling
            ~profiling ~transaction ~options ~workers ~updates env ~files_to_focus
        )
      )
    )
  in
  (** TODO: update log to reflect current terminology **)
  FlowEventLogger.recheck ~modified ~deleted ~dependent_files ~profiling ~skipped_count;

  let duration = Profiling_js.get_profiling_duration profiling in
  let dependent_file_count = Utils_js.FilenameSet.cardinal dependent_files in
  let changed_file_count = (Utils_js.FilenameSet.cardinal modified)
    + (Utils_js.FilenameSet.cardinal deleted) in
  let top_cycle = Core_list.fold cycle_leaders ~init:None ~f:(fun top (f2, count2) ->
    match top with
    | Some (f1, count1) -> if f2 > f1 then Some (f2, count2) else Some (f1, count1)
    | None -> Some (f2, count2)) in
  let summary = ServerStatus.({
      duration;
      info = RecheckSummary {dependent_file_count; changed_file_count; top_cycle}; }) in
  Lwt.return (profiling, summary, env)

(* creates a closure that lists all files in the given root, returned in chunks *)
let make_next_files ~libs ~file_options root =
  let make_next_raw =
    Files.make_next_files ~root ~all:false ~subdir:None ~options:file_options ~libs in
  let total = ref 0 in
  fun () ->
    let files = make_next_raw () in

    let finished = !total in
    let length = List.length files in
    MonitorRPC.status_update ServerStatus.(Parsing_progress {
        finished;
        total = None;
    });
    total := finished + length;

    files
    |> List.map (Files.filename_from_string ~options:file_options)
    |> Bucket.of_list

let init_from_saved_state ~profiling ~workers ~saved_state options =
  Transaction.with_transaction @@ fun transaction ->

  let file_options = Options.file_options options in
  (* We don't want to walk the file system for the checked in files. But we still need to find the
   * flowlibs *)
  let ordered_flowlib_libs, _ = Files.init ~flowlibs_only:true file_options in

  let { Saved_state.
    flowconfig_hash=_;
    parsed_heaps;
    unparsed_heaps;
    ordered_non_flowlib_libs;
    local_errors;
    node_modules_containers;
  } = saved_state in

  Files.node_modules_containers := node_modules_containers;

  Hh_logger.info "Restoring heaps";
  let%lwt () = with_timer_lwt ~options "RestoreHeaps" profiling (fun () ->
    let%lwt () = MultiWorkerLwt.call workers
      ~job:(List.fold_left (fun () (fn, parsed_file_data) ->
        (* Every package.json file should have a Package_json.t. Use those to restore the
         * PackageHeap and the ReversePackageHeap *)
        begin match fn with
        | File_key.JsonFile str when Filename.basename str = "package.json" ->
          begin match parsed_file_data.Saved_state.package with
          | None -> failwith (Printf.sprintf "Saved state for `%s` missing Package_json.t data" str)
          | Some package -> Module_heaps.Package_heap_mutator.add_package_json str package
          end
        | _ -> ()
        end;

        (* Restore the FileSigHeap *)
        Parsing_heaps.From_saved_state.add_file_sig fn parsed_file_data.Saved_state.file_sig;

        (* Restore the FileHashHeap *)
        Parsing_heaps.From_saved_state.add_file_hash fn parsed_file_data.Saved_state.hash;

        (* Restore the ResolvedRequiresHeap *)
        Module_heaps.From_saved_state.add_resolved_requires
          fn parsed_file_data.Saved_state.resolved_requires
      ))
      ~merge:(fun () () -> ())
      ~neutral:()
      ~next:(MultiWorkerLwt.next workers (FilenameMap.bindings parsed_heaps))
    in

    MultiWorkerLwt.call workers
      ~job:(List.fold_left (fun () (fn, unparsed_file_data) ->
        (* Restore the FileHashHeap *)
        let hash = unparsed_file_data.Saved_state.unparsed_hash in
        Parsing_heaps.From_saved_state.add_file_hash fn hash;
      ))
      ~merge:(fun () () -> ())
      ~neutral:()
      ~next:(MultiWorkerLwt.next workers (FilenameMap.bindings unparsed_heaps))
  ) in

  Hh_logger.info "Loading libraries";
  (* We actually parse and typecheck the libraries, even though we're loading from saved state.
   * We'd need to check them anyway, as soon as any file is checked, since we don't track
   * dependents for libraries. And we don't really support incrementally checking libraries
   *
   * The order of libraries is significant. If two libraries define the same thing, the one
   * merged later wins. For this reason, the saved state stores the order in which the non-flowlib
   * libraries were merged. So all we need to guarantee here is:
   *
   * 1. The builtin libraries are merged first
   * 2. The non-builtin libraries are merged in the same order as before
   *)
  let ordered_libs = List.rev_append (List.rev ordered_flowlib_libs) ordered_non_flowlib_libs in
  let libs = SSet.of_list ordered_libs in

  let%lwt libs_ok, local_errors, suppressions, severity_cover_set =
    let suppressions = Error_suppressions.empty in
    let severity_cover_set = FilenameMap.empty in
    init_libs ~options ~profiling ~local_errors ~suppressions ~severity_cover_set ordered_libs
  in

  Hh_logger.info "Resolving dependencies";
  MonitorRPC.status_update ServerStatus.Resolving_dependencies_progress;

  let%lwt parsed_set, unparsed_set, all_files, parsed, unparsed =
    with_timer_lwt ~options "PrepareCommitModules" profiling (fun () ->
      let parsed_set = parsed_heaps |> FilenameMap.keys |> FilenameSet.of_list in
      let unparsed_set = unparsed_heaps |> FilenameMap.keys |> FilenameSet.of_list in
      let all_files = FilenameSet.union parsed_set unparsed_set in
      let parsed = FilenameMap.fold
        (fun fn data acc -> (fn, data.Saved_state.info)::acc) parsed_heaps []
      in
      let unparsed = FilenameMap.fold
        (fun fn data acc -> (fn, data.Saved_state.unparsed_info)::acc) unparsed_heaps []
      in
      Lwt.return (parsed_set, unparsed_set, all_files, parsed, unparsed)
    )
  in

  let all_providers_mutator = Module_hashtables.All_providers_mutator.create transaction in

  (* This will restore InfoHeap, NameHeap, & all_providers hashtable *)
  let%lwt _ =
    commit_modules_from_saved_state
      ~transaction
      ~all_providers_mutator
      ~options
      ~is_init:true
      ~profiling
      ~workers
      ~parsed
      ~parsed_set
      ~unparsed
      ~unparsed_set
      ~old_modules:[]
      ~deleted:FilenameSet.empty
      ~local_errors
      ~new_or_changed:all_files
  in

  let errors = { ServerEnv.
    local_errors;
    merge_errors = FilenameMap.empty;
    suppressions;
    severity_cover_set;
  } in

  let%lwt dependency_graph = with_timer_lwt ~options "CalcDepsTypecheck" profiling (fun () ->
    Dep_service.calc_dependency_graph workers ~parsed:parsed_set
  ) in

  Lwt.return (parsed_set, unparsed_set, dependency_graph, ordered_libs, libs, libs_ok, errors)

let init ~profiling ~workers options =
  let file_options = Options.file_options options in

  Transaction.with_transaction @@ fun transaction ->
  (* TODO - explicitly order the libs.
   *
   * Should we let the filesystem dictate the order that we merge libs? Are we sheep? No! We are
   * totally humans and we should deterministically order our library definitions. The order that
   * they are merged determines priority. We should be able to guarantee
   *
   * flowlibs are merged first (therefore have the lowest priority)
   * other libs are merged second (and therefore have the highest priority)
   *
   * However making this change is likely going to be a breaking change for people with conflicting
   * libraries
   *)
  let ordered_libs, libs = Files.init file_options in
  let next_files = make_next_files ~libs ~file_options (Options.root options) in

  Hh_logger.info "Parsing";
  let%lwt parsed, unparsed, unchanged, local_errors =
    parse ~options ~profiling ~workers next_files in

  assert (FilenameSet.is_empty unchanged);

  Hh_logger.info "Building package heap";
  let%lwt () = init_package_heap ~options ~profiling parsed in

  Hh_logger.info "Loading libraries";
  let%lwt libs_ok, local_errors, suppressions, (severity_cover_set: ExactCover.lint_severity_cover Utils_js.FilenameMap.t) =
    let suppressions = Error_suppressions.empty in
    let severity_cover_set = FilenameMap.empty in
    init_libs ~options ~profiling ~local_errors ~suppressions ~severity_cover_set ordered_libs in

  Hh_logger.info "Resolving dependencies";
  MonitorRPC.status_update ServerStatus.Resolving_dependencies_progress;

  let all_files, unparsed_set = List.fold_left (fun (all_files, unparsed_set) (filename, _) ->
    FilenameSet.add filename all_files, (FilenameSet.add filename unparsed_set)
  ) (parsed, FilenameSet.empty) unparsed in

  let all_providers_mutator = Module_hashtables.All_providers_mutator.create transaction in

  let%lwt _, errors =
    let errors = { ServerEnv.
      local_errors;
      merge_errors = FilenameMap.empty;
      suppressions;
      severity_cover_set;
    } in
    commit_modules_and_resolve_requires
      ~transaction
      ~all_providers_mutator
      ~options
      ~profiling
      ~workers
      ~old_modules:[]
      ~parsed_set:parsed
      ~unparsed
      ~unparsed_set
      ~new_or_changed:all_files
      ~deleted:FilenameSet.empty
      ~errors
      ~is_init:true
  in
  let%lwt dependency_graph = with_timer_lwt ~options "CalcDepsTypecheck" profiling (fun () ->
    Dep_service.calc_dependency_graph workers ~parsed
  ) in
  Lwt.return (parsed, unparsed_set, dependency_graph, ordered_libs, libs, libs_ok, errors)

(* Does a best-effort job to load a saved state. If it fails, returns None *)
let load_saved_state ~profiling ~workers options =
  let%lwt fetch_profiling, fetch_result = match Options.saved_state_fetcher options with
    | Options.Dummy_fetcher -> Saved_state_dummy_fetcher.fetch ~options
    | Options.Local_fetcher -> Saved_state_local_fetcher.fetch ~options
    | Options.Fb_fetcher -> Saved_state_fb_fetcher.fetch ~options
  in
  Profiling_js.merge ~from:fetch_profiling ~into:profiling;
  match fetch_result with
  | Saved_state_fetcher.No_saved_state ->
    Hh_logger.info "No saved state available";
    Lwt.return_none
  | Saved_state_fetcher.Saved_state { saved_state_filename; changed_files; } ->
    with_timer_lwt ~options "LoadSavedState" profiling (fun () ->
      let changed_files_count = SSet.cardinal changed_files in
      try%lwt
        let%lwt saved_state = Saved_state.load ~workers ~saved_state_filename ~options in
        let updates = Recheck_updates.process_updates
          ~options
          ~libs:(SSet.of_list saved_state.Saved_state.ordered_non_flowlib_libs)
          changed_files
        in
        let updates = match updates with
        | Core_result.Error ({ Recheck_updates.msg; _; }) ->
          Hh_logger.error "The saved state is no longer valid due to file changes: %s" msg;
          raise Saved_state.(Invalid_saved_state Changed_files)
        | Core_result.Ok updates -> updates in
        Hh_logger.info "Saved state script reports %d files changed & we care about %d of them"
          (SSet.cardinal changed_files)
          (FilenameSet.cardinal updates);
        FlowEventLogger.set_saved_state_filename (Path.to_string saved_state_filename);
        FlowEventLogger.load_saved_state_success ~changed_files_count;
        Lwt.return_some (saved_state, updates)
      with Saved_state.Invalid_saved_state invalid_reason ->
        let invalid_reason = Saved_state.invalid_reason_to_string invalid_reason in
        FlowEventLogger.load_saved_state_error
          ~saved_state_filename:(Path.to_string saved_state_filename)
          ~changed_files_count
          ~invalid_reason;
        if Options.saved_state_no_fallback options
        then
          let msg =
            spf "Failed to load saved state: %s" invalid_reason
          in
          FlowExitStatus.exit ~msg FlowExitStatus.Invalid_saved_state
        else Lwt.return_none
    )

let init ~profiling ~workers options =
  let%lwt updates, (parsed, unparsed, dependency_graph, ordered_libs, libs, libs_ok, errors) =
    match%lwt load_saved_state ~profiling ~workers options with
    | None ->
      (* Either there is no saved state or we failed to load it for some reason *)
      let%lwt init_ret = init ~profiling ~workers options in
      Lwt.return (FilenameSet.empty, init_ret)
    | Some (saved_state, updates) ->
      (* We loaded a saved state successfully! We are awesome! *)
      let%lwt init_ret = init_from_saved_state ~profiling ~workers ~saved_state options in
      Lwt.return (updates, init_ret)
  in

  let env = { ServerEnv.
    files = parsed;
    unparsed;
    dependency_graph;
    checked_files = CheckedSet.empty;
    ordered_libs;
    libs;
    errors;
    collated_errors = ref None;
    connections = Persistent_connection.empty;
  } in

  (* Don't recheck if the libs are not ok *)
  if FilenameSet.is_empty updates || not libs_ok
  then Lwt.return (libs_ok, env)
  else begin
    let%lwt recheck_profiling, _summary, env =
      recheck ~options ~workers ~updates env ~files_to_focus:FilenameSet.empty
    in
    Profiling_js.merge ~from:recheck_profiling ~into:profiling;
    Lwt.return (true, env)
  end

let full_check ~profiling ~options ~workers ~focus_targets env =
  let { ServerEnv.files = parsed; dependency_graph; errors; _; } = env in
  let%lwt (checked_files, _, errors, _skipped_count) = Transaction.with_transaction (fun transaction ->
    let%lwt infer_input = files_to_infer
      ~options ~focused:focus_targets ~profiling ~parsed ~dependency_graph in

    let unchanged_checked = CheckedSet.empty in
    let%lwt to_merge, components, recheck_map =
      include_dependencies_and_dependents
        ~options ~profiling
        ~unchanged_checked
        ~infer_input
        ~dependency_graph
        ~all_dependent_files:FilenameSet.empty
        ~direct_dependent_files:FilenameSet.empty
    in

    let%lwt () = ensure_parsed ~options ~profiling ~workers to_merge in

    merge
      ~transaction
      ~options
      ~profiling
      ~workers
      ~errors
      ~unchanged_checked
      ~to_merge
      ~components
      ~recheck_map
      ~dependency_graph
      ~deleted:FilenameSet.empty
      ~persistent_connections:None
      ~prep_merge:None
  ) in
  Lwt.return { env with ServerEnv.checked_files; errors }
