#region Apache License
//
// Licensed to the Apache Software Foundation (ASF) under one or more 
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership. 
// The ASF licenses this file to you under the Apache License, Version 2.0
// (the "License"); you may not use this file except in compliance with 
// the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

using System;
using System.Diagnostics.CodeAnalysis;
using System.IO;

using log4net.Core;

namespace log4net.Layout.Pattern;

/// <summary>
/// Converter to output the relative time of the event
/// </summary>
/// <remarks>
/// <para>
/// Converter to output the time of the event relative to the start of the program.
/// </para>
/// </remarks>
/// <author>Nicko Cadell</author>
[SuppressMessage("Microsoft.Performance", "CA1812:AvoidUninstantiatedInternalClasses", Justification = "Reflection")]
internal sealed class RelativeTimePatternConverter : PatternLayoutConverter
{
  /// <summary>
  /// Write the relative time to the output
  /// </summary>
  /// <param name="writer"><see cref="TextWriter" /> that will receive the formatted result.</param>
  /// <param name="loggingEvent">the event being logged</param>
  /// <remarks>
  /// <para>
  /// Writes out the relative time of the event in milliseconds.
  /// That is the number of milliseconds between the event <see cref="LoggingEvent.TimeStamp"/>
  /// and the <see cref="LoggingEvent.StartTime"/>.
  /// </para>
  /// </remarks>
  protected override void Convert(TextWriter writer, LoggingEvent loggingEvent) 
    => writer.Write(TimeDifferenceInMillis(LoggingEvent.StartTimeUtc, loggingEvent.TimeStampUtc)
             .ToString(System.Globalization.NumberFormatInfo.InvariantInfo));

  /// <summary>
  /// Helper method to get the time difference between two DateTime objects
  /// </summary>
  /// <param name="start">start time (in the current local time zone)</param>
  /// <param name="end">end time (in the current local time zone)</param>
  /// <returns>the time difference in milliseconds</returns>
  private static long TimeDifferenceInMillis(DateTime start, DateTime end)
    // We must convert all times to UTC before performing any mathematical
    // operations on them. This allows use to take into account discontinuities
    // caused by daylight savings time transitions.
    => (long)(end.ToUniversalTime() - start.ToUniversalTime()).TotalMilliseconds;
}
