/*
 * 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.
 */
package org.apache.logging.log4j.core.layout;

import static org.apache.logging.log4j.util.Chars.LF;
import static org.apache.logging.log4j.util.Chars.NUL;

import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.zip.DeflaterOutputStream;
import java.util.zip.GZIPOutputStream;
import org.apache.logging.log4j.Level;
import org.apache.logging.log4j.core.Layout;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.config.Configuration;
import org.apache.logging.log4j.core.config.Node;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.config.plugins.PluginAttribute;
import org.apache.logging.log4j.core.config.plugins.PluginBuilderAttribute;
import org.apache.logging.log4j.core.config.plugins.PluginBuilderFactory;
import org.apache.logging.log4j.core.config.plugins.PluginElement;
import org.apache.logging.log4j.core.layout.internal.ExcludeChecker;
import org.apache.logging.log4j.core.layout.internal.IncludeChecker;
import org.apache.logging.log4j.core.layout.internal.ListChecker;
import org.apache.logging.log4j.core.lookup.StrSubstitutor;
import org.apache.logging.log4j.core.net.Severity;
import org.apache.logging.log4j.core.util.JsonUtils;
import org.apache.logging.log4j.core.util.KeyValuePair;
import org.apache.logging.log4j.core.util.NetUtils;
import org.apache.logging.log4j.core.util.Patterns;
import org.apache.logging.log4j.message.MapMessage;
import org.apache.logging.log4j.message.Message;
import org.apache.logging.log4j.status.StatusLogger;
import org.apache.logging.log4j.util.StringBuilderFormattable;
import org.apache.logging.log4j.util.Strings;
import org.apache.logging.log4j.util.TriConsumer;

/**
 * Lays out events in the Graylog Extended Log Format (GELF) 1.1.
 * <p>
 * This layout compresses JSON to GZIP or ZLIB (the {@code compressionType}) if
 * log event data is larger than 1024 bytes (the {@code compressionThreshold}).
 * This layout does not implement chunking.
 * </p>
 *
 * @see <a href="https://go2docs.graylog.org/current/getting_in_log_data/gelf.html#GELFPayloadSpecification">GELF specification</a>
 */
@Plugin(name = "GelfLayout", category = Node.CATEGORY, elementType = Layout.ELEMENT_TYPE, printObject = true)
public final class GelfLayout extends AbstractStringLayout {

    public enum CompressionType {
        GZIP {
            @Override
            public DeflaterOutputStream createDeflaterOutputStream(final OutputStream os) throws IOException {
                return new GZIPOutputStream(os);
            }
        },
        ZLIB {
            @Override
            public DeflaterOutputStream createDeflaterOutputStream(final OutputStream os) throws IOException {
                return new DeflaterOutputStream(os);
            }
        },
        OFF {
            @Override
            public DeflaterOutputStream createDeflaterOutputStream(final OutputStream os) throws IOException {
                return null;
            }
        };

        public abstract DeflaterOutputStream createDeflaterOutputStream(OutputStream os) throws IOException;
    }

    private static final char C = ',';
    private static final int COMPRESSION_THRESHOLD = 1024;
    private static final char Q = '\"';
    private static final String QC = "\",";
    private static final String QU = "\"_";

    private final KeyValuePair[] additionalFields;
    private final int compressionThreshold;
    private final CompressionType compressionType;
    private final String host;
    private final boolean includeStacktrace;
    private final boolean includeThreadContext;
    private final boolean includeMapMessage;
    private final boolean includeNullDelimiter;
    private final boolean includeNewLineDelimiter;
    private final boolean omitEmptyFields;
    private final PatternLayout layout;
    private final FieldWriter mdcWriter;
    private final FieldWriter mapWriter;

    public static class Builder<B extends Builder<B>> extends AbstractStringLayout.Builder<B>
            implements org.apache.logging.log4j.core.util.Builder<GelfLayout> {

        @PluginBuilderAttribute
        private String host;

        @PluginElement("AdditionalField")
        private KeyValuePair[] additionalFields;

        @PluginBuilderAttribute
        private CompressionType compressionType = CompressionType.GZIP;

        @PluginBuilderAttribute
        private int compressionThreshold = COMPRESSION_THRESHOLD;

        @PluginBuilderAttribute
        private boolean includeStacktrace = true;

        @PluginBuilderAttribute
        private boolean includeThreadContext = true;

        @PluginBuilderAttribute
        private boolean includeNullDelimiter;

        @PluginBuilderAttribute
        private boolean includeNewLineDelimiter;

        @PluginBuilderAttribute
        private String threadContextIncludes;

        @PluginBuilderAttribute
        private String threadContextExcludes;

        @PluginBuilderAttribute
        private String mapMessageIncludes;

        @PluginBuilderAttribute
        private String mapMessageExcludes;

        @PluginBuilderAttribute
        private boolean includeMapMessage = true;

        @PluginBuilderAttribute
        private boolean omitEmptyFields;

        @PluginBuilderAttribute
        private String messagePattern;

        @PluginBuilderAttribute
        private String threadContextPrefix = "";

        @PluginBuilderAttribute
        private String mapPrefix = "";

        @PluginElement("PatternSelector")
        private PatternSelector patternSelector;

        public Builder() {
            setCharset(StandardCharsets.UTF_8);
        }

        @Override
        public GelfLayout build() {
            final ListChecker mdcChecker = createChecker(threadContextExcludes, threadContextIncludes);
            final ListChecker mapChecker = createChecker(mapMessageExcludes, mapMessageIncludes);
            PatternLayout patternLayout = null;
            if (messagePattern != null && patternSelector != null) {
                LOGGER.error("A message pattern and PatternSelector cannot both be specified on GelfLayout, "
                        + "ignoring message pattern");
                messagePattern = null;
            }
            if (messagePattern != null) {
                patternLayout = PatternLayout.newBuilder()
                        .withPattern(messagePattern)
                        .withAlwaysWriteExceptions(includeStacktrace)
                        .withConfiguration(getConfiguration())
                        .build();
            }
            if (patternSelector != null) {
                patternLayout = PatternLayout.newBuilder()
                        .withPatternSelector(patternSelector)
                        .withAlwaysWriteExceptions(includeStacktrace)
                        .withConfiguration(getConfiguration())
                        .build();
            }
            return new GelfLayout(
                    getConfiguration(),
                    host,
                    additionalFields,
                    compressionType,
                    compressionThreshold,
                    includeStacktrace,
                    includeThreadContext,
                    includeMapMessage,
                    includeNullDelimiter,
                    includeNewLineDelimiter,
                    omitEmptyFields,
                    mdcChecker,
                    mapChecker,
                    patternLayout,
                    threadContextPrefix,
                    mapPrefix);
        }

        private ListChecker createChecker(final String excludes, final String includes) {
            ListChecker checker = null;
            if (excludes != null) {
                final String[] array = excludes.split(Patterns.COMMA_SEPARATOR);
                if (array.length > 0) {
                    final List<String> excludeList = new ArrayList<>(array.length);
                    for (final String str : array) {
                        excludeList.add(str.trim());
                    }
                    checker = new ExcludeChecker(excludeList);
                }
            }
            if (includes != null) {
                final String[] array = includes.split(Patterns.COMMA_SEPARATOR);
                if (array.length > 0) {
                    final List<String> includeList = new ArrayList<>(array.length);
                    for (final String str : array) {
                        includeList.add(str.trim());
                    }
                    checker = new IncludeChecker(includeList);
                }
            }
            if (checker == null) {
                checker = ListChecker.NOOP_CHECKER;
            }
            return checker;
        }

        public String getHost() {
            return host;
        }

        public CompressionType getCompressionType() {
            return compressionType;
        }

        public int getCompressionThreshold() {
            return compressionThreshold;
        }

        public boolean isIncludeStacktrace() {
            return includeStacktrace;
        }

        public boolean isIncludeThreadContext() {
            return includeThreadContext;
        }

        public boolean isIncludeNullDelimiter() {
            return includeNullDelimiter;
        }

        public boolean isIncludeNewLineDelimiter() {
            return includeNewLineDelimiter;
        }

        public KeyValuePair[] getAdditionalFields() {
            return additionalFields;
        }

        /**
         * The value of the <code>host</code> property (optional, defaults to local host name).
         *
         * @return this builder
         */
        public B setHost(final String host) {
            this.host = host;
            return asBuilder();
        }

        /**
         * Compression to use (optional, defaults to GZIP).
         *
         * @return this builder
         */
        public B setCompressionType(final CompressionType compressionType) {
            this.compressionType = compressionType;
            return asBuilder();
        }

        /**
         * Compress if data is larger than this number of bytes (optional, defaults to 1024).
         *
         * @return this builder
         */
        public B setCompressionThreshold(final int compressionThreshold) {
            this.compressionThreshold = compressionThreshold;
            return asBuilder();
        }

        /**
         * Whether to include full stacktrace of logged Throwables (optional, default to true).
         * If set to false, only the class name and message of the Throwable will be included.
         *
         * @return this builder
         */
        public B setIncludeStacktrace(final boolean includeStacktrace) {
            this.includeStacktrace = includeStacktrace;
            return asBuilder();
        }

        /**
         * Whether to include thread context as additional fields (optional, default to true).
         *
         * @return this builder
         */
        public B setIncludeThreadContext(final boolean includeThreadContext) {
            this.includeThreadContext = includeThreadContext;
            return asBuilder();
        }

        /**
         * Whether to include NULL byte as delimiter after each event (optional, default to false).
         * Useful for Graylog GELF TCP input.
         *
         * @return this builder
         */
        public B setIncludeNullDelimiter(final boolean includeNullDelimiter) {
            this.includeNullDelimiter = includeNullDelimiter;
            return asBuilder();
        }

        /**
         * Whether to include newline (LF) as delimiter after each event (optional, default to false).
         *
         * @return this builder
         */
        public B setIncludeNewLineDelimiter(final boolean includeNewLineDelimiter) {
            this.includeNewLineDelimiter = includeNewLineDelimiter;
            return asBuilder();
        }

        /**
         * Additional fields to set on each log event.
         *
         * @return this builder
         */
        public B setAdditionalFields(final KeyValuePair[] additionalFields) {
            this.additionalFields = additionalFields;
            return asBuilder();
        }

        /**
         * The pattern to use to format the message.
         * @param pattern the pattern string.
         * @return this builder
         */
        public B setMessagePattern(final String pattern) {
            this.messagePattern = pattern;
            return asBuilder();
        }

        /**
         * The PatternSelector to use to format the message.
         * @param patternSelector the PatternSelector.
         * @return this builder
         */
        public B setPatternSelector(final PatternSelector patternSelector) {
            this.patternSelector = patternSelector;
            return asBuilder();
        }

        /**
         * A comma separated list of thread context keys to include;
         * @param mdcIncludes the list of keys.
         * @return this builder
         */
        public B setMdcIncludes(final String mdcIncludes) {
            this.threadContextIncludes = mdcIncludes;
            return asBuilder();
        }

        /**
         * A comma separated list of thread context keys to include;
         * @param mdcExcludes the list of keys.
         * @return this builder
         */
        public B setMdcExcludes(final String mdcExcludes) {
            this.threadContextExcludes = mdcExcludes;
            return asBuilder();
        }

        /**
         * Whether to include MapMessage fields as additional fields (optional, default to true).
         *
         * @return this builder
         */
        public B setIncludeMapMessage(final boolean includeMapMessage) {
            this.includeMapMessage = includeMapMessage;
            return asBuilder();
        }

        /**
         * A comma separated list of thread context keys to include;
         * @param mapMessageIncludes the list of keys.
         * @return this builder
         */
        public B setMapMessageIncludes(final String mapMessageIncludes) {
            this.mapMessageIncludes = mapMessageIncludes;
            return asBuilder();
        }

        /**
         * A comma separated list of MapMessage keys to exclude;
         * @param mapMessageExcludes the list of keys.
         * @return this builder
         */
        public B setMapMessageExcludes(final String mapMessageExcludes) {
            this.mapMessageExcludes = mapMessageExcludes;
            return asBuilder();
        }

        /**
         * The String to prefix the ThreadContext attributes.
         * @param prefix The prefix value. Null values will be ignored.
         * @return this builder.
         */
        public B setThreadContextPrefix(final String prefix) {
            if (prefix != null) {
                this.threadContextPrefix = prefix;
            }
            return asBuilder();
        }

        /**
         * The String to prefix the MapMessage attributes.
         * @param prefix The prefix value. Null values will be ignored.
         * @return this builder.
         */
        public B setMapPrefix(final String prefix) {
            if (prefix != null) {
                this.mapPrefix = prefix;
            }
            return asBuilder();
        }
    }

    /**
     * @deprecated Use {@link #newBuilder()} instead
     */
    @Deprecated
    public GelfLayout(
            final String host,
            final KeyValuePair[] additionalFields,
            final CompressionType compressionType,
            final int compressionThreshold,
            final boolean includeStacktrace) {
        this(
                null,
                host,
                additionalFields,
                compressionType,
                compressionThreshold,
                includeStacktrace,
                true,
                true,
                false,
                false,
                false,
                null,
                null,
                null,
                "",
                "");
    }

    private GelfLayout(
            final Configuration config,
            final String host,
            final KeyValuePair[] additionalFields,
            final CompressionType compressionType,
            final int compressionThreshold,
            final boolean includeStacktrace,
            final boolean includeThreadContext,
            final boolean includeMapMessage,
            final boolean includeNullDelimiter,
            final boolean includeNewLineDelimiter,
            final boolean omitEmptyFields,
            final ListChecker mdcChecker,
            final ListChecker mapChecker,
            final PatternLayout patternLayout,
            final String mdcPrefix,
            final String mapPrefix) {
        super(config, StandardCharsets.UTF_8, null, null);
        this.host = host != null ? host : NetUtils.getLocalHostname();
        this.additionalFields = additionalFields != null ? additionalFields : KeyValuePair.EMPTY_ARRAY;
        if (config == null) {
            for (final KeyValuePair additionalField : this.additionalFields) {
                if (valueNeedsLookup(additionalField.getValue())) {
                    throw new IllegalArgumentException(
                            "configuration needs to be set when there are additional fields with variables");
                }
            }
        }
        this.compressionType = compressionType;
        this.compressionThreshold = compressionThreshold;
        this.includeStacktrace = includeStacktrace;
        this.includeThreadContext = includeThreadContext;
        this.includeMapMessage = includeMapMessage;
        this.includeNullDelimiter = includeNullDelimiter;
        this.includeNewLineDelimiter = includeNewLineDelimiter;
        this.omitEmptyFields = omitEmptyFields;
        if (includeNullDelimiter && compressionType != CompressionType.OFF) {
            throw new IllegalArgumentException("null delimiter cannot be used with compression");
        }
        this.mdcWriter = new FieldWriter(mdcChecker, mdcPrefix);
        this.mapWriter = new FieldWriter(mapChecker, mapPrefix);
        this.layout = patternLayout;
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder();
        sb.append("host=").append(host);
        sb.append(", compressionType=").append(compressionType.toString());
        sb.append(", compressionThreshold=").append(compressionThreshold);
        sb.append(", includeStackTrace=").append(includeStacktrace);
        sb.append(", includeThreadContext=").append(includeThreadContext);
        sb.append(", includeNullDelimiter=").append(includeNullDelimiter);
        sb.append(", includeNewLineDelimiter=").append(includeNewLineDelimiter);
        final String threadVars = mdcWriter.getChecker().toString();
        if (threadVars.length() > 0) {
            sb.append(", ").append(threadVars);
        }
        final String mapVars = mapWriter.getChecker().toString();
        if (mapVars.length() > 0) {
            sb.append(", ").append(mapVars);
        }
        if (layout != null) {
            sb.append(", PatternLayout{").append(layout.toString()).append("}");
        }
        return sb.toString();
    }

    /**
     * @deprecated Use {@link #newBuilder()} instead
     */
    @Deprecated
    public static GelfLayout createLayout(
            // @formatter:off
            @PluginAttribute("host") final String host,
            @PluginElement("AdditionalField") final KeyValuePair[] additionalFields,
            @PluginAttribute(value = "compressionType", defaultString = "GZIP") final CompressionType compressionType,
            @PluginAttribute(value = "compressionThreshold", defaultInt = COMPRESSION_THRESHOLD)
                    final int compressionThreshold,
            @PluginAttribute(value = "includeStacktrace", defaultBoolean = true) final boolean includeStacktrace) {
        // @formatter:on
        return new GelfLayout(
                null,
                host,
                additionalFields,
                compressionType,
                compressionThreshold,
                includeStacktrace,
                true,
                true,
                false,
                false,
                false,
                null,
                null,
                null,
                "",
                "");
    }

    @PluginBuilderFactory
    public static <B extends Builder<B>> B newBuilder() {
        return new Builder<B>().asBuilder();
    }

    @Override
    public Map<String, String> getContentFormat() {
        return Collections.emptyMap();
    }

    @Override
    public String getContentType() {
        return JsonLayout.CONTENT_TYPE + "; charset=" + this.getCharset();
    }

    @Override
    public byte[] toByteArray(final LogEvent event) {
        final StringBuilder text = toText(event, getStringBuilder(), false);
        final byte[] bytes = getBytes(text.toString());
        return compressionType != CompressionType.OFF && bytes.length > compressionThreshold ? compress(bytes) : bytes;
    }

    @Override
    public void encode(final LogEvent event, final ByteBufferDestination destination) {
        if (compressionType != CompressionType.OFF) {
            super.encode(event, destination);
            return;
        }
        final StringBuilder text = toText(event, getStringBuilder(), true);
        final Encoder<StringBuilder> helper = getStringBuilderEncoder();
        helper.encode(text, destination);
    }

    @Override
    public boolean requiresLocation() {
        return Objects.nonNull(layout) && layout.requiresLocation();
    }

    private byte[] compress(final byte[] bytes) {
        try {
            final ByteArrayOutputStream baos = new ByteArrayOutputStream(compressionThreshold / 8);
            try (final DeflaterOutputStream stream = compressionType.createDeflaterOutputStream(baos)) {
                if (stream == null) {
                    return bytes;
                }
                stream.write(bytes);
                stream.finish();
            }
            return baos.toByteArray();
        } catch (final IOException e) {
            StatusLogger.getLogger().error(e);
            return bytes;
        }
    }

    @Override
    public String toSerializable(final LogEvent event) {
        final StringBuilder text = toText(event, getStringBuilder(), false);
        return text.toString();
    }

    private StringBuilder toText(final LogEvent event, final StringBuilder builder, final boolean gcFree) {
        builder.append('{');
        builder.append("\"version\":\"1.1\",");
        builder.append("\"host\":\"");
        JsonUtils.quoteAsString(toNullSafeString(host), builder);
        builder.append(QC);
        builder.append("\"timestamp\":")
                .append(formatTimestamp(event.getTimeMillis()))
                .append(C);
        builder.append("\"level\":").append(formatLevel(event.getLevel())).append(C);
        if (event.getThreadName() != null) {
            builder.append("\"_thread\":\"");
            JsonUtils.quoteAsString(event.getThreadName(), builder);
            builder.append(QC);
        }
        if (event.getLoggerName() != null) {
            builder.append("\"_logger\":\"");
            JsonUtils.quoteAsString(event.getLoggerName(), builder);
            builder.append(QC);
        }
        if (additionalFields.length > 0) {
            final StrSubstitutor strSubstitutor = getConfiguration().getStrSubstitutor();
            for (final KeyValuePair additionalField : additionalFields) {
                final String value = valueNeedsLookup(additionalField.getValue())
                        ? strSubstitutor.replace(event, additionalField.getValue())
                        : additionalField.getValue();
                if (Strings.isNotEmpty(value) || !omitEmptyFields) {
                    builder.append(QU);
                    JsonUtils.quoteAsString(additionalField.getKey(), builder);
                    builder.append("\":\"");
                    JsonUtils.quoteAsString(toNullSafeString(value), builder);
                    builder.append(QC);
                }
            }
        }
        if (includeThreadContext) {
            event.getContextData().forEach(mdcWriter, builder);
        }
        if (includeMapMessage && event.getMessage() instanceof MapMessage) {
            ((MapMessage<?, Object>) event.getMessage()).forEach((key, value) -> mapWriter.accept(key, value, builder));
        }

        if (event.getThrown() != null || layout != null) {
            builder.append("\"full_message\":\"");
            if (layout != null) {
                final StringBuilder messageBuffer = getMessageStringBuilder();
                layout.serialize(event, messageBuffer);
                JsonUtils.quoteAsString(messageBuffer, builder);
            } else if (includeStacktrace) {
                JsonUtils.quoteAsString(formatThrowable(event.getThrown()), builder);
            } else {
                JsonUtils.quoteAsString(event.getThrown().toString(), builder);
            }
            builder.append(QC);
        }

        builder.append("\"short_message\":\"");
        final Message message = event.getMessage();
        if (message instanceof CharSequence) {
            JsonUtils.quoteAsString(((CharSequence) message), builder);
        } else if (gcFree && message instanceof StringBuilderFormattable) {
            final StringBuilder messageBuffer = getMessageStringBuilder();
            try {
                ((StringBuilderFormattable) message).formatTo(messageBuffer);
                JsonUtils.quoteAsString(messageBuffer, builder);
            } finally {
                trimToMaxSize(messageBuffer);
            }
        } else {
            JsonUtils.quoteAsString(toNullSafeString(message.getFormattedMessage()), builder);
        }
        builder.append(Q);
        builder.append('}');
        if (includeNullDelimiter) {
            builder.append(NUL);
        }
        if (includeNewLineDelimiter) {
            builder.append(LF);
        }
        return builder;
    }

    private static boolean valueNeedsLookup(final String value) {
        return value != null && value.contains("${");
    }

    private class FieldWriter implements TriConsumer<String, Object, StringBuilder> {
        private final ListChecker checker;
        private final String prefix;

        FieldWriter(final ListChecker checker, final String prefix) {
            this.checker = checker;
            this.prefix = prefix;
        }

        @Override
        public void accept(final String key, final Object value, final StringBuilder stringBuilder) {
            final String stringValue = String.valueOf(value);
            if (checker.check(key) && (Strings.isNotEmpty(stringValue) || !omitEmptyFields)) {
                stringBuilder.append(QU);
                JsonUtils.quoteAsString(Strings.concat(prefix, key), stringBuilder);
                stringBuilder.append("\":\"");
                JsonUtils.quoteAsString(toNullSafeString(stringValue), stringBuilder);
                stringBuilder.append(QC);
            }
        }

        public ListChecker getChecker() {
            return checker;
        }
    }

    private static final ThreadLocal<StringBuilder> messageStringBuilder = new ThreadLocal<>();

    private static StringBuilder getMessageStringBuilder() {
        StringBuilder result = messageStringBuilder.get();
        if (result == null) {
            result = new StringBuilder(DEFAULT_STRING_BUILDER_SIZE);
            messageStringBuilder.set(result);
        }
        result.setLength(0);
        return result;
    }

    private static CharSequence toNullSafeString(final CharSequence s) {
        return s == null ? Strings.EMPTY : s;
    }

    /**
     * Non-private to make it accessible from unit test.
     */
    static CharSequence formatTimestamp(final long timeMillis) {
        if (timeMillis < 1000) {
            return "0";
        }
        final StringBuilder builder = getTimestampStringBuilder();
        builder.append(timeMillis);
        builder.insert(builder.length() - 3, '.');
        return builder;
    }

    private static final ThreadLocal<StringBuilder> timestampStringBuilder = new ThreadLocal<>();

    private static StringBuilder getTimestampStringBuilder() {
        StringBuilder result = timestampStringBuilder.get();
        if (result == null) {
            result = new StringBuilder(20);
            timestampStringBuilder.set(result);
        }
        result.setLength(0);
        return result;
    }

    /**
     * http://en.wikipedia.org/wiki/Syslog#Severity_levels
     */
    private int formatLevel(final Level level) {
        return Severity.getSeverity(level).getCode();
    }

    /**
     * Non-private to make it accessible from unit test.
     */
    @SuppressFBWarnings(
            value = "INFORMATION_EXPOSURE_THROUGH_AN_ERROR_MESSAGE",
            justification = "Log4j prints stacktraces only to logs, which should be private.")
    static CharSequence formatThrowable(final Throwable throwable) {
        // stack traces are big enough to provide a reasonably large initial capacity here
        final StringWriter sw = new StringWriter(2048);
        final PrintWriter pw = new PrintWriter(sw);
        throwable.printStackTrace(pw);
        pw.flush();
        return sw.getBuffer();
    }
}
