Emitter.cs

// This file is part of YamlDotNet - A .NET library for YAML.
// Copyright (c) Antoine Aubry and contributors
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of
// this software and associated documentation files (the "Software"), to deal in
// the Software without restriction, including without limitation the rights to
// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
// of the Software, and to permit persons to whom the Software is furnished to do
// so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Text;
using System.Text.RegularExpressions;
using YamlDotNet.Core.Events;
using YamlDotNet.Core.ObjectPool;
using ParsingEvent = YamlDotNet.Core.Events.ParsingEvent;
using TagDirective = YamlDotNet.Core.Tokens.TagDirective;
using VersionDirective = YamlDotNet.Core.Tokens.VersionDirective;

namespace YamlDotNet.Core
{
    /// <summary>
    /// Emits YAML streams.
    /// </summary>
    public class Emitter : IEmitter
    {
        private static readonly Regex UriReplacer = new Regex(@"[^0-9A-Za-z_\-;?@=$~\\\)\]/:&+,\.\*\(\[!]",
            StandardRegexOptions.Compiled | RegexOptions.Singleline);

        private static readonly string[] NewLineSeparators = ["\r\n", "\r", "\n"];

        private readonly TextWriter output;
        private readonly bool outputUsesUnicodeEncoding;

        private readonly int maxSimpleKeyLength;
        private readonly bool isCanonical;
        private readonly bool skipAnchorName;
        private readonly int bestIndent;
        private readonly int bestWidth;
        private EmitterState state;

        private readonly Stack<EmitterState> states = new Stack<EmitterState>();
        private readonly Queue<ParsingEvent> events = new Queue<ParsingEvent>();
        private readonly Stack<int> indents = new Stack<int>();
        private readonly TagDirectiveCollection tagDirectives = [];
        private int indent;
        private int flowLevel;
        private bool isMappingContext;
        private bool isSimpleKeyContext;

        private int column;
        private bool isWhitespace;
        private bool isIndentation;
        private readonly bool forceIndentLess;
        private readonly bool useUtf16SurrogatePair;

        private bool isDocumentEndWritten;

        private readonly AnchorData anchorData = new AnchorData();
        private readonly TagData tagData = new TagData();
        private readonly ScalarData scalarData = new ScalarData();

        private class AnchorData
        {
            public AnchorName Anchor;
            public bool IsAlias;
        }

        private class TagData
        {
            public string? Handle;
            public string? Suffix;
        }

        private class ScalarData
        {
            public string Value = string.Empty;
            public bool IsMultiline;
            public bool IsFlowPlainAllowed;
            public bool IsBlockPlainAllowed;
            public bool IsSingleQuotedAllowed;
            public bool IsBlockAllowed;
            public bool HasSingleQuotes;
            public ScalarStyle Style;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Emitter"/> class.
        /// </summary>
        /// <param name="output">The <see cref="TextWriter"/> where the emitter will write.</param>
        public Emitter(TextWriter output)
            : this(output, EmitterSettings.Default)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Emitter"/> class.
        /// </summary>
        /// <param name="output">The <see cref="TextWriter"/> where the emitter will write.</param>
        /// <param name="bestIndent">The preferred indentation.</param>
        public Emitter(TextWriter output, int bestIndent)
            : this(output, bestIndent, int.MaxValue)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Emitter"/> class.
        /// </summary>
        /// <param name="output">The <see cref="TextWriter"/> where the emitter will write.</param>
        /// <param name="bestIndent">The preferred indentation.</param>
        /// <param name="bestWidth">The preferred text width.</param>
        public Emitter(TextWriter output, int bestIndent, int bestWidth)
            : this(output, bestIndent, bestWidth, false)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Emitter"/> class.
        /// </summary>
        /// <param name="output">The <see cref="TextWriter"/> where the emitter will write.</param>
        /// <param name="bestIndent">The preferred indentation.</param>
        /// <param name="bestWidth">The preferred text width.</param>
        /// <param name="isCanonical">If true, write the output in canonical form.</param>
        public Emitter(TextWriter output, int bestIndent, int bestWidth, bool isCanonical)
            : this(output, new EmitterSettings(bestIndent, bestWidth, isCanonical, 1024))
        {
        }

        public Emitter(TextWriter output, EmitterSettings settings)
        {
            this.bestIndent = settings.BestIndent;
            this.bestWidth = settings.BestWidth;
            this.isCanonical = settings.IsCanonical;
            this.maxSimpleKeyLength = settings.MaxSimpleKeyLength;
            this.skipAnchorName = settings.SkipAnchorName;
            this.forceIndentLess = !settings.IndentSequences;
            this.useUtf16SurrogatePair = settings.UseUtf16SurrogatePairs;

            this.output = output;
            this.output.NewLine = settings.NewLine;
            this.outputUsesUnicodeEncoding = IsUnicode(output.Encoding);
        }

        /// <summary>
        /// Emit an evt.
        /// </summary>
        public void Emit(ParsingEvent @event)
        {
            events.Enqueue(@event);

            while (!NeedMoreEvents())
            {
                var current = events.Peek();
                try
                {

                    AnalyzeEvent(current);
                    StateMachine(current);
                }
                finally
                {
                    // Only dequeue after calling state_machine because it checks how many events are in the queue.
                    // Todo: well, move into StateMachine() then
                    events.Dequeue();
                }
            }
        }

        /// <summary>
        /// Check if we need to accumulate more events before emitting.
        ///
        /// We accumulate extra
        ///  - 1 event for DOCUMENT-START
        ///  - 2 events for SEQUENCE-START
        ///  - 3 events for MAPPING-START
        /// </summary>
        private bool NeedMoreEvents()
        {
            if (events.Count == 0)
            {
                return true;
            }

            int accumulate;
            switch (events.Peek().Type)
            {
                case EventType.DocumentStart:
                    accumulate = 1;
                    break;

                case EventType.SequenceStart:
                    accumulate = 2;
                    break;

                case EventType.MappingStart:
                    accumulate = 3;
                    break;

                default:
                    return false;
            }

            if (events.Count > accumulate)
            {
                return false;
            }

            var level = 0;
            foreach (var evt in events)
            {
                switch (evt.Type)
                {
                    case EventType.DocumentStart:
                    case EventType.SequenceStart:
                    case EventType.MappingStart:
                        ++level;
                        break;

                    case EventType.DocumentEnd:
                    case EventType.SequenceEnd:
                    case EventType.MappingEnd:
                        --level;
                        break;
                }
                if (level == 0)
                {
                    return false;
                }
            }

            return true;
        }

        private void AnalyzeEvent(ParsingEvent evt)
        {
            anchorData.Anchor = AnchorName.Empty;
            tagData.Handle = null;
            tagData.Suffix = null;

            if (evt is AnchorAlias alias)
            {
                AnalyzeAnchor(alias.Value, true);
                return;
            }

            if (evt is NodeEvent nodeEvent)
            {
                if (evt is Scalar scalar)
                {
                    AnalyzeScalar(scalar);
                }

                AnalyzeAnchor(nodeEvent.Anchor, false);

                if (!nodeEvent.Tag.IsEmpty && (isCanonical || nodeEvent.IsCanonical))
                {
                    AnalyzeTag(nodeEvent.Tag);
                }
            }
        }

        private void AnalyzeAnchor(AnchorName anchor, bool isAlias)
        {
            anchorData.Anchor = anchor;
            anchorData.IsAlias = isAlias;
        }

        private void AnalyzeScalar(Scalar scalar)
        {
            var value = scalar.Value;
            scalarData.Value = value;

            if (value.Length == 0)
            {
                if (scalar.Tag == "tag:yaml.org,2002:null")
                {
                    scalarData.IsMultiline = false;
                    scalarData.IsFlowPlainAllowed = false;
                    scalarData.IsBlockPlainAllowed = true;
                    scalarData.IsSingleQuotedAllowed = false;
                    scalarData.IsBlockAllowed = false;
                }
                else
                {
                    scalarData.IsMultiline = false;
                    scalarData.IsFlowPlainAllowed = false;
                    scalarData.IsBlockPlainAllowed = false;
                    scalarData.IsSingleQuotedAllowed = true;
                    scalarData.IsBlockAllowed = false;
                }
                return;
            }

            var flowIndicators = false;
            var blockIndicators = false;
            if (value.StartsWith("---", StringComparison.Ordinal) || value.StartsWith("...", StringComparison.Ordinal))
            {
                flowIndicators = true;
                blockIndicators = true;
            }

            using var slab = StringLookAheadBufferPool.Rent(value);
            var buffer = new CharacterAnalyzer<StringLookAheadBuffer>(slab.Buffer);
            var preceededByWhitespace = true;
            var followedByWhitespace = buffer.IsWhiteBreakOrZero(1);

            var leadingSpace = false;
            var leadingBreak = false;
            var trailingSpace = false;
            var trailingBreak = false;
            var leadingQuote = false;

            var breakSpace = false;
            var spaceBreak = false;
            var previousSpace = false;
            var previousBreak = false;
            var lineOfSpaces = false;

            var lineBreaks = false;

            var specialCharacters = !ValueIsRepresentableInOutputEncoding(value);
            var singleQuotes = false;
            var linesOfSpaces = false;

            var isFirst = true;
            while (!buffer.EndOfInput)
            {
                if (isFirst)
                {
                    if (buffer.Check(@"#,[]{}&*!|>""%@`'"))
                    {
                        flowIndicators = true;
                        blockIndicators = true;
                        leadingQuote = buffer.Check('\'');
                        singleQuotes |= buffer.Check('\'');
                    }

                    if (buffer.Check("?:"))
                    {
                        flowIndicators = true;
                        if (followedByWhitespace)
                        {
                            blockIndicators = true;
                        }
                    }

                    if (buffer.Check('-') && followedByWhitespace)
                    {
                        flowIndicators = true;
                        blockIndicators = true;
                    }
                }
                else
                {
                    if (buffer.Check(",?[]{}"))
                    {
                        flowIndicators = true;
                    }

                    if (buffer.Check(':'))
                    {
                        flowIndicators = true;
                        if (followedByWhitespace)
                        {
                            blockIndicators = true;
                        }
                    }

                    if (buffer.Check('#') && preceededByWhitespace)
                    {
                        flowIndicators = true;
                        blockIndicators = true;
                    }

                    singleQuotes |= buffer.Check('\'');
                }

                if (!specialCharacters && !buffer.IsPrintable())
                {
                    specialCharacters = true;
                }

                if (buffer.IsBreak())
                {
                    lineBreaks = true;
                }

                if (buffer.IsSpace())
                {
                    if (isFirst)
                    {
                        leadingSpace = true;
                    }

                    if (buffer.Buffer.Position >= buffer.Buffer.Length - 1)
                    {
                        trailingSpace = true;
                    }

                    if (previousBreak)
                    {
                        breakSpace = true;
                        lineOfSpaces = true;
                    }

                    previousSpace = true;
                    previousBreak = false;
                }
                else if (buffer.IsBreak())
                {
                    if (isFirst)
                    {
                        leadingBreak = true;
                    }

                    if (buffer.Buffer.Position >= buffer.Buffer.Length - 1)
                    {
                        trailingBreak = true;
                    }

                    if (previousSpace)
                    {
                        spaceBreak = true;
                    }

                    if (lineOfSpaces)
                    {
                        linesOfSpaces = true;
                    }

                    previousSpace = false;
                    previousBreak = true;
                }
                else
                {
                    previousSpace = false;
                    previousBreak = false;
                    lineOfSpaces = false;
                }

                preceededByWhitespace = buffer.IsWhiteBreakOrZero();
                buffer.Skip(1);
                if (!buffer.EndOfInput)
                {
                    followedByWhitespace = buffer.IsWhiteBreakOrZero(1);
                }

                isFirst = false;
            }

            scalarData.IsFlowPlainAllowed = true;
            scalarData.IsBlockPlainAllowed = true;
            scalarData.IsSingleQuotedAllowed = true;
            scalarData.IsBlockAllowed = true;

            if (leadingSpace || leadingBreak || trailingSpace || trailingBreak || leadingQuote)
            {
                scalarData.IsFlowPlainAllowed = false;
                scalarData.IsBlockPlainAllowed = false;
            }

            if (trailingSpace)
            {
                scalarData.IsBlockAllowed = false;
            }

            if (breakSpace)
            {
                scalarData.IsFlowPlainAllowed = false;
                scalarData.IsBlockPlainAllowed = false;
                scalarData.IsSingleQuotedAllowed = false;
            }

            if (spaceBreak || specialCharacters)
            {
                scalarData.IsFlowPlainAllowed = false;
                scalarData.IsBlockPlainAllowed = false;
                scalarData.IsSingleQuotedAllowed = false;
            }
            if (linesOfSpaces)
            {
                scalarData.IsBlockAllowed = false;
            }

            scalarData.IsMultiline = lineBreaks;
            if (lineBreaks)
            {
                scalarData.IsFlowPlainAllowed = false;
                scalarData.IsBlockPlainAllowed = false;
            }

            if (flowIndicators)
            {
                scalarData.IsFlowPlainAllowed = false;
            }

            if (blockIndicators)
            {
                scalarData.IsBlockPlainAllowed = false;
            }

            scalarData.HasSingleQuotes = singleQuotes;
        }

        private bool ValueIsRepresentableInOutputEncoding(string value)
        {
            if (outputUsesUnicodeEncoding)
            {
                return true;
            }

            try
            {
                var encodedBytes = output.Encoding.GetBytes(value);
                var decodedString = output.Encoding.GetString(encodedBytes, 0, encodedBytes.Length);
                return decodedString.Equals(value);
            }
            catch (EncoderFallbackException)
            {
                return false;
            }
            catch (ArgumentOutOfRangeException)
            {
                return false;
            }
        }

        private static bool IsUnicode(Encoding encoding)
        {
            return encoding is UTF8Encoding ||
                   encoding is UnicodeEncoding ||
                   encoding is UTF7Encoding;
        }

        private void AnalyzeTag(TagName tag)
        {
            tagData.Handle = tag.Value;
            foreach (var tagDirective in tagDirectives)
            {
                if (tag.Value.StartsWith(tagDirective.Prefix, StringComparison.Ordinal))
                {
                    tagData.Handle = tagDirective.Handle;
                    tagData.Suffix = tag.Value.Substring(tagDirective.Prefix.Length);
                    break;
                }
            }
        }

        private void StateMachine(ParsingEvent evt)
        {
            if (evt is Comment comment)
            {
                EmitComment(comment);
                return;
            }

            switch (state)
            {
                case EmitterState.StreamStart:
                    EmitStreamStart(evt);
                    break;

                case EmitterState.FirstDocumentStart:
                    EmitDocumentStart(evt, true);
                    break;

                case EmitterState.DocumentStart:
                    EmitDocumentStart(evt, false);
                    break;

                case EmitterState.DocumentContent:
                    EmitDocumentContent(evt);
                    break;

                case EmitterState.DocumentEnd:
                    EmitDocumentEnd(evt);
                    break;

                case EmitterState.FlowSequenceFirstItem:
                    EmitFlowSequenceItem(evt, true);
                    break;

                case EmitterState.FlowSequenceItem:
                    EmitFlowSequenceItem(evt, false);
                    break;

                case EmitterState.FlowMappingFirstKey:
                    EmitFlowMappingKey(evt, true);
                    break;

                case EmitterState.FlowMappingKey:
                    EmitFlowMappingKey(evt, false);
                    break;

                case EmitterState.FlowMappingSimpleValue:
                    EmitFlowMappingValue(evt, true);
                    break;

                case EmitterState.FlowMappingValue:
                    EmitFlowMappingValue(evt, false);
                    break;

                case EmitterState.BlockSequenceFirstItem:
                    EmitBlockSequenceItem(evt, true);
                    break;

                case EmitterState.BlockSequenceItem:
                    EmitBlockSequenceItem(evt, false);
                    break;

                case EmitterState.BlockMappingFirstKey:
                    EmitBlockMappingKey(evt, true);
                    break;

                case EmitterState.BlockMappingKey:
                    EmitBlockMappingKey(evt, false);
                    break;

                case EmitterState.BlockMappingSimpleValue:
                    EmitBlockMappingValue(evt, true);
                    break;

                case EmitterState.BlockMappingValue:
                    EmitBlockMappingValue(evt, false);
                    break;

                case EmitterState.StreamEnd:
                    throw new YamlException("Expected nothing after STREAM-END");

                default:
                    throw new InvalidOperationException();
            }
        }

        private void EmitComment(Comment comment)
        {
            // If we're in flow mode or about to enter it: Skip comments.
            if (flowLevel > 0 || state == EmitterState.FlowMappingFirstKey || state == EmitterState.FlowSequenceFirstItem)
            {
                return;
            }

            var lines = comment.Value.Split(NewLineSeparators, StringSplitOptions.None);

            if (comment.IsInline)
            {
                Write(" # ");
                Write(string.Join(" ", lines));
            }
            else
            {
                // If we're about to enter a YAML block we need to manually increase the indent for the comment and then decrease again.
                var isFirst = state == EmitterState.BlockMappingFirstKey;

                if (isFirst)
                {
                    IncreaseIndent(false, false);
                }

                foreach (var line in lines)
                {
                    WriteIndent();
                    Write("# ");
                    Write(line);
                    WriteBreak();
                }

                if (isFirst)
                {
                    indent = indents.Pop();
                }
            }

            isIndentation = true;
        }

        /// <summary>
        /// Expect STREAM-START.
        /// </summary>
        private void EmitStreamStart(ParsingEvent evt)
        {
            if (!(evt is StreamStart))
            {
                throw new ArgumentException("Expected STREAM-START.", nameof(evt));
            }

            indent = -1;
            column = 0;
            isWhitespace = true;
            isIndentation = true;

            state = EmitterState.FirstDocumentStart;
        }

        /// <summary>
        /// Expect DOCUMENT-START or STREAM-END.
        /// </summary>
        private void EmitDocumentStart(ParsingEvent evt, bool isFirst)
        {
            if (evt is DocumentStart documentStart)
            {
                var isImplicit = documentStart.IsImplicit
                    && isFirst
                    && !isCanonical;

                var documentTagDirectives = NonDefaultTagsAmong(documentStart.Tags);

                if (!isFirst && !isDocumentEndWritten && (documentStart.Version != null || documentTagDirectives.Count > 0))
                {
                    isDocumentEndWritten = false;
                    WriteIndicator("...", true, false, false);
                    WriteIndent();
                }

                if (documentStart.Version != null)
                {
                    Emitter.AnalyzeVersionDirective(documentStart.Version);

                    var documentVersion = documentStart.Version.Version;
                    isImplicit = false;
                    WriteIndicator("%YAML", true, false, false);
                    WriteIndicator(string.Format(CultureInfo.InvariantCulture,
                        "{0}.{1}", documentVersion.Major, documentVersion.Minor),
                        true, false, false);
                    WriteIndent();
                }

                foreach (var tagDirective in documentTagDirectives)
                {
                    AppendTagDirectiveTo(tagDirective, false, tagDirectives);
                }

                foreach (var tagDirective in Constants.DefaultTagDirectives)
                {
                    AppendTagDirectiveTo(tagDirective, true, tagDirectives);
                }

                if (documentTagDirectives.Count > 0)
                {
                    isImplicit = false;
                    foreach (var tagDirective in Constants.DefaultTagDirectives)
                    {
                        AppendTagDirectiveTo(tagDirective, true, documentTagDirectives);
                    }

                    foreach (var tagDirective in documentTagDirectives)
                    {
                        WriteIndicator("%TAG", true, false, false);
                        WriteTagHandle(tagDirective.Handle);
                        WriteTagContent(tagDirective.Prefix, true);
                        WriteIndent();
                    }
                }

                if (CheckEmptyDocument())
                {
                    isImplicit = false;
                }

                if (!isImplicit)
                {
                    WriteIndent();
                    WriteIndicator("---", true, false, false);
                    if (isCanonical)
                    {
                        WriteIndent();
                    }
                }

                state = EmitterState.DocumentContent;
            }

            else if (evt is StreamEnd)
            {
                state = EmitterState.StreamEnd;
            }
            else
            {
                throw new YamlException("Expected DOCUMENT-START or STREAM-END");
            }
        }

        private static TagDirectiveCollection NonDefaultTagsAmong(IEnumerable<TagDirective>? tagCollection)
        {
            var directives = new TagDirectiveCollection();
            if (tagCollection == null)
            {
                return directives;
            }

            foreach (var tagDirective in tagCollection)
            {
                AppendTagDirectiveTo(tagDirective, false, directives);
            }
            foreach (var tagDirective in Constants.DefaultTagDirectives)
            {
                directives.Remove(tagDirective);
            }
            return directives;
        }

        private static void AnalyzeVersionDirective(VersionDirective versionDirective)
        {
            if (versionDirective.Version.Major != Constants.MajorVersion || versionDirective.Version.Minor > Constants.MinorVersion)
            {
                throw new YamlException("Incompatible %YAML directive");
            }
        }

        private static void AppendTagDirectiveTo(TagDirective value, bool allowDuplicates, TagDirectiveCollection tagDirectives)
        {
            if (tagDirectives.Contains(value))
            {
                if (!allowDuplicates)
                {
                    throw new YamlException("Duplicate %TAG directive.");
                }
            }
            else
            {
                tagDirectives.Add(value);
            }
        }

        /// <summary>
        /// Expect the root node.
        /// </summary>
        private void EmitDocumentContent(ParsingEvent evt)
        {
            states.Push(EmitterState.DocumentEnd);
            EmitNode(evt, false, false);
        }

        /// <summary>
        /// Expect a node.
        /// </summary>
        private void EmitNode(ParsingEvent evt, bool isMapping, bool isSimpleKey)
        {
            isMappingContext = isMapping;
            isSimpleKeyContext = isSimpleKey;

            switch (evt.Type)
            {
                case EventType.Alias:
                    EmitAlias();
                    break;

                case EventType.Scalar:
                    EmitScalar(evt);
                    break;

                case EventType.SequenceStart:
                    EmitSequenceStart(evt);
                    break;

                case EventType.MappingStart:
                    EmitMappingStart(evt);
                    break;

                default:
                    throw new YamlException($"Expected SCALAR, SEQUENCE-START, MAPPING-START, or ALIAS, got {evt.Type}");
            }
        }

        /// <summary>
        /// Expect ALIAS.
        /// </summary>
        private void EmitAlias()
        {
            ProcessAnchor();
            state = states.Pop();
        }

        /// <summary>
        /// Expect SCALAR.
        /// </summary>
        private void EmitScalar(ParsingEvent evt)
        {
            SelectScalarStyle(evt);
            ProcessAnchor();
            ProcessTag();
            IncreaseIndent(true, false);
            ProcessScalar();

            indent = indents.Pop();
            state = states.Pop();
        }

        private void SelectScalarStyle(ParsingEvent evt)
        {
            var scalar = (Scalar)evt;

            var style = scalar.Style;
            var noTag = tagData.Handle == null && tagData.Suffix == null;

            if (noTag && !scalar.IsPlainImplicit && !scalar.IsQuotedImplicit)
            {
                throw new YamlException("Neither tag nor isImplicit flags are specified.");
            }

            if (style == ScalarStyle.Any)
            {
                style = scalarData.IsMultiline ? ScalarStyle.Folded : ScalarStyle.Plain;
            }

            if (isCanonical)
            {
                style = ScalarStyle.DoubleQuoted;
            }

            if (isSimpleKeyContext && scalarData.IsMultiline)
            {
                style = ScalarStyle.DoubleQuoted;
            }

            if (style == ScalarStyle.Plain)
            {
                if ((flowLevel != 0 && !scalarData.IsFlowPlainAllowed) || (flowLevel == 0 && !scalarData.IsBlockPlainAllowed))
                {
                    style = (scalarData.IsSingleQuotedAllowed && !scalarData.HasSingleQuotes) ? ScalarStyle.SingleQuoted : ScalarStyle.DoubleQuoted;
                }
                if (string.IsNullOrEmpty(scalarData.Value) && (flowLevel != 0 || isSimpleKeyContext))
                {
                    style = ScalarStyle.SingleQuoted;
                }
                if (noTag && !scalar.IsPlainImplicit)
                {
                    style = ScalarStyle.SingleQuoted;
                }
            }

            if (style == ScalarStyle.SingleQuoted)
            {
                if (!scalarData.IsSingleQuotedAllowed)
                {
                    style = ScalarStyle.DoubleQuoted;
                }
            }

            if (style == ScalarStyle.Literal || style == ScalarStyle.Folded)
            {
                if (!scalarData.IsBlockAllowed || flowLevel != 0 || isSimpleKeyContext)
                {
                    style = ScalarStyle.DoubleQuoted;
                }
            }

            if (style == ScalarStyle.ForcePlain)
            {
                style = ScalarStyle.Plain;
            }

            scalarData.Style = style;
        }

        private void ProcessScalar()
        {
            switch (scalarData.Style)
            {
                case ScalarStyle.Plain:
                    WritePlainScalar(scalarData.Value, !isSimpleKeyContext);
                    break;

                case ScalarStyle.SingleQuoted:
                    WriteSingleQuotedScalar(scalarData.Value, !isSimpleKeyContext);
                    break;

                case ScalarStyle.DoubleQuoted:
                    WriteDoubleQuotedScalar(scalarData.Value, !isSimpleKeyContext);
                    break;

                case ScalarStyle.Literal:
                    WriteLiteralScalar(scalarData.Value);
                    break;

                case ScalarStyle.Folded:
                    WriteFoldedScalar(scalarData.Value);
                    break;

                default:
                    throw new InvalidOperationException();
            }
        }

        #region Write scalar Methods

        private void WritePlainScalar(string value, bool allowBreaks)
        {
            if (!isWhitespace)
            {
                Write(' ');
            }

            var previousSpace = false;
            var previousBreak = false;
            for (var index = 0; index < value.Length; ++index)
            {
                var character = value[index];
                if (IsSpace(character))
                {
                    if (allowBreaks && !previousSpace && column > bestWidth && index + 1 < value.Length && value[index + 1] != ' ')
                    {
                        WriteIndent();
                    }
                    else
                    {
                        Write(character);
                    }
                    previousSpace = true;
                }
                else if (IsBreak(character, out var breakCharacter))
                {
                    if (!previousBreak && character == '\n')
                    {
                        WriteBreak();
                    }
                    WriteBreak(breakCharacter);
                    isIndentation = true;
                    previousBreak = true;
                }
                else
                {
                    if (previousBreak)
                    {
                        WriteIndent();
                    }
                    Write(character);
                    isIndentation = false;
                    previousSpace = false;
                    previousBreak = false;
                }
            }

            isWhitespace = false;
            isIndentation = false;
        }

        private void WriteSingleQuotedScalar(string value, bool allowBreaks)
        {
            WriteIndicator("'", true, false, false);

            var previousSpace = false;
            var previousBreak = false;

            for (var index = 0; index < value.Length; ++index)
            {
                var character = value[index];
                if (character == ' ')
                {
                    if (allowBreaks && !previousSpace && column > bestWidth && index != 0 && index + 1 < value.Length &&
                        value[index + 1] != ' ')
                    {
                        WriteIndent();
                    }
                    else
                    {
                        Write(character);
                    }
                    previousSpace = true;
                }
                else if (IsBreak(character, out var breakCharacter))
                {
                    if (!previousBreak && character == '\n')
                    {
                        WriteBreak();
                    }
                    WriteBreak(breakCharacter);
                    isIndentation = true;
                    previousBreak = true;
                }
                else
                {
                    if (previousBreak)
                    {
                        WriteIndent();
                    }
                    if (character == '\'')
                    {
                        Write(character);
                    }
                    Write(character);
                    isIndentation = false;
                    previousSpace = false;
                    previousBreak = false;
                }
            }

            WriteIndicator("'", false, false, false);

            isWhitespace = false;
            isIndentation = false;
        }

        private void WriteDoubleQuotedScalar(string value, bool allowBreaks)
        {
            WriteIndicator("\"", true, false, false);

            var previousSpace = false;
            for (var index = 0; index < value.Length; ++index)
            {
                var character = value[index];
                if (!IsPrintable(character) || IsBreak(character, out _) || character == '"' || character == '\\')
                {
                    Write('\\');

                    switch (character)
                    {
                        case '\0':
                            Write('0');
                            break;

                        case '\x7':
                            Write('a');
                            break;

                        case '\x8':
                            Write('b');
                            break;

                        case '\x9':
                            Write('t');
                            break;

                        case '\xA':
                            Write('n');
                            break;

                        case '\xB':
                            Write('v');
                            break;

                        case '\xC':
                            Write('f');
                            break;

                        case '\xD':
                            Write('r');
                            break;

                        case '\x1B':
                            Write('e');
                            break;

                        case '\x22':
                            Write('"');
                            break;

                        case '\x5C':
                            Write('\\');
                            break;

                        case '\x85':
                            Write('N');
                            break;

                        case '\xA0':
                            Write('_');
                            break;

                        case '\x2028':
                            Write('L');
                            break;

                        case '\x2029':
                            Write('P');
                            break;

                        default:
                            var code = (ushort)character;
                            if (code <= 0xFF)
                            {
                                Write('x');
                                Write(code.ToString("X02", CultureInfo.InvariantCulture));
                            }
                            else if (IsHighSurrogate(character))
                            {
                                if (index + 1 < value.Length && IsLowSurrogate(value[index + 1]))
                                {
                                    if (useUtf16SurrogatePair)
                                    {
                                        Write('u');
                                        Write(code.ToString("X04", CultureInfo.InvariantCulture));
                                        Write('\\');
                                        Write('u');
                                        Write(((ushort)value[index + 1]).ToString("X04", CultureInfo.InvariantCulture));
                                    }
                                    else
                                    {
                                        Write('U');
                                        Write(char.ConvertToUtf32(character, value[index + 1]).ToString("X08", CultureInfo.InvariantCulture));
                                    }

                                    index++;
                                }
                                else
                                {
                                    throw new SyntaxErrorException("While writing a quoted scalar, found an orphaned high surrogate.");
                                }
                            }
                            else
                            {
                                Write('u');
                                Write(code.ToString("X04", CultureInfo.InvariantCulture));
                            }
                            break;
                    }
                    previousSpace = false;
                }
                else if (character == ' ')
                {
                    if (allowBreaks && !previousSpace && column > bestWidth && index > 0 && index + 1 < value.Length)
                    {
                        WriteIndent();
                        if (value[index + 1] == ' ')
                        {
                            Write('\\');
                        }
                    }
                    else
                    {
                        Write(character);
                    }
                    previousSpace = true;
                }
                else
                {
                    Write(character);
                    previousSpace = false;
                }
            }

            WriteIndicator("\"", false, false, false);

            isWhitespace = false;
            isIndentation = false;
        }

        private void WriteLiteralScalar(string value)
        {
            var previousBreak = true;

            WriteIndicator("|", true, false, false);
            WriteBlockScalarHints(value);
            WriteBreak();

            isIndentation = true;
            isWhitespace = true;

            for (var i = 0; i < value.Length; ++i)
            {
                var character = value[i];
                if (character == '\r' && (i + 1) < value.Length && value[i + 1] == '\n')
                {
                    continue;
                }

                if (IsBreak(character, out var breakCharacter))
                {
                    WriteBreak(breakCharacter);
                    isIndentation = true;
                    previousBreak = true;
                }
                else
                {
                    if (previousBreak)
                    {
                        WriteIndent();
                    }
                    Write(character);
                    isIndentation = false;
                    previousBreak = false;
                }
            }
        }

        private void WriteFoldedScalar(string value)
        {
            var previousBreak = true;
            var leadingSpaces = true;

            WriteIndicator(">", true, false, false);
            WriteBlockScalarHints(value);
            WriteBreak();

            isIndentation = true;
            isWhitespace = true;

            for (var i = 0; i < value.Length; ++i)
            {
                var character = value[i];
                if (IsBreak(character, out var breakCharacter))
                {
                    if (character == '\r' && (i + 1) < value.Length && value[i + 1] == '\n')
                    {
                        continue;
                    }

                    if (!previousBreak && !leadingSpaces && breakCharacter == '\n')
                    {
                        var k = 0;
                        while (i + k < value.Length && IsBreak(value[i + k], out _))
                        {
                            ++k;
                        }
                        if (i + k < value.Length && !(IsBlank(value[i + k]) || IsBreak(value[i + k], out _)))
                        {
                            WriteBreak();
                        }
                    }

                    WriteBreak(breakCharacter);
                    isIndentation = true;
                    previousBreak = true;
                }
                else
                {
                    if (previousBreak)
                    {
                        WriteIndent();
                        leadingSpaces = IsBlank(character);
                    }
                    if (!previousBreak && character == ' ' && i + 1 < value.Length && value[i + 1] != ' ' && column > bestWidth)
                    {
                        WriteIndent();
                    }
                    else
                    {
                        Write(character);
                    }
                    isIndentation = false;
                    previousBreak = false;
                }
            }
        }

        // Todo: isn't this what CharacterAnalyser is for?
        private static bool IsSpace(char character)
        {
            return character == ' ';
        }

        private static bool IsBreak(char character, out char breakChar)
        {
            switch (character)
            {
                case '\r':
                case '\n':
                case '\x85':
                    breakChar = '\n';
                    return true;

                case '\x2028':
                case '\x2029':
                    breakChar = character;
                    return true;

                default:
                    breakChar = '\0';
                    return false;
            }
        }

        private static bool IsBlank(char character)
        {
            return character == ' ' || character == '\t';
        }

        private static bool IsPrintable(char character)
        {
            return
                character == '\x9' ||
                    character == '\xA' ||
                    character == '\xD' ||
                    (character >= '\x20' && character <= '\x7E') ||
                    character == '\x85' ||
                    (character >= '\xA0' && character <= '\xD7FF') ||
                    (character >= '\xE000' && character <= '\xFFFD');
        }

        private static bool IsHighSurrogate(char c)
        {
            return 0xD800 <= c && c <= 0xDBFF;
        }

        private static bool IsLowSurrogate(char c)
        {
            return 0xDC00 <= c && c <= 0xDFFF;
        }

        #endregion

        /// <summary>
        /// Expect SEQUENCE-START.
        /// </summary>
        private void EmitSequenceStart(ParsingEvent evt)
        {
            ProcessAnchor();
            ProcessTag();

            var sequenceStart = (SequenceStart)evt;

            if (flowLevel != 0 || isCanonical || sequenceStart.Style == SequenceStyle.Flow || CheckEmptySequence())
            {
                state = EmitterState.FlowSequenceFirstItem;
            }
            else
            {
                state = EmitterState.BlockSequenceFirstItem;
            }
        }

        /// <summary>
        /// Expect MAPPING-START.
        /// </summary>
        private void EmitMappingStart(ParsingEvent evt)
        {
            ProcessAnchor();
            ProcessTag();

            var mappingStart = (MappingStart)evt;

            if (flowLevel != 0 || isCanonical || mappingStart.Style == MappingStyle.Flow || CheckEmptyMapping())
            {
                state = EmitterState.FlowMappingFirstKey;
            }
            else
            {
                state = EmitterState.BlockMappingFirstKey;
            }
        }

        private void ProcessAnchor()
        {
            if (!anchorData.Anchor.IsEmpty && !skipAnchorName)
            {
                WriteIndicator(anchorData.IsAlias ? "*" : "&", true, false, false);
                WriteAnchor(anchorData.Anchor);
            }
        }

        private void ProcessTag()
        {
            if (tagData.Handle == null && tagData.Suffix == null)
            {
                return;
            }

            if (tagData.Handle != null)
            {
                WriteTagHandle(tagData.Handle);
                if (tagData.Suffix != null)
                {
                    WriteTagContent(tagData.Suffix, false);
                }
            }
            else
            {
                WriteIndicator("!<", true, false, false);
                WriteTagContent(tagData.Suffix!, false);
                WriteIndicator(">", false, false, false);
            }
        }

        /// <summary>
        /// Expect DOCUMENT-END.
        /// </summary>
        private void EmitDocumentEnd(ParsingEvent evt)
        {
            if (evt is DocumentEnd documentEnd)
            {
                WriteIndent();
                if (!documentEnd.IsImplicit)
                {
                    WriteIndicator("...", true, false, false);
                    WriteIndent();
                    isDocumentEndWritten = true;
                }

                state = EmitterState.DocumentStart;

                tagDirectives.Clear();
            }
            else
            {
                throw new YamlException("Expected DOCUMENT-END.");
            }
        }

        /// <summary>
        /// Expect a flow item node.
        /// </summary>
        private void EmitFlowSequenceItem(ParsingEvent evt, bool isFirst)
        {
            if (isFirst)
            {
                WriteIndicator("[", true, true, false);
                IncreaseIndent(true, false);
                ++flowLevel;
            }

            if (evt is SequenceEnd)
            {
                --flowLevel;
                indent = indents.Pop();
                if (isCanonical && !isFirst)
                {
                    WriteIndicator(",", false, false, false);
                    WriteIndent();
                }
                WriteIndicator("]", false, false, false);
                state = states.Pop();
                return;
            }

            if (!isFirst)
            {
                WriteIndicator(",", false, false, false);
            }

            if (isCanonical || column > bestWidth)
            {
                WriteIndent();
            }

            states.Push(EmitterState.FlowSequenceItem);

            EmitNode(evt, false, false);
        }

        /// <summary>
        /// Expect a flow key node.
        /// </summary>
        private void EmitFlowMappingKey(ParsingEvent evt, bool isFirst)
        {
            if (isFirst)
            {
                WriteIndicator("{", true, true, false);
                IncreaseIndent(true, false);
                ++flowLevel;
            }

            if (evt is MappingEnd)
            {
                --flowLevel;
                indent = indents.Pop();
                if (isCanonical && !isFirst)
                {
                    WriteIndicator(",", false, false, false);
                    WriteIndent();
                }
                WriteIndicator("}", false, false, false);
                state = states.Pop();
                return;
            }

            if (!isFirst)
            {
                WriteIndicator(",", false, false, false);
            }
            if (isCanonical || column > bestWidth)
            {
                WriteIndent();
            }

            if (!isCanonical && CheckSimpleKey())
            {
                states.Push(EmitterState.FlowMappingSimpleValue);
                EmitNode(evt, true, true);
            }
            else
            {
                WriteIndicator("?", true, false, false);
                states.Push(EmitterState.FlowMappingValue);
                EmitNode(evt, true, false);
            }
        }

        /// <summary>
        /// Expect a flow value node.
        /// </summary>
        private void EmitFlowMappingValue(ParsingEvent evt, bool isSimple)
        {
            if (isSimple)
            {
                WriteIndicator(":", false, false, false);
            }
            else
            {
                if (isCanonical || column > bestWidth)
                {
                    WriteIndent();
                }
                WriteIndicator(":", true, false, false);
            }
            states.Push(EmitterState.FlowMappingKey);
            EmitNode(evt, true, false);
        }

        /// <summary>
        /// Expect a block item node.
        /// </summary>
        private void EmitBlockSequenceItem(ParsingEvent evt, bool isFirst)
        {
            if (isFirst)
            {
                IncreaseIndent(false, (isMappingContext && !isIndentation));
            }

            if (evt is SequenceEnd)
            {
                indent = indents.Pop();
                state = states.Pop();
                return;
            }

            WriteIndent();
            WriteIndicator("-", true, false, true);
            states.Push(EmitterState.BlockSequenceItem);

            EmitNode(evt, false, false);
        }

        /// <summary>
        /// Expect a block key node.
        /// </summary>
        private void EmitBlockMappingKey(ParsingEvent evt, bool isFirst)
        {
            if (isFirst)
            {
                IncreaseIndent(false, false);
            }

            if (evt is MappingEnd)
            {
                indent = indents.Pop();
                state = states.Pop();
                return;
            }

            WriteIndent();

            if (CheckSimpleKey())
            {
                states.Push(EmitterState.BlockMappingSimpleValue);
                EmitNode(evt, true, true);
                WriteIndicator(":", false, false, false);
            }
            else
            {
                WriteIndicator("?", true, false, true);
                states.Push(EmitterState.BlockMappingValue);
                EmitNode(evt, true, false);
            }
        }

        /// <summary>
        /// Expect a block value node.
        /// </summary>
        private void EmitBlockMappingValue(ParsingEvent evt, bool isSimple)
        {
            if (!isSimple)
            {
                WriteIndent();
                WriteIndicator(":", true, false, true);
            }
            states.Push(EmitterState.BlockMappingKey);
            EmitNode(evt, true, false);
        }

        private void IncreaseIndent(bool isFlow, bool isIndentless)
        {
            indents.Push(indent);

            if (indent < 0)
            {
                indent = isFlow ? bestIndent : 0;
            }
            else if (!isIndentless || !forceIndentLess)
            {
                indent += bestIndent;
            }
        }

        #region Check Methods

        /// <summary>
        /// Check if the document content is an empty scalar.
        /// </summary>
        private bool CheckEmptyDocument()
        {
            var index = 0;
            foreach (var parsingEvent in events)
            {
                index++;
                if (index == 2)
                {
                    if (parsingEvent is Scalar scalar)
                    {
                        return string.IsNullOrEmpty(scalar.Value);
                    }
                    break;
                }
            }

            return false;
        }

        /// <summary>
        /// Check if the next node can be expressed as a simple key.
        /// </summary>
        private bool CheckSimpleKey()
        {
            if (events.Count < 1)
            {
                return false;
            }

            int length;
            switch (events.Peek().Type)
            {
                case EventType.Alias:
                    length = Emitter.AnchorNameLength(anchorData.Anchor);
                    break;

                case EventType.Scalar:
                    if (scalarData.IsMultiline)
                    {
                        return false;
                    }

                    length =
                        Emitter.AnchorNameLength(anchorData.Anchor) +
                            SafeStringLength(tagData.Handle) +
                            SafeStringLength(tagData.Suffix) +
                            SafeStringLength(scalarData.Value);
                    break;

                case EventType.SequenceStart:
                    if (!CheckEmptySequence())
                    {
                        return false;
                    }
                    length =
                        Emitter.AnchorNameLength(anchorData.Anchor) +
                            SafeStringLength(tagData.Handle) +
                            SafeStringLength(tagData.Suffix);
                    break;

                case EventType.MappingStart:
                    if (!CheckEmptySequence())
                    {
                        return false;
                    }
                    length =
                        Emitter.AnchorNameLength(anchorData.Anchor) +
                            SafeStringLength(tagData.Handle) +
                            SafeStringLength(tagData.Suffix);
                    break;

                default:
                    return false;
            }

            return length <= maxSimpleKeyLength;
        }

        private static int AnchorNameLength(AnchorName value)
        {
            return value.IsEmpty ? 0 : value.Value.Length;
        }

        private static int SafeStringLength(string? value)
        {
            return value?.Length ?? 0;
        }

        private bool CheckEmptySequence() => CheckEmptyStructure<SequenceStart, SequenceEnd>();
        private bool CheckEmptyMapping() => CheckEmptyStructure<MappingStart, MappingEnd>();

        private bool CheckEmptyStructure<TStart, TEnd>()
            where TStart : NodeEvent
            where TEnd : ParsingEvent
        {
            if (events.Count < 2)
            {
                return false;
            }

            using var enumerator = events.GetEnumerator();
            return enumerator.MoveNext()
                && enumerator.Current is TStart
                && enumerator.MoveNext()
                && enumerator.Current is TEnd;
        }
        #endregion

        #region Write Methods

        private void WriteBlockScalarHints(string value)
        {
            using var slab = StringLookAheadBufferPool.Rent(value);
            var analyzer = new CharacterAnalyzer<StringLookAheadBuffer>(slab.Buffer);

            if (analyzer.IsSpace() || analyzer.IsBreak())
            {
                var indentHint = bestIndent.ToString(CultureInfo.InvariantCulture);
                WriteIndicator(indentHint, false, false, false);
            }

            string? chompHint = null;
            if (value.Length == 0 || !analyzer.IsBreak(value.Length - 1))
            {
                chompHint = "-";
            }
            else if (value.Length >= 2 && analyzer.IsBreak(value.Length - 2))
            {
                chompHint = "+";
            }

            if (chompHint != null)
            {
                WriteIndicator(chompHint, false, false, false);
            }
        }

        private void WriteIndicator(string indicator, bool needWhitespace, bool whitespace, bool indentation)
        {
            if (needWhitespace && !isWhitespace)
            {
                Write(' ');
            }

            Write(indicator);

            isWhitespace = whitespace;
            isIndentation &= indentation;
        }

        private void WriteIndent()
        {
            var currentIndent = Math.Max(indent, 0);

            var isBreakRequired = !isIndentation
                || column > currentIndent
                || (column == currentIndent && !isWhitespace);

            if (isBreakRequired)
            {
                WriteBreak();
            }

            while (column < currentIndent)
            {
                Write(' ');
            }

            isWhitespace = true;
            isIndentation = true;
        }

        private void WriteAnchor(AnchorName value)
        {
            Write(value.Value);

            isWhitespace = false;
            isIndentation = false;
        }

        private void WriteTagHandle(string value)
        {
            if (!isWhitespace)
            {
                Write(' ');
            }

            Write(value);

            isWhitespace = false;
            isIndentation = false;
        }

        private void WriteTagContent(string value, bool needsWhitespace)
        {
            if (needsWhitespace && !isWhitespace)
            {
                Write(' ');
            }

            Write(UrlEncode(value));

            isWhitespace = false;
            isIndentation = false;
        }

        private static string UrlEncode(string text)
        {
            return UriReplacer.Replace(text, delegate (Match match)
            {
                using var bufferBuilder = StringBuilderPool.Rent();
                var buffer = bufferBuilder.Builder;
                foreach (var toEncode in Encoding.UTF8.GetBytes(match.Value))
                {
                    buffer.AppendFormat(CultureInfo.InvariantCulture, "%{0:X02}", toEncode);
                }
                return buffer.ToString();
            });
        }

        private void Write(char value)
        {
            output.Write(value);
            ++column;
        }

        private void Write(string value)
        {
            output.Write(value);
            column += value.Length;
        }

        private void WriteBreak(char breakCharacter = '\n')
        {
            if (breakCharacter == '\n')
            {
                output.WriteLine();
            }
            else
            {
                output.Write(breakCharacter);
            }
            column = 0;
        }

        #endregion
    }
}