part of dart._engine;
// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.





/// Performs layout on a [CanvasParagraph].
///
/// It uses a [html.CanvasElement] to measure text.
class TextLayoutService {
  TextLayoutService(this.paragraph);

  final CanvasParagraph paragraph;

  final html.CanvasRenderingContext2D context = html.CanvasElement().context2D;

  // *** Results of layout *** //

  // Look at the Paragraph class for documentation of the following properties.

  double width = -1.0;

  double height = 0.0;

  EngineLineMetrics? longestLine;

  double minIntrinsicWidth = 0.0;

  double maxIntrinsicWidth = 0.0;

  double alphabeticBaseline = -1.0;

  double ideographicBaseline = -1.0;

  bool didExceedMaxLines = false;

  final List<EngineLineMetrics> lines = <EngineLineMetrics>[];

  // *** Convenient shortcuts used during layout *** //

  int? get maxLines => paragraph.paragraphStyle.maxLines;
  bool get unlimitedLines => maxLines == null;

  String? get ellipsis => paragraph.paragraphStyle.ellipsis;
  bool get hasEllipsis => ellipsis != null;

  /// Performs the layout on a paragraph given the [constraints].
  ///
  /// The function starts by resetting all layout-related properties. Then it
  /// starts looping through the paragraph to calculate all layout metrics.
  ///
  /// It uses a [Spanometer] to perform measurements within spans of the
  /// paragraph. It also uses [LineBuilders] to generate [EngineLineMetrics] as
  /// it iterates through the paragraph.
  ///
  /// The main loop keeps going until:
  ///
  /// 1. The end of the paragraph is reached (i.e. LineBreakType.endOfText).
  /// 2. Enough lines have been computed to satisfy [maxLines].
  /// 3. An ellipsis is appended because of an overflow.
  void performLayout(ui.ParagraphConstraints constraints) {
    final int spanCount = paragraph.spans.length;

    // Reset results from previous layout.
    width = constraints.width;
    height = 0.0;
    longestLine = null;
    minIntrinsicWidth = 0.0;
    maxIntrinsicWidth = 0.0;
    didExceedMaxLines = false;
    lines.clear();

    if (spanCount == 0) {
      return;
    }

    final Spanometer spanometer = Spanometer(paragraph, context);

    int spanIndex = 0;
    LineBuilder currentLine =
        LineBuilder.first(paragraph, spanometer, maxWidth: constraints.width);

    // The only way to exit this while loop is by hitting one of the `break;`
    // statements (e.g. when we reach `endOfText`, when ellipsis has been
    // appended).
    while (true) {
      // ************************** //
      // *** HANDLE END OF TEXT *** //
      // ************************** //

      // All spans have been consumed.
      final bool reachedEnd = spanIndex == spanCount;
      if (reachedEnd) {
        // In some cases, we need to extend the line to the end of text and
        // build it:
        //
        // 1. Line is not empty. This could happen when the last span is a
        //    placeholder.
        //
        // 2. We haven't reached `LineBreakType.endOfText` yet. This could
        //    happen when the last character is a new line.
        if (currentLine.isNotEmpty || currentLine.end.type != LineBreakType.endOfText) {
          currentLine.extendToEndOfText();
          lines.add(currentLine.build());
        }
        break;
      }

      // ********************************* //
      // *** THE MAIN MEASUREMENT PART *** //
      // ********************************* //

      ParagraphSpan span = paragraph.spans[spanIndex];

      if (span is PlaceholderSpan) {
        if (currentLine.widthIncludingSpace + span.width <= constraints.width) {
          // The placeholder fits on the current line.
          currentLine.addPlaceholder(span);
        } else {
          // The placeholder can't fit on the current line.
          if (currentLine.isNotEmpty) {
            lines.add(currentLine.build());
            currentLine = currentLine.nextLine();
          }
          currentLine.addPlaceholder(span);
        }
        spanIndex++;
      } else if (span is FlatTextSpan) {
        spanometer.currentSpan = span;
        final DirectionalPosition nextBreak = currentLine.findNextBreak();
        final double additionalWidth =
            currentLine.getAdditionalWidthTo(nextBreak.lineBreak);

        if (currentLine.width + additionalWidth <= constraints.width) {
          // The line can extend to `nextBreak` without overflowing.
          currentLine.extendTo(nextBreak);
          if (nextBreak.type == LineBreakType.mandatory) {
            lines.add(currentLine.build());
            currentLine = currentLine.nextLine();
          }
        } else {
          // The chunk of text can't fit into the current line.
          final bool isLastLine =
              (hasEllipsis && unlimitedLines) || lines.length + 1 == maxLines;

          if (isLastLine && hasEllipsis) {
            // We've reached the line that requires an ellipsis to be appended
            // to it.

            currentLine.forceBreak(
              nextBreak,
              allowEmpty: true,
              ellipsis: ellipsis,
            );
            lines.add(currentLine.build(ellipsis: ellipsis));
            break;
          } else if (currentLine.isNotBreakable) {
            // The entire line is unbreakable, which means we are dealing
            // with a single block of text that doesn't fit in a single line.
            // We need to force-break it without adding an ellipsis.

            currentLine.forceBreak(nextBreak, allowEmpty: false);
            lines.add(currentLine.build());
            currentLine = currentLine.nextLine();
          } else {
            // Normal line break.
            currentLine.revertToLastBreakOpportunity();
            // If a revert had occurred in the line, we need to revert the span
            // index accordingly.
            //
            // If no revert occurred, then `revertedToSpan` will be equal to
            // `span` and the following while loop won't do anything.
            final ParagraphSpan revertedToSpan = currentLine.lastSegment.span;
            while (span != revertedToSpan) {
              span = paragraph.spans[--spanIndex];
            }
            lines.add(currentLine.build());
            currentLine = currentLine.nextLine();
          }
        }

        // Only go to the next span if we've reached the end of this span.
        if (currentLine.end.index >= span.end) {
          currentLine.createBox();
          ++spanIndex;
        }
      } else {
        throw UnimplementedError('Unknown span type: ${span.runtimeType}');
      }

      if (lines.length == maxLines) {
        break;
      }
    }

    // ************************************************** //
    // *** PARAGRAPH BASELINE & HEIGHT & LONGEST LINE *** //
    // ************************************************** //

    for (final EngineLineMetrics line in lines) {
      height += line.height;
      if (alphabeticBaseline == -1.0) {
        alphabeticBaseline = line.baseline;
        ideographicBaseline = alphabeticBaseline * baselineRatioHack;
      }
      final double longestLineWidth = longestLine?.width ?? 0.0;
      if (longestLineWidth < line.width) {
        longestLine = line;
      }
    }

    // ******************************** //
    // *** MAX/MIN INTRINSIC WIDTHS *** //
    // ******************************** //

    spanIndex = 0;
    currentLine =
        LineBuilder.first(paragraph, spanometer, maxWidth: constraints.width);

    while (spanIndex < spanCount) {
      final ParagraphSpan span = paragraph.spans[spanIndex];
      bool breakToNextLine = false;

      if (span is PlaceholderSpan) {
        currentLine.addPlaceholder(span);
        spanIndex++;
      } else if (span is FlatTextSpan) {
        spanometer.currentSpan = span;
        final DirectionalPosition nextBreak = currentLine.findNextBreak();

        // For the purpose of max intrinsic width, we don't care if the line
        // fits within the constraints or not. So we always extend it.
        currentLine.extendTo(nextBreak);
        if (nextBreak.type == LineBreakType.mandatory) {
          // We don't want to break the line now because we want to update
          // min/max intrinsic widths below first.
          breakToNextLine = true;
        }

        // Only go to the next span if we've reached the end of this span.
        if (currentLine.end.index >= span.end) {
          spanIndex++;
        }
      }

      final double widthOfLastSegment = currentLine.lastSegment.width;
      if (minIntrinsicWidth < widthOfLastSegment) {
        minIntrinsicWidth = widthOfLastSegment;
      }

      // Max intrinsic width includes the width of trailing spaces.
      if (maxIntrinsicWidth < currentLine.widthIncludingSpace) {
        maxIntrinsicWidth = currentLine.widthIncludingSpace;
      }

      if (breakToNextLine) {
        currentLine = currentLine.nextLine();
      }
    }
  }

  List<ui.TextBox> getBoxesForPlaceholders() {
    final List<ui.TextBox> boxes = <ui.TextBox>[];
    for (final EngineLineMetrics line in lines) {
      for (final RangeBox box in line.boxes) {
        if (box is PlaceholderBox) {
          boxes.add(box.toTextBox(line));
        }
      }
    }
    return boxes;
  }

  List<ui.TextBox> getBoxesForRange(
    int start,
    int end,
    ui.BoxHeightStyle boxHeightStyle,
    ui.BoxWidthStyle boxWidthStyle,
  ) {
    // Zero-length ranges and invalid ranges return an empty list.
    if (start >= end || start < 0 || end < 0) {
      return <ui.TextBox>[];
    }

    final int length = paragraph.toPlainText().length;
    // Ranges that are out of bounds should return an empty list.
    if (start > length || end > length) {
      return <ui.TextBox>[];
    }

    final List<ui.TextBox> boxes = <ui.TextBox>[];

    for (final EngineLineMetrics line in lines) {
      if (line.overlapsWith(start, end)) {
        for (final RangeBox box in line.boxes) {
          if (box is SpanBox && box.overlapsWith(start, end)) {
            boxes.add(box.intersect(line, start, end));
          }
        }
      }
    }
    return boxes;
  }

  ui.TextPosition getPositionForOffset(ui.Offset offset) {
    // After layout, each line has boxes that contain enough information to make
    // it possible to do hit testing. Once we find the box, we look inside that
    // box to find where exactly the `offset` is located.

    final EngineLineMetrics line = _findLineForY(offset.dy);
    // [offset] is to the left of the line.
    if (offset.dx <= line.left) {
      return ui.TextPosition(
        offset: line.startIndex,
        affinity: ui.TextAffinity.downstream,
      );
    }

    // [offset] is to the right of the line.
    if (offset.dx >= line.left + line.widthWithTrailingSpaces) {
      return ui.TextPosition(
        offset: line.endIndexWithoutNewlines,
        affinity: ui.TextAffinity.upstream,
      );
    }

    final double dx = offset.dx - line.left;
    for (final RangeBox box in line.boxes) {
      if (box.left <= dx && dx <= box.right) {
        return box.getPositionForX(dx);
      }
    }
    // Is this ever reachable?
    return ui.TextPosition(offset: line.startIndex);
  }

  EngineLineMetrics _findLineForY(double y) {
    // We could do a binary search here but it's not worth it because the number
    // of line is typically low, and each iteration is a cheap comparison of
    // doubles.
    for (final EngineLineMetrics line in lines) {
      if (y <= line.height) {
        return line;
      }
      y -= line.height;
    }
    return lines.last;
  }
}

/// Represents a box inside a paragraph span with the range of [start] to [end].
///
/// The box's coordinates are all relative to the line it belongs to. For
/// example, [left] is the distance from the left edge of the line to the left
/// edge of the box.
///
/// This is what the various measurements/coordinates look like for a box in an
/// LTR paragraph:
///
///          *------------------------lineWidth------------------*
///                            *--width--*
///          ┌─────────────────┬─────────┬───────────────────────┐
///          │                 │---BOX---│                       │
///          └─────────────────┴─────────┴───────────────────────┘
///          *---startOffset---*
///          *------left-------*
///          *--------endOffset----------*
///          *----------right------------*
///
///
/// And in an RTL paragraph, [startOffset] and [endOffset] are flipped because
/// the line starts from the right. Here's what they look like:
///
///          *------------------------lineWidth------------------*
///                            *--width--*
///          ┌─────────────────┬─────────┬───────────────────────┐
///          │                 │---BOX---│                       │
///          └─────────────────┴─────────┴───────────────────────┘
///                                      *------startOffset------*
///          *------left-------*
///                            *-----------endOffset-------------*
///          *----------right------------*
///
abstract class RangeBox {
  RangeBox(
    this.start,
    this.end,
    this.width,
    this.paragraphDirection,
    this.boxDirection,
  );

  final LineBreakResult start;
  final LineBreakResult end;

  /// The distance from the beginning of the line to the beginning of the box.
  late final double startOffset;

  /// The distance from the beginning of the line to the end of the box.
  double get endOffset => startOffset + width;

  /// The distance from the left edge of the line to the left edge of the box.
  double get left => paragraphDirection == ui.TextDirection.ltr
      ? startOffset
      : lineWidth - endOffset;

  /// The distance from the left edge of the line to the right edge of the box.
  double get right => paragraphDirection == ui.TextDirection.ltr
      ? endOffset
      : lineWidth - startOffset;

  /// The distance from the left edge of the box to the right edge of the box.
  final double width;

  /// The width of the line that this box belongs to.
  late final double lineWidth;

  /// The text direction of the paragraph that this box belongs to.
  final ui.TextDirection paragraphDirection;

  /// Indicates how this box flows among other boxes.
  ///
  /// Example: In an LTR paragraph, the text "ABC hebrew_word 123 DEF" is shown
  /// visually in the following order:
  ///
  ///                +-------------------------------+
  ///                | ABC | 123 | drow_werbeh | DEF |
  ///                +-------------------------------+
  /// box direction:   LTR   RTL       RTL       LTR
  ///                 ----> <---- <------------  ---->
  ///
  /// (In the above example, we are ignoring whitespace to simplify).
  final ui.TextDirection boxDirection;

  /// Returns a [ui.TextBox] representing this range box in the given [line].
  ///
  /// The coordinates of the resulting [ui.TextBox] are relative to the
  /// paragraph, not to the line.
  ui.TextBox toTextBox(EngineLineMetrics line);

  /// Returns the text position within this box's range that's closest to the
  /// given [x] offset.
  ///
  /// The [x] offset is expected to be relative to the left edge of the line,
  /// just like the coordinates of this box.
  ui.TextPosition getPositionForX(double x);
}

/// Represents a box for a [PlaceholderSpan].
class PlaceholderBox extends RangeBox {
  PlaceholderBox(
    this.placeholder, {
    required LineBreakResult index,
    required ui.TextDirection paragraphDirection,
    required ui.TextDirection boxDirection,
  }) : super(index, index, placeholder.width, paragraphDirection, boxDirection);

  final PlaceholderSpan placeholder;

  @override
  ui.TextBox toTextBox(EngineLineMetrics line) {
    final double left = line.left + this.left;
    final double right = line.left + this.right;

    final double lineTop = line.baseline - line.ascent;

    final double top;
    switch (placeholder.alignment) {
      case ui.PlaceholderAlignment.top:
        top = lineTop;
        break;

      case ui.PlaceholderAlignment.middle:
        top = lineTop + (line.height - placeholder.height) / 2;
        break;

      case ui.PlaceholderAlignment.bottom:
        top = lineTop + line.height - placeholder.height;
        break;

      case ui.PlaceholderAlignment.aboveBaseline:
        top = line.baseline - placeholder.height;
        break;

      case ui.PlaceholderAlignment.belowBaseline:
        top = line.baseline;
        break;

      case ui.PlaceholderAlignment.baseline:
        top = line.baseline - placeholder.baselineOffset;
        break;
    }

    return ui.TextBox.fromLTRBD(
      left,
      top,
      right,
      top + placeholder.height,
      paragraphDirection,
    );
  }

  @override
  ui.TextPosition getPositionForX(double x) {
    // See if `x` is closer to the left edge or the right edge of the box.
    final bool closerToLeft = x - left < right - x;
    return ui.TextPosition(
      offset: start.index,
      affinity: closerToLeft ? ui.TextAffinity.upstream : ui.TextAffinity.downstream,
    );
  }
}

/// Represents a box in a [FlatTextSpan].
class SpanBox extends RangeBox {
  SpanBox(
    this.spanometer, {
    required LineBreakResult start,
    required LineBreakResult end,
    required double width,
    required ui.TextDirection paragraphDirection,
    required ui.TextDirection boxDirection,
    required this.contentDirection,
    required this.isSpaceOnly,
  })  : span = spanometer.currentSpan,
        height = spanometer.height,
        baseline = spanometer.ascent,
        super(start, end, width, paragraphDirection, boxDirection);


  final Spanometer spanometer;
  final FlatTextSpan span;

  /// The direction of the text inside this box.
  ///
  /// To illustrate the difference between [boxDirection] and [contentDirection]
  /// here's an example:
  ///
  /// In an LTR paragraph, the text "ABC hebrew_word 123 DEF" is rendered as
  /// follows:
  ///
  ///                     ----> <---- <------------  ---->
  ///     box direction:   LTR   RTL       RTL       LTR
  ///                    +-------------------------------+
  ///                    | ABC | 123 | drow_werbeh | DEF |
  ///                    +-------------------------------+
  /// content direction:   LTR   LTR       RTL       LTR
  ///                     ----> ----> <------------  ---->
  ///
  /// Notice the box containing "123" flows in the RTL direction (because it
  /// comes after an RTL box), while the content of the box flows in the LTR
  /// direction (i.e. the text is shown as "123" not "321").
  final ui.TextDirection contentDirection;

  /// Whether this box is made of only white space.
  final bool isSpaceOnly;

  /// Whether the contents of this box flow in the left-to-right direction.
  bool get isContentLtr => contentDirection == ui.TextDirection.ltr;

  /// Whether the contents of this box flow in the right-to-left direction.
  bool get isContentRtl => !isContentLtr;

  /// The distance from the top edge to the bottom edge of the box.
  final double height;

  /// The distance from the top edge of the box to the alphabetic baseline of
  /// the box.
  final double baseline;

  /// Whether this box's range overlaps with the range from [startIndex] to
  /// [endIndex].
  bool overlapsWith(int startIndex, int endIndex) {
    return startIndex < end.index && start.index < endIndex;
  }

  /// Returns the substring of the paragraph that's represented by this box.
  ///
  /// Trailing newlines are omitted, if any.
  String toText() {
    return spanometer.paragraph.toPlainText().substring(start.index, end.indexWithoutTrailingNewlines);
  }

  /// Returns a [ui.TextBox] representing this range box in the given [line].
  ///
  /// The coordinates of the resulting [ui.TextBox] are relative to the
  /// paragraph, not to the line.
  @override
  ui.TextBox toTextBox(EngineLineMetrics line) {
    return intersect(line, start.index, end.index);
  }

  /// Performs the intersection of this box with the range given by [start] and
  /// [end] indices, and returns a [ui.TextBox] representing that intersection.
  ///
  /// The coordinates of the resulting [ui.TextBox] are relative to the
  /// paragraph, not to the line.
  ui.TextBox intersect(EngineLineMetrics line, int start, int end) {
    final double top = line.baseline - baseline;

    final double before;
    if (start <= this.start.index) {
      before = 0.0;
    } else {
      spanometer.currentSpan = span;
      before = spanometer._measure(this.start.index, start);
    }

    final double after;
    if (end >= this.end.indexWithoutTrailingNewlines) {
      after = 0.0;
    } else {
      spanometer.currentSpan = span;
      after = spanometer._measure(end, this.end.indexWithoutTrailingNewlines);
    }

    final double left, right;
    if (isContentLtr) {
      // Example: let's say the text is "Loremipsum" and we want to get the box
      // for "rem". In this case, `before` is the width of "Lo", and `after`
      // is the width of "ipsum".
      //
      // Here's how the measurements/coordinates look like:
      //
      //              before         after
      //              |----|     |----------|
      //              +---------------------+
      //              | L o r e m i p s u m |
      //              +---------------------+
      //    this.left ^                     ^ this.right
      left = this.left + before;
      right = this.right - after;
    } else {
      // Example: let's say the text is "txet_werbeH" ("Hebrew_text" flowing from
      // right to left). Say we want to get the box for "brew". The `before` is
      // the width of "He", and `after` is the width of "_text".
      //
      //                 after           before
      //              |----------|       |----|
      //              +-----------------------+
      //              | t x e t _ w e r b e H |
      //              +-----------------------+
      //    this.left ^                       ^ this.right
      //
      // Notice how `before` and `after` are reversed in the RTL example. That's
      // because the text flows from right to left.
      left = this.left + after;
      right = this.right - before;
    }

    // The [RangeBox]'s left and right edges are relative to the line. In order
    // to make them relative to the paragraph, we need to add the left edge of
    // the line.
    return ui.TextBox.fromLTRBD(
      line.left + left,
      top,
      line.left + right,
      top + height,
      contentDirection,
    );
  }

  /// Transforms the [x] coordinate to be relative to this box and matches the
  /// flow of content.
  ///
  /// In LTR paragraphs, the [startOffset] and [endOffset] of an RTL box
  /// indicate the visual beginning and end of the box. But the text inside the
  /// box flows in the opposite direction (from [endOffset] to [startOffset]).
  ///
  /// The X (input) is relative to the line, and always from left-to-right
  /// independent of paragraph and content direction.
  ///
  /// Here's how it looks for a box with LTR content:
  ///
  ///          *------------------------lineWidth------------------*
  ///          *---------------X (input)
  ///          ┌───────────┬────────────────────────┬───────────────┐
  ///          │           │ --content-direction--> │               │
  ///          └───────────┴────────────────────────┴───────────────┘
  ///                      *---X' (output)
  ///          *---left----*
  ///          *---------------right----------------*
  ///
  ///
  /// And here's how it looks for a box with RTL content:
  ///
  ///          *------------------------lineWidth------------------*
  ///          *----------------X (input)
  ///          ┌───────────┬────────────────────────┬───────────────┐
  ///          │           │ <--content-direction-- │               │
  ///          └───────────┴────────────────────────┴───────────────┘
  ///                  (output) X'------------------*
  ///          *---left----*
  ///          *---------------right----------------*
  ///
  double _makeXRelativeToContent(double x) {
    return isContentRtl ? right - x : x - left;
  }

  @override
  ui.TextPosition getPositionForX(double x) {
    spanometer.currentSpan = span;

    x = _makeXRelativeToContent(x);

    final int startIndex = start.index;
    final int endIndex = end.indexWithoutTrailingNewlines;
    // The resulting `cutoff` is the index of the character where the `x` offset
    // falls. We should return the text position of either `cutoff` or
    // `cutoff + 1` depending on which one `x` is closer to.
    //
    //   offset x
    //      ↓
    // "A B C D E F"
    //     ↑
    //   cutoff
    final int cutoff = spanometer.forceBreak(
      startIndex,
      endIndex,
      availableWidth: x,
      allowEmpty: true,
    );

    if (cutoff == endIndex) {
      return ui.TextPosition(
        offset: cutoff,
        affinity: ui.TextAffinity.upstream,
      );
    }

    final double lowWidth = spanometer._measure(startIndex, cutoff);
    final double highWidth = spanometer._measure(startIndex, cutoff + 1);

    // See if `x` is closer to `cutoff` or `cutoff + 1`.
    if (x - lowWidth < highWidth - x) {
      // The offset is closer to cutoff.
      return ui.TextPosition(
        offset: cutoff,
        affinity: ui.TextAffinity.downstream,
      );
    } else {
      // The offset is closer to cutoff + 1.
      return ui.TextPosition(
        offset: cutoff + 1,
        affinity: ui.TextAffinity.upstream,
      );
    }
  }
}

/// Represents a segment in a line of a paragraph.
///
/// For example, this line: "Lorem ipsum dolor sit" is broken up into the
/// following segments:
///
/// - "Lorem "
/// - "ipsum "
/// - "dolor "
/// - "sit"
class LineSegment {
  LineSegment({
    required this.span,
    required this.start,
    required this.end,
    required this.width,
    required this.widthIncludingSpace,
  });

  /// The span that this segment belongs to.
  final ParagraphSpan span;

  /// The index of the beginning of the segment in the paragraph.
  final LineBreakResult start;

  /// The index of the end of the segment in the paragraph.
  final LineBreakResult end;

  /// The width of the segment excluding any trailing white space.
  final double width;

  /// The width of the segment including any trailing white space.
  final double widthIncludingSpace;

  /// The width of the trailing white space in the segment.
  double get widthOfTrailingSpace => widthIncludingSpace - width;

  /// Whether this segment is made of only white space.
  ///
  /// We rely on the [width] to determine this because relying on incides
  /// doesn't work well for placeholders (they are zero-length strings).
  bool get isSpaceOnly => width == 0;
}

/// Builds instances of [EngineLineMetrics] for the given [paragraph].
///
/// Usage of this class starts by calling [LineBuilder.first] to start building
/// the first line of the paragraph.
///
/// Then new line breaks can be found by calling [LineBuilder.findNextBreak].
///
/// The line can be extended one or more times before it's built by calling
/// [LineBuilder.build] which generates the [EngineLineMetrics] instace.
///
/// To start building the next line, simply call [LineBuilder.nextLine] which
/// creates a new [LineBuilder] that can be extended and built and so on.
class LineBuilder {
  LineBuilder._(
    this.paragraph,
    this.spanometer, {
    required this.maxWidth,
    required this.start,
    required this.lineNumber,
    required this.accumulatedHeight,
  }) : _end = start;

  /// Creates a [LineBuilder] for the first line in a paragraph.
  factory LineBuilder.first(
    CanvasParagraph paragraph,
    Spanometer spanometer, {
    required double maxWidth,
  }) {
    return LineBuilder._(
      paragraph,
      spanometer,
      maxWidth: maxWidth,
      lineNumber: 0,
      start: const LineBreakResult.sameIndex(0, LineBreakType.prohibited),
      accumulatedHeight: 0.0,
    );
  }

  final List<LineSegment> _segments = <LineSegment>[];
  final List<RangeBox> _boxes = <RangeBox>[];

  final double maxWidth;
  final CanvasParagraph paragraph;
  final Spanometer spanometer;
  final LineBreakResult start;
  final int lineNumber;

  /// The accumulated height of all preceding lines, excluding the current line.
  final double accumulatedHeight;

  /// The index of the end of the line so far.
  LineBreakResult get end => _end;
  LineBreakResult _end;
  set end(LineBreakResult value) {
    if (value.type != LineBreakType.prohibited) {
      isBreakable = true;
    }
    _end = value;
  }

  /// The width of the line so far, excluding trailing white space.
  double width = 0.0;

  /// The width of the line so far, including trailing white space.
  double widthIncludingSpace = 0.0;

  /// The width of trailing white space in the line.
  double get widthOfTrailingSpace => widthIncludingSpace - width;

  /// The distance from the top of the line to the alphabetic baseline.
  double ascent = 0.0;

  /// The distance from the bottom of the line to the alphabetic baseline.
  double descent = 0.0;

  /// The height of the line so far.
  double get height => ascent + descent;

  /// The last segment in this line.
  LineSegment get lastSegment => _segments.last;

  /// Returns true if there is at least one break opportunity in the line.
  bool isBreakable = false;

  /// Returns true if there's no break opportunity in the line.
  bool get isNotBreakable => !isBreakable;

  /// Whether the end of this line is a prohibited break.
  bool get isEndProhibited => end.type == LineBreakType.prohibited;

  int _spaceBoxCount = 0;

  bool get isEmpty => _segments.isEmpty;
  bool get isNotEmpty => _segments.isNotEmpty;

  /// The horizontal offset necessary for the line to be correctly aligned.
  double get alignOffset {
    final double emptySpace = maxWidth - width;
    final ui.TextAlign textAlign = paragraph.paragraphStyle.effectiveTextAlign;

    switch (textAlign) {
      case ui.TextAlign.center:
        return emptySpace / 2.0;
      case ui.TextAlign.right:
        return emptySpace;
      case ui.TextAlign.start:
        return _paragraphDirection == ui.TextDirection.rtl ? emptySpace : 0.0;
      case ui.TextAlign.end:
        return _paragraphDirection == ui.TextDirection.rtl ? 0.0 : emptySpace;
      default:
        return 0.0;
    }
  }

  /// Measures the width of text between the end of this line and [newEnd].
  double getAdditionalWidthTo(LineBreakResult newEnd) {
    // If the extension is all made of space characters, it shouldn't add
    // anything to the width.
    if (end.index == newEnd.indexWithoutTrailingSpaces) {
      return 0.0;
    }

    return widthOfTrailingSpace + spanometer.measure(end, newEnd);
  }

  bool get _isLastBoxAPlaceholder {
    if (_boxes.isEmpty) {
      return false;
    }
    return _boxes.last is PlaceholderBox;
  }

  ui.TextDirection get _paragraphDirection =>
      paragraph.paragraphStyle.effectiveTextDirection;

  late ui.TextDirection _currentBoxDirection = _paragraphDirection;

  late ui.TextDirection _currentContentDirection = _paragraphDirection;

  bool _shouldCreateBoxBeforeExtendingTo(DirectionalPosition newEnd) {
    // When the direction changes, we need to make sure to put them in separate
    // boxes.
    return newEnd.isSpaceOnly || _currentBoxDirection != newEnd.textDirection || _currentContentDirection != newEnd.textDirection;
  }

  /// Extends the line by setting a [newEnd].
  void extendTo(DirectionalPosition newEnd) {
    ascent = math.max(ascent, spanometer.ascent);
    descent = math.max(descent, spanometer.descent);

    // When the direction changes, we need to make sure to put them in separate
    // boxes.
    if (_shouldCreateBoxBeforeExtendingTo(newEnd)) {
      createBox();
    }
    _currentBoxDirection = newEnd.textDirection ?? _currentBoxDirection;
    _currentContentDirection = newEnd.textDirection ?? ui.TextDirection.ltr;

    _addSegment(_createSegment(newEnd.lineBreak));
    if (newEnd.isSpaceOnly) {
      // Whitespace sequences go in their own boxes.
      createBox(isSpaceOnly: true);
    }
  }

  /// Extends the line to the end of the paragraph.
  void extendToEndOfText() {
    if (end.type == LineBreakType.endOfText) {
      return;
    }

    final LineBreakResult endOfText = LineBreakResult.sameIndex(
      paragraph.toPlainText().length,
      LineBreakType.endOfText,
    );

    // The spanometer may not be ready in some cases. E.g. when the paragraph
    // is made up of only placeholders and no text.
    if (spanometer.isReady) {
      ascent = math.max(ascent, spanometer.ascent);
      descent = math.max(descent, spanometer.descent);
      _addSegment(_createSegment(endOfText));
    } else {
      end = endOfText;
    }
  }

  void addPlaceholder(PlaceholderSpan placeholder) {
    // Increase the line's height to fit the placeholder, if necessary.
    final double ascent, descent;
    switch (placeholder.alignment) {
      case ui.PlaceholderAlignment.top:
        // The placeholder is aligned to the top of text, which means it has the
        // same `ascent` as the remaining text. We only need to extend the
        // `descent` enough to fit the placeholder.
        ascent = this.ascent;
        descent = placeholder.height - this.ascent;
        break;

      case ui.PlaceholderAlignment.bottom:
        // The opposite of `top`. The `descent` is the same, but we extend the
        // `ascent`.
        ascent = placeholder.height - this.descent;
        descent = this.descent;
        break;

      case ui.PlaceholderAlignment.middle:
        final double textMidPoint = height / 2;
        final double placeholderMidPoint = placeholder.height / 2;
        final double diff = placeholderMidPoint - textMidPoint;
        ascent = this.ascent + diff;
        descent = this.descent + diff;
        break;

      case ui.PlaceholderAlignment.aboveBaseline:
        ascent = placeholder.height;
        descent = 0.0;
        break;

      case ui.PlaceholderAlignment.belowBaseline:
        ascent = 0.0;
        descent = placeholder.height;
        break;

      case ui.PlaceholderAlignment.baseline:
        ascent = placeholder.baselineOffset;
        descent = placeholder.height - ascent;
        break;
    }

    this.ascent = math.max(this.ascent, ascent);
    this.descent = math.max(this.descent, descent);

    _addSegment(LineSegment(
      span: placeholder,
      start: end,
      end: end,
      width: placeholder.width,
      widthIncludingSpace: placeholder.width,
    ));

    // Add the placeholder box.
    _boxes.add(PlaceholderBox(
      placeholder,
      index: _currentBoxStart,
      paragraphDirection: _paragraphDirection,
      boxDirection: _currentBoxDirection,
    ));
    _currentBoxStartOffset = widthIncludingSpace;
    // Breaking is always allowed after a placeholder.
    isBreakable = true;
  }

  /// Creates a new segment to be appended to the end of this line.
  LineSegment _createSegment(LineBreakResult segmentEnd) {
    // The segment starts at the end of the line.
    final LineBreakResult segmentStart = end;
    return LineSegment(
      span: spanometer.currentSpan,
      start: segmentStart,
      end: segmentEnd,
      width: spanometer.measure(segmentStart, segmentEnd),
      widthIncludingSpace:
          spanometer.measureIncludingSpace(segmentStart, segmentEnd),
    );
  }

  /// Adds a segment to this line.
  ///
  /// It adjusts the width properties to accommodate the new segment. It also
  /// sets the line end to the end of the segment.
  void _addSegment(LineSegment segment) {
    _segments.add(segment);

    // Adding a space-only segment has no effect on `width` because it doesn't
    // include trailing white space.
    if (!segment.isSpaceOnly) {
      // Add the width of previous trailing space.
      width += widthOfTrailingSpace + segment.width;
    }
    widthIncludingSpace += segment.widthIncludingSpace;
    end = segment.end;
  }

  /// Removes the latest [LineSegment] added by [_addSegment].
  ///
  /// It re-adjusts the width properties and the end of the line.
  LineSegment _popSegment() {
    final LineSegment poppedSegment = _segments.removeLast();

    if (_segments.isEmpty) {
      width = 0.0;
      widthIncludingSpace = 0.0;
      end = start;
    } else {
      widthIncludingSpace -= poppedSegment.widthIncludingSpace;
      end = lastSegment.end;

      // Now, let's figure out what to do with `width`.

      // Popping a space-only segment has no effect on `width`.
      if (!poppedSegment.isSpaceOnly) {
        // First, we subtract the width of the popped segment.
        width -= poppedSegment.width;

        // Second, we subtract all trailing spaces from `width`. There could be
        // multiple trailing segments that are space-only.
        double widthOfTrailingSpace = 0.0;
        int i = _segments.length - 1;
        while (i >= 0 && _segments[i].isSpaceOnly) {
          // Since the segment is space-only, `widthIncludingSpace` contains
          // the width of the space and nothing else.
          widthOfTrailingSpace += _segments[i].widthIncludingSpace;
          i--;
        }
        if (i >= 0) {
          // Having `i >= 0` means in the above loop we stopped at a
          // non-space-only segment. We should also subtract its trailing spaces.
          widthOfTrailingSpace += _segments[i].widthOfTrailingSpace;
        }
        width -= widthOfTrailingSpace;
      }
    }

    // Now let's fixes boxes if they need fixing.
    //
    // If we popped a segment of an already created box, we should pop the box
    // too.
    if (_currentBoxStart.index > poppedSegment.start.index) {
      final RangeBox poppedBox = _boxes.removeLast();
      _currentBoxStartOffset -= poppedBox.width;
      if (poppedBox is SpanBox && poppedBox.isSpaceOnly) {
        _spaceBoxCount--;
      }
    }

    return poppedSegment;
  }

  /// Force-breaks the line in order to fit in [maxWidth] while trying to extend
  /// to [nextBreak].
  ///
  /// This should only be called when there isn't enough width to extend to
  /// [nextBreak], and either of the following is true:
  ///
  /// 1. An ellipsis is being appended to this line, OR
  /// 2. The line doesn't have any line break opportunities and has to be
  ///    force-broken.
  void forceBreak(
    DirectionalPosition nextBreak, {
    required bool allowEmpty,
    String? ellipsis,
  }) {
    if (ellipsis == null) {
      final double availableWidth = maxWidth - widthIncludingSpace;
      final int breakingPoint = spanometer.forceBreak(
        end.index,
        nextBreak.lineBreak.indexWithoutTrailingSpaces,
        availableWidth: availableWidth,
        allowEmpty: allowEmpty,
      );

      // This condition can be true in the following case:
      // 1. Next break is only one character away, with zero or many spaces. AND
      // 2. There isn't enough width to fit the single character. AND
      // 3. `allowEmpty` is false.
      if (breakingPoint == nextBreak.lineBreak.indexWithoutTrailingSpaces) {
        // In this case, we just extend to `nextBreak` instead of creating a new
        // artificial break. It's safe (and better) to do so, because we don't
        // want the trailing white space to go to the next line.
        extendTo(nextBreak);
      } else {
        extendTo(nextBreak.copyWithIndex(breakingPoint));
      }
      return;
    }

    // For example: "foo bar baz". Let's say all characters have the same width, and
    // the constraint width can only fit 9 characters "foo bar b". So if the
    // paragraph has an ellipsis, we can't just remove the last segment "baz"
    // and replace it with "..." because that would overflow.
    //
    // We need to keep popping segments until we are able to fit the "..."
    // without overflowing. In this example, that would be: "foo ba..."

    final double ellipsisWidth = spanometer.measureText(ellipsis);
    final double availableWidth = maxWidth - ellipsisWidth;

    // First, we create the new segment until `nextBreak`.
    LineSegment segmentToBreak = _createSegment(nextBreak.lineBreak);

    // Then, we keep popping until we find the segment that has to be broken.
    // After the loop ends, two things are correct:
    // 1. All remaining segments in `_segments` can fit within constraints.
    // 2. Adding `segmentToBreak` causes the line to overflow.
    while (_segments.isNotEmpty && widthIncludingSpace > availableWidth) {
      segmentToBreak = _popSegment();
    }

    spanometer.currentSpan = segmentToBreak.span as FlatTextSpan;
    final double availableWidthForSegment =
        availableWidth - widthIncludingSpace;
    final int breakingPoint = spanometer.forceBreak(
      segmentToBreak.start.index,
      segmentToBreak.end.index,
      availableWidth: availableWidthForSegment,
      allowEmpty: allowEmpty,
    );

    // There's a possibility that the end of line has moved backwards, so we
    // need to remove some boxes in that case.
    while (_boxes.isNotEmpty && _boxes.last.end.index > breakingPoint) {
      _boxes.removeLast();
    }
    _currentBoxStartOffset = widthIncludingSpace;

    extendTo(nextBreak.copyWithIndex(breakingPoint));
  }

  /// Looks for the last break opportunity in the line and reverts the line to
  /// that point.
  ///
  /// If the line already ends with a break opportunity, this method does
  /// nothing.
  void revertToLastBreakOpportunity() {
    assert(isBreakable);
    while (isEndProhibited) {
      _popSegment();
    }
    // Make sure the line is not empty and still breakable after popping a few
    // segments.
    assert(isNotEmpty);
    assert(isBreakable);
  }

  LineBreakResult get _currentBoxStart {
    if (_boxes.isEmpty) {
      return start;
    }
    // The end of the last box is the start of the new box.
    return _boxes.last.end;
  }

  double _currentBoxStartOffset = 0.0;

  double get _currentBoxWidth => widthIncludingSpace - _currentBoxStartOffset;

  /// Cuts a new box in the line.
  ///
  /// If this is the first box in the line, it'll start at the beginning of the
  /// line. Else, it'll start at the end of the last box.
  ///
  /// A box should be cut whenever the end of line is reached, when switching
  /// from one span to another, or when switching text direction.
  ///
  /// [isSpaceOnly] indicates that the box contains nothing but whitespace
  /// characters.
  void createBox({bool isSpaceOnly = false}) {
    final LineBreakResult boxStart = _currentBoxStart;
    final LineBreakResult boxEnd = end;
    // Avoid creating empty boxes. This could happen when the end of a span
    // coincides with the end of a line. In this case, `createBox` is called twice.
    if (boxStart.index == boxEnd.index) {
      return;
    }

    _boxes.add(SpanBox(
      spanometer,
      start: boxStart,
      end: boxEnd,
      width: _currentBoxWidth,
      paragraphDirection: _paragraphDirection,
      boxDirection: _currentBoxDirection,
      contentDirection: _currentContentDirection,
      isSpaceOnly: isSpaceOnly,
    ));

    if (isSpaceOnly) {
      _spaceBoxCount++;
    }

    _currentBoxStartOffset = widthIncludingSpace;
  }

  /// Builds the [EngineLineMetrics] instance that represents this line.
  EngineLineMetrics build({String? ellipsis}) {
    // At the end of each line, we cut the last box of the line.
    createBox();
    _positionBoxes();

    final double ellipsisWidth =
        ellipsis == null ? 0.0 : spanometer.measureText(ellipsis);

    final int endIndexWithoutNewlines = math.max(start.index, end.indexWithoutTrailingNewlines);
    final bool hardBreak;
    if (end.type != LineBreakType.endOfText && _isLastBoxAPlaceholder) {
      hardBreak = false;
    } else {
      hardBreak = end.isHard;
    }
    return EngineLineMetrics.rich(
      lineNumber,
      ellipsis: ellipsis,
      startIndex: start.index,
      endIndex: end.index,
      endIndexWithoutNewlines: endIndexWithoutNewlines,
      hardBreak: hardBreak,
      width: width + ellipsisWidth,
      widthWithTrailingSpaces: widthIncludingSpace + ellipsisWidth,
      left: alignOffset,
      height: height,
      baseline: accumulatedHeight + ascent,
      ascent: ascent,
      descent: descent,
      boxes: _boxes,
      spaceBoxCount: _spaceBoxCount,
    );
  }

  /// Positions the boxes and takes into account their directions, and the
  /// paragraph's direction.
  void _positionBoxes() {
    final List<RangeBox> boxes = _boxes;

    int i = 0;
    double cumulativeWidth = 0.0;
    while (i < boxes.length) {
      final RangeBox box = boxes[i];
      if (box.boxDirection == _paragraphDirection) {
        // The box is in the same direction as the paragraph.
        box.startOffset = cumulativeWidth;
        box.lineWidth = width;

        cumulativeWidth += box.width;
        i++;
        continue;
      }

      // At this point, we found a box that has the opposite direction to the
      // paragraph. This could be a sequence of one or more boxes.
      //
      // These boxes should flow in the opposite direction. So we need to
      // position them in reverse order.
      //
      // If the last box in the sequence is a space-only box (contains only
      // whitespace characters), it should be excluded from the sequence.
      //
      // Example: an LTR paragraph with the contents:
      //
      // "ABC rtl1 rtl2 rtl3 XYZ"
      //     ^    ^    ^    ^
      //    SP1  SP2  SP3  SP4
      //
      //
      // box direction:    LTR           RTL               LTR
      //                |------>|<-----------------------|------>
      //                +----------------------------------------+
      //                | ABC | | rtl3 | | rtl2 | | rtl1 | | XYZ |
      //                +----------------------------------------+
      //                       ^        ^        ^        ^
      //                      SP1      SP3      SP2      SP4
      //
      // Notice how SP2 and SP3 are flowing in the RTL direction because of the
      // surrounding RTL words. SP4 is also preceded by an RTL word, but it marks
      // the end of the RTL sequence, so it goes back to flowing in the paragraph
      // direction (LTR).

      final int first = i;
      int lastNonSpaceBox = first;
      i++;
      while (i < boxes.length && boxes[i].boxDirection != _paragraphDirection) {
        final RangeBox box = boxes[i];
        if (box is SpanBox && box.isSpaceOnly) {
          // Do nothing.
        } else {
          lastNonSpaceBox = i;
        }
        i++;
      }
      final int last = lastNonSpaceBox;
      i = lastNonSpaceBox + 1;

      // The range (first:last) is the entire sequence of boxes that have the
      // opposite direction to the paragraph.
      final double sequenceWidth =
          _positionBoxesInReverse(boxes, first, last, startOffset: cumulativeWidth);
      cumulativeWidth += sequenceWidth;
    }
  }

  /// Positions a sequence of boxes in the direction opposite to the paragraph
  /// text direction.
  ///
  /// This is needed when a right-to-left sequence appears in the middle of a
  /// left-to-right paragraph, or vice versa.
  ///
  /// Returns the total width of all the positioned boxes in the sequence.
  ///
  /// [first] and [last] are expected to be inclusive.
  double _positionBoxesInReverse(
    List<RangeBox> boxes,
    int first,
    int last, {
    required double startOffset,
  }) {
    double cumulativeWidth = 0.0;
    for (int i = last; i >= first; i--) {
      // Update the visual position of each box.
      final RangeBox box = boxes[i];
      assert(box.boxDirection != _paragraphDirection);
      box.startOffset = startOffset + cumulativeWidth;
      box.lineWidth = width;

      cumulativeWidth += box.width;
    }
    return cumulativeWidth;
  }

  LineBreakResult? _cachedNextBreak;

  /// Finds the next line break after the end of this line.
  DirectionalPosition findNextBreak() {
    LineBreakResult? nextBreak = _cachedNextBreak;
    final String text = paragraph.toPlainText();
    // Don't recompute the `nextBreak` until the line has reached the previously
    // computed `nextBreak`.
    if (nextBreak == null || end.index >= nextBreak.index) {
      final int maxEnd = spanometer.currentSpan.end;
      nextBreak = nextLineBreak(text, end.index, maxEnd: maxEnd);
      _cachedNextBreak = nextBreak;
    }
    // The current end of the line is the beginning of the next block.
    return getDirectionalBlockEnd(text, end, nextBreak);
  }

  /// Creates a new [LineBuilder] to build the next line in the paragraph.
  LineBuilder nextLine() {
    return LineBuilder._(
      paragraph,
      spanometer,
      maxWidth: maxWidth,
      start: end,
      lineNumber: lineNumber + 1,
      accumulatedHeight: accumulatedHeight + height,
    );
  }
}

/// Responsible for taking measurements within spans of a paragraph.
///
/// Can't perform measurements across spans. To measure across spans, multiple
/// measurements have to be taken.
///
/// Before performing any measurement, the [currentSpan] has to be set. Once
/// it's set, the [Spanometer] updates the underlying [context] so that
/// subsequent measurements use the correct styles.
class Spanometer {
  Spanometer(this.paragraph, this.context);

  final CanvasParagraph paragraph;
  final html.CanvasRenderingContext2D context;

  static RulerHost _rulerHost = RulerHost();

  static Map<TextHeightStyle, TextHeightRuler> _rulers =
      <TextHeightStyle, TextHeightRuler>{};

  
  static Map<TextHeightStyle, TextHeightRuler> get rulers => _rulers;

  /// Clears the cache of rulers that are used for measuring text height and
  /// baseline metrics.
  static void clearRulersCache() {
    _rulers.forEach((TextHeightStyle style, TextHeightRuler ruler) {
      ruler.dispose();
    });
    _rulers.clear();
  }

  String _cssFontString = '';

  double? get letterSpacing => currentSpan.style.letterSpacing;

  TextHeightRuler? _currentRuler;
  FlatTextSpan? _currentSpan;

  FlatTextSpan get currentSpan => _currentSpan!;
  set currentSpan(FlatTextSpan? span) {
    if (span == _currentSpan) {
      return;
    }
    _currentSpan = span;

    // No need to update css font string when `span` is null.
    if (span == null) {
      _currentRuler = null;
      return;
    }

    // Update the height ruler.
    // If the ruler doesn't exist in the cache, create a new one and cache it.
    final TextHeightStyle heightStyle = span.style.heightStyle;
    TextHeightRuler? ruler = _rulers[heightStyle];
    if (ruler == null) {
      ruler = TextHeightRuler(heightStyle, _rulerHost);
      _rulers[heightStyle] = ruler;
    }
    _currentRuler = ruler;

    // Update the font string if it's different from the previous span.
    final String cssFontString = span.style.cssFontString;
    if (_cssFontString != cssFontString) {
      _cssFontString = cssFontString;
      context.font = cssFontString;
    }
  }

  /// Whether the spanometer is ready to take measurements.
  bool get isReady => _currentSpan != null;

  /// The distance from the top of the current span to the alphabetic baseline.
  double get ascent => _currentRuler!.alphabeticBaseline;

  /// The distance from the bottom of the current span to the alphabetic baseline.
  double get descent => height - ascent;

  /// The line height of the current span.
  double get height => _currentRuler!.height;

  /// Measures the width of text between two line breaks.
  ///
  /// Doesn't include the width of any trailing white space.
  double measure(LineBreakResult start, LineBreakResult end) {
    return _measure(start.index, end.indexWithoutTrailingSpaces);
  }

  /// Measures the width of text between two line breaks.
  ///
  /// Includes the width of trailing white space, if any.
  double measureIncludingSpace(LineBreakResult start, LineBreakResult end) {
    return _measure(start.index, end.indexWithoutTrailingNewlines);
  }

  double measureText(String text) {
    return measureSubstring(context, text, 0, text.length);
  }

  /// In a continuous, unbreakable block of text from [start] to [end], finds
  /// the point where text should be broken to fit in the given [availableWidth].
  ///
  /// The [start] and [end] indices have to be within the same text span.
  ///
  /// When [allowEmpty] is true, the result is guaranteed to be at least one
  /// character after [start]. But if [allowEmpty] is false and there isn't
  /// enough [availableWidth] to fit the first character, then [start] is
  /// returned.
  ///
  /// See also:
  /// - [LineBuilder.forceBreak].
  int forceBreak(
    int start,
    int end, {
    required double availableWidth,
    required bool allowEmpty,
  }) {
    assert(_currentSpan != null);

    final FlatTextSpan span = currentSpan;

    // Make sure the range is within the current span.
    assert(start >= span.start && start <= span.end);
    assert(end >= span.start && end <= span.end);

    if (availableWidth <= 0.0) {
      return allowEmpty ? start : start + 1;
    }

    int low = start;
    int high = end;
    do {
      final int mid = (low + high) ~/ 2;
      final double width = _measure(start, mid);
      if (width < availableWidth) {
        low = mid;
      } else if (width > availableWidth) {
        high = mid;
      } else {
        low = high = mid;
      }
    } while (high - low > 1);

    if (low == start && !allowEmpty) {
      low++;
    }
    return low;
  }

  double _measure(int start, int end) {
    assert(_currentSpan != null);
    final FlatTextSpan span = currentSpan;

    // Make sure the range is within the current span.
    assert(start >= span.start && start <= span.end);
    assert(end >= span.start && end <= span.end);

    final String text = paragraph.toPlainText();
    return measureSubstring(
      context,
      text,
      start,
      end,
      letterSpacing: letterSpacing,
    );
  }
}
