Avalonia/src/Skia/Avalonia.Skia/FormattedTextImpl.cs

// Copyright (c) The Avalonia Project. All rights reserved.
// Licensed under the MIT license. See licence.md file in the project root for full license information.

using Avalonia.Media;
using Avalonia.Platform;
using SkiaSharp;
using System;
using System.Collections.Generic;
using System.Linq;

namespace Avalonia.Skia
{
    public class FormattedTextImpl : IFormattedTextImpl
    {
        public FormattedTextImpl(
            string text,
            Typeface typeface,
            TextAlignment textAlignment,
            TextWrapping wrapping,
            Size constraint,
            IReadOnlyList<FormattedTextStyleSpan> spans)
        {
            Text = text ?? string.Empty;

            // Replace 0 characters with zero-width spaces (200B)
            Text = Text.Replace((char)0, (char)0x200B);

            var skiaTypeface = TypefaceCache.GetTypeface(
                typeface?.FontFamilyName ?? "monospace",
                typeface?.Style ?? FontStyle.Normal,
                typeface?.Weight ?? FontWeight.Normal);

            _paint = new SKPaint();

            //currently Skia does not measure properly with Utf8 !!!
            //Paint.TextEncoding = SKTextEncoding.Utf8;
            _paint.TextEncoding = SKTextEncoding.Utf16;
            _paint.IsStroke = false;
            _paint.IsAntialias = true;            
            _paint.LcdRenderText = true;            
            _paint.SubpixelText = true;
            _paint.Typeface = skiaTypeface;
            _paint.TextSize = (float)(typeface?.FontSize ?? 12);
            _paint.TextAlign = textAlignment.ToSKTextAlign();
            _paint.BlendMode = SKBlendMode.Src;

            _wrapping = wrapping;
            _constraint = constraint;

            if (spans != null)
            {
                foreach (var span in spans)
                {
                    if (span.ForegroundBrush != null)
                    {
                        SetForegroundBrush(span.ForegroundBrush, span.StartIndex, span.Length);
                    }
                }
            }

            Rebuild();
        }

        public Size Constraint => _constraint;

        public Size Size => _size;

        public IEnumerable<FormattedTextLine> GetLines()
        {
            return _lines;
        }

        public TextHitTestResult HitTestPoint(Point point)
        {
            float y = (float)point.Y;
            var line = _skiaLines.Find(l => l.Top <= y && (l.Top + l.Height) > y);

            if (!line.Equals(default(AvaloniaFormattedTextLine)))
            {
                var rects = GetRects();

                for (int c = line.Start; c < line.Start + line.TextLength; c++)
                {
                    var rc = rects[c];
                    if (rc.Contains(point))
                    {
                        return new TextHitTestResult
                        {
                            IsInside = !(line.TextLength > line.Length),
                            TextPosition = c,
                            IsTrailing = (point.X - rc.X) > rc.Width / 2
                        };
                    }
                }

                int offset = 0;

                if (point.X >= (rects[line.Start].X + line.Width) / 2 && line.Length > 0)
                {
                    offset = line.TextLength > line.Length ?
                                    line.Length : (line.Length - 1);
                }

                return new TextHitTestResult
                {
                    IsInside = false,
                    TextPosition = line.Start + offset,
                    IsTrailing = Text.Length == (line.Start + offset + 1)
                };
            }

            bool end = point.X > _size.Width || point.Y > _lines.Sum(l => l.Height);

            return new TextHitTestResult()
            {
                IsInside = false,
                IsTrailing = end,
                TextPosition = end ? Text.Length - 1 : 0
            };
        }

        public Rect HitTestTextPosition(int index)
        {
            var rects = GetRects();

            if (index < 0 || index >= rects.Count)
            {
                var r = rects.LastOrDefault();
                return new Rect(r.X + r.Width, r.Y, 0, _lineHeight);
            }

            if (rects.Count == 0)
            {
                return new Rect(0, 0, 1, _lineHeight);
            }

            if (index == rects.Count)
            {
                var lr = rects[rects.Count - 1];
                return new Rect(new Point(lr.X + lr.Width, lr.Y), rects[index - 1].Size);
            }

            return rects[index];
        }

        public IEnumerable<Rect> HitTestTextRange(int index, int length)
        {
            List<Rect> result = new List<Rect>();

            var rects = GetRects();

            int lastIndex = index + length - 1;

            foreach (var line in _skiaLines.Where(l =>
                                                    (l.Start + l.Length) > index &&
                                                    lastIndex >= l.Start))
            {
                int lineEndIndex = line.Start + (line.Length > 0 ? line.Length - 1 : 0);

                double left = rects[line.Start > index ? line.Start : index].X;
                double right = rects[lineEndIndex > lastIndex ? lastIndex : lineEndIndex].Right;

                result.Add(new Rect(left, line.Top, right - left, line.Height));
            }

            return result;
        }

        public override string ToString()
        {
            return Text;
        }

        internal void Draw(DrawingContextImpl context,
                           SKCanvas canvas, SKPoint origin,
                           DrawingContextImpl.PaintWrapper foreground)
        {
            /* TODO: This originated from Native code, it might be useful for debugging character positions as
             * we improve the FormattedText support. Will need to port this to C# obviously. Rmove when
             * not needed anymore.

                SkPaint dpaint;
                ctx->Canvas->save();
                ctx->Canvas->translate(origin.fX, origin.fY);
                for (int c = 0; c < Lines.size(); c++)
                {
                    dpaint.setARGB(255, 0, 0, 0);
                    SkRect rc;
                    rc.fLeft = 0;
                    rc.fTop = Lines[c].Top;
                    rc.fRight = Lines[c].Width;
                    rc.fBottom = rc.fTop + LineOffset;
                    ctx->Canvas->drawRect(rc, dpaint);
                }
                for (int c = 0; c < Length; c++)
                {
                    dpaint.setARGB(255, c % 10 * 125 / 10 + 125, (c * 7) % 10 * 250 / 10, (c * 13) % 10 * 250 / 10);
                    dpaint.setStyle(SkPaint::kFill_Style);
                    ctx->Canvas->drawRect(Rects[c], dpaint);
                }
                ctx->Canvas->restore();
            */
            SKPaint paint = _paint;
            IDisposable currd = null;
            var currentWrapper = foreground;

            try
            {
                SKPaint currFGPaint = ApplyWrapperTo(ref foreground, ref currd, paint);
                bool hasCusomFGBrushes = _foregroundBrushes.Any();

                for (int c = 0; c < _skiaLines.Count; c++)
                {
                    AvaloniaFormattedTextLine line = _skiaLines[c];

                    float x = TransformX(origin.X, 0, paint.TextAlign);

                    if (!hasCusomFGBrushes)
                    {
                        var subString = Text.Substring(line.Start, line.Length);
                        canvas.DrawText(subString, x, origin.Y + line.Top + _lineOffset, paint);
                    }
                    else
                    {
                        float currX = x;
                        string subStr;
                        int len;

                        for (int i = line.Start; i < line.Start + line.Length;)
                        {
                            var fb = GetNextForegroundBrush(ref line, i, out len);

                            if (fb != null)
                            {
                                //TODO: figure out how to get the brush size
                                currentWrapper = context.CreatePaint(fb, new Size());
                            }
                            else
                            {
                                if (!currentWrapper.Equals(foreground)) currentWrapper.Dispose();
                                currentWrapper = foreground;
                            }

                            subStr = Text.Substring(i, len);

                            if (currFGPaint != currentWrapper.Paint)
                            {
                                currFGPaint = ApplyWrapperTo(ref currentWrapper, ref currd, paint);
                            }

                            canvas.DrawText(subStr, currX, origin.Y + line.Top + _lineOffset, paint);

                            i += len;
                            currX += paint.MeasureText(subStr);
                        }
                    }
                }
            }
            finally
            {
                if (!currentWrapper.Equals(foreground)) currentWrapper.Dispose();
                currd?.Dispose();
            }
        }

        private const float MAX_LINE_WIDTH = 10000;

        private readonly List<KeyValuePair<FBrushRange, IBrush>> _foregroundBrushes =
                                                new List<KeyValuePair<FBrushRange, IBrush>>();
        private readonly List<FormattedTextLine> _lines = new List<FormattedTextLine>();
        private readonly SKPaint _paint;
        private readonly List<Rect> _rects = new List<Rect>();
        public string Text { get; }
        private readonly TextWrapping _wrapping;
        private Size _constraint = new Size(double.PositiveInfinity, double.PositiveInfinity);
        private float _lineHeight = 0;
        private float _lineOffset = 0;
        private Size _size;
        private List<AvaloniaFormattedTextLine> _skiaLines;

        private static SKPaint ApplyWrapperTo(ref DrawingContextImpl.PaintWrapper wrapper,
                                                ref IDisposable curr, SKPaint paint)
        {
            curr?.Dispose();
            curr = wrapper.ApplyTo(paint);
            return wrapper.Paint;
        }

        private static bool IsBreakChar(char c)
        {
            //white space or zero space whitespace
            return char.IsWhiteSpace(c) || c == '\u200B';
        }

        private static int LineBreak(string textInput, int textIndex, int stop,
                                     SKPaint paint, float maxWidth,
                                     out int trailingCount)
        {
            int lengthBreak;
            if (maxWidth == -1)
            {
                lengthBreak = stop - textIndex;
            }
            else
            {
                float measuredWidth;
                string subText = textInput.Substring(textIndex, stop - textIndex);
                lengthBreak = (int)paint.BreakText(subText, maxWidth, out measuredWidth) / 2;
            }

            //Check for white space or line breakers before the lengthBreak
            int startIndex = textIndex;
            int index = textIndex;
            int word_start = textIndex;
            bool prevBreak = true;

            trailingCount = 0;

            while (index < stop)
            {
                int prevText = index;
                char currChar = textInput[index++];
                bool currBreak = IsBreakChar(currChar);

                if (!currBreak && prevBreak)
                {
                    word_start = prevText;
                }

                prevBreak = currBreak;

                if (index > startIndex + lengthBreak)
                {
                    if (currBreak)
                    {
                        // eat the rest of the whitespace
                        while (index < stop && IsBreakChar(textInput[index]))
                        {
                            index++;
                        }

                        trailingCount = index - prevText;
                    }
                    else
                    {
                        // backup until a whitespace (or 1 char)
                        if (word_start == startIndex)
                        {
                            if (prevText > startIndex)
                            {
                                index = prevText;
                            }
                        }
                        else
                        {
                            index = word_start;
                        }
                    }
                    break;
                }

                if ('\n' == currChar)
                {
                    int ret = index - startIndex;
                    int lineBreakSize = 1;
                    if (index < stop)
                    {
                        currChar = textInput[index++];
                        if ('\r' == currChar)
                        {
                            ret = index - startIndex;
                            ++lineBreakSize;
                        }
                    }

                    trailingCount = lineBreakSize;

                    return ret;
                }

                if ('\r' == currChar)
                {
                    int ret = index - startIndex;
                    int lineBreakSize = 1;
                    if (index < stop)
                    {
                        currChar = textInput[index++];
                        if ('\n' == currChar)
                        {
                            ret = index - startIndex;
                            ++lineBreakSize;
                        }
                    }

                    trailingCount = lineBreakSize;

                    return ret;
                }
            }

            return index - startIndex;
        }

        private void BuildRects()
        {
            // Build character rects
            var fm = _paint.FontMetrics;
            SKTextAlign align = _paint.TextAlign;

            for (int li = 0; li < _skiaLines.Count; li++)
            {
                var line = _skiaLines[li];
                float prevRight = TransformX(0, line.Width, align);
                double nextTop = line.Top + line.Height;

                if (li + 1 < _skiaLines.Count)
                {
                    nextTop = _skiaLines[li + 1].Top;
                }

                for (int i = line.Start; i < line.Start + line.TextLength; i++)
                {
                    float w = _paint.MeasureText(Text[i].ToString());

                    _rects.Add(new Rect(
                        prevRight,
                        line.Top,
                        w,
                        nextTop - line.Top));
                    prevRight += w;
                }
            }
        }

        private IBrush GetNextForegroundBrush(ref AvaloniaFormattedTextLine line, int index, out int length)
        {
            IBrush result = null;
            int len = length = line.Start + line.Length - index;

            if (_foregroundBrushes.Any())
            {
                var bi = _foregroundBrushes.FindIndex(b =>
                                                        b.Key.StartIndex <= index &&
                                                        b.Key.EndIndex > index
                                                        );

                if (bi > -1)
                {
                    var match = _foregroundBrushes[bi];

                    len = match.Key.EndIndex - index + 1;
                    result = match.Value;

                    if (len > 0 && len < length)
                    {
                        length = len;
                    }
                }

                int endIndex = index + length;
                int max = bi == -1 ? _foregroundBrushes.Count : bi;
                var next = _foregroundBrushes.Take(max)
                                                .Where(b => b.Key.StartIndex < endIndex &&
                                                            b.Key.StartIndex > index)
                                                .OrderBy(b => b.Key.StartIndex)
                                                .FirstOrDefault();

                if (next.Value != null)
                {
                    length = next.Key.StartIndex - index;
                }
            }

            return result;
        }

        private List<Rect> GetRects()
        {
            if (Text.Length > _rects.Count)
            {
                BuildRects();
            }

            return _rects;
        }

        private void Rebuild()
        {
            var length = Text.Length;

            _lines.Clear();
            _rects.Clear();
            _skiaLines = new List<AvaloniaFormattedTextLine>();

            int curOff = 0;
            float curY = 0;

            var metrics = _paint.FontMetrics;
            var mTop = metrics.Top;  // The greatest distance above the baseline for any glyph (will be <= 0).
            var mBottom = metrics.Bottom;  // The greatest distance below the baseline for any glyph (will be >= 0).
            var mLeading = metrics.Leading;  // The recommended distance to add between lines of text (will be >= 0).
            var mDescent = metrics.Descent;  //The recommended distance below the baseline. Will be >= 0.
            var mAscent = metrics.Ascent;    //The recommended distance above the baseline. Will be <= 0.
            var lastLineDescent = mBottom - mDescent;

            // This seems like the best measure of full vertical extent
            // matches Direct2D line height
            _lineHeight = mDescent - mAscent;

            // Rendering is relative to baseline
            _lineOffset = (-metrics.Ascent);

            string subString;

            float widthConstraint = (_constraint.Width != double.PositiveInfinity)
                                        ? (float)_constraint.Width
                                        : -1;

            for (int c = 0; curOff < length; c++)
            {
                float lineWidth = -1;
                int measured;
                int trailingnumber = 0;

                subString = Text.Substring(curOff);

                float constraint = -1;

                if (_wrapping == TextWrapping.Wrap)
                {
                    constraint = widthConstraint <= 0 ? MAX_LINE_WIDTH : widthConstraint;
                    if (constraint > MAX_LINE_WIDTH)
                        constraint = MAX_LINE_WIDTH;
                }

                measured = LineBreak(Text, curOff, length, _paint, constraint, out trailingnumber);

                AvaloniaFormattedTextLine line = new AvaloniaFormattedTextLine();
                line.TextLength = measured;

                subString = Text.Substring(line.Start, line.TextLength);
                lineWidth = _paint.MeasureText(subString);
                line.Start = curOff;
                line.Length = measured - trailingnumber;
                line.Width = lineWidth;
                line.Height = _lineHeight;
                line.Top = curY;

                _skiaLines.Add(line);

                curY += _lineHeight;

                curY += mLeading;

                curOff += measured;
            }

            // Now convert to Avalonia data formats
            _lines.Clear();
            float maxX = 0;

            for (var c = 0; c < _skiaLines.Count; c++)
            {
                var w = _skiaLines[c].Width;
                if (maxX < w)
                    maxX = w;

                _lines.Add(new FormattedTextLine(_skiaLines[c].TextLength, _skiaLines[c].Height));
            }

            if (_skiaLines.Count == 0)
            {
                _lines.Add(new FormattedTextLine(0, _lineHeight));
                _size = new Size(0, _lineHeight);
            }
            else
            {
                var lastLine = _skiaLines[_skiaLines.Count - 1];
                _size = new Size(maxX, lastLine.Top + lastLine.Height);
            }
        }

        private float TransformX(float originX, float lineWidth, SKTextAlign align)
        {
            float x = 0;

            if (align == SKTextAlign.Left)
            {
                x = originX;
            }
            else
            {
                double width = Constraint.Width > 0 && !double.IsPositiveInfinity(Constraint.Width) ?
                                Constraint.Width :
                                _size.Width;

                switch (align)
                {
                    case SKTextAlign.Center: x = originX + (float)(width - lineWidth) / 2; break;
                    case SKTextAlign.Right: x = originX + (float)(width - lineWidth); break;
                }
            }

            return x;
        }

        private void SetForegroundBrush(IBrush brush, int startIndex, int length)
        {
            var key = new FBrushRange(startIndex, length);
            int index = _foregroundBrushes.FindIndex(v => v.Key.Equals(key));

            if (index > -1)
            {
                _foregroundBrushes.RemoveAt(index);
            }

            if (brush != null)
            {
                _foregroundBrushes.Insert(0, new KeyValuePair<FBrushRange, IBrush>(key, brush));
            }
        }

        private struct AvaloniaFormattedTextLine
        {
            public float Height;
            public int Length;
            public int Start;
            public int TextLength;
            public float Top;
            public float Width;
        };

        private struct FBrushRange
        {
            public FBrushRange(int startIndex, int length)
            {
                StartIndex = startIndex;
                Length = length;
            }

            public int EndIndex => StartIndex + Length - 1;

            public int Length { get; private set; }

            public int StartIndex { get; private set; }

            public bool Intersects(int index, int len) =>
                (index + len) > StartIndex &&
                (StartIndex + Length) > index;

            public override string ToString()
            {
                return $"{StartIndex}-{EndIndex}";
            }
        }
    }
}