TextShaper.cs

// RichTextKit
// Copyright © 2019-2020 Topten Software. All Rights Reserved.
// 
// Licensed under the Apache License, Version 2.0 (the "License"); you may 
// not use this product except in compliance with the License. You may obtain 
// a copy of the License at
// 
// http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software 
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 
// License for the specific language governing permissions and limitations 
// under the License.

using HarfBuzzSharp;
using SkiaSharp;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using Topten.RichTextKit.Utils;

namespace Topten.RichTextKit
{
    /// <summary>
    /// Helper class for shaping text
    /// </summary>
    internal class TextShaper : IDisposable
    {
        /// <summary>
        /// Cache of shapers for typefaces
        /// </summary>
        static Dictionary<SKTypeface, TextShaper> _shapers = new Dictionary<SKTypeface, TextShaper>();

        /// <summary>
        /// Get the text shaper for a particular type face
        /// </summary>
        /// <param name="typeface">The typeface being queried for</param>
        /// <returns>A TextShaper</returns>
        public static TextShaper ForTypeface(SKTypeface typeface)
        {
            lock (_shapers)
            {
                if (!_shapers.TryGetValue(typeface, out var shaper))
                {
                    shaper = new TextShaper(typeface);
                    _shapers.Add(typeface, shaper);
                }

                return shaper;
            }
        }

        /// <summary>
        /// Constructs a new TextShaper 
        /// </summary>
        /// <param name="typeface">The typeface of this shaper</param>
        private TextShaper(SKTypeface typeface)
        {
            // Store typeface
            _typeface = typeface;

            // Load the typeface stream to a HarfBuzz font
            int index;
            using (var blob = GetHarfBuzzBlob(typeface.OpenStream(out index)))
            using (var face = new Face(blob, (uint)index))
            {
                face.UnitsPerEm = typeface.UnitsPerEm;

                _font = new HarfBuzzSharp.Font(face);
                _font.SetScale(overScale, overScale);
                _font.SetFunctionsOpenType();
            }

            // Get font metrics for this typeface
            using (var paint = new SKPaint())
            {
                paint.Typeface = typeface;
                paint.TextSize = overScale;
                _fontXMin = paint.FontMetrics.XMin;
                _verticalMetrics = FontMetricsMapper.Default.GetVerticalMetrics(typeface, overScale);

                // This is a temporary hack until SkiaSharp exposes
                // a way to check if a font is fixed pitch.  For now
                // we just measure and `i` and a `w` and see if they're
                // the same width.
                float[] widths = paint.GetGlyphWidths("iw", out var rects);
                _isFixedPitch = widths != null && widths.Length > 1 && widths[0] == widths[1];
                if (_isFixedPitch)
                    _fixedCharacterWidth = widths[0];

            }
        }

        /// <summary>
        /// Dispose this text shaper
        /// </summary>
        public void Dispose()
        {
            if (_font != null)
            {
                _font.Dispose();
                _font = null;
            }
        }

        /// <summary>
        /// The HarfBuzz font for this shaper
        /// </summary>
        HarfBuzzSharp.Font _font;

        /// <summary>
        /// The typeface for this shaper
        /// </summary>
        SKTypeface _typeface;

        /// <summary>
        /// The minimum bounding box x value for the font.
        /// </summary>
        float _fontXMin;

        /// <summary>
        /// The vertical font metrics as provided by the metrics mapper.
        /// </summary>
        readonly VerticalFontMetrics _verticalMetrics;

        /// <summary>
        /// True if this font face is fixed pitch
        /// </summary>
        bool _isFixedPitch;

        /// <summary>
        /// Fixed pitch character width
        /// </summary>
        float _fixedCharacterWidth;

        /// <summary>
        /// A set of re-usable result buffers to store the result of text shaping operation
        /// </summary>
        public class ResultBufferSet
        {
            public void Clear()
            {
                GlyphIndicies.Clear();
                GlyphPositions.Clear();
                Clusters.Clear();
                CodePointXCoords.Clear();
            }

            public Buffer<ushort> GlyphIndicies = new Buffer<ushort>();
            public Buffer<SKPoint> GlyphPositions = new Buffer<SKPoint>();
            public Buffer<int> Clusters = new Buffer<int>();
            public Buffer<float> CodePointXCoords = new Buffer<float>();
        }

        /// <summary>
        /// Returned as the result of a text shaping operation
        /// </summary>
        public struct Result
        {
            /// <summary>
            /// The glyph indicies of all glyphs required to render the shaped text
            /// </summary>
            public Slice<ushort> GlyphIndicies;

            /// <summary>
            /// The position of each glyph
            /// </summary>
            public Slice<SKPoint> GlyphPositions;

            /// <summary>
            /// One entry for each glyph, showing the code point index
            /// of the characters it was derived from
            /// </summary>
            public Slice<int> Clusters;

            /// <summary>
            /// The end position of the rendered text
            /// </summary>
            public SKPoint EndXCoord;

            /// <summary>
            /// The X-Position of each passed code point
            /// </summary>
            public Slice<float> CodePointXCoords;

            /// <summary>
            /// The ascent of the font
            /// </summary>
            public float Ascent;

            /// <summary>
            /// The descent of the font
            /// </summary>
            public float Descent;

            /// <summary>
            /// The leading of the font
            /// </summary>
            public float Leading;

            /// <summary>
            /// The XMin for the font
            /// </summary>
            public float XMin;
        }

        /// <summary>
        /// Over scale used for all font operations
        /// </summary>
        const int overScale = 512;



        /// <summary>
        /// Shape an array of utf-32 code points replacing each grapheme cluster with a replacement character
        /// </summary>
        /// <param name="bufferSet">A re-usable text shaping buffer set that results will be allocated from</param>
        /// <param name="codePoints">The utf-32 code points to be shaped</param>
        /// <param name="style">The user style for the text</param>
        /// <param name="clusterAdjustment">A value to add to all reported cluster numbers</param>
        /// <returns>A TextShaper.Result representing the shaped text</returns>
        public Result ShapeReplacement(ResultBufferSet bufferSet, Slice<int> codePoints, IStyle style, int clusterAdjustment)
        {
            var clusters = GraphemeClusterAlgorithm.GetBoundaries(codePoints).ToArray();
            var glyph = _typeface.GetGlyph(style.ReplacementCharacter);
            var font = new SKFont(_typeface, overScale);
            float glyphScale = style.FontSize / overScale;

            float[] widths = new float[1];
            SKRect[] bounds = new SKRect[1];
            font.GetGlyphWidths((new ushort[] { glyph }).AsSpan(), widths.AsSpan(), bounds.AsSpan());

            var r = new Result();
            r.GlyphIndicies = bufferSet.GlyphIndicies.Add((int)clusters.Length-1, false);
            r.GlyphPositions = bufferSet.GlyphPositions.Add((int)clusters.Length-1, false);
            r.Clusters = bufferSet.Clusters.Add((int)clusters.Length-1, false);
            r.CodePointXCoords = bufferSet.CodePointXCoords.Add(codePoints.Length, false);
            r.CodePointXCoords.Fill(0);

            float xCoord = 0;
            for (int i = 0; i < clusters.Length-1; i++)
            {
                r.GlyphPositions[i].X = xCoord * glyphScale;
                r.GlyphPositions[i].Y = 0;
                r.GlyphIndicies[i] = codePoints[clusters[i]] == 0x2029 ? (ushort)0 : glyph;
                r.Clusters[i] = clusters[i] + clusterAdjustment;

                for (int j = clusters[i]; j < clusters[i + 1]; j++)
                {
                    r.CodePointXCoords[j] = r.GlyphPositions[i].X;
                }

                xCoord += widths[0] + style.LetterSpacing / glyphScale; 
            }

            // Also return the end cursor position
            r.EndXCoord = new SKPoint(xCoord * glyphScale, 0);
            
            ApplyFontMetrics(ref r, style.FontSize);

            return r;
        }


        /// <summary>
        /// Shape an array of utf-32 code points
        /// </summary>
        /// <param name="bufferSet">A re-usable text shaping buffer set that results will be allocated from</param>
        /// <param name="codePoints">The utf-32 code points to be shaped</param>
        /// <param name="style">The user style for the text</param>
        /// <param name="direction">LTR or RTL direction</param>
        /// <param name="clusterAdjustment">A value to add to all reported cluster numbers</param>
        /// <param name="asFallbackFor">The type face this font is a fallback for</param>
        /// <param name="textAlignment">The text alignment of the paragraph, used to control placement of glyphs within character cell when letter spacing used</param>
        /// <returns>A TextShaper.Result representing the shaped text</returns>
        public Result Shape(ResultBufferSet bufferSet, Slice<int> codePoints, IStyle style, TextDirection direction, int clusterAdjustment, SKTypeface asFallbackFor, TextAlignment textAlignment)
        {
            // Work out if we need to force this to a fixed pitch and if
            // so the unscale character width we need to use
            float forceFixedPitchWidth = 0;
            if (asFallbackFor != _typeface && asFallbackFor != null)
            {
                var originalTypefaceShaper = ForTypeface(asFallbackFor);
                if (originalTypefaceShaper._isFixedPitch)
                {
                    forceFixedPitchWidth = originalTypefaceShaper._fixedCharacterWidth;
                }
            }

            // Work out how much to shift glyphs in the character cell when using letter spacing
            // The idea here is to align the glyphs within the character cell the same way as the
            // text block alignment so that left/right aligned text still aligns with the margin
            // and centered text is still centered (and not shifted slightly due to the extra 
            // space that would be at the right with normal letter spacing).
            float glyphLetterSpacingAdjustment = 0;
            switch (textAlignment)
            {
                case TextAlignment.Right:
                    glyphLetterSpacingAdjustment = style.LetterSpacing;
                    break;

                case TextAlignment.Center:
                    glyphLetterSpacingAdjustment = style.LetterSpacing / 2;
                    break;
            }


            using (var buffer = new HarfBuzzSharp.Buffer())
            {
                // Setup buffer
                buffer.AddUtf32(codePoints.AsSpan(), 0, -1);

                // Setup directionality (if supplied)
                switch (direction)
                {
                    case TextDirection.LTR:
                        buffer.Direction = Direction.LeftToRight;
                        break;

                    case TextDirection.RTL:
                        buffer.Direction = Direction.RightToLeft;
                        break;

                    default:
                        throw new ArgumentException(nameof(direction));
                }

                // Guess other attributes
                buffer.GuessSegmentProperties();

                // Shape it
                _font.Shape(buffer);

                // RTL?
                bool rtl = buffer.Direction == Direction.RightToLeft;

                // Work out glyph scaling and offsetting for super/subscript
                float glyphScale = style.FontSize / overScale;
                float glyphVOffset = 0;
                if (style.FontVariant == FontVariant.SuperScript)
                {
                    glyphScale *= 0.65f;
                    glyphVOffset -= style.FontSize * 0.35f;
                }
                if (style.FontVariant == FontVariant.SubScript)
                {
                    glyphScale *= 0.65f;
                    glyphVOffset += style.FontSize * 0.1f;
                }

                // Create results and get buffes
                var r = new Result();
                r.GlyphIndicies = bufferSet.GlyphIndicies.Add((int)buffer.Length, false);
                r.GlyphPositions = bufferSet.GlyphPositions.Add((int)buffer.Length, false);
                r.Clusters = bufferSet.Clusters.Add((int)buffer.Length, false);
                r.CodePointXCoords = bufferSet.CodePointXCoords.Add(codePoints.Length, false);
                r.CodePointXCoords.Fill(0);
                
                // Convert points
                var gp = buffer.GlyphPositions;
                var gi = buffer.GlyphInfos;
                float cursorX = 0;
                float cursorY = 0;
                float cursorXCluster = 0;
                for (int i = 0; i < buffer.Length; i++)
                {
                    r.GlyphIndicies[i] = (ushort)gi[i].Codepoint;
                    r.Clusters[i] = (int)gi[i].Cluster + clusterAdjustment;

                    
                    // Update code point positions
                    if (!rtl)
                    {
                        // First cluster, different cluster, or same cluster with lower x-coord
                        if ( i == 0 ||
                            (r.Clusters[i] != r.Clusters[i - 1]) || 
                            (cursorX < r.CodePointXCoords[r.Clusters[i] - clusterAdjustment]))
                        {
                            r.CodePointXCoords[r.Clusters[i] - clusterAdjustment] = cursorX;
                        }
                    }
    
                    // Get the position
                    var pos = gp[i];

                    // Update glyph position
                    r.GlyphPositions[i] = new SKPoint(
                        cursorX + pos.XOffset * glyphScale + glyphLetterSpacingAdjustment,
                        cursorY - pos.YOffset * glyphScale + glyphVOffset
                        );

                    // Update cursor position
                    cursorX += pos.XAdvance * glyphScale;
                    cursorY += pos.YAdvance * glyphScale;

                    // Ensure paragraph separator character (0x2029) has some
                    // width so it can be seen as part of the selection in the editor.
                    if (pos.XAdvance == 0 && codePoints[(int)gi[i].Cluster] == 0x2029)
                    {
                        cursorX += style.FontSize * 2 / 3;
                    }

                    if (i+1 == gi.Length || gi[i].Cluster != gi[i+1].Cluster)
                    {
                        cursorX += style.LetterSpacing;
                    }

                    // Are we falling back for a fixed pitch font and is the next character a 
                    // new cluster?  If so advance by the width of the original font, not this
                    // fallback font
                    if (forceFixedPitchWidth != 0)
                    {
                        // New cluster?
                        if (i + 1 >= buffer.Length || gi[i].Cluster != gi[i + 1].Cluster)
                        {
                            // Work out fixed pitch position of next cluster
                            cursorXCluster += forceFixedPitchWidth * glyphScale;
                            if (cursorXCluster > cursorX)
                            {
                                // Nudge characters to center them in the fixed pitch width
                                if (i == 0 || gi[i - 1].Cluster != gi[i].Cluster)
                                {
                                    r.GlyphPositions[i].X += (cursorXCluster - cursorX)/ 2;
                                }

                                // Use fixed width character position
                                cursorX = cursorXCluster;
                            }
                            else
                            {
                                // Character is wider (probably an emoji) so we 
                                // allow it to exceed the fixed pitch character width
                                cursorXCluster = cursorX;
                            }
                        }
                    }

                    // Store RTL cursor position
                    if (rtl)
                    {
                        // First cluster, different cluster, or same cluster with lower x-coord
                        if (i == 0 ||
                            (r.Clusters[i] != r.Clusters[i - 1]) ||
                            (cursorX > r.CodePointXCoords[r.Clusters[i] - clusterAdjustment]))
                        {
                            r.CodePointXCoords[r.Clusters[i] - clusterAdjustment] = cursorX;
                        }
                    }
                }

                // Finalize cursor positions by filling in any that weren't
                // referenced by a cluster
                if (rtl)
                {
                    r.CodePointXCoords[0] = cursorX;
                    for (int i = codePoints.Length - 2;  i >= 0; i--)
                    {
                        if (r.CodePointXCoords[i] == 0)
                            r.CodePointXCoords[i] = r.CodePointXCoords[i + 1];
                    }
                }
                else
                {
                    for (int i = 1; i < codePoints.Length; i++)
                    {
                        if (r.CodePointXCoords[i] == 0)
                            r.CodePointXCoords[i] = r.CodePointXCoords[i - 1];
                    }
                }

                // Also return the end cursor position
                r.EndXCoord = new SKPoint(cursorX, cursorY);

                // And some other useful metrics
                ApplyFontMetrics(ref r, style.FontSize);

                // Done
                return r;
            }
        }

        private void ApplyFontMetrics(ref Result result, float fontSize)
        {
            // And some other useful metrics
            result.Ascent = _verticalMetrics.Ascent * fontSize / overScale;
            result.Descent = _verticalMetrics.Descent * fontSize / overScale;
            result.Leading = _verticalMetrics.Leading * fontSize / overScale;
            result.XMin = _fontXMin * fontSize / overScale;
        }

        private static Blob GetHarfBuzzBlob(SKStreamAsset asset)
        {
            if (asset == null)
                throw new ArgumentNullException(nameof(asset));

            Blob blob;

            var size = asset.Length;
            var memoryBase = asset.GetMemoryBase();
            if (memoryBase != IntPtr.Zero)
            {
                // the underlying stream is really a mamory block
                // so save on copying and just use that directly
                blob = new Blob(memoryBase, size, MemoryMode.ReadOnly, () => asset.Dispose());
            }
            else
            {
                // this could be a forward-only stream, so we must copy
                var ptr = Marshal.AllocCoTaskMem(size);
                asset.Read(ptr, size);
                blob = new Blob(ptr, size, MemoryMode.ReadOnly, () => Marshal.FreeCoTaskMem(ptr));
            }

            // make immutable for performance?
            blob.MakeImmutable();

            return blob;
        }
    }
}