TextLine.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 SkiaSharp;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Topten.RichTextKit.Utils;

namespace Topten.RichTextKit
{
    /// <summary>
    /// Represents a laid out line of text.
    /// </summary>
    public class TextLine
    {
        /// <summary>
        /// Constructs a new TextLine.
        /// </summary>
        public TextLine()
        {
        }

        /// <summary>
        /// Gets the set of text runs comprising this line.
        /// </summary>
        /// <remarks>
        /// Font runs are order logically (ie: in code point index order)
        /// but may have unordered <see cref="FontRun.XCoord"/>'s when right to
        /// left text is in use.
        /// </remarks>
        public IReadOnlyList<FontRun> Runs => RunsInternal;

        /// <summary>
        /// Gets the text block that owns this line.
        /// </summary>
        public TextBlock TextBlock
        {
            get;
            internal set;
        }

        /// <summary>
        /// Gets the next line in this text block, or null if this is the last line.
        /// </summary>
        public TextLine NextLine
        {
            get
            {
                int index = (TextBlock.Lines as List<TextLine>).IndexOf(this);
                if (index < 0 || index + 1 >= TextBlock.Lines.Count)
                    return null;
                return TextBlock.Lines[index + 1];
            }
        }

        /// <summary>
        /// Gets the previous line in this text block, or null if this is the first line.
        /// </summary>
        public TextLine PreviousLine
        {
            get
            {
                int index = (TextBlock.Lines as List<TextLine>).IndexOf(this);
                if (index <= 0)
                    return null;
                return TextBlock.Lines[index - 1];
            }
        }

        /// <summary>
        /// Gets the y-coordinate of the top of this line, relative to the top of the text block.
        /// </summary>
        public float YCoord
        {
            get;
            internal set;
        }

        /// <summary>
        /// Gets the base line of this line (relative to <see cref="YCoord"/>)
        /// </summary>
        public float BaseLine
        {
            get;
            internal set;
        }

        /// <summary>
        /// Gets the maximum magnitude ascent of all font runs in this line.
        /// </summary>
        /// <remarks>
        /// The ascent is reported as a negative value from the base line.
        /// </remarks>
        public float MaxAscent
        {
            get;
            internal set;
        }


        /// <summary>
        /// Gets the maximum descent of all font runs in this line.
        /// </summary>
        /// <remarks>
        /// The descent is reported as a positive value from the base line.
        /// </remarks>
        public float MaxDescent
        {
            get;
            internal set;
        }

        /// <summary>
        /// Gets the text height of this line.
        /// </summary>
        /// <remarks>
        /// The text height of a line is the sum of the ascent and desent.
        /// </remarks>
        public float TextHeight => -MaxAscent + MaxDescent;

        /// <summary>
        /// Gets the height of this line
        /// </summary>
        /// <remarks>
        /// The height of a line is based on the font and <see cref="IStyle.LineHeight"/>
        /// value of all runs in this line.
        /// </remarks>
        public float Height
        {
            get;
            internal set;
        }

        /// <summary>
        /// The width of the content on this line, excluding trailing whitespace and overhang.
        /// </summary>
        public float Width
        {
            get;
            internal set;
        }

        /// <summary>
        /// Paint this line
        /// </summary>
        /// <param name="ctx">The paint context</param>
        internal void Paint(PaintTextContext ctx)
        {
            foreach (var r in Runs)
            {
                r.PaintBackground(ctx);
            }
            
            foreach (var r in Runs)
            {
                r.Paint(ctx);
            }
        }

        /// <summary>
        /// Code point index of start of this line
        /// </summary>
        public int Start
        {
            get
            {
                var pl = PreviousLine;
                return PreviousLine == null ? 0 : PreviousLine.End;
            }
        }

        /// <summary>
        /// The length of this line in codepoints
        /// </summary>
        public int Length => End - Start;

        /// <summary>
        /// The code point index of the first character after this line
        /// </summary>
        public int End
        {
            get
            {
                // Get the last run that's not an ellipsis
                var lastRun = this.Runs.LastOrDefault(x => x.RunKind != FontRunKind.Ellipsis);

                // If last run found, then it's the end of the run, other wise it's the start index
                return lastRun == null ? Start : lastRun.End;
            }
        }

        /// <summary>
        /// Hit test this line, working out the cluster the x position is over
        /// and closest to.
        /// </summary>
        /// <remarks>
        /// This method only populates the code point indicies in the returned result
        /// and the line indicies will be -1
        /// </remarks>
        /// <param name="x">The xcoord relative to the text block</param>
        public HitTestResult HitTest(float x)
        {
            var htr = new HitTestResult();
            htr.OverLine = -1;
            htr.ClosestLine = -1;
            HitTest(x, ref htr);
            return htr;
        }


        /// <summary>
        /// Hit test this line, working out the cluster the x position is over
        /// and closest to.
        /// </summary>
        /// <param name="x">The xcoord relative to the text block</param>
        /// <param name="htr">HitTestResult to be filled out</param>
        internal void HitTest(float x, ref HitTestResult htr)
        {
            // Working variables
            float closestXPosition = 0;
            int closestCodePointIndex = -1;

            if (Runs.Count > 0)
            {
                // If caret is beyond the end of the line...
                var lastRun = Runs[Runs.Count - 1];
                if ((lastRun.Direction == TextDirection.LTR && x >= lastRun.XCoord + lastRun.Width) ||
                    (lastRun.Direction == TextDirection.RTL && x < lastRun.XCoord))
                {
                    // Special handling for clicking after a soft line break ('\n') in which case
                    // the caret should be positioned before the new line character, not after it 
                    // as this would cause the cursor to appear on the next line).
                    if (lastRun.RunKind == FontRunKind.TrailingWhitespace || lastRun.RunKind == FontRunKind.Ellipsis)
                    {
                        if (lastRun.CodePoints.Length > 0 && 
                            (lastRun.CodePoints[lastRun.CodePoints.Length - 1] == '\n' ||
                             lastRun.CodePoints[lastRun.CodePoints.Length - 1] == 0x2029))
                        {
                            htr.ClosestCodePointIndex = lastRun.End - 1;
                            return;
                        }
                    }
                }
            }

            // Check all runs
            foreach (var r in Runs)
            {
                // Ignore ellipsis runs
                if (r.RunKind == FontRunKind.Ellipsis)
                    continue;

                if (x < r.XCoord)
                {
                    // Before the run...
                    updateClosest(r.XCoord, r.Direction == TextDirection.LTR ? r.Start : r.End, r.Direction);
                }
                else if (x >= r.XCoord + r.Width)
                {
                    // After the run...
                    updateClosest(r.XCoord + r.Width, r.Direction == TextDirection.RTL ? r.Start : r.End, r.Direction);
                }
                else
                {
                    // Inside the run
                    for (int i = 0; i < r.Clusters.Length;)
                    {
                        // Get the xcoord of this cluster
                        var codePointIndex = r.Clusters[i];
                        var xcoord1 = r.GetXCoordOfCodePointIndex(codePointIndex);

                        // Find the code point of the next cluster
                        var j = i;
                        while (j < r.Clusters.Length && r.Clusters[j] == r.Clusters[i])
                            j++;

                        // Get the xcoord of other side of this cluster
                        int codePointIndexOther;
                        if (r.Direction == TextDirection.LTR)
                        {
                            if (j == r.Clusters.Length)
                            {
                                codePointIndexOther = r.End;
                            }
                            else
                            {
                                codePointIndexOther = r.Clusters[j];
                            }
                        }
                        else
                        {
                            if (i > 0)
                            {
                                codePointIndexOther = r.Clusters[i - 1];
                            }
                            else
                            {
                                codePointIndexOther = r.End;
                            }
                        }

                        // Gethte xcoord of the other side of the cluster
                        var xcoord2 = r.GetXCoordOfCodePointIndex(codePointIndexOther);

                        // Ensure order correct for easier in-range check
                        if (xcoord1 > xcoord2)
                        {
                            var temp = xcoord1;
                            xcoord1 = xcoord2;
                            xcoord2 = temp;
                        }

                        // On the character?
                        if (x >= xcoord1 && x < xcoord2)
                        {
                            // Store this as the cluster the point is over
                            htr.OverCodePointIndex = codePointIndex;

                            // Don't move to the rhs (or lhs) of a line break
                            if (r.CodePoints[codePointIndex - r.Start] == '\n')
                            {
                                htr.ClosestCodePointIndex = codePointIndex;
                            }
                            else
                            {
                                // Work out if position is closer to the left or right side of the cluster
                                if (x < (xcoord1 + xcoord2) / 2)
                                {
                                    htr.ClosestCodePointIndex = r.Direction == TextDirection.LTR ? codePointIndex : codePointIndexOther;
                                }
                                else
                                {
                                    htr.ClosestCodePointIndex = r.Direction == TextDirection.LTR ? codePointIndexOther : codePointIndex;
                                }
                            }
                            if (htr.ClosestCodePointIndex == End)
                            {
                                htr.AltCaretPosition = true;
                            }
                            return;
                        }

                        // Move to the next cluster
                        i = j;
                    }
                }
            }

            // Store closest character
            htr.ClosestCodePointIndex = closestCodePointIndex;

            if (htr.ClosestCodePointIndex == End)
            {
                htr.AltCaretPosition = true;
            }

            // Helper for updating closest caret position
            void updateClosest(float xPosition, int codePointIndex, TextDirection dir)
            {
                if (closestCodePointIndex == -1 || Math.Abs(xPosition - x) < Math.Abs(closestXPosition - x))
                {
                    closestXPosition = xPosition;
                    closestCodePointIndex = codePointIndex;
                }
            }
        }

        internal void UpdateOverhang(float right, bool updateTop, bool updateBottom, ref float leftOverhang, ref float rightOverhang, ref float topOverhang, ref float bottomOverhang)
        {
            foreach (var r in Runs)
            {
                r.UpdateOverhang(right, updateTop, updateBottom, ref leftOverhang, ref rightOverhang, ref topOverhang, ref bottomOverhang);
            }
        }

        /// <summary>
        /// Internal List of runs
        /// </summary>
        internal List<FontRun> RunsInternal = new List<FontRun>();

        internal static ThreadLocal<ObjectPool<TextLine>> Pool = new ThreadLocal<ObjectPool<TextLine>>(() => new ObjectPool<TextLine>()
        {
            Cleaner = (r) =>
            {
                r.TextBlock = null;
                r.RunsInternal.Clear();
            }
        });
    }
}