Fonts/src/SixLabors.Fonts/Tables/General/Kern/KerningSubTable.cs at 8cf0e58a641453599f4bede3e330ad5d3bc30466 · SixLabors/Fonts · GitHub

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// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System.Numerics;

namespace SixLabors.Fonts.Tables.General.Kern;

internal abstract class KerningSubTable
{
    private readonly KerningCoverage coverage;

    public KerningSubTable(KerningCoverage coverage)
        => this.coverage = coverage;

    public static KerningSubTable? Load(BigEndianBinaryReader reader)
    {
        // Kerning subtables will share the same header format.
        // This header is used to identify the format of the subtable and the kind of information it contains:
        // +--------+----------+----------------------------------------------------------+
        // | Type   | Field    | Description                                              |
        // +========+==========+==========================================================+
        // | uint16 | version  | Kern subtable version number                             |
        // +--------+----------+----------------------------------------------------------+
        // | uint16 | length   | Length of the subtable, in bytes(including this header). |
        // +--------+----------+----------------------------------------------------------+
        // | uint16 | coverage | What type of information is contained in this table.     |
        // +--------+----------+----------------------------------------------------------+
        ushort subVersion = reader.ReadUInt16();
        ushort length = reader.ReadUInt16();
        KerningCoverage coverage = KerningCoverage.Read(reader);
        if (coverage.Format == 0)
        {
            return Format0SubTable.Load(reader, coverage);
        }
        else
        {
            // we don't support versions other than 'Format 0' same as Windows
            return null;
        }
    }

    protected abstract bool TryGetOffset(ushort index1, ushort index2, out short offset);

    public bool TryApplyOffset(ushort index1, ushort index2, ref Vector2 result)
    {
        if (this.TryGetOffset(index1, index2, out short offset))
        {
            if (this.coverage.Horizontal)
            {
                // apply to X
                if (this.coverage.OverrideAccumulator)
                {
                    result.X = offset;
                }
                else
                {
                    result.X += offset;
                }
            }
            else
            {
                // apply to Y
                if (this.coverage.OverrideAccumulator)
                {
                    result.Y = offset;
                }
                else
                {
                    result.Y += offset;
                }
            }

            return true;
        }

        return false;
    }
}