tiny.filigree.font.kern

1const std = @import("std");2const binary = @import("binary.zig");3const model = @import("model.zig");45const FontError = model.FontError;6const Table = binary.Table;7const readI16 = binary.readI16;8const readU16 = binary.readU16;910pub const Kern = struct {11    subtable: ?KernSubtable,1213    pub fn init(data: []const u8, kern_table: Table) FontError!Kern {14        if (kern_table.len < 4) return error.InvalidKern;15        const version = try readU16(data, kern_table.offset);16        if (version != 0) return error.InvalidKern;17        const count = try readU16(data, kern_table.offset + 2);18        var offset = kern_table.offset + 4;19        const end = kern_table.offset + kern_table.len;20        var first: ?KernSubtable = null;2122        for (0..count) |_| {23            if (offset > end or end - offset < 6) return error.InvalidKern;24            const subtable_start = offset;25            const subtable_version = try readU16(data, offset);26            const len: usize = try readU16(data, offset + 2);27            const coverage = try readU16(data, offset + 4);28            if (len < 6 or len > end - offset) return error.InvalidKern;29            offset += len;3031            if (subtable_version != 0) continue;32            const format: u8 = @intCast(coverage >> 8);33            const horizontal = (coverage & 0x0001) != 0;34            const minimum = (coverage & 0x0002) != 0;35            const cross_stream = (coverage & 0x0004) != 0;36            if (format != 0 or !horizontal or minimum or cross_stream) continue;37            const subtable = try validateKernFormat0(data, .{ .offset = subtable_start, .len = len });38            if (first == null) first = subtable;39        }4041        return .{ .subtable = first };42    }4344    pub fn value(self: Kern, data: []const u8, left_glyph: u32, right_glyph: u32) FontError!i16 {45        if (self.subtable) |subtable| {46            if (try subtable.value(data, left_glyph, right_glyph)) |pair_value| {47                return pair_value;48            }49        }50        return 0;51    }52};5354const KernSubtable = struct {55    pairs_offset: usize,56    pair_count: u16,5758    fn value(self: KernSubtable, data: []const u8, left_glyph: u32, right_glyph: u32) FontError!?i16 {59        if (left_glyph > std.math.maxInt(u16) or right_glyph > std.math.maxInt(u16)) return null;60        const key = (@as(u32, @intCast(left_glyph)) << 16) | @as(u32, @intCast(right_glyph));6162        var left: usize = 0;63        var right: usize = self.pair_count;64        while (left < right) {65            const mid = left + (right - left) / 2;66            const record = self.pairs_offset + mid * 6;67            const candidate = (@as(u32, try readU16(data, record)) << 16) | @as(u32, try readU16(data, record + 2));68            if (key < candidate) {69                right = mid;70            } else if (key > candidate) {71                left = mid + 1;72            } else {73                return try readI16(data, record + 4);74            }75        }76        return null;77    }78};7980fn validateKernFormat0(data: []const u8, subtable: Table) FontError!KernSubtable {81    if (subtable.len < 14) return error.InvalidKern;82    const pairs_offset = subtable.offset + 14;83    const pair_count = try readU16(data, subtable.offset + 6);84    const pairs_len = @as(usize, pair_count) * 6;85    if (pairs_len > subtable.len - 14) return error.InvalidKern;86    return .{87        .pairs_offset = pairs_offset,88        .pair_count = pair_count,89    };90}

1pub const kern = @import("kern.zig");