From af5f035830394d083fab9c6da4445e048bbc7808 Mon Sep 17 00:00:00 2001
From: Szymon Nowakowski <renati@renati.me>
Date: Sun, 20 Sep 2026 15:42:19 +0200
Subject: [PATCH] media: stbtt kerning table iterator

---
 packages/media/src/stbtt.zig            | 34 +++++++++++++++++++
 packages/media/src/stbtt/stb_truetype.c | 43 +++++++++++++++++++++++++
 2 files changed, 77 insertions(+)

diff --git a/packages/media/src/stbtt.zig b/packages/media/src/stbtt.zig
index 5dd0df3..40ad53d 100644
--- a/packages/media/src/stbtt.zig
+++ b/packages/media/src/stbtt.zig
@@ -100,6 +100,12 @@ const import = struct {
         padding: u8,
     };
 
+    const stbtt_kerning_table_iterator = extern struct {
+        data: [*]u8,
+        k: c_int,
+        length: c_int,
+    };
+
     extern fn stbtt_GetNumberOfFonts(data: [*]const u8) c_int;
     extern fn stbtt_GetFontOffsetForIndex(index: c_int, data: [*]const u8) c_int;
     extern fn stbtt_InitFont(info: *stbtt_fontinfo, data: [*]const u8, offset: c_int) c_int;
@@ -117,6 +123,8 @@ const import = struct {
 
     extern fn stbtt_UnitsPerEm(info: *const stbtt_fontinfo) c_int;
     extern fn stbtt_GetFontCapHeight(info: *const stbtt_fontinfo, capHeight: ?*c_int) c_int;
+    extern fn stbtt_KerningTableIterator(info: *const stbtt_fontinfo, it: *stbtt_kerning_table_iterator) void;
+    extern fn stbtt_KerningTableNext(it: *stbtt_kerning_table_iterator, entry: *stbtt_kerningentry) c_int;
 };
 
 pub fn initFont(self: *Self, data: []const u8) !*Font {
@@ -212,6 +220,13 @@ pub const Font = opaque {
         return null;
     }
 
+    pub fn kerningTableIterator(self: *const Font) KerningTableIterator {
+        const info = @as(*const import.stbtt_fontinfo, @ptrCast(@alignCast(self)));
+        var it: import.stbtt_kerning_table_iterator = undefined;
+        import.stbtt_KerningTableIterator(info, &it);
+        return .{ .it = it };
+    }
+
     test "refAllDecls" {
         std.testing.refAllDecls(@This());
     }
@@ -229,6 +244,25 @@ pub const GlyphMetrics = struct {
     left_side_bearing: i16,
 };
 
+pub const KerningTableEntry = struct {
+    glyph_prev: u16,
+    glyph_next: u16,
+    advance: i16,
+};
+
+pub const KerningTableIterator = struct {
+    it: import.stbtt_kerning_table_iterator,
+
+    pub fn next(self: *KerningTableIterator) ?KerningTableEntry {
+        var entry: import.stbtt_kerningentry = undefined;
+        return if (import.stbtt_KerningTableNext(&self.it, &entry) != 0) .{
+            .glyph_prev = @intCast(entry.glyph1),
+            .glyph_next = @intCast(entry.glyph2),
+            .advance = @intCast(entry.advance),
+        } else null;
+    }
+};
+
 // --- MALLOC INTERFACE --------------------------------------------------------
 
 export fn castle_media_stbtt_malloc(self: *Self, size: usize) callconv(.c) VoidPtr {
diff --git a/packages/media/src/stbtt/stb_truetype.c b/packages/media/src/stbtt/stb_truetype.c
index 45b3952..09f983f 100644
--- a/packages/media/src/stbtt/stb_truetype.c
+++ b/packages/media/src/stbtt/stb_truetype.c
@@ -22,3 +22,46 @@ STBTT_DEF int stbtt_GetFontCapHeight(const stbtt_fontinfo *info, int *capHeight)
    if (capHeight) *capHeight = ttSHORT(info->data+tab + 88);
    return 1;
 }
+
+typedef struct stbtt_kerning_table_iterator {
+   stbtt_uint8 *data;
+   int k;
+   int length;
+} stbtt_kerning_table_iterator;
+
+STBTT_DEF void stbtt_KerningTableIterator(const stbtt_fontinfo *info, stbtt_kerning_table_iterator *it)
+{
+   stbtt_uint8 *data = info->data + info->kern;
+
+   it->data = data;
+   it->k = 0;
+   it->length = 0;
+
+   if (!info->kern)
+      return;
+   if (ttUSHORT(data+2) < 1) // number of tables, need at least 1
+      return;
+   if (ttUSHORT(data+8) != 1) // horizontal flag must be set in format
+      return;
+
+   it->length = ttUSHORT(data+10);
+}
+
+STBTT_DEF int stbtt_KerningTableNext(stbtt_kerning_table_iterator *it, stbtt_kerningentry *entry)
+{
+   stbtt_uint8 *data = it->data;
+   int k = it->k;
+   int length = it->length;
+
+   if (k < length)
+   {
+      entry->glyph1 = ttUSHORT(data+18+(k*6));
+      entry->glyph2 = ttUSHORT(data+20+(k*6));
+      entry->advance = ttSHORT(data+22+(k*6));
+
+      it->k += 1;
+      return 1;
+   }
+
+   return 0;
+}