Add build script with PNG encoder

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/build b/build
new file mode 100755
index 0000000..73b5823
--- /dev/null
+++ b/build
@@ -0,0 +1,70 @@
+#!/usr/bin/env python3
+# build: assemble the installable GRUB theme into dist/udt/.
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+# Licensed under the GNU General Public License v2 only.
+"""Assemble dist/udt/ from the theme.txt udt-palette rendered.
+
+Usage: build [--selftest]
+
+Writes the PNG slices, the .pf2 fonts and a GRUB-readable copy of the
+current wallpaper next to theme.txt. Everything is staged in a temporary
+directory and swapped in at the end, so a failed run leaves the previous
+dist/udt/ untouched. Never touches /boot: root copies dist/udt/ there.
+"""
+import struct
+import sys
+import zlib
+
+PNG_SIG = b"\x89PNG\r\n\x1a\n"
+
+
+def chunk(tag, data):
+    return (struct.pack(">I", len(data)) + tag + data
+            + struct.pack(">I", zlib.crc32(tag + data)))
+
+
+def png(w, h, rgba):
+    """A w x h PNG of one colour: 8-bit RGBA, not interlaced, which GRUB reads."""
+    row = b"\0" + bytes(rgba) * w  # filter byte 0, then the pixels
+    return (PNG_SIG
+            + chunk(b"IHDR", struct.pack(">IIBBBBB", w, h, 8, 6, 0, 0, 0))
+            + chunk(b"IDAT", zlib.compress(row * h))
+            + chunk(b"IEND", b""))
+
+
+def unpng(data):
+    """Decode what png() writes: (width, height, set of RGBA pixels)."""
+    assert data[:8] == PNG_SIG
+    pos, idat = 8, b""
+    while pos < len(data):
+        n, tag = struct.unpack(">I4s", data[pos:pos + 8])
+        body = data[pos + 8:pos + 8 + n]
+        assert struct.unpack(">I", data[pos + 8 + n:pos + 12 + n])[0] == zlib.crc32(tag + body)
+        if tag == b"IHDR":
+            w, h = struct.unpack(">II", body[:8])
+        elif tag == b"IDAT":
+            idat += body
+        pos += 12 + n
+    raw = zlib.decompress(idat)
+    stride = 1 + 4 * w
+    px = set()
+    for y in range(h):
+        row = raw[y * stride:(y + 1) * stride]
+        assert row[0] == 0
+        px |= {tuple(row[1 + 4 * x:5 + 4 * x]) for x in range(w)}
+    return w, h, px
+
+
+def selftest():
+    w, h, px = unpng(png(3, 2, (0x12, 0x34, 0x56, 0xd9)))
+    assert (w, h, px) == (3, 2, {(0x12, 0x34, 0x56, 0xd9)}), (w, h, px)
+    print("selftest ok")
+
+
+def main():
+    if sys.argv[1:] == ["--selftest"]:
+        return selftest()
+
+
+if __name__ == "__main__":
+    main()