feat: skip PNG renders whose source has not changed (#147)

* feat: skip PNG renders whose source has not changed

* chore: re-render every PNG once to stamp its source hash

Author: lesnik512

brand/apparel/back-lockup-2400.png

@@ -1,21 +1,41 @@
+import hashlib
 import shutil
 import subprocess
 from pathlib import Path
 
+try:
+    from PIL import Image, PngImagePlugin
+except ModuleNotFoundError:
+    Image = PngImagePlugin = None
+
 _PNG_COLORS = 32  # palette size; flat art needs few (8 already looked clean; 32 = headroom)
+_SOURCE_KEY = "source-sha256"
+
+
+def _source_hash(svg_path: Path, width: int | None, height: int | None) -> str:
+    h = hashlib.sha256(svg_path.read_bytes())
+    h.update(f"|{width}|{height}|{_PNG_COLORS}".encode())
+    return h.hexdigest()
+
+
+def _stored_hash(png_path: Path) -> str | None:
+    if Image is None or not png_path.is_file():
+        return None
+    with Image.open(png_path) as im:
+        return im.info.get(_SOURCE_KEY)
 
 
-def _quantize_png(path: Path, colors: int = _PNG_COLORS) -> None:
+def _quantize_png(path: Path, source_hash: str, colors: int = _PNG_COLORS) -> None:
     """Re-save a PNG as an indexed-palette image — visually lossless for flat
     art. FASTOCTREE preserves alpha, so it is correct for both the opaque
     social cards and the transparent marks. No-op if Pillow is unavailable."""
-    try:
-        from PIL import Image
-    except ModuleNotFoundError:
+    if Image is None:
         return
     im = Image.open(path).convert("RGBA")
     q = im.quantize(colors=colors, method=Image.Quantize.FASTOCTREE)
-    q.save(path, format="PNG", optimize=True)
+    info = PngImagePlugin.PngInfo()
+    info.add_text(_SOURCE_KEY, source_hash)
+    q.save(path, format="PNG", optimize=True, pnginfo=info)
 
 
 def export_png(
@@ -28,12 +48,15 @@ def export_png(
     exe = shutil.which("rsvg-convert")
     if exe is None:
         return False
+    source_hash = _source_hash(svg_path, width, height)
+    if _stored_hash(png_path) == source_hash:
+        return True
     args = [exe]
     if width is not None:
         args += ["-w", str(width)]
     if height is not None:
         args += ["-h", str(height)]
     args += [str(svg_path), "-o", str(png_path)]
     subprocess.run(args, check=True)
-    _quantize_png(png_path)
+    _quantize_png(png_path, source_hash)
     return True