feat: extract wallpaper accent without touching the pywal cache

Calls pywal.backends.colorz.get() directly rather than running `wal -i`. That
function returns a list and writes nothing, so the preset-driven cache that
kitty, dunst and neovim read is left alone. `wal -i` has no isolation flag and
would rewrite all of ~/.cache/wal, reintroducing the unreadable-terminal
problem the preset was adopted to fix.

Writes atomically, so a rofi launch concurrent with a wallpaper change cannot
read a half-written theme. A missing or broken image falls back to mauve rather
than leaving no accent at all.

Verified across six wallpapers with colors.json byte-identical afterward.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G6aeE37K4GHBsaM51yLTTq

1 files changed, 58 insertions, 0 deletions

diff --git a/bin/udt-accent b/bin/udt-accent
index e459a35..a47460e 100755
--- a/bin/udt-accent
+++ b/bin/udt-accent
@@ -13,7 +13,10 @@ including the terminal's ANSI colors. See the design spec for why that matters.
 """
 
 import math
+import os
 import sys
+import tempfile
+from pathlib import Path
 
 # The nine candidate accents. rosewater and flamingo are excluded as
 # near-neutral tints that capture saturated inputs; maroon, sapphire and
@@ -33,6 +36,8 @@ ACCENTS = {
 FALLBACK = "mauve"
 MIN_CHROMA = 10.0
 
+OUTPUT = Path.home() / ".cache" / "wal" / "udt-accent.rasi"
+
 
 def _to_lab(hexval):
     """Convert #rrggbb to CIELAB. sRGB D65, the standard conversion."""
@@ -80,6 +85,54 @@ def snap(hexval):
     return min(ACCENTS, key=distance)
 
 
+def signature_color(image):
+    """Extract the image's most chromatic mid-tone color.
+
+    Calls the colorz backend directly. It returns a list and writes nothing,
+    which is what keeps the pywal cache (and so the terminal) untouched.
+    """
+    from pywal.backends import colorz
+
+    colors = colorz.get(str(image), 16)
+    # Slot 0 trends near-black and the upper slots near-white; the signature
+    # color of an image lives in the middle.
+    return max(colors[1:7], key=_chroma)
+
+
+def write_accent(name):
+    """Write the accent rasi file atomically."""
+    hexval = ACCENTS[name]
+    content = (
+        "/* Generated by udt-accent. Do not edit. */\n"
+        f"* {{ accent: {hexval}ff; }}\n"
+    )
+
+    OUTPUT.parent.mkdir(parents=True, exist_ok=True)
+    # Write-then-rename: a rofi launch during a wallpaper change must never
+    # read a half-written file.
+    fd, tmp = tempfile.mkstemp(dir=str(OUTPUT.parent), suffix=".tmp")
+    try:
+        with os.fdopen(fd, "w") as handle:
+            handle.write(content)
+        os.replace(tmp, OUTPUT)
+    except BaseException:
+        if os.path.exists(tmp):
+            os.unlink(tmp)
+        raise
+
+
+def main(image):
+    try:
+        name = snap(signature_color(image))
+    except Exception as exc:
+        # A broken image must still leave a working theme.
+        print(f"udt-accent: {exc}, falling back to {FALLBACK}", file=sys.stderr)
+        name = FALLBACK
+
+    write_accent(name)
+    print(f"{name} {ACCENTS[name]}")
+
+
 def selftest():
     # Every accent must snap to itself, or the metric is not self-consistent.
     for name, hexval in ACCENTS.items():
@@ -99,3 +152,8 @@ def selftest():
 if __name__ == "__main__":
     if len(sys.argv) == 2 and sys.argv[1] == "--selftest":
         selftest()
+    elif len(sys.argv) == 2:
+        main(sys.argv[1])
+    else:
+        print(__doc__, file=sys.stderr)
+        sys.exit(2)