feat: drive the Hyprland border gradient from the wallpaper accent

Replaces the six-stop RGB rainbow with the current accent plus its two
neighbours on the perceptual hue wheel. The borderangle animation rotates a
gradient, so it needs more than one colour to show motion; neighbouring hues
keep that motion legible without leaving the palette.

Emitted as Lua rather than a .conf snippet. Hyprland's Lua parser refuses
hyprctl keyword ("keyword can't work with non-legacy parsers") and has no
source directive, so look_and_feel.lua reads the generated table with dofile()
and falls back to a fixed Macchiato triple when it is absent.

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

1 files changed, 55 insertions, 1 deletions

diff --git a/bin/udt-accent b/bin/udt-accent
index a47460e..fbd6a65 100755
--- a/bin/udt-accent
+++ b/bin/udt-accent
@@ -14,6 +14,7 @@ including the terminal's ANSI colors. See the design spec for why that matters.
 
 import math
 import os
+import subprocess
 import sys
 import tempfile
 from pathlib import Path
@@ -37,6 +38,7 @@ FALLBACK = "mauve"
 MIN_CHROMA = 10.0
 
 OUTPUT = Path.home() / ".cache" / "wal" / "udt-accent.rasi"
+BORDER_OUTPUT = Path.home() / ".cache" / "wal" / "udt-border.lua"
 
 
 def _to_lab(hexval):
@@ -121,6 +123,45 @@ def write_accent(name):
         raise
 
 
+def hue_neighbours(name):
+    """The accent either side of `name` on the perceptual hue wheel.
+
+    The animated border rotates a gradient, so it needs more than one colour to
+    show motion. Using the accent's own neighbours keeps the movement visible
+    while staying inside one region of the palette.
+    """
+    order = sorted(ACCENTS, key=lambda n: _hue(ACCENTS[n]))
+    i = order.index(name)
+    return order[(i - 1) % len(order)], order[(i + 1) % len(order)]
+
+
+def write_border(name):
+    """Write the Hyprland border gradient, atomically.
+
+    Emitted as Lua rather than a .conf snippet: Hyprland's Lua parser refuses
+    `hyprctl keyword` ("keyword can't work with non-legacy parsers") and has no
+    source directive, so the config reads this file with dofile() instead.
+    """
+    before, after = hue_neighbours(name)
+    stops = ", ".join(f'"rgb({ACCENTS[n].lstrip("#")})"' for n in (before, name, after))
+
+    content = (
+        "-- Generated by udt-accent. Do not edit.\n"
+        f"return {{ {stops} }}\n"
+    )
+
+    BORDER_OUTPUT.parent.mkdir(parents=True, exist_ok=True)
+    fd, tmp = tempfile.mkstemp(dir=str(BORDER_OUTPUT.parent), suffix=".tmp")
+    try:
+        with os.fdopen(fd, "w") as handle:
+            handle.write(content)
+        os.replace(tmp, BORDER_OUTPUT)
+    except BaseException:
+        if os.path.exists(tmp):
+            os.unlink(tmp)
+        raise
+
+
 def main(image):
     try:
         name = snap(signature_color(image))
@@ -130,7 +171,13 @@ def main(image):
         name = FALLBACK
 
     write_accent(name)
-    print(f"{name} {ACCENTS[name]}")
+    write_border(name)
+
+    # Hyprland only rereads its config on request, and may not be running.
+    subprocess.run(["hyprctl", "reload"], capture_output=True, check=False)
+
+    before, after = hue_neighbours(name)
+    print(f"{name} {ACCENTS[name]} (border: {before} .. {name} .. {after})")
 
 
 def selftest():
@@ -146,6 +193,13 @@ def selftest():
     # A near-grey has an unstable hue angle and must take the fallback.
     assert snap("#888888") == FALLBACK, snap("#888888")
 
+    # Border neighbours must be distinct from the accent and from each other,
+    # or the gradient has nothing to animate between.
+    for name in ACCENTS:
+        before, after = hue_neighbours(name)
+        assert len({before, name, after}) == 3, (name, before, after)
+    assert hue_neighbours("peach") == ("red", "yellow"), hue_neighbours("peach")
+
     print("selftest OK")