fix badly implemented loop, loop less, cache common results; this makes requests by new people >10x faster when there are a lot of existing people

website/utils/colour.py

@@ -1,12 +1,25 @@
 import hashlib
 from website.constants import BACKGROUND_COLOUR
+from functools import lru_cache
+
+# functions for calculating constrast between two colours
+# https://www2.cs.sfu.ca/CourseCentral/820/li/material/RGB-YUV.pdf
+# https://www.accessibility-developer-guide.com/knowledge/colours-and-contrast/how-to-calculate/
+
+def _contrast_numerator(c1):
+    l1 = (0.299 * c1[0] + 0.587 * c1[1] + 0.114 * c1[2]) / 256
+    return l1 + 0.05
+
+# this is being cached because it is always called with the colours of existing
+# users, and we don't need to do the same float division thousands of times
+# just for gen_colour to return a single colour
+@lru_cache(maxsize=256)
+def _contrast_denominator(c2):
+    l2 = (0.299 * c2[0] + 0.587 * c2[1] + 0.114 * c2[2]) / 256
+    return 1 / (l2 + 0.05)
 
 def _contrast(c1, c2):
-    # https://www2.cs.sfu.ca/CourseCentral/820/li/material/RGB-YUV.pdf
-    l1 = (0.299 * c1[0] + 0.587 * c1[1] + 0.114 * c1[2]) / 256
-    l2 = (0.299 * c2[0] + 0.587 * c2[1] + 0.114 * c2[2]) / 256
-    # https://www.accessibility-developer-guide.com/knowledge/colours-and-contrast/how-to-calculate/
-    return (l1 + 0.05) / (l2 + 0.05)
+    return _contrast_numerator(c1) * _contrast_denominator(c2)
 
 def _distance_sq(c1, c2):
     return sum((i / 256 - j / 256) ** 2 for i, j in zip(c1, c2))
@@ -15,12 +28,13 @@ def _gen_colour(seed, background=BACKGROUND_COLOUR):
     '''
     Returns a colour with sufficient contrast to the background colour
     '''
-    while True:
+    for _ in range(16): # this loop exits on the first iteration 99.99% of the time (literally)
         seed = hashlib.sha256(seed).digest()
         for i in range(0, len(seed) - len(seed) % 3, 3):
             colour = seed[i:i+3]
             if 1.5 < _contrast(colour, background) < 3:
-                return colour
+                break
+    return colour, seed
 
 def gen_colour(seed, background=BACKGROUND_COLOUR, *avoid):
     '''
@@ -29,22 +43,20 @@ def gen_colour(seed, background=BACKGROUND_COLOUR, *avoid):
     This function hasn't been analysed for efficiency or anything
     '''
     best_colour, best_score = None, None
-    for _ in range(16384):
-        colour = _gen_colour(seed, background)
-        score = float('inf') if len(avoid) == 0 else sum(_contrast(colour, c) for c in avoid) / len(avoid)
-        if colour in avoid:
+    for _ in range(1024):
+        colour, seed = _gen_colour(seed, background)
+        if len(avoid) == 0:
+            score = float('inf')
+        elif colour in avoid:
             score = float('-inf')
+        else:
+            score = sum(_contrast(colour, c) for c in avoid) / len(avoid)
         if 2.5 < score:
             return colour
         if best_score == None or score > best_score:
-            best_colour = colour
+            best_colour, best_score = colour, score
     return best_colour
 
-# old-style tag generation: similar colours make similar tags
-#def tag(colour):
-#    tag = ((colour[2] & 0xf0) >> 4) | (colour[1] & 0xf0) | ((colour[0] & 0xf0) << 4)
-#    return f'#{tag:03x}'
-
 def tag(token, length=3):
     '''
     Generates a deterministic pseudorandom tag from a given token