From fbcb30bf9870aa2b4d75376b3a02703576a715e4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Mon, 13 Jun 2022 13:11:56 +0200
Subject: [PATCH] vt: improve key calculation for compose sequences
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* Don’t assume 32 bits when rotating the old key. Use the number of
  actual bits available, as determined by CELL_COMB_CHARS_{HI,LO}
* Multiply with magic hash constant

This greatly reduces the number of collisions seen. For example, the
Emoji test file (from the Unicode specification), now has zero
collisions.
---
 vt.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/vt.c b/vt.c
index 9746364b..54fd643c 100644
--- a/vt.c
+++ b/vt.c
@@ -622,8 +622,21 @@ action_put(struct terminal *term, uint8_t c)
 static inline uint32_t
 chain_key(uint32_t old_key, uint32_t new_wc)
 {
-    /* Rotate left 8 bits, xor with new char */
-    return ((old_key << 8) | (old_key >> (32 - 8))) ^ new_wc;
+    unsigned bits = 32 - __builtin_clz(CELL_COMB_CHARS_HI - CELL_COMB_CHARS_LO);
+
+    /* Rotate old key 8 bits */
+    uint32_t new_key = (old_key << 8) | (old_key >> (bits - 8));
+
+    /* xor with new char */
+    new_key ^= new_wc;
+
+    /* Multiply with magic hash constant */
+    new_key *= 2654435761;
+
+    /* And mask, to ensure the new value is within range */
+    new_key &= CELL_COMB_CHARS_HI - CELL_COMB_CHARS_LO;
+
+    return new_key;
 }
 
 static void
@@ -668,8 +681,6 @@ action_utf8_print(struct terminal *term, char32_t wc)
         } else
             key = chain_key(base, wc);
 
-        key &= CELL_COMB_CHARS_HI - CELL_COMB_CHARS_LO;
-
 #if defined(FOOT_GRAPHEME_CLUSTERING)
         if (grapheme_clustering) {
             /* Check if we're on a grapheme cluster break */
@@ -767,6 +778,10 @@ action_utf8_print(struct terminal *term, char32_t wc)
                     cc->count != wanted_count ||
                     cc->chars[wanted_count - 1] != wc)
                 {
+#if 0
+                    LOG_WARN("COLLISION: base: %04x/%04x, count: %d/%zu, last: %04x/%04x",
+                             cc->chars[0], base, cc->count, wanted_count, cc->chars[wanted_count - 1], wc);
+#endif
                     key++;
                     collision_count++;
                     continue;