From b67cb4643cc837728f5c527dbc5bf59958039735 Mon Sep 17 00:00:00 2001
From: Bruce Hill <bruce@bruce-hill.com>
Date: Fri, 1 Oct 2021 18:18:22 -0700
Subject: Use chained scatter table

---
 match.c | 77 +++++++++++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 53 insertions(+), 24 deletions(-)

diff --git a/match.c b/match.c
index aa00592..cd284a2 100644
--- a/match.c
+++ b/match.c
@@ -17,14 +17,16 @@
 
 #define MAX_CACHE_SIZE (1<<14)
 
-typedef struct {
+// Cache datastructures
+typedef struct cache_hit_s {
     pat_t *pat;
     const char *start, *end;
+    // Cache entries use a chained scatter approach modeled after Lua's tables
+    struct cache_hit_s *next_probe;
 } cache_hit_t;
 
-// Cache datastructure
 typedef struct {
-    size_t size, occupancy;
+    unsigned int size, occupancy, next_free;
     cache_hit_t *hits;
 } cache_t;
 
@@ -88,14 +90,46 @@ static inline size_t hash(const char *str, size_t pat_id)
 // given definitions. If a result has been memoized, set *result to the
 // memoized value and return true, otherwise return false.
 //
-static cache_hit_t cache_get(match_ctx_t *ctx, const char *str, pat_t *pat)
+static cache_hit_t *cache_get(match_ctx_t *ctx, const char *str, pat_t *pat)
 {
-    if (!ctx->cache->hits) return (cache_hit_t){0};
-    size_t h = hash(str, pat->id) & (ctx->cache->size-1);
-    cache_hit_t c = ctx->cache->hits[h];
-    if (c.pat == pat && c.start == str)
-        return c;
-    return (cache_hit_t){0};
+    if (!ctx->cache->hits) return NULL;
+    for (cache_hit_t *hit = &ctx->cache->hits[hash(str, pat->id) & (ctx->cache->size-1)]; hit; hit = hit->next_probe) {
+        if (hit->pat == pat && hit->start == str)
+            return hit;
+    }
+    return NULL;
+}
+
+static void _hash_insert(cache_t *cache, cache_hit_t hit)
+{
+    size_t h = hash(hit.start, hit.pat->id) & (cache->size-1);
+    cache_hit_t collision = cache->hits[h];
+    if (collision.pat == NULL) { // No collision
+        hit.next_probe = NULL;
+        cache->hits[h] = hit;
+        ++cache->occupancy;
+        return;
+    }
+
+    if (collision.pat == hit.pat && collision.start == hit.start)
+        return; // Duplicate entry, just leave it be
+
+    // Shuffle the collision along to a free space:
+    while (cache->hits[cache->next_free].pat) ++cache->next_free;
+    cache->hits[cache->next_free] = collision;
+
+    size_t hcol = hash(collision.start, collision.pat->id) & (cache->size-1);
+    if (hcol == h) { // Chain `collision` after `hit`
+        hit.next_probe = &cache->hits[cache->next_free];
+    } else { // Keep `collision` in its own chain
+        cache_hit_t *prev = &cache->hits[hcol]; // Where `collision` wanted to be originally
+        while (prev->next_probe != &cache->hits[h]) prev = prev->next_probe;
+        prev->next_probe = &cache->hits[cache->next_free];
+        hit.next_probe = NULL;
+    }
+    cache->hits[h] = hit;
+    ++cache->next_free;
+    ++cache->occupancy;
 }
 
 //
@@ -104,27 +138,23 @@ static cache_hit_t cache_get(match_ctx_t *ctx, const char *str, pat_t *pat)
 static void cache_save(match_ctx_t *ctx, const char *str, pat_t *pat, match_t *m)
 {
     cache_t *cache = ctx->cache;
-    if (cache->occupancy+1 > (cache->size*1)/5) {
+    // Grow the hash if needed (>99% utilization):
+    if (cache->occupancy+1 > (cache->size*99)/100) {
         cache_hit_t *old_hits = cache->hits;
         size_t old_size = cache->size;
         cache->size = old_size == 0 ? 16 : 2*old_size;
         cache->hits = new(cache_hit_t[cache->size]);
+        cache->next_free = 0;
 
         // Rehash:
         for (size_t i = 0; i < old_size; i++) {
-            if (old_hits[i].pat) {
-                size_t h = hash(old_hits[i].start, old_hits[i].pat->id) & (cache->size-1);
-                cache->hits[h] = old_hits[i];
-            }
+            if (old_hits[i].pat)
+                _hash_insert(cache, old_hits[i]);
         }
         if (old_hits) delete(&old_hits);
     }
 
-    size_t h = hash(str, pat->id) & (cache->size-1);
-    if (!cache->hits[h].start) ++cache->occupancy;
-    cache->hits[h].start = str;
-    cache->hits[h].end = m ? m->end : NULL;
-    cache->hits[h].pat = pat;
+    _hash_insert(cache, (cache_hit_t){.pat = pat, .start = str, .end = m ? m->end : NULL});
 }
 
 //
@@ -134,8 +164,7 @@ void cache_destroy(match_ctx_t *ctx)
 {
     cache_t *cache = ctx->cache;
     if (cache->hits) delete(&cache->hits);
-    cache->occupancy = 0;
-    cache->size = 0;
+    memset(cache, 0, sizeof(cache_t));
 }
 
 //
@@ -561,8 +590,8 @@ static match_t *match(match_ctx_t *ctx, const char *str, pat_t *pat)
         return new_match(pat, str, p ? p->end : str, MATCHES(p));
     }
     case BP_REF: {
-        cache_hit_t hit = cache_get(ctx, str, pat);
-        if (hit.start && !hit.end)
+        cache_hit_t *hit = cache_get(ctx, str, pat);
+        if (hit && !hit->end)
             return NULL;
 
         pat_t *ref = lookup(ctx, pat->args.ref.name, pat->args.ref.len);
-- 
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