journal: introduce entry array chain cache

When traversing entry array chains for a bisection or for retrieving an
item by index we previously always started at the beginning of the
chain. Since we tend to look at the same chains repeatedly, let's cache
where we have been the last time, and maybe we can skip ahead with this
the next time.

This turns most bisections and index lookups from O(log(n)*log(n)) into
O(log(n)). More importantly however, we seek around on disk much less,
which is good to reduce buffer cache and seek times on rotational disks.

src/shared/hashmap.c

@@ -147,6 +147,25 @@ int trivial_compare_func(const void *a, const void *b) {
         return a < b ? -1 : (a > b ? 1 : 0);
 }
 
+unsigned uint64_hash_func(const void *p) {
+        uint64_t u;
+
+        assert_cc(sizeof(uint64_t) == 2*sizeof(unsigned));
+
+        u = *(const uint64_t*) p;
+
+        return (unsigned) ((u >> 32) ^ u);
+}
+
+int uint64_compare_func(const void *_a, const void *_b) {
+        uint64_t a, b;
+
+        a = *(const uint64_t*) _a;
+        b = *(const uint64_t*) _b;
+
+        return a < b ? -1 : (a > b ? 1 : 0);
+}
+
 Hashmap *hashmap_new(hash_func_t hash_func, compare_func_t compare_func) {
         bool b;
         Hashmap *h;

src/shared/hashmap.h

@@ -44,6 +44,9 @@ int string_compare_func(const void *a, const void *b);
 unsigned trivial_hash_func(const void *p);
 int trivial_compare_func(const void *a, const void *b);
 
+unsigned uint64_hash_func(const void *p);
+int uint64_compare_func(const void *a, const void *b);
+
 Hashmap *hashmap_new(hash_func_t hash_func, compare_func_t compare_func);
 void hashmap_free(Hashmap *h);
 void hashmap_free_free(Hashmap *h);