hash: provide default callback functions

* lib/hash.c (raw_hasher, raw_comparator): New functions.
(hash_initialize): Use them as defaults.
* tests/test-hash.c (main): Test this.

Signed-off-by: Eric Blake <ebb9@byu.net>

1 files changed, 28 insertions, 4 deletions

diff --git a/lib/hash.c b/lib/hash.c
index e464ce3904..59f1ff0d07 100644
--- a/lib/hash.c
+++ b/lib/hash.c
@@ -479,6 +479,28 @@ hash_reset_tuning (Hash_tuning *tuning)
   *tuning = default_tuning;
 }
 
+/* If the user passes a NULL hasher, we hash the raw pointer.  */
+static size_t
+raw_hasher (const void *data, size_t n)
+{
+  /* When hashing unique pointers, it is often the case that they were
+     generated by malloc and thus have the property that the low-order
+     bits are 0.  As this tends to give poorer performance with small
+     tables, we rotate the pointer value before performing division,
+     in an attempt to improve hash quality.  */
+  size_t val = (size_t) data;
+  val = ((val >> 3) | (val << (CHAR_BIT * sizeof val - 3))) & SIZE_MAX;
+  return val % n;
+}
+
+/* If the user passes a NULL comparator, we use pointer comparison.  */
+static bool
+raw_comparator (const void *a, const void *b)
+{
+  return a == b;
+}
+
+
 /* For the given hash TABLE, check the user supplied tuning structure for
    reasonable values, and return true if there is no gross error with it.
    Otherwise, definitively reset the TUNING field to some acceptable default
@@ -527,12 +549,12 @@ check_tuning (Hash_table *table)
    provided but the values requested are out of bounds or might cause
    rounding errors, return NULL.
 
-   The user-supplied HASHER function should be provided.  It accepts two
+   The user-supplied HASHER function, when not NULL, accepts two
    arguments ENTRY and TABLE_SIZE.  It computes, by hashing ENTRY contents, a
    slot number for that entry which should be in the range 0..TABLE_SIZE-1.
    This slot number is then returned.
 
-   The user-supplied COMPARATOR function should be provided.  It accepts two
+   The user-supplied COMPARATOR function, when not NULL, accepts two
    arguments pointing to user data, it then returns true for a pair of entries
    that compare equal, or false otherwise.  This function is internally called
    on entries which are already known to hash to the same bucket index,
@@ -553,8 +575,10 @@ hash_initialize (size_t candidate, const Hash_tuning *tuning,
 {
   Hash_table *table;
 
-  if (hasher == NULL || comparator == NULL)
-    return NULL;
+  if (hasher == NULL)
+    hasher = raw_hasher;
+  if (comparator == NULL)
+    comparator = raw_comparator;
 
   table = malloc (sizeof *table);
   if (table == NULL)