allow to optionally use linear probing

diff --git a/khash.h b/khash.h
index 9b877e98b724b44c8f4b3065e73501b735fa43cf..5936be3eb6e6ba122bf485ebcdac4e84ccfb1f8a 100644 (file)
--- a/khash.h
+++ b/khash.h
@@ -55,7 +55,12 @@ int main() {
           - http://code.google.com/p/ulib/
           - http://nothings.org/computer/judy/
 
-       * Added Wang's integer hash function (not used by default)
+       * Allow to optionally use linear probing which usually has better
+         performance for random input. Double hashing is still the default as it
+         is more robust to certain non-random input.
+
+       * Added Wang's integer hash function (not used by default). This hash
+         function is more robust to certain non-random input.
 
   2011-02-14 (0.2.5):
 
@@ -137,7 +142,12 @@ typedef khint_t khiter_t;
 #define __ac_set_isboth_false(flag, i) (flag[i>>4]&=~(3ul<<((i&0xfU)<<1)))
 #define __ac_set_isdel_true(flag, i) (flag[i>>4]|=1ul<<((i&0xfU)<<1))
 
-#define __ac_inc(k) ((k) | 1)
+#ifdef KHASH_LINEAR
+#define __ac_inc(k, m) 1
+#else
+#define __ac_inc(k, m) ((k)>>3 | 1) & (m)
+#endif
+
 #define __ac_fsize(m) ((m) < 16? 1 : (m)>>4)
 
 #ifndef kroundup32
@@ -192,7 +202,7 @@ static const double __ac_HASH_UPPER = 0.77;
                        khint_t inc, k, i, last, mask;                                                          \
                        mask = h->n_buckets - 1;                                                                        \
                        k = __hash_func(key); i = k & mask;                                                     \
-                       inc = __ac_inc(k) & mask; last = i; /* inc==1 for linear probing */ \
+                       inc = __ac_inc(k, mask); last = i; /* inc==1 for linear probing */ \
                        while (!__ac_isempty(h->flags, i) && (__ac_isdel(h->flags, i) || !__hash_equal(h->keys[i], key))) { \
                                i = (i + inc) & mask;                                                                   \
                                if (i == last) return h->n_buckets;                                             \
@@ -230,7 +240,7 @@ static const double __ac_HASH_UPPER = 0.77;
                                                khint_t inc, k, i;                                                              \
                                                k = __hash_func(key);                                                   \
                                                i = k & new_mask;                                                               \
-                                               inc = __ac_inc(k) & new_mask;                                   \
+                                               inc = __ac_inc(k, new_mask);                                    \
                                                while (!__ac_isempty(new_flags, i)) i = (i + inc) & new_mask; \
                                                __ac_set_isempty_false(new_flags, i);                   \
                                                if (i < h->n_buckets && __ac_iseither(h->flags, i) == 0) { /* kick out the existing element */ \
@@ -268,7 +278,7 @@ static const double __ac_HASH_UPPER = 0.77;
                        x = site = h->n_buckets; k = __hash_func(key); i = k & mask; \
                        if (__ac_isempty(h->flags, i)) x = i; /* for speed up */        \
                        else {                                                                                                          \
-                               inc = __ac_inc(k) & mask; last = i;                                             \
+                               inc = __ac_inc(k, mask); last = i;                                              \
                                while (!__ac_isempty(h->flags, i) && (__ac_isdel(h->flags, i) || !__hash_equal(h->keys[i], key))) { \
                                        if (__ac_isdel(h->flags, i)) site = i;                          \
                                        i = (i + inc) & mask;                                                           \