using murmur2 for probing step

pandas/_libs/src/klib/khash.h

@@ -143,10 +143,38 @@ typedef khint_t khiter_t;
 #define __ac_set_isboth_false(flag, i) __ac_set_isempty_false(flag, i)
 #define __ac_set_isdel_true(flag, i) ((void)0)
 
+
+// specializations of https://github.com/aappleby/smhasher/blob/master/src/MurmurHash2.cpp
+khint32_t PANDAS_INLINE murmur2_32to32(khint32_t k){
+    const khint32_t SEED = 0xc70f6907UL;
+    // 'm' and 'r' are mixing constants generated offline.
+    // They're not really 'magic', they just happen to work well.
+    const khint32_t M_32 = 0x5bd1e995;
+    const int R_32 = 24;
+
+    // Initialize the hash to a 'random' value
+    khint32_t h = SEED ^ 4;
+
+    //handle 4 bytes:
+    k *= M_32;
+    k ^= k >> R_32;
+    k *= M_32;
+
+    h *= M_32;
+    h ^= k;
+
+    // Do a few final mixes of the hash to ensure the "last few
+    // bytes" are well-incorporated. (Really needed here?)
+    h ^= h >> 13;
+    h *= M_32;
+    h ^= h >> 15;
+    return h;
+}
+
 #ifdef KHASH_LINEAR
 #define __ac_inc(k, m) 1
 #else
-#define __ac_inc(k, m) (((k)>>3 ^ (k)<<3) | 1) & (m)
+#define __ac_inc(k, m) (murmur2_32to32(k) | 1) & (m)
 #endif
 
 #define __ac_fsize(m) ((m) < 32? 1 : (m)>>5)