Add hashlittle2() and ensure the hash is never 0

It's probably time for a faster hash algorithm, but this gives us
the free 64-bit hashing that things like the xattr code can use.

diff --git a/hashtable.c b/hashtable.c
index e272f439db5e81c2d8e42772eea7c3258a333eae..04543f60cbfacef85ed6f24d90964aa5b0da6a33 100644 (file)
--- a/hashtable.c
+++ b/hashtable.c
@@ -350,6 +350,9 @@ void *hashtable_find(struct hashtable *tbl, int64 key, void *data_when_new)
  -------------------------------------------------------------------------------
 */
 
+#define NON_ZERO_32(x) ((x) ? (x) : (uint32_t)1)
+#define NON_ZERO_64(x, y) ((x) || (y) ? ((int64)(x) << 32) | (y) : (int64)1)
+
 uint32_t hashlittle(const void *key, size_t length)
 {
   uint32_t a,b,c;                                          /* internal state */
@@ -390,7 +393,7 @@ uint32_t hashlittle(const void *key, size_t length)
     case 3 : a+=((uint32_t)k8[2])<<16;   /* fall through */
     case 2 : a+=((uint32_t)k8[1])<<8;    /* fall through */
     case 1 : a+=k8[0]; break;
-    case 0 : return c;
+    case 0 : return NON_ZERO_32(c);
     }
   } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) {
     const uint16_t *k = (const uint16_t *)key;         /* read 16-bit chunks */
@@ -436,7 +439,7 @@ uint32_t hashlittle(const void *key, size_t length)
              break;
     case 1 : a+=k8[0];
              break;
-    case 0 : return c;                     /* zero length requires no mixing */
+    case 0 : return NON_ZERO_32(c);         /* zero length requires no mixing */
     }
 
   } else {                        /* need to read the key one byte at a time */
@@ -489,10 +492,171 @@ uint32_t hashlittle(const void *key, size_t length)
             /* FALLTHROUGH */
     case 1 : a+=k[0];
              break;
-    case 0 : return c;
+    case 0 : return NON_ZERO_32(c);
     }
   }
 
   final(a,b,c);
-  return c;
+  return NON_ZERO_32(c);
 }
+
+#if SIZEOF_INT64 >= 8
+/*
+ * hashlittle2: return 2 32-bit hash values joined into an int64.
+ *
+ * This is identical to hashlittle(), except it returns two 32-bit hash
+ * values instead of just one.  This is good enough for hash table
+ * lookup with 2^^64 buckets, or if you want a second hash if you're not
+ * happy with the first, or if you want a probably-unique 64-bit ID for
+ * the key.  *pc is better mixed than *pb, so use *pc first.  If you want
+ * a 64-bit value do something like "*pc + (((uint64_t)*pb)<<32)".
+ */
+int64 hashlittle2(const void *key, size_t length)
+{
+  uint32_t a,b,c;                                          /* internal state */
+  union { const void *ptr; size_t i; } u;     /* needed for Mac Powerbook G4 */
+
+  /* Set up the internal state */
+  a = b = c = 0xdeadbeef + ((uint32_t)length);
+
+  u.ptr = key;
+  if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) {
+    const uint32_t *k = (const uint32_t *)key;         /* read 32-bit chunks */
+    const uint8_t  *k8;
+
+    /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */
+    while (length > 12)
+    {
+      a += k[0];
+      b += k[1];
+      c += k[2];
+      mix(a,b,c);
+      length -= 12;
+      k += 3;
+    }
+
+    /*----------------------------- handle the last (probably partial) block */
+    k8 = (const uint8_t *)k;
+    switch(length)
+    {
+    case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
+    case 11: c+=((uint32_t)k8[10])<<16;  /* fall through */
+    case 10: c+=((uint32_t)k8[9])<<8;    /* fall through */
+    case 9 : c+=k8[8];                   /* fall through */
+    case 8 : b+=k[1]; a+=k[0]; break;
+    case 7 : b+=((uint32_t)k8[6])<<16;   /* fall through */
+    case 6 : b+=((uint32_t)k8[5])<<8;    /* fall through */
+    case 5 : b+=k8[4];                   /* fall through */
+    case 4 : a+=k[0]; break;
+    case 3 : a+=((uint32_t)k8[2])<<16;   /* fall through */
+    case 2 : a+=((uint32_t)k8[1])<<8;    /* fall through */
+    case 1 : a+=k8[0]; break;
+    case 0 : return NON_ZERO_64(b, c);
+    }
+  } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) {
+    const uint16_t *k = (const uint16_t *)key;         /* read 16-bit chunks */
+    const uint8_t  *k8;
+
+    /*--------------- all but last block: aligned reads and different mixing */
+    while (length > 12)
+    {
+      a += k[0] + (((uint32_t)k[1])<<16);
+      b += k[2] + (((uint32_t)k[3])<<16);
+      c += k[4] + (((uint32_t)k[5])<<16);
+      mix(a,b,c);
+      length -= 12;
+      k += 6;
+    }
+
+    /*----------------------------- handle the last (probably partial) block */
+    k8 = (const uint8_t *)k;
+    switch(length)
+    {
+    case 12: c+=k[4]+(((uint32_t)k[5])<<16);
+             b+=k[2]+(((uint32_t)k[3])<<16);
+             a+=k[0]+(((uint32_t)k[1])<<16);
+             break;
+    case 11: c+=((uint32_t)k8[10])<<16;     /* fall through */
+    case 10: c+=k[4];
+             b+=k[2]+(((uint32_t)k[3])<<16);
+             a+=k[0]+(((uint32_t)k[1])<<16);
+             break;
+    case 9 : c+=k8[8];                      /* fall through */
+    case 8 : b+=k[2]+(((uint32_t)k[3])<<16);
+             a+=k[0]+(((uint32_t)k[1])<<16);
+             break;
+    case 7 : b+=((uint32_t)k8[6])<<16;      /* fall through */
+    case 6 : b+=k[2];
+             a+=k[0]+(((uint32_t)k[1])<<16);
+             break;
+    case 5 : b+=k8[4];                      /* fall through */
+    case 4 : a+=k[0]+(((uint32_t)k[1])<<16);
+             break;
+    case 3 : a+=((uint32_t)k8[2])<<16;      /* fall through */
+    case 2 : a+=k[0];
+             break;
+    case 1 : a+=k8[0];
+             break;
+    case 0 : return NON_ZERO_64(b, c);  /* zero length strings require no mixing */
+    }
+
+  } else {                        /* need to read the key one byte at a time */
+    const uint8_t *k = (const uint8_t *)key;
+
+    /*--------------- all but the last block: affect some 32 bits of (a,b,c) */
+    while (length > 12)
+    {
+      a += k[0];
+      a += ((uint32_t)k[1])<<8;
+      a += ((uint32_t)k[2])<<16;
+      a += ((uint32_t)k[3])<<24;
+      b += k[4];
+      b += ((uint32_t)k[5])<<8;
+      b += ((uint32_t)k[6])<<16;
+      b += ((uint32_t)k[7])<<24;
+      c += k[8];
+      c += ((uint32_t)k[9])<<8;
+      c += ((uint32_t)k[10])<<16;
+      c += ((uint32_t)k[11])<<24;
+      mix(a,b,c);
+      length -= 12;
+      k += 12;
+    }
+
+    /*-------------------------------- last block: affect all 32 bits of (c) */
+    switch(length)                   /* all the case statements fall through */
+    {
+    case 12: c+=((uint32_t)k[11])<<24;
+            /* FALLTHROUGH */
+    case 11: c+=((uint32_t)k[10])<<16;
+            /* FALLTHROUGH */
+    case 10: c+=((uint32_t)k[9])<<8;
+            /* FALLTHROUGH */
+    case 9 : c+=k[8];
+            /* FALLTHROUGH */
+    case 8 : b+=((uint32_t)k[7])<<24;
+            /* FALLTHROUGH */
+    case 7 : b+=((uint32_t)k[6])<<16;
+            /* FALLTHROUGH */
+    case 6 : b+=((uint32_t)k[5])<<8;
+            /* FALLTHROUGH */
+    case 5 : b+=k[4];
+            /* FALLTHROUGH */
+    case 4 : a+=((uint32_t)k[3])<<24;
+            /* FALLTHROUGH */
+    case 3 : a+=((uint32_t)k[2])<<16;
+            /* FALLTHROUGH */
+    case 2 : a+=((uint32_t)k[1])<<8;
+            /* FALLTHROUGH */
+    case 1 : a+=k[0];
+             break;
+    case 0 : return NON_ZERO_64(b, c);
+    }
+  }
+
+  final(a,b,c);
+  return NON_ZERO_64(b, c);
+}
+#else
+#define hashlittle2(key, len) hashlittle(key, len)
+#endif