* You can do whatever you want with this file.
*/
-#ifndef _WIMLIB_LZ_HASH_H
-#define _WIMLIB_LZ_HASH_H
+#ifndef _LZ_HASH_H
+#define _LZ_HASH_H
#include "wimlib/unaligned.h"
-/* Constant for the multiplicative hash function. */
-#define LZ_HASH_MULTIPLIER 0x1E35A7BD
-
-static inline u32
-loaded_u32_to_u24(u32 v)
-{
- if (CPU_IS_LITTLE_ENDIAN)
- return v & 0xFFFFFF;
- else
- return v >> 8;
-}
-
+/*
+ * The hash function: given a sequence prefix held in the low-order bits of a
+ * 32-bit value, multiply by a carefully-chosen large constant. Discard any
+ * bits of the product that don't fit in a 32-bit value, but take the
+ * next-highest @num_bits bits of the product as the hash value, as those have
+ * the most randomness.
+ */
static inline u32
-load_u24_unaligned(const u8 *p)
+lz_hash(u32 seq, unsigned num_bits)
{
- if (UNALIGNED_ACCESS_IS_FAST)
- return loaded_u32_to_u24(load_u32_unaligned(p));
- else
- return ((u32)p[0] << 0) | ((u32)p[1] << 8) | ((u32)p[2] << 16);
+ return (u32)(seq * 0x1E35A7BD) >> (32 - num_bits);
}
+/*
+ * Hash the 2-byte sequence beginning at @p, producing a hash of length
+ * @num_bits bits. At least 2 bytes of data must be available at @p.
+ */
static inline u32
-lz_hash(u32 str, unsigned num_bits)
+lz_hash_2_bytes(const u8 *p, unsigned num_bits)
{
- return (u32)(str * LZ_HASH_MULTIPLIER) >> (32 - num_bits);
+ u32 seq = load_u16_unaligned(p);
+ if (num_bits >= 16)
+ return seq;
+ return lz_hash(seq, num_bits);
}
/*
- * Hash the next 3-byte sequence in the window, producing a hash of length
- * 'num_bits' bits. At least LZ_HASH_REQUIRED_NBYTES must be available at 'p';
- * this might be 4 bytes rather than 3 because an unaligned load is faster on
- * some architectures.
+ * Hash the 3-byte sequence beginning at @p, producing a hash of length
+ * @num_bits bits. At least LZ_HASH3_REQUIRED_NBYTES bytes of data must be
+ * available at @p; note that this may be more than 3.
*/
static inline u32
lz_hash_3_bytes(const u8 *p, unsigned num_bits)
{
- return lz_hash(load_u24_unaligned(p), num_bits);
+ u32 seq = load_u24_unaligned(p);
+ if (num_bits >= 24)
+ return seq;
+ return lz_hash(seq, num_bits);
}
-/* Number of bytes the hash function actually requires be available, due to the
- * possibility of an unaligned load. */
-#define LZ_HASH_REQUIRED_NBYTES (UNALIGNED_ACCESS_IS_FAST ? 4 : 3)
+#define LZ_HASH3_REQUIRED_NBYTES LOAD_U24_REQUIRED_NBYTES
-#endif /* _WIMLIB_LZ_HASH_H */
+#endif /* _LZ_HASH_H */