Merge LZX compression updates

[wimlib] / include / wimlib / lz_hash.h

diff --git a/include/wimlib/lz_hash.h b/include/wimlib/lz_hash.h
new file mode 100644 (file)
index 0000000..dbab589
--- /dev/null
+++ b/include/wimlib/lz_hash.h
@@ -0,0 +1,61 @@
+/*
+ * lz_hash.h
+ *
+ * Hashing for Lempel-Ziv matchfinding.
+ *
+ * Author:     Eric Biggers
+ * Year:       2014, 2015
+ *
+ * The author dedicates this file to the public domain.
+ * You can do whatever you want with this file.
+ */
+
+#ifndef _WIMLIB_LZ_HASH_H
+#define _WIMLIB_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;
+}
+
+static inline u32
+load_u24_unaligned(const u8 *p)
+{
+       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);
+}
+
+static inline u32
+lz_hash(u32 str, unsigned num_bits)
+{
+       return (u32)(str * LZ_HASH_MULTIPLIER) >> (32 - 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.
+ */
+static inline u32
+lz_hash_3_bytes(const u8 *p, unsigned num_bits)
+{
+       return lz_hash(load_u24_unaligned(p), 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)
+
+#endif /* _WIMLIB_LZ_HASH_H */