integrate lzma

[zpackage] / lzma / lzma / fastpos.h

diff --git a/lzma/lzma/fastpos.h b/lzma/lzma/fastpos.h
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+++ b/lzma/lzma/fastpos.h
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+///////////////////////////////////////////////////////////////////////////////
+//
+/// \file       fastpos.h
+/// \brief      Kind of two-bit version of bit scan reverse
+///
+//  Authors:    Igor Pavlov
+//              Lasse Collin
+//
+//  This file has been put into the public domain.
+//  You can do whatever you want with this file.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef LZMA_FASTPOS_H
+#define LZMA_FASTPOS_H
+
+// LZMA encodes match distances by storing the highest two bits using
+// a six-bit value [0, 63], and then the missing lower bits.
+// Dictionary size is also stored using this encoding in the .xz
+// file format header.
+//
+// fastpos.h provides a way to quickly find out the correct six-bit
+// values. The following table gives some examples of this encoding:
+//
+//     dist   return
+//       0       0
+//       1       1
+//       2       2
+//       3       3
+//       4       4
+//       5       4
+//       6       5
+//       7       5
+//       8       6
+//      11       6
+//      12       7
+//     ...      ...
+//      15       7
+//      16       8
+//      17       8
+//     ...      ...
+//      23       8
+//      24       9
+//      25       9
+//     ...      ...
+//
+//
+// Provided functions or macros
+// ----------------------------
+//
+// get_dist_slot(dist) is the basic version. get_dist_slot_2(dist)
+// assumes that dist >= FULL_DISTANCES, thus the result is at least
+// FULL_DISTANCES_BITS * 2. Using get_dist_slot(dist) instead of
+// get_dist_slot_2(dist) would give the same result, but get_dist_slot_2(dist)
+// should be tiny bit faster due to the assumption being made.
+//
+//
+// Size vs. speed
+// --------------
+//
+// With some CPUs that have fast BSR (bit scan reverse) instruction, the
+// size optimized version is slightly faster than the bigger table based
+// approach. Such CPUs include Intel Pentium Pro, Pentium II, Pentium III
+// and Core 2 (possibly others). AMD K7 seems to have slower BSR, but that
+// would still have speed roughly comparable to the table version. Older
+// x86 CPUs like the original Pentium have very slow BSR; on those systems
+// the table version is a lot faster.
+//
+// On some CPUs, the table version is a lot faster when using position
+// dependent code, but with position independent code the size optimized
+// version is slightly faster. This occurs at least on 32-bit SPARC (no
+// ASM optimizations).
+//
+// I'm making the table version the default, because that has good speed
+// on all systems I have tried. The size optimized version is sometimes
+// slightly faster, but sometimes it is a lot slower.
+
+#ifdef HAVE_SMALL
+#      define get_dist_slot(dist) \
+               ((dist) <= 4 ? (dist) : get_dist_slot_2(dist))
+
+static inline uint32_t
+get_dist_slot_2(uint32_t dist)
+{
+       const uint32_t i = bsr32(dist);
+       return (i + i) + ((dist >> (i - 1)) & 1);
+}
+
+
+#else
+
+#define FASTPOS_BITS 13
+
+extern const uint8_t lzma_fastpos[1 << FASTPOS_BITS];
+
+
+#define fastpos_shift(extra, n) \
+       ((extra) + (n) * (FASTPOS_BITS - 1))
+
+#define fastpos_limit(extra, n) \
+       (UINT32_C(1) << (FASTPOS_BITS + fastpos_shift(extra, n)))
+
+#define fastpos_result(dist, extra, n) \
+       lzma_fastpos[(dist) >> fastpos_shift(extra, n)] \
+                       + 2 * fastpos_shift(extra, n)
+
+
+static inline uint32_t
+get_dist_slot(uint32_t dist)
+{
+       // If it is small enough, we can pick the result directly from
+       // the precalculated table.
+       if (dist < fastpos_limit(0, 0))
+               return lzma_fastpos[dist];
+
+       if (dist < fastpos_limit(0, 1))
+               return fastpos_result(dist, 0, 1);
+
+       return fastpos_result(dist, 0, 2);
+}
+
+
+#ifdef FULL_DISTANCES_BITS
+static inline uint32_t
+get_dist_slot_2(uint32_t dist)
+{
+       assert(dist >= FULL_DISTANCES);
+
+       if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 0))
+               return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 0);
+
+       if (dist < fastpos_limit(FULL_DISTANCES_BITS - 1, 1))
+               return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 1);
+
+       return fastpos_result(dist, FULL_DISTANCES_BITS - 1, 2);
+}
+#endif
+
+#endif
+
+#endif