Externals: Add libLZMA.

Externals/liblzma/rangecoder/range_common.h

@@ -0,0 +1,71 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+/// \file       range_common.h
+/// \brief      Common things for range encoder and decoder
+///
+//  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_RANGE_COMMON_H
+#define LZMA_RANGE_COMMON_H
+
+#include "common.h"
+
+
+///////////////
+// Constants //
+///////////////
+
+#define RC_SHIFT_BITS 8
+#define RC_TOP_BITS 24
+#define RC_TOP_VALUE (UINT32_C(1) << RC_TOP_BITS)
+#define RC_BIT_MODEL_TOTAL_BITS 11
+#define RC_BIT_MODEL_TOTAL (UINT32_C(1) << RC_BIT_MODEL_TOTAL_BITS)
+#define RC_MOVE_BITS 5
+
+
+////////////
+// Macros //
+////////////
+
+// Resets the probability so that both 0 and 1 have probability of 50 %
+#define bit_reset(prob) \
+	prob = RC_BIT_MODEL_TOTAL >> 1
+
+// This does the same for a complete bit tree.
+// (A tree represented as an array.)
+#define bittree_reset(probs, bit_levels) \
+	for (uint32_t bt_i = 0; bt_i < (1 << (bit_levels)); ++bt_i) \
+		bit_reset((probs)[bt_i])
+
+
+//////////////////////
+// Type definitions //
+//////////////////////
+
+/// \brief      Type of probabilities used with range coder
+///
+/// This needs to be at least 12-bit integer, so uint16_t is a logical choice.
+/// However, on some architecture and compiler combinations, a bigger type
+/// may give better speed, because the probability variables are accessed
+/// a lot. On the other hand, bigger probability type increases cache
+/// footprint, since there are 2 to 14 thousand probability variables in
+/// LZMA (assuming the limit of lc + lp <= 4; with lc + lp <= 12 there
+/// would be about 1.5 million variables).
+///
+/// With malicious files, the initialization speed of the LZMA decoder can
+/// become important. In that case, smaller probability variables mean that
+/// there is less bytes to write to RAM, which makes initialization faster.
+/// With big probability type, the initialization can become so slow that it
+/// can be a problem e.g. for email servers doing virus scanning.
+///
+/// I will be sticking to uint16_t unless some specific architectures
+/// are *much* faster (20-50 %) with uint32_t.
+typedef uint16_t probability;
+
+#endif