initial commit, 4.5 stable

thirdparty/brotli/common/context.h

@@ -0,0 +1,113 @@
+/* Copyright 2013 Google Inc. All Rights Reserved.
+
+   Distributed under MIT license.
+   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
+*/
+
+/* Lookup table to map the previous two bytes to a context id.
+
+  There are four different context modeling modes defined here:
+    CONTEXT_LSB6: context id is the least significant 6 bits of the last byte,
+    CONTEXT_MSB6: context id is the most significant 6 bits of the last byte,
+    CONTEXT_UTF8: second-order context model tuned for UTF8-encoded text,
+    CONTEXT_SIGNED: second-order context model tuned for signed integers.
+
+  If |p1| and |p2| are the previous two bytes, and |mode| is current context
+  mode, we calculate the context as:
+
+    context = ContextLut(mode)[p1] | ContextLut(mode)[p2 + 256].
+
+  For CONTEXT_UTF8 mode, if the previous two bytes are ASCII characters
+  (i.e. < 128), this will be equivalent to
+
+    context = 4 * context1(p1) + context2(p2),
+
+  where context1 is based on the previous byte in the following way:
+
+    0  : non-ASCII control
+    1  : \t, \n, \r
+    2  : space
+    3  : other punctuation
+    4  : " '
+    5  : %
+    6  : ( < [ {
+    7  : ) > ] }
+    8  : , ; :
+    9  : .
+    10 : =
+    11 : number
+    12 : upper-case vowel
+    13 : upper-case consonant
+    14 : lower-case vowel
+    15 : lower-case consonant
+
+  and context2 is based on the second last byte:
+
+    0 : control, space
+    1 : punctuation
+    2 : upper-case letter, number
+    3 : lower-case letter
+
+  If the last byte is ASCII, and the second last byte is not (in a valid UTF8
+  stream it will be a continuation byte, value between 128 and 191), the
+  context is the same as if the second last byte was an ASCII control or space.
+
+  If the last byte is a UTF8 lead byte (value >= 192), then the next byte will
+  be a continuation byte and the context id is 2 or 3 depending on the LSB of
+  the last byte and to a lesser extent on the second last byte if it is ASCII.
+
+  If the last byte is a UTF8 continuation byte, the second last byte can be:
+    - continuation byte: the next byte is probably ASCII or lead byte (assuming
+      4-byte UTF8 characters are rare) and the context id is 0 or 1.
+    - lead byte (192 - 207): next byte is ASCII or lead byte, context is 0 or 1
+    - lead byte (208 - 255): next byte is continuation byte, context is 2 or 3
+
+  The possible value combinations of the previous two bytes, the range of
+  context ids and the type of the next byte is summarized in the table below:
+
+  |--------\-----------------------------------------------------------------|
+  |         \                         Last byte                              |
+  | Second   \---------------------------------------------------------------|
+  | last byte \    ASCII            |   cont. byte        |   lead byte      |
+  |            \   (0-127)          |   (128-191)         |   (192-)         |
+  |=============|===================|=====================|==================|
+  |  ASCII      | next: ASCII/lead  |  not valid          |  next: cont.     |
+  |  (0-127)    | context: 4 - 63   |                     |  context: 2 - 3  |
+  |-------------|-------------------|---------------------|------------------|
+  |  cont. byte | next: ASCII/lead  |  next: ASCII/lead   |  next: cont.     |
+  |  (128-191)  | context: 4 - 63   |  context: 0 - 1     |  context: 2 - 3  |
+  |-------------|-------------------|---------------------|------------------|
+  |  lead byte  | not valid         |  next: ASCII/lead   |  not valid       |
+  |  (192-207)  |                   |  context: 0 - 1     |                  |
+  |-------------|-------------------|---------------------|------------------|
+  |  lead byte  | not valid         |  next: cont.        |  not valid       |
+  |  (208-)     |                   |  context: 2 - 3     |                  |
+  |-------------|-------------------|---------------------|------------------|
+*/
+
+#ifndef BROTLI_COMMON_CONTEXT_H_
+#define BROTLI_COMMON_CONTEXT_H_
+
+#include <brotli/port.h>
+#include <brotli/types.h>
+
+typedef enum ContextType {
+  CONTEXT_LSB6 = 0,
+  CONTEXT_MSB6 = 1,
+  CONTEXT_UTF8 = 2,
+  CONTEXT_SIGNED = 3
+} ContextType;
+
+/* "Soft-private", it is exported, but not "advertised" as API. */
+/* Common context lookup table for all context modes. */
+BROTLI_COMMON_API extern const uint8_t _kBrotliContextLookupTable[2048];
+
+typedef const uint8_t* ContextLut;
+
+/* typeof(MODE) == ContextType; returns ContextLut */
+#define BROTLI_CONTEXT_LUT(MODE) (&_kBrotliContextLookupTable[(MODE) << 9])
+
+/* typeof(LUT) == ContextLut */
+#define BROTLI_CONTEXT(P1, P2, LUT) ((LUT)[P1] | ((LUT) + 256)[P2])
+
+#endif  /* BROTLI_COMMON_CONTEXT_H_ */