/* Copyright 2013 Google Inc. All Rights Reserved.

	   Licensed under the Apache License, Version 2.0 (the "License");
	   you may not use this file except in compliance with the License.
	   You may obtain a copy of the License at

	   http://www.apache.org/licenses/LICENSE-2.0

	   Unless required by applicable law or agreed to in writing, software
	   distributed under the License is distributed on an "AS IS" BASIS,
	   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	   See the License for the specific language governing permissions and
	   limitations under the License.

	   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.

	   The context id for the UTF8 context model is calculated as follows. If p1
	   and p2 are the previous two bytes, we calcualte the context as

	     context = kContextLookup[p1] | kContextLookup[p2 + 256].

	   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     |                  |
	   |-------------|-------------------|---------------------|------------------|

	   The context id for the signed context mode is calculated as:

	     context = (kContextLookup[512 + p1] << 3) | kContextLookup[512 + p2].

	   For any context modeling modes, the context ids can be calculated by |-ing
	   together two lookups from one table using context model dependent offsets:

	     context = kContextLookup[offset1 + p1] | kContextLookup[offset2 + p2].

	   where offset1 and offset2 are dependent on the context mode.
	*/