OpenJPH: VLC decoding tables

OpenJPH Open-source implementation of JPEG2000 Part-15

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VLC decoding tables

## Functions
static bool | ojph::local::uvlc_init_tables ()
| Initializes uvlc_tbl0 and uvlc_tbl1 tables.
static bool | ojph::local::uvlc_init_wide_table ()
| Initializes uvlc_tbl1_wide: wider UVLC table for non-initial rows. Index = mode(2b) * 1024 + vlc(10b). Entry packs total_bits[4:0], u_q0[12:5], u_q1[20:13]. total_bits == 0x1F signals fallback to original decode.

## Variables
ui16 | ojph::local::uvlc_tbl0 [256+64] = { 0 }
| uvlc_tbl0 contains decoding information for initial row of quads
ui16 | ojph::local::uvlc_tbl1 [256] = { 0 }
| uvlc_tbl1 contains decoding information for non-initial row of quads
ui32 | ojph::local::uvlc_tbl1_wide [4096] = { 0 }
| uvlc_tbl1_wide: wider UVLC table for non-initial rows. Index = mode(2 bits) * 1024 + vlc_data(10 bits) = 12 bits. Entry bits: [4:0]=total_bits, [12:5]=u_q0, [20:13]=u_q1. total_bits == 0x1F means fallback to original decode path.
ui8 | ojph::local::uvlc_bias [256+64] = { 0 }
| uvlc_bias contains decoding info. for initial row of quads
static bool | ojph::local::uvlc_tables_initialized = uvlc_init_tables()
| Initializes UVLC tables uvlc_tbl0 and uvlc_tbl1.
static bool | ojph::local::uvlc_wide_initialized = uvlc_init_wide_table()
| Initializes wide UVLC table uvlc_tbl1_wide.

## Detailed Description

UVLC decoding tables used to partially decode u values from UVLC
codewords.
The table index is 8 (or 9) bits and composed of two parts:
The 6 LSBs carries the head of the VLC to be decoded. Up to 6 bits to be used; these are uvlc prefix code for quad 0 and 1
The 2 (or 3) MSBs contain u_off of quad 0 + 2 * o_off quad 1

- 4 * mel event for initial row of quads when needed
  Each entry contains, starting from the LSB
  - `total` total prefix length for quads 0 and 1 (3 bits)
  - `total` total suffix length for quads 0 and 1 (4 bits)
  - `suffix` length for quad 0 (3 bits)
  - `prefix` for quad 0 (3 bits)
  - `prefix` for quad 1 (3 bits)
    Another table is uvlc_bias, which is needed to correctly decode the extension u_ext for initial row of quads. Under certain condition, we deduct 1 or 2 from u_q0 and u_q1 before encoding them; so for us to know that decoding u_ext is needed, we recreate the u_q0 and u_q1 that we actually encoded.
    For simplicity, we use the same index as before
  - `u_q0` bias is 2 bits
  - `u_q1` bias is 2 bits

## Function Documentation

## ◆ uvlc_init_tables()

bool ojph::local::uvlc_init_tables | ( |  | ) |
| static

`#include <ojph_block_common.cpp>`

Initializes uvlc_tbl0 and uvlc_tbl1 tables.

Definition at line 196 of file ojph_block_common.cpp.

References uvlc_bias, uvlc_tbl0, and uvlc_tbl1.

Referenced by initialize_block_encoder_tables().

## ◆ uvlc_init_wide_table()

bool ojph::local::uvlc_init_wide_table | ( |  | ) |
| static

`#include <ojph_block_common.cpp>`

Initializes uvlc_tbl1_wide: wider UVLC table for non-initial rows. Index = mode(2b) * 1024 + vlc(10b). Entry packs total_bits[4:0], u_q0[12:5], u_q1[20:13]. total_bits == 0x1F signals fallback to original decode.

Definition at line 346 of file ojph_block_common.cpp.

References uvlc_tbl1_wide.

## Variable Documentation

## ◆ uvlc_bias

ui8 ojph::local::uvlc_bias = { 0 }

`#include <ojph_block_common.cpp>`

uvlc_bias contains decoding info. for initial row of quads

Definition at line 116 of file ojph_block_common.cpp.

Referenced by ojph_decode_codeblock64(), and uvlc_init_tables().

## ◆ uvlc_tables_initialized

bool ojph::local::uvlc_tables_initialized = uvlc_init_tables()
| static

`#include <ojph_block_common.cpp>`

Initializes UVLC tables uvlc_tbl0 and uvlc_tbl1.

Definition at line 428 of file ojph_block_common.cpp.

## ◆ uvlc_tbl0

ui16 ojph::local::uvlc_tbl0 = { 0 }

`#include <ojph_block_common.cpp>`

uvlc_tbl0 contains decoding information for initial row of quads

Definition at line 106 of file ojph_block_common.cpp.

Referenced by ojph_decode_codeblock32(), ojph_decode_codeblock64(), ojph_decode_codeblock_wasm(), and uvlc_init_tables().

## ◆ uvlc_tbl1

ui16 ojph::local::uvlc_tbl1 = { 0 }

`#include <ojph_block_common.cpp>`

uvlc_tbl1 contains decoding information for non-initial row of quads

Definition at line 109 of file ojph_block_common.cpp.

Referenced by ojph_decode_codeblock32(), ojph_decode_codeblock64(), ojph_decode_codeblock_wasm(), and uvlc_init_tables().

## ◆ uvlc_tbl1_wide

ui32 ojph::local::uvlc_tbl1_wide = { 0 }

`#include <ojph_block_common.cpp>`

uvlc_tbl1_wide: wider UVLC table for non-initial rows. Index = mode(2 bits) * 1024 + vlc_data(10 bits) = 12 bits. Entry bits: [4:0]=total_bits, [12:5]=u_q0, [20:13]=u_q1. total_bits == 0x1F means fallback to original decode path.

Definition at line 114 of file ojph_block_common.cpp.

Referenced by uvlc_init_wide_table().

## ◆ uvlc_wide_initialized

bool ojph::local::uvlc_wide_initialized = uvlc_init_wide_table()
| static

`#include <ojph_block_common.cpp>`

Initializes wide UVLC table uvlc_tbl1_wide.

Definition at line 434 of file ojph_block_common.cpp.