av/codec/h264/h264dec/cavlc.go

/*
DESCRIPTION
  cavlc.go provides utilities for context-adaptive variable-length coding
  for the parsing of H.264 syntax structure fields.

AUTHORS
  Saxon A. Nelson-Milton <saxon@ausocean.org>

LICENSE
  Copyright (C) 2019 the Australian Ocean Lab (AusOcean).

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

package h264dec

import (
	"errors"
	"fmt"

	"bitbucket.org/ausocean/av/codec/h264/h264dec/bits"
)

// parseLevelPrefix parses the level_prefix variable as specified by the process
// outlined in section 9.2.2.1 in the specifications.
func parseLevelPrefix(br *bits.BitReader) (int, error) {
	zeros := -1
	for b := 0; b != 1; zeros++ {
		_b, err := br.ReadBits(1)
		if err != nil {
			return -1, fmt.Errorf("could not read bit, failed with error: %v", err)
		}
		b = int(_b)
	}
	return zeros, nil
}

// parseLevelInformation parses level information and returns the resultant
// levelVal list using the process defined by section 9.2.2 in the specifications.
func parseLevelInformation(br *bits.BitReader, totalCoeff, trailingOnes int) ([]int, error) {
	var levelVal []int
	var i int
	for ; i < trailingOnes; i++ {
		b, err := br.ReadBits(1)
		if err != nil {
			return nil, fmt.Errorf("could not read trailing_ones_sign_flag, failed with error: %v", err)
		}
		levelVal = append(levelVal, 1-int(b)*2)
	}

	var suffixLen int
	switch {
	case totalCoeff > 10 && trailingOnes < 3:
		suffixLen = 1
	case totalCoeff <= 10 || trailingOnes == 3:
		suffixLen = 0
	default:
		return nil, errors.New("invalid TotalCoeff and TrailingOnes combination")
	}

	for j := 0; j < totalCoeff-trailingOnes; j++ {
		levelPrefix, err := parseLevelPrefix(br)
		if err != nil {
			return nil, fmt.Errorf("could not parse level prefix, failed with error: %v", err)
		}

		var levelSuffixSize int
		switch {
		case levelPrefix == 14 && suffixLen == 0:
			levelSuffixSize = 4
		case levelPrefix >= 15:
			levelSuffixSize = levelPrefix - 3
		default:
			levelSuffixSize = suffixLen
		}

		var levelSuffix int
		if levelSuffixSize > 0 {
			b, err := br.ReadBits(levelSuffixSize)
			if err != nil {
				return nil, fmt.Errorf("could not parse levelSuffix, failed with error: %v", err)
			}
			levelSuffix = int(b)
		} else {
			levelSuffix = 0
		}

		levelCode := (mini(15, levelPrefix) << uint(suffixLen)) + levelSuffix

		if levelPrefix >= 15 && suffixLen == 0 {
			levelCode += 15
		}

		if levelPrefix >= 16 {
			levelCode += (1 << uint(levelPrefix-3)) - 4096
		}

		if i == trailingOnes && trailingOnes < 3 {
			levelCode += 2
		}

		if levelCode%2 == 0 {
			levelVal = append(levelVal, (levelCode+2)>>1)
		} else {
			levelVal = append(levelVal, (-levelCode-1)>>1)
		}

		if suffixLen == 0 {
			suffixLen = 1
		}

		if absi(levelVal[i]) > (3<<uint(suffixLen-1)) && suffixLen < 6 {
			suffixLen++
		}
		i++
	}
	return levelVal, nil
}

// combineLevelRunInfo combines the level and run information obtained prior
// using the process defined in section 9.2.4 of the specifications and returns
// the corresponding coeffLevel list.
func combineLevelRunInfo(levelVal, runVal []int, totalCoeff int) []int {
	coeffNum := -1
	i := totalCoeff - 1
	var coeffLevel []int
	for j := 0; j < totalCoeff; j++ {
		coeffNum += runVal[i] + 1
		if coeffNum >= len(coeffLevel) {
			coeffLevel = append(coeffLevel, make([]int, (coeffNum+1)-len(coeffLevel))...)
		}
		coeffLevel[coeffNum] = levelVal[i]
		i++
	}
	return coeffLevel
}