From ce803ba4883c7f7b668c363ec9aacc072f010579 Mon Sep 17 00:00:00 2001
From: Saxon <saxon@ausocean.org>
Date: Fri, 6 Sep 2019 17:45:23 +0930
Subject: [PATCH] codec/h264/h264dec: add function for parsing level
 information using process in section 9.2.2 of specifications

---
 codec/h264/h264dec/cavlc.go | 91 +++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)

diff --git a/codec/h264/h264dec/cavlc.go b/codec/h264/h264dec/cavlc.go
index c477a490..0dcbc64b 100644
--- a/codec/h264/h264dec/cavlc.go
+++ b/codec/h264/h264dec/cavlc.go
@@ -26,6 +26,7 @@ LICENSE
 package h264dec
 
 import (
+	"errors"
 	"fmt"
 
 	"bitbucket.org/ausocean/av/codec/h264/h264dec/bits"
@@ -44,3 +45,93 @@ func parseLevelPrefix(br *bits.BitReader) (int, error) {
 	}
 	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
+	if trailingOnes != 0 {
+		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)
+			}
+			if b == 0 {
+				levelVal = append(levelVal, 1)
+				continue
+			}
+			levelVal = append(levelVal, -1)
+		}
+	}
+
+	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")
+	}
+
+	if totalCoeff-trailingOnes != 0 {
+		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
+}