codec/h264/decode/mbtype.go: improve MbPartPredMode switch logic

[Saxon Milton]

The function here has some switch logic that could be simplified.

[kortschak]

I would do it this way:

func MbPartPredMode(data *SliceData, sliceType string, mbType, partition int) (mbPartPredMode, error) {
    if partition != 0 {
        return naMbPartPredMode, errPartition
    }
    switch sliceType {
    case "I":
        if mbType == 0 {
            if data.TransformSize8x8Flag {
                return intra8x8, nil
            }
            return intra4x4, nil
        }
        if 0 < mbType && mbType < 25 {
            return intra16x16, nil
        }
        return naMbPartPredMode, errNaMode
    case "SI":
        if mbType != 0 {
            return naMbPartPredMode, errNaMode
        }
        return intra4x4, nil
    case "P", "SP":
        switch {
        case 0 <= mbType && mbType < 3:
            return predL0, nil
        case mbType == 3 || mbType == 4:
            return naMbPartPredMode, errNaMode
        default:
            return predL0, nil
        }
    case "B":
        switch mbType {
        case 0:
            return direct, nil
        case 3:
            return biPred, nil
        case 1, 4, 5, 8, 9, 12, 13:
            return predL0, nil
        case 2, 6, 7, 10, 11, 14, 15:
            return predL1, nil
        case 22:
            return naMbPartPredMode, errNaMode
        default:
            if mbType > 15 && mbType < 22 {
                return biPred, nil
            }
            return direct, nil
        }
    default:
        return naMbPartPredMode, errSliceType
    }
}

I was just going through the steps to send that as a PR, but that part of the tree is not connected to the client code and the paths look wrong (the path is .../decode, but the package is h264).

[Saxon Milton]

@kortschak Yes, it didn’t occur to me I’d have to change the package for every file. I’ll do that now.

[kortschak]

It's not just that, the imports still point to the github repo and the go.mod file exists (it shouldn't). The path name terminal should be something more descriptive than "decode".

[Saxon Milton]

@kortschak were you still going to look into this ?

[kortschak]

Is the import graph fixed?

[Saxon Milton]

yes

[Saxon Milton]

H.264 decoder work is on hold.