[Cryptography Team] interleaving indexing has me scrambled

[Robert Withers]

In particular the tempIndex of the second scan goes out of range and I 
scratching my head. If anyone actually wants to look at how this method 
works and can help, awesome! :)

best,
robert



---

interleave: symbols blockCount: blockCount

	| symbolsPerChunk table interleaved interleavedBlock fourChunk 
symbolsPerBlock |
	((symbols size \\ 4) > 0) ifTrue: [self error: 'interleaving bytes size 
not a multiple of 4'].
	symbolsPerChunk := symbols size // blockCount.
	symbolsPerBlock := symbols size // (blockCount // 4).
	table := OrderedCollection new: 4 withAll: 0.
	table at: 1 put: (ByteArray new: symbolsPerChunk withAll: 0).
	table at: 2 put: (ByteArray new: symbolsPerChunk withAll: 0).
	table at: 3 put: (ByteArray new: symbolsPerChunk withAll: 0).
	table at: 4 put: (ByteArray new: symbolsPerChunk withAll: 0).
	interleavedBlock := ByteArray new: (symbols size / blockCount) withAll: 0.
	interleaved := ByteArray new: (symbols size + 1) withAll: 0.
	interleaved at: 1 put: blockCount.

	0 to: blockCount - 1 do: [ :blockIndex |
		| end start |
		start := blockIndex * symbolsPerBlock + 1.
		end := (blockIndex + 1) * symbolsPerBlock.
		fourChunk := symbols copyFrom: start to: end.
		"first scan: bytes to populate the table"
		0 to: 3 do: [ :chunkIndex |
			| chunk slice symbol |
			chunk := fourChunk copyFrom: (chunkIndex * symbolsPerChunk + 1) to: 
((chunkIndex + 1) * symbolsPerChunk).
			 1 to: symbolsPerChunk do: [ :index |
				slice := table at: (chunkIndex + 1).
				symbol := chunk at: index.
				slice at: index put: symbol]].
		"second scan: table to populate the interleaved bytes"
		1 to: symbolsPerChunk do: [ :index |
			0 to: 3 do: [ :chunkIndex |
				| localIndex offset tempIndex symbol |
				offset := symbolsPerBlock * blockIndex.
				tempIndex := index * 4 + chunkIndex - 3.
				localIndex := offset + tempIndex.
				symbol := (table at: (chunkIndex + 1)) at: index.
				interleavedBlock at: localIndex put: symbol]].
		start := symbols size / blockCount * blockIndex + 1.
		end := symbols size / blockCount * blockIndex + interleavedBlock size.
		interleaved replaceFrom: (start + 1) to: (end + 1) with: 
interleavedBlock startingAt: 1].
	^ interleaved