canonicalise cyphers

NOTES

For simplicity, the cypher spec uses a pair of bytes for each entry: opcode, and (optional) argument

simplify.go

func (w *wrapper) simplify() {
	idx := 0

	remove := func(idx int) {
		copy(w.cypher[idx:], w.cypher[idx+2:])
		w.cypher = w.cypher[:len(w.cypher)-2]
	}

	write := func(idx int, bs ...byte) {
		for i2, b := range bs {
			w.cypher[idx+i2] = b
		}
	}

loop:
	for idx < len(w.cypher) {
		// Simplify the cypher - we could scan forwards rather than backwards; either works
		prev, pAux := byte(0), byte(0)
		if idx > 0 {
			prev = w.cypher[idx-2]
			pAux = w.cypher[idx-1]
		}
		curr, aux := w.cypher[idx], w.cypher[idx+1]

		switch curr {
		case reverseBits:
			if prev == reverseBits {
				remove(idx - 2)
				remove(idx - 2)
				idx -= 2 // Back up
				continue loop
			}
		case xorN:
			if aux == 0 {
				remove(idx)
				continue loop
			}
			switch prev {
			case reverseBits:
				write(idx-2, curr, revBits[aux], reverseBits, 0)
				idx -= 2
				continue loop
			case xorN:
				write(idx-2, xorN, pAux^aux)
				remove(idx)
				idx -= 2
				continue loop
			case xorPos:
				// These two can be swapped harmlessly
				write(idx-2, xorN, aux, xorPos, 0)
				idx -= 2
				continue loop
			}
		case addN:
			if aux == 0 {
				remove(idx)
				continue loop
			}
			switch prev {
			case addN:
				write(idx-2, addN, pAux+aux)
				remove(idx)
				idx -= 2
				continue loop
			}
		case addPos:
		case xorPos:
			switch prev {
			case xorPos:
				remove(idx - 2)
				remove(idx - 2)
				idx -= 2
				continue loop
			}
		}
		idx += 2
	}
}