'From Pharo1.1a of ''19 October 2009'' [Latest update: #11010] on 21 October 2009 at 2:13:43 am'!

!BitBltSimulation methodsFor: 'combination rules' stamp: 'HenrikSperreJohansen 10/21/2009 02:13'!
partitionedAdd: word1 to: word2 nBits: nBits nPartitions: nParts
	"Add word1 to word2 as nParts partitions of nBits each.
	This is useful for packed pixels, or packed colors"
	| mask sum result |
	mask := maskTable at: nBits.  "partition mask starts at the right"
	result := 0.
	1 to: nParts do:
		[:i |
		i = nParts ifFalse: [
		sum := (word1 bitAnd: mask) + (word2 bitAnd: mask).
		sum <= mask  "result must not carry out of partition"
			ifTrue: [result := result bitOr: sum]
			ifFalse: [result := result bitOr: mask].
		mask := mask << nBits  "slide left to next partition"] 
		"Avoid overflow when nBits*nParts = sizeOf: Integer"
			ifTrue: [|w1 w2|
		w1 := (word1 bitAnd: mask) >> nBits.
		w2 := (word2 bitAnd: mask) >> nBits.
		sum := w1 + w2.
		sum <= (mask >> nBits)  "result must not carry out of partition"
			ifTrue: [result := result bitOr: (sum <<nBits)]
			ifFalse: [result := result bitOr: mask].	
				
			]
	]
		.
	^ result
! !