[Pharo-project] Issue 3702 in pharo: squeeze out a bit more speed from TextDiffBuilder

pharo at googlecode.com pharo at googlecode.com
Mon Feb 14 10:25:31 CET 2011


Updates:
	Labels: Difficulty-Easy

Comment #1 on issue 3702 by stephane... at gmail.com: squeeze out a bit more  
speed from TextDiffBuilder
http://code.google.com/p/pharo/issues/detail?id=3702

to create a cs

lcsFor: xFilteredLines and: yFilteredLines
	"I find one of the longest common subsequences of my the arguments. I  
assume that none of my arguments are empty. I return nil or an Array which  
represents a list. The first two elements are the matching line numbers,  
the last is the next node in the list or nil if there are no more elements.  
The list containts the longest common subsequence. I'm a modified version  
of the greedy lcs algorithm from the 6th page of 'An O(ND) Difference  
Algorithm and Its Variations (1986)' by Eugene W. Myers"

	| n m v lcss max |
	n := xFilteredLines size.
	m := yFilteredLines size.
	max := m + n.
	v := Array new: 2 * max + 1.
	v at: max + 2 put: 0.
	lcss := Array new: 2 * max + 1.
	0 to: max do: [ :d |
		d negated to: d by: 2 do: [ :k |
			| index lcs x y |
			index := max + k.
			(k + d = 0 or: [ k ~= d and: [ (v at: index) < (v at: index + 2) ] ])
				ifTrue: [ x := v at: (index := index + 2) ]
				ifFalse: [ x := (v at: index) + 1 ].
			y := x - k.
			lcs := lcss at: index.
			[ x < n and: [ y < m and: [ (xFilteredLines at: x + 1) = (yFilteredLines  
at: y + 1) ] ] ]
				whileTrue: [
					lcs := { x := x + 1. y := y + 1. lcs } ].
			(x >= n and: [ y >= m ]) ifTrue: [ ^lcs ].
			v at: max + k + 1 put: x.
			lcss at: max + k + 1 put: lcs ] ].
	self error

and

findMatches
	"I find the matching pairs of xLines and yLines. First I filter out all  
lines that can't have a pair, then I find the longest common subsequence of  
the remaining elements. Finally I mark the matching pairs."

	| temp lcs xFilteredLines yFilteredLines xNumbers yNumbers |
	"Filter out all lines that can't have a pair."
	temp := yLines asSet.
	xFilteredLines := xLines select: [ :each |
		temp includes: each ].
	xFilteredLines size = 0 ifTrue: [ ^self ].
	temp := xLines asSet.
	yFilteredLines := yLines select: [ :each |
		temp includes: each ].
	yFilteredLines size = 0 ifTrue: [ ^self ].
	"Map all lines to SmallIntegers, because they can be compared faster."
	temp := Dictionary new.
	xNumbers := xFilteredLines collect: [ :each |
		temp at: each ifAbsentPut: [ temp size ] ].
	yNumbers := yFilteredLines collect: [ :each |
		temp at: each ifAbsentPut: [ temp size ] ].
	temp := nil.
	"Find the longest common subsequence."
	lcs := self lcsFor: xNumbers and: yNumbers.
	"Mark the matching pairs."
	[ lcs == nil ] whileFalse: [
		(xFilteredLines at: (lcs at: 1)) matches: (yFilteredLines at: (lcs at:  
2)).
		lcs := lcs at: 3 ]





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