[Pharo-dev] String deduplication

Philippe Marschall philippe.marschall at netcetera.ch
Fri May 30 03:39:43 EDT 2014


This is an idea I stole from somebody else. The assumption is that you 
have a lot of Strings in the image that are equal. We could therefore 
remove the duplicates and make all the objects refer to the same instance.

However it's not a simple as that. The main issue is that String has two 
responsibilities. The first is as an immutable value object. The second 
is as a mutable character buffer for building immutable value objects. 
We must not deduplicate the second kind. Unfortunately it's not straight 
forward to figure out which kind a string is. The approach I took is 
looking at whether it contains any 0 characters. An other option would 
be to check whether any WirteStreams are referring to it.
Also, since there are behavioral differences between String and Symbol 
besides #= we must exclude Symbols (eg. there is #'hello' and 'hello' in 
the heap and they compare #= true but we must not make anybody who 
refers to 'hello' suddenly refer to #'hello').

Anyway here's the code, this saves about 2 MB in a fairly stock Pharo 3 
image. Sorry for the bad variable names.

| b d m |
b := Bag new.
d := OrderedCollection new.
m := Dictionary new.
"count all string instances"
String allSubInstancesDo: [ :s |
     s isSymbol ifFalse: [
         b add: s ] ].
"find the ones that have no duplicates or are likely buffers"
b doWithOccurrences: [ :s :i |
     (i = 1 or: [ s anySatisfy: [ :c | c codePoint = 0 ] ]) ifTrue: [
         d add: s -> i ] ].
"remove the ones that have no duplicates or are likely buffers"
d do: [ :a |
     a value timesRepeat: [
         b remove: a key ]  ].
"map all duplicate strings to their duplicates"
String allSubInstancesDo: [ :s |
     s isSymbol ifFalse: [
         (b includes: s) ifTrue: [
             | l |
             l := m at: s ifAbsentPut: [ OrderedCollection new ].
             l add: s  ] ].
"remove the duplicates"
m keysAndValues do [ :k :v |
     | f |
     f := v at: 1.
     2 to: v size do: [ :i |
         (v at: i) becomeForward: f ] ]


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