13 lines
538 B
Standard ML
13 lines
538 B
Standard ML
val Entropy = fn input =>
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let
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val N = Real.fromInt (String.size input) ;
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val term = fn a => Math.ln (a/N) * a / ( Math.ln 2.0 * N ) ;
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val v0 = Vector.tabulate (255,fn i=>0) ;
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val freq = Vector.map Real.fromInt (* List.foldr: count occurrences *)
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(List.foldr (fn (i,v) => Vector.update( v, ord i, Vector.sub(v,ord i) + 1) ) v0 (explode input) )
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in
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~ (Vector.foldr (fn (a,s) => if a > 0.0 then term a + s else s) 0.0 freq )
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end ;
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