2c2fcff4f8
This PR removes all uses of `String.Iterator` from core, preferring `String.ValidPos` instead. In an upcoming PR, `String.Iterator` will be renamed to `String.Legacy.Iterator`.
57 lines
1.7 KiB
Text
57 lines
1.7 KiB
Text
/-
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Copyright (c) 2024-2025 Lean FRO, LLC. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: David Thrane Christiansen
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-/
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module
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prelude
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public import Init.Data.String.Basic
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import Init.Data.Vector.Basic
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set_option linter.missingDocs true
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namespace Lean.EditDistance
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/--
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Computes the Levenshtein distance between two strings, up to some cutoff.
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If the return value is `none`, then the distance is certainly greater than the cutoff value, but a
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returned `some` does not necessarily indicate that the edit distance is less than or equal to the
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cutoff.
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-/
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public def levenshtein (str1 str2 : String) (cutoff : Nat) : Option Nat := Id.run do
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let len1 := str1.length
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let len2 := str2.length
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-- The lower bound on the Levenshtein distance is the difference in lengths
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if max len1 len2 - min len1 len2 > cutoff then return none
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let mut v0 := Vector.replicate (len2 + 1) 0
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let mut v1 := v0
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for h : i in [0:v0.size] do
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v0 := v0.set i i
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let mut iter1 := str1.startValidPos
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let mut i := 0
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while h1 : ¬iter1.IsAtEnd do
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v1 := v1.set 0 (i+1)
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let mut iter2 := str2.startValidPos
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let mut j : Fin (len2 + 1) := 0
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while h2 : ¬iter2.IsAtEnd do
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let j' : Fin _ := j + 1
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let deletionCost := v0[j'] + 1
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let insertionCost := v1[j] + 1
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let substCost :=
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if iter1.get h1 == iter2.get h2 then v0[j]
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else v0[j] + 1
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let cost := min (min deletionCost insertionCost) substCost
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v1 := v1.set j' cost
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iter2 := iter2.next h2
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j := j + 1
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iter1 := iter1.next h1
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i := i + 1
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-- Terminate early if it's impossible that the result is below the cutoff
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if v1.all (· > cutoff) then return none
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v0 := v1
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some v0[len2]
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