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Document the early return.

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Rob Rix 2015-10-29 13:16:16 -04:00
parent 17d0582331
commit 73f094cc29
1 changed files with 3 additions and 0 deletions
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3
prototype/Doubt/SES.swift
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@ -34,6 +34,9 @@ public func SES<Term, Leaf, Annotation>(a: [Term], _ b: [Term], cost: Free<Leaf,
if let right = right, down = down, diagonal = diagonal {
let here = recur(a[i], b[j])
// If the diff at this vertex is zero-cost, we’re not going to find a cheaper one either rightwards or downwards. We can therefore short-circuit selecting the best outgoing edge and save ourselves evaluating the entire row rightwards and the entire column downwards from this point.
//
// Thus, in the best case (two equal sequences), we now complete in O(n + m). However, this optimization only applies to equalities at the beginning of the edit graph; once inequalities are encountered, the remainder of the diff is effectively O(nm).
if let here = here where cost(here) == 0 { return cons(here, rest: diagonal) }
let right = (right, Diff.Delete(a[i]), costOfStream(right))
let down = (down, Diff.Insert(b[j]), costOfStream(down))