github/semantic
1
1
Fork 0
mirror of https://github.com/github/semantic.git synced 2024-11-29 11:02:26 +03:00
Code Issues Projects Releases Wiki Activity

Explain SES implementation details a little.

Browse Source
This commit is contained in:
Rob Rix 2015-10-06 17:36:49 -04:00
parent 6030731cc8
commit d097f14fcb
1 changed files with 6 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
prototype/Doubt/SES.swift
View File

@ -34,6 +34,12 @@ public func SES<A>(a: [Fix<A>], _ b: [Fix<A>], equals: (A, A) -> Bool, recur: (F
// A matrix whose values are streams representing paths through the edit graph, carrying both the diff & the cost of the remainder of the path.
var matrix: Matrix<Stream<(Diff, Int)>>!
matrix = Matrix(width: a.count + 1, height: b.count + 1) { i, j in
// Some explanation is warranted:
//
// 1. `matrix` captures itself during construction, because each vertex in the edit graph depends on other vertices. This is safe, because a) `Matrix` populates its fields lazily, and b) vertices only depend on those vertices downwards and rightwards of them.
//
// 2. `matrix` is sized bigger than `a.count` x `b.count`. This is safe, because a) we only get a[i]/b[j] when right/down are non-nil (respectively), and b) right/down are found by looking up elements (i + 1, j) & (i, j + 1) in the matrix, which returns `nil` when out of bounds. So we only access a[i] and b[j] when i and j are in bounds.
let right = matrix[i + 1, j]
let down = matrix[i, j + 1]
let diagonal = matrix[i + 1, j + 1]