hasChanged() returns true for an unchanged string when an edit spans a chunk boundary #332

hasChanged() reports a change on a string that is byte-identical to the original, when an overwrite covers more than one chunk.

Version

master @ e324bc4. Not present in the published 1.2.1.

Reproduction

import MagicString from 'magic-string'

const s = new MagicString('abcdefghij')
s.appendLeft(3, '')          // splits the chunk at 3
s.overwrite(0, 5, 'abcde')   // spans the split

s.toString()   // 'abcdefghij'
s.hasChanged() // true

The split does not have to be explicit. Two overlapping overwrites create one on their own:

const t = new MagicString('abcdefghij')
t.overwrite(2, 6, 'cdef')
t.overwrite(2, 7, 'cdefg')

t.toString()   // 'abcdefghij'
t.hasChanged() // true

Both return false on 1.2.1 from npm.

Cause

#328 replaced original !== toString() with a walk over the chunk list that compares each edited chunk against original.slice(chunk.start, chunk.end). That assumes a chunk's content is the replacement for its own range. An overwrite spanning several chunks stores the whole replacement on the first chunk and empties the rest, so the first chunk's content is longer than the range it covers and the comparison fails even though the concatenation is unchanged.

Scope

Every case I found is a false positive, hasChanged() returning true on an unchanged string. I did not find one where it returns false on a changed string. Comparing against the old implementation over 20,880 random operation sequences drawn from remove, overwrite, update, reset, move, indent and the four append and prepend methods, 775 disagree, all in that direction.

#328 has not been released, so this is on master only.