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Add comments explaining how ctySequenceDiff works

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The logic behind this code took me a while to understand, so I wrote
down what I understand to be the reasoning behind how it works. The
trickiest part is rendering changing objects as updates. I think the
other pieces are fairly common to LCS sequence diff rendering, so I
didn't explain those in detail.
This commit is contained in:
Alisdair McDiarmid 2021-07-09 13:14:20 -04:00
parent d0d9ee1e62
commit ef0181cfbd
1 changed files with 25 additions and 0 deletions
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25
internal/command/format/diff.go
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@ -1814,7 +1814,27 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
lcs := objchange.LongestCommonSubsequence(old, new)
var oldI, newI, lcsI int
for oldI < len(old) || newI < len(new) || lcsI < len(lcs) {
// We first process items in the old and new sequences which are not
// equal to the current common sequence item. Old items are marked as
// deletions, and new items are marked as additions.
//
// There is an exception for deleted & created object items, which we
// try to render as updates where that makes sense.
for oldI < len(old) && (lcsI >= len(lcs) || !old[oldI].RawEquals(lcs[lcsI])) {
// Render this as an object update if all of these are true:
//
// - the current old item is an object;
// - there's a current new item which is also an object;
// - either there are no common items left, or the current new item
// doesn't equal the current common item.
//
// Why do we need the the last clause? If we have current items in all
// three sequences, and the current new item is equal to a common item,
// then we should just need to advance the old item list and we'll
// eventually find a common item matching both old and new.
//
// This combination of conditions allows us to render an object update
// diff instead of a combination of delete old & create new.
isObjectDiff := old[oldI].Type().IsObjectType() && newI < len(new) && new[newI].Type().IsObjectType() && (lcsI >= len(lcs) || !new[newI].RawEquals(lcs[lcsI]))
if isObjectDiff {
ret = append(ret, &plans.Change{
@ -1827,6 +1847,8 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
continue
}
// Otherwise, this item is not part of the common sequence, so
// render as a deletion.
ret = append(ret, &plans.Change{
Action: plans.Delete,
Before: old[oldI],
@ -1842,6 +1864,9 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
})
newI++
}
// When we've exhausted the old & new sequences of items which are not
// in the common subsequence, we render a common item and continue.
if lcsI < len(lcs) {
ret = append(ret, &plans.Change{
Action: plans.NoOp,