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alisdairalisdair
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Add comments explaining how ctySequenceDiff works
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.
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‎internal/command/format/diff.go‎

Lines changed: 25 additions & 0 deletions
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@@ -1814,7 +1814,27 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
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lcs := objchange.LongestCommonSubsequence(old, new)
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var oldI, newI, lcsI int
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for oldI < len(old) || newI < len(new) || lcsI < len(lcs) {
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// We first process items in the old and new sequences which are not
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// equal to the current common sequence item. Old items are marked as
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// deletions, and new items are marked as additions.
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//
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// There is an exception for deleted & created object items, which we
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// try to render as updates where that makes sense.
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for oldI < len(old) && (lcsI >= len(lcs) || !old[oldI].RawEquals(lcs[lcsI])) {
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// Render this as an object update if all of these are true:
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//
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// - the current old item is an object;
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// - there's a current new item which is also an object;
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// - either there are no common items left, or the current new item
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// doesn't equal the current common item.
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//
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// Why do we need the the last clause? If we have current items in all
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// three sequences, and the current new item is equal to a common item,
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// then we should just need to advance the old item list and we'll
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// eventually find a common item matching both old and new.
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//
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// This combination of conditions allows us to render an object update
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// diff instead of a combination of delete old & create new.
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isObjectDiff := old[oldI].Type().IsObjectType() && newI < len(new) && new[newI].Type().IsObjectType() && (lcsI >= len(lcs) || !new[newI].RawEquals(lcs[lcsI]))
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if isObjectDiff {
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ret = append(ret, &plans.Change{
@@ -1827,6 +1847,8 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
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continue
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}
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// Otherwise, this item is not part of the common sequence, so
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// render as a deletion.
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ret = append(ret, &plans.Change{
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Action: plans.Delete,
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Before: old[oldI],
@@ -1842,6 +1864,9 @@ func ctySequenceDiff(old, new []cty.Value) []*plans.Change {
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})
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newI++
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}
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// When we've exhausted the old & new sequences of items which are not
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// in the common subsequence, we render a common item and continue.
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if lcsI < len(lcs) {
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ret = append(ret, &plans.Change{
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Action: plans.NoOp,

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