fix(tree): stop swapping Offset start and end#676
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Tree.Offset(start, end) is documented (via list.Offset) so that start counts from the beginning and end counts from the end, e.g. Offset(1, -1) on [A,B,C,D] shows [B,C,D]. The previous implementation swapped start and end whenever start > end, which made Offset(2, 1) on the duckduckgoose example render the first two Ducks instead of the third Duck and the Goose. The two arguments aren't interchangeable coordinates, so the swap was wrong. Closes charmbracelet#535. Also tightened the bounds handling: start and end are now both clamped to [0, length], and end=-1 (the "skip nothing from end" idiom shown in the docs) is normalized to 0 instead of being treated as "skip everything" which had its own off-by-everything bug. Added two unit tests, one for the asymmetric case from the issue and one for the negative-clamp invariant. Signed-off-by: Charlie Tonneslan <[email protected]>
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Closes #535.
`Tree.Offset(start, end)` is documented (via `list.Offset`) such that `start` counts from the beginning and `end` counts from the end:
The implementation swapped `start` and `end` whenever `start > end`, so `Offset(2, 1)` on the duckduckgoose example (["Duck", "Duck", "Duck", "Goose", "Duck"]) ended up acting like `Offset(1, 2)` and rendered the first two Ducks instead of the third Duck plus the Goose. The two arguments aren't interchangeable coordinates, so the swap was wrong.
While I was in there I also tightened the bounds: `start` and `end` are both clamped to `[0, length]`, and `end = -1` (the "skip nothing from end" idiom in the docs example) is normalized to `0` instead of clamping to `length` (which previously made the loop in render iterate over zero elements).
Test plan
```
go test ./...
```
Added two new unit tests in `tree/tree_test.go`: