fix(casing): stop transforming an index after its part is removed

[youdie006]

casing.Camel panics when a transform removes the first part.

drop := func(p string) string { if p == "and" { return "" }; return p }
casing.Camel("and-one-two", drop)

panic: runtime error: index out of range [-1]
github.com/danielgtaylor/huma/v2/casing.Join(...) casing/casing.go:214
github.com/danielgtaylor/huma/v2/casing.Camel(...) casing/casing.go:296

Same root cause, silently wrong value when the removed part is not first:

suffix := func(s string) string { return s + "x" }
casing.Join([]string{"one", "and", "two"}, "_", drop, suffix)
// "onexx_twox"   want "onex_twox"  -  "one" gets suffixed twice

Cause

casing/casing.go:212:

for i := 0; i < len(parts); i++ {
    for _, t := range transform {
        parts[i] = t(parts[i])

        if parts[i] == "" {
            // Transformer completely removed this part.
            parts = append(parts[:i], parts[i+1:]...)
            i--
        }
    }
}

i-- is the outer loop's correction, but it is applied while the inner transform loop is
still running. The next transform in the list then reads parts[i] at the decremented index — the
element before the one that was removed, which has already been transformed. With the removed part
first, i becomes -1.

Camel (casing/casing.go:295) always does append(transform, strings.Title), so every
Camel/LowerCamel call has at least two transforms and a single removing transform is enough to
reach it.

The change

break out of the inner loop after removing the part, so the outer i-- and i++ land on the
element that shifted into this position and it starts its transform chain from the beginning.

Why the tests missed it

TestRemovePart (casing/casing_test.go:82) exercises removal with exactly one transform, via
Snake. It still gives "one_two", unchanged by this PR. The next step — a second transform, or
Camel, which supplies one implicitly — is where it breaks.

Tests

TestRemovePartWithMultipleTransforms, covering both the panic and the double-transform value.
Mutation-checked in both directions:

• Revert the fix (drop break): panic: runtime error: index out of range [-1]
• Over-correct (keep break, drop i--): expected: "OneTwo" actual: "oneTwo" and
  expected: "onex_twox" — so the test pins the boundary from both sides rather than just one.

go test -race ./... passes across all 21 packages. gofmt -l casing/ empty, go vet ./casing/
clean.

Observable change: only on inputs that currently panic or double-transform. No existing test
changes result.

One more, not bundled

LowerCamel("") panics too, at casing/casing.go:300, which indexes runes[0] without a length
check — also reachable from any all-punctuation input, since Split("_") returns an empty slice. A
separate root cause, so I left it out to keep this PR to one; happy to fold it in or send it after.

---

Disclosure: found and prepared with AI assistance (Claude). Every figure above is from a run on this
branch, and the root cause is one I traced and verified by hand.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-04T03:25:00.799857Z	65c31a6	PR opened

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[Copilot]

🟢 Approval recommended

The fix is minimal, directly addresses the documented root cause, and is backed by a focused regression test covering both the panic and incorrect-output scenarios.

Pull request overview

This PR fixes a panic and incorrect output in casing.Join/casing.Camel when a transform removes a part while multiple transforms are applied, by preventing subsequent transforms from running on a shifted index.

Changes:

• Stop applying remaining transforms to an index after a part is removed (break out of the inner transform loop).
• Add a regression test covering both the panic case (removed first part) and the “double-transform” incorrect output case (removed middle part).

File summaries

File	Description
casing/casing.go	Breaks out of the inner transform loop after removing a part to avoid transforming the wrong shifted element (and negative index).
casing/casing_test.go	Adds a test that exercises part removal with multiple transforms to prevent panic and double-application regressions.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (ad801e0) to head (65c31a6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1113   +/-   ##
=======================================
  Coverage   93.20%   93.20%           
=======================================
  Files          23       23           
  Lines        4988     4989    +1     
=======================================
+ Hits         4649     4650    +1     
  Misses        272      272           
  Partials       67       67           

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