dualfroz
e2e5b5495f
The center branch of resize() added the odd-padding space whenever the input was non-empty, because the parity check omitted the source length (wantLen - padLn*2 instead of wantLen - sLen - padLn*2). This made Resize, Align and their utf8 variants overflow wantLen by one for every non-empty string with an even amount of padding. Subtract sLen so the extra space is only added when the total padding is actually odd.
74 lines
2.3 KiB
Markdown
74 lines
2.3 KiB
Markdown
# fix(strutil): center-aligned Resize/Align overflows the requested length
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## Problem
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`strutil.Resize` / `strutil.Align` (and their utf8 variants) with
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`comdef.Center` alignment produce strings longer than the requested length for
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almost every non-empty input:
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```go
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strutil.Resize("ab", 6, comdef.Center) // " ab " (len 7), want " ab " (len 6)
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strutil.Resize("ab", 4, comdef.Center) // " ab " (len 5), want " ab " (len 4)
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strutil.Resize("abc", 7, comdef.Center) // " abc "(len 8), want " abc "(len 7)
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```
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The extra space that centers an odd amount of padding is added unconditionally
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whenever the input is non-empty, not only when the padding is actually odd.
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## Root cause
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`strutil/padding.go`, `resize`:
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```go
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if align == comdef.Center {
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padLn := (wantLen - sLen) / 2
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if diff = wantLen - padLn*2; diff > 0 { // wrong: missing "- sLen"
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s += " "
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}
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...
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}
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```
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The condition is meant to detect an odd amount of total padding
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(`wantLen - sLen`). Simplifying the buggy expression:
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```
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wantLen - 2*padLn = wantLen - 2*floor((wantLen-sLen)/2)
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= sLen + ((wantLen - sLen) mod 2)
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```
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so `diff > 0` is true whenever `sLen > 0`, regardless of the padding parity.
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The result is one space too long for every non-empty string with even padding.
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The existing `TestResize("title", 20, PosMiddle)` case did not catch this
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because its padding (15) is odd, where the extra space is legitimately needed.
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## Fix
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Compare against the actual leftover padding by subtracting `sLen`:
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```go
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if diff = wantLen - sLen - padLn*2; diff > 0 {
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s += " "
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}
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```
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This adds the extra space only when `(wantLen - sLen)` is odd, keeping the odd
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case (e.g. the "title" test) unchanged.
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## Test
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Added `TestResize_center_length` in `strutil/padding_test.go` asserting both the
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exact result and `len(result) == wantLen` for even- and odd-padding cases.
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- Gate: `go test ./strutil/ -run TestResize` -> ok (PASS)
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- `go vet ./strutil/` -> exit 0
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- `go build ./...` -> exit 0
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Counterfactual: reverting the change makes the new test fail with
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`actual: int(7)` / `actual: " ab "` vs the expected length 6 / `" ab "`.
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Note: `TestDateSNv3/dateLen4_base62` in `strutil/gensn_test.go` is flaky and
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already fails on the unmodified base commit (it asserts uniqueness of randomly
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generated serial numbers); it is unrelated to this change.
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