fix: use codepoint count for std.format width padding - #1091
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Motivation:
`std.format("%5s", "😀")` produced 3 spaces instead of 4 because width
was computed using String.length (UTF-16 code units), counting the
surrogate pair as 2. Python's % formatting defines width as codepoint
count; go-jsonnet uses `len([]rune(str))`. Both yield 4 spaces.
Modification:
In `widen`, use `codePointCount` for non-numeric (string) paths where
supplementary characters can appear. Numeric paths (%d/%f/%g/%x/%o)
retain O(1) `.length` since their output is provably ASCII. This avoids
O(n) scans on every numeric format call while fixing the string case.
Result:
Width padding counts supplementary characters as 1 codepoint, matching
go-jsonnet and Python. Numeric formatting performance is unchanged.
References:
- Python docs: width is "minimum field width" in characters (codepoints)
- go-jsonnet: uses len([]rune(str)) for width calculation
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Ref: databricks/sjsonnet#1091 Change-Id: Iaf87474402255661590bc69989c801626a6a6964 Signed-off-by: Yaroslav Bolyukin <iam@lach.pw>
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Motivation
std.format("%5s", "😀")produced 3 spaces instead of 4 because width was computed usingString.length(UTF-16 code units), counting the surrogate pair as 2. Python's%formatting defines width as codepoint count; go-jsonnet useslen([]rune(str)). Both yield 4 spaces.Mathematical basis: Python 3 strings are sequences of Unicode codepoints. The
%operator's width field specifies "minimum field width" measured in characters (codepoints), not UTF-16 code units or bytes.Modification
In
widen, usecodePointCountfor non-numeric (string) paths where supplementary characters can appear. Numeric paths (%d/%f/%g/%x/%o) retain O(1).lengthsince their output is provably ASCII — avoiding O(n) scans on every numeric format call.Result
Width padding counts supplementary characters as 1 codepoint, matching go-jsonnet and Python. Numeric formatting performance is unchanged.
Behavior comparison
"%5s" % "😀"" 😀"(4 spaces)" 😀"(1 space, byte-len)" 😀"(4 spaces)" 😀"(3 spaces)" 😀"(4 spaces)"%-5s" % "😀""😀 "(4 spaces)"😀 "(1 space)"😀 "(4 spaces)"😀 "(3 spaces)"😀 "(4 spaces)"%5s" % "a😀b"" a😀b"(2 spaces)" a😀b"(2 spaces)" a😀b"(1 space)" a😀b"(2 spaces)"%5s" % "hello""hello""hello""hello""hello""hello"(unchanged)"%05d" % 42"00042""00042""00042""00042""00042"(unchanged)Note: jrsonnet uses UTF-8 byte length (a third behavior); go-jsonnet is the reference implementation.
Test plan
format_width_codepoint.jsonnetregression test with golden file"%5s" % "😀"→ 4 spaces + emoji (matches go-jsonnet/Python)"%5.1s" % "😀x"→ 4 spaces + emoji (precision + width combined).length)