GH-50136: [C++][Gandiva] Enhance CHR to work with unicode

[lriggs]

Rationale for this change

CHR(n) only worked for ASCII (0–127). Values ≥ 128 emitted a single raw byte
(invalid UTF‑8), causing "Error during planning". Goal: emit the proper
multi‑byte UTF‑8 encoding of the Unicode code point, consistent with
PostgreSQL/Snowflake.

What changes are included in this PR?

Arrow (C++ / Gandiva)

File	Change
cpp/src/gandiva/precompiled/string_ops.cc	chr_int64 rewritten to UTF‑8‑encode the code point (1–4 bytes) and error on invalid input (negative, > 0x10FFFF, surrogate range 0xD800–0xDFFF). chr_int32 now delegates to it.
cpp/src/gandiva/precompiled/string_ops_test.cc	TestChrBigInt rewritten for UTF‑8 semantics: every byte‑length boundary (1/2/3/4‑byte, low+high), í/€/日/😀, and the three invalid‑input error cases.

Are these changes tested?

Yes, unit tests.

Are there any user-facing changes?

Yes, the CHR gandiva function now supports unicode characters.

• GitHub Issue: [C++][Gandiva] Enhance CHR to work with unicode. #50136

[github-actions]

⚠️ GitHub issue #50136 has been automatically assigned in GitHub to PR creator.

[selvaganesang]

LGTM

[lriggs]

@kou any chance this can be merged?

[Copilot]

Pull request overview

Updates Gandiva’s CHR(n) implementation to correctly emit UTF-8 for Unicode code points (instead of raw bytes), aligning behavior with common SQL engines and preventing invalid UTF-8 from causing planning failures.

Changes:

• Rewrites chr_int64 to UTF-8 encode valid Unicode code points (1–4 bytes) and reject invalid inputs (negative, > 0x10FFFF, surrogate range).
• Makes chr_int32 delegate to chr_int64 for consistent semantics.
• Updates/extends unit and integration-style tests to validate UTF-8 boundaries, representative characters, and invalid-input error behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cpp/src/gandiva/precompiled/string_ops.cc	Implements UTF-8 encoding and validation for chr_int64; chr_int32 delegates.
cpp/src/gandiva/precompiled/string_ops_test.cc	Expands CHR unit tests to cover UTF-8 boundaries, sample Unicode characters, and invalid code points.
cpp/src/gandiva/tests/projector_test.cc	Updates projector-level CHR test for UTF-8 output and adds an invalid-input evaluation test.

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[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 57139b0.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 159 possible false positives for unstable benchmarks that are known to sometimes produce them.