Optimize SparkHex function using lookup table #19585
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Replace slow write!() format-based hex encoding with a
pre-computed lookup table for significant performance improvement.
The previous implementation used write!(&mut s, "{b:02x}") for each
byte, which has format string parsing overhead. The new implementation
uses const lookup tables (HEX_UPPER/HEX_LOWER) that map each byte
value directly to its two-character hex representation.
Closes apache#15986
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Which issue does this PR close?
Closes #15986
Rationale for this change
The current
hexfunction implementation useswrite!(&mut s, "{b:02x}")for each byte, which has significant overhead from format string parsing.What changes are included in this PR?
Replaced the slow format-based hex encoding with pre-computed lookup tables (HEX_UPPER/HEX_LOWER). Each byte value maps directly to its two-character hex representation via simple array indexing.
Benefits:
How are these changes tested?
All existing tests pass:
Are these changes safe?
Yes - the lookup tables contain only valid ASCII hex characters, so the conversion is safe. The behavior is identical to before, just faster.