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https://issues.apache.org/jira/browse/SPARK-59379?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated SPARK-59379:
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Labels: pull-request-available (was: )
> Add ASCII fast-path to UTF8String character/byte position lookups
> -----------------------------------------------------------------
>
> Key: SPARK-59379
> URL: https://issues.apache.org/jira/browse/SPARK-59379
> Project: Spark
> Issue Type: Improvement
> Components: Spark Core
> Affects Versions: 4.1.0
> Reporter: David Mollitor
> Priority: Minor
> Labels: pull-request-available
>
> h2. Summary
> Several {{UTF8String}} methods convert a character index to/from a byte
> offset by walking the string one code point at a time ({{{}i +=
> numBytesForFirstByte(getByte(i{}}}). For a full-ASCII string every code point
> is exactly one byte, so *character index == byte index* and that scan can be
> replaced with O(1) arithmetic.
> This adds an ASCII fast-path to the four char<->byte "locate" methods:
> {{{}substring(int, int){}}}, {{{}getChar(int){}}},
> {{{}charPosToByte(int){}}}, and {{{}bytePosToChar(int){}}}.
> h2. Details
> The fast-path is gated on the cached ASCII flag being {*}already warm{*},
> read without forcing a scan:
> {code:java}
> if (isFullAscii == IsFullAscii.FULL_ASCII) {
> // ASCII: char index == byte index, so locate directly.
> ...
> }
> {code}
> * This is a pure win with {*}no regression{*}: when the flag is cold
> ({{{}UNKNOWN{}}}) the methods fall back to the existing per-code-point scan.
> It deliberately does NOT call {{{}isFullAscii(){}}}, which would force a full
> scan and could regress cold long strings (e.g. {{{}SUBSTR(s, 1, 5){}}}).
> * Arithmetic replicates the scan's behavior exactly: a negative index clamps
> to 0, the {{until == Integer.MAX_VALUE}} sentinel maps to {{{}numBytes{}}},
> and endpoints clamp to {{{}numBytes{}}}. For example {{substring}} computes
> {{j = max(start, 0)}} and {{{}i = (until == Integer.MAX_VALUE) ? numBytes :
> min(until, numBytes){}}}; {{getChar}} returns {{codePointFrom(charIndex)}}
> directly (the existing {{Objects.checkIndex}} guarantees the index is in
> range).
> * Non-ASCII and invalid-UTF-8 strings have the flag set to {{NOT_ASCII}}
> when warmed, so they take the unchanged scan path.
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