Micro optimization for whitespace trimming of strings

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This is a micro optimization to make String/removingLeadingWhitespace() and String/removingTrailingWhitespace() faster. This difference is only measurable in micro benchmarks.

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I stumbled upon this code while reviewing something else and the repeated String copying irked me.

Using micro benchmarks I could confirm that using firstIndex(where:)/ lastIndex(where:) is faster for all amounts of leading and trailing whitespace. In the graphs below, all measurements are divided by the string length.

1 leading and 1 trailing space

" \(String(repeating: "A", count: n)) "

n leading and n trailing spaces

"\(String(repeating: " ", count: n))A\(String(repeating: " ", count: n))"

0 leading and 0 trailing spaces

String(repeating: "A", count: n)

Note that green compares to red and yellow compares to blue.

Dependencies

None

Testing

None. This isn't a change in behavior.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [x] Added tests
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test