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Performance optimization for case_fold #326

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Performance optimization for case_fold #326

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11 changes: 7 additions & 4 deletions include/boost/parser/detail/case_fold.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -47,12 +47,15 @@ namespace boost::parser::detail {
// One-byte fast path.
if (cp < 0x100) {
// ASCII letter fast path.
if (0x61 <= cp && cp <= 0x7a) {

@tzlaine tzlaine Oct 18, 2026

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Adding the if (cp < 0xb5) case below seems good to me, but removing the fast-path for the already-folded region [0x61, 0x7a] seems wrong. Is there a reason for it, or was it a mistake?

@andreasbuhr andreasbuhr Oct 22, 2026

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Actually I thought about it and it was a deliberate choice to remove it. But it's not important. Important is that there is no more std::lower_bound call for each space, brace, etc, as it was in the past. I am fine both with the fast path and without it.

If you are interested, here is my reasoning. Only read if you really want to know:

I considered six cases and counted the number of comparisons for each case.

Three different classes of characters:

  1. uppercase A-Z [0x41, 0x5a]
  2. lowercase a-z [0x61, 0x7a]
  3. punctuation/whitespace ()*+.. [0x21,0x40]

And two code options:
A: with fast-path for [0x61,0x7a]
B: without fast-path for [0x61,0x7a]

Now let's count the comparisons:

First uppercase A-Z:
Case 1-A:
0x61(false),0x41(true),0x5a(true)
Case 1-B:
0x41(true),0x5a(true)
-> The fast path for lowercase a-z costs us one comparison for each uppercase A-Z

Then lowercase a-z:
Case 2-A:
0x61(true),0x7a(true)
Case 2-B:
0x41(true),0x5a(false),0xb5(true)
-> The fast path for lowercase a-z saves us one comparison for each lowercase a-z

Then punctuation/whitespace:
Case 3-A:
0x61(false),0x41(false),0xb5(true)
Case 3-B:
0x41(false),0xb5(true)
-> The fast path for lowercase a-z costs us one comparison for each punctuation or whitespace

Conclusion:
The fast path for a-z saves us one comparison for each lowercase a-z character, but costs us one comparison for every uppercase character, every punctuation, brace, space etc. Looking at my vba code, I had the impression that there are not more a-z characters than other characters. Combined with my fundamental believe "less code is better" I decided to remove the fast-path.

@andreasbuhr andreasbuhr Oct 23, 2026

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reading my own comment I realized we can do even better:
#335

*out++ = cp;
return out;
} else if (0x41 <= cp && cp <= 0x5a) {
if (0x41 <= cp && cp <= 0x5a) {
// upper case ASCII letters
*out++ = cp + 0x20;
return out;
} else if (cp < 0xb5) {
// nothing else is mapped before 0xb5
// https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt
*out++ = cp;
return out;
} else if (cp == 0x00DF) {
// The lone multi-mapping below 0x100.
*out++ = 0x0073;
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