[JRuby] Optimize scan(): Remove duplicate if (restLen() < patternsize()) return context.nil; checks in !headonly.

[naitoh]

• before: [JRuby] Optimize scan(): Use strBL.getBegin() + curr instead of currPtr() #109

Why?

strscan/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java

Lines 371 to 373 in d31274f

	privateintrestLen() {
	returnstr.size() - curr;
	}

This means the following :

if (str.size() - curr < pattern.size()) return context.nil;

A similar check is made within StringSupport#index() within !headonly.

https://github.com/jruby/jruby/blob/be7815ec02356a58891c8727bb448f0c6a826d96/core/src/main/java/org/jruby/util/StringSupport.java#L1706-L1720

publicstaticintindex(ByteListsource, ByteListother, intoffset, Encodingenc) {
intsourceLen = source.realSize();
intsourceBegin = source.begin();
intotherLen = other.realSize();
if (otherLen == 0) returnoffset;
if (sourceLen - offset < otherLen) return -1;

• source = strBL
• other = patternBL
• offset = strBeg + curr

This means the following :
if (strBL.realSize() - (strBeg + curr) < patternBL.realSize()) return -1;

Both checks are the same.

Benchmark

It shows String as a pattern is 2.40x faster than Regexp as a pattern.

$ benchmark-driver benchmark/check_until.yaml
Warming up --------------------------------------
regexp 7.613M i/s - 7.593M times in 0.997350s (131.35ns/i)
regexp_var 7.793M i/s - 7.772M times in 0.997364s (128.32ns/i)
string 13.222M i/s - 13.199M times in 0.998297s (75.63ns/i)
string_var 15.283M i/s - 15.216M times in 0.995667s (65.43ns/i)
Calculating -------------------------------------
regexp 10.003M i/s - 22.840M times in 2.283361s (99.97ns/i)
regexp_var 9.991M i/s - 23.378M times in 2.340019s (100.09ns/i)
string 23.454M i/s - 39.666M times in 1.691221s (42.64ns/i)
string_var 23.998M i/s - 45.848M times in 1.910447s (41.67ns/i)
Comparison:
string_var: 23998466.3 i/s
string: 23453777.5 i/s - 1.02x slower
regexp: 10002809.4 i/s - 2.40x slower
regexp_var: 9990580.1 i/s - 2.40x slower

[kou]

Thanks.

It's better that we compare performance change between string/string_var instead of string/regexp with/without this change.