[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

	private int restLen() {
	return str.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

    public static int index(ByteList source, ByteList other, int offset, Encoding enc) {
        int sourceLen = source.realSize();
        int sourceBegin = source.begin();
        int otherLen = other.realSize();

        if (otherLen == 0) return offset;
        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.