Hankaku.groovy

@Grab('com.ibm.icu:icu4j:4.8.1.1')
import com.ibm.icu.text.Transliterator

class StringCategory {
    static hankaku = Transliterator.getInstance('Fullwidth-Halfwidth').&transliterate
    static String div(String self, int divisor) {
        divisor == 2 ? hankaku(self) : self
    }
}

use (StringCategory) {
    assert '這いよれ！　ニャル子さん ４' / 2 == '這いよれ! ﾆｬﾙ子さん 4'
    assert 'ＡＢＣ' / 2 == 'ABC'
    assert 'あ　い　う　え　お' / 2 == 'あ い う え お'
}