package peter;

import java.util.Comparator;

public enum DecimalComparator implements Comparator<CharSequence> {
    LEADING_ZEROES_GREATER(1),
    LEADING_ZEROES_EQUAL(0),
    LEADING_ZEROES_LESS(-1);

    private final int leadingZeroesOrder;

    DecimalComparator(int leadingZeroesOrder) {
        this.leadingZeroesOrder = leadingZeroesOrder;
    }

    @Override
    public final int compare(CharSequence cs1, CharSequence cs2) {
        // count leading zeroes
        int len1 = cs1.length();
        int z1 = leadingZeroes(cs1, len1);
        int len2 = cs2.length();
        int z2 = leadingZeroes(cs2, len2);

        // compare length of the rest of sequences with leading zeroes stripped
        int cmp = Integer.compare(len1 - z1, len2 - z2);
        if (cmp != 0) return cmp;

        // stripped lengths match -> compare digits
        int i1 = z1;
        int i2 = z2;
        while (i1 < len1) {
            // assert i2 < l2;
            cmp = digit(cs1.charAt(i1)) - digit(cs2.charAt(i2));
            if (cmp != 0) return cmp;
            i1++;
            i2++;
        }

        // digits match -> compare lengths of leading zeroes
        return Integer.compare(z1, z2) * leadingZeroesOrder;
    }

    private static int leadingZeroes(CharSequence cs, int len) {
        int i = 0;
        while (i < len && digit(cs.charAt(i)) == 0) i++;
        return i;
    }

    private static int digit(char c) {
        return (c >= '0' && c <= '9')
               ? (c - '0') // optimization for common ASCII case
               : Character.digit(c, 10);
    }
}