IrohaComparer.cs

using System;
using System.Collections.Generic;
using System.Globalization;

public class IrohaComparer : IComparer<string>
{
    private string iroha = "ｨィぃｲイいﾛロろﾊハはバばパぱﾆニにﾎホほボぼポぽﾍヘへベべペぺﾄトとドどﾁチちヂぢﾘリりﾇヌぬﾙルるｦヲをヺヮゎﾜワわヷヵゕｶカかガがｮョょﾖヨよﾀタたダだﾚレれｿソそゾぞｯッっﾂツつヅづﾈネねﾅナなﾗラらﾑムむｩゥぅｳウうヴゔヰゐヸﾉノのｫォぉｵオおｸクくグぐｬャゃﾔヤやﾏマまヶゖｹケけゲげﾌフふブぶプぷｺコこゴごｪェぇｴエえﾃテてデでｧァぁｱアあｻサさザざｷキきギぎｭュゅﾕユゆﾒメめﾐミみｼシしジじヱゑヹﾋヒひビびピぴﾓモもｾセせゼぜｽスすズずﾝンん";

    public int Compare(string x, string y)
    {
        // null の方が小さい
        if (x == null && y == null) return 0;
        if (x == null) return -1;
        if (y == null) return 1;

        // 1文字ずつ比較
        var xsi = new StringInfo(x);
        var ysi = new StringInfo(y);
        var xlen = xsi.LengthInTextElements;
        var ylen = ysi.LengthInTextElements;
        var xidx = StringInfo.ParseCombiningCharacters(x);
        var yidx = StringInfo.ParseCombiningCharacters(y);

        for (int i = 0; i < Math.Min(xlen, ylen); i++)
        {
            var xc = StringInfo.GetNextTextElement(x, xidx[i]);
            var yc = StringInfo.GetNextTextElement(y, yidx[i]);
            if (xc == yc) continue;

            var xi = iroha.IndexOf(xc);
            if (xi < 0) return xc.CompareTo(yc);
            var yi = iroha.IndexOf(yc);
            if (yi < 0) return xc.CompareTo(yc);

            if (xi < yi) return -1;
            if (xi > yi) return 1;
        }

        // 文字数の短い方が小さい
        if (xlen < ylen) return -1;
        if (xlen > ylen) return 1;
        return 0;
    }
}