求指教这段 JS 代码是在做什么

如题：

__d('BitMap', [], (function a(b, c, d, e, f, g) {
    var h = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_';

    function i() {
        'use strict';
        this.$BitMap1 = [];
    }

    i.prototype.set = function(l) {
        'use strict';
        this.$BitMap1[l] = 1;
        return this;
    };

    i.prototype.toString = function() {
        'use strict';
        var l = [];
        for (var m = 0; m < this.$BitMap1.length; m++)
            l.push(this.$BitMap1[m] ? 1 : 0);
        return l.length ? k(l.join('')) : '';
    };

    i.prototype.toCompressedString = function() {
        'use strict';
        if (this.$BitMap1.length === 0) return '';
        var l = [],
            m = 1,
            n = this.$BitMap1[0] || 0,
            o = n.toString(2);
        for (var p = 1; p < this.$BitMap1.length; p++) {
            var q = this.$BitMap1[p] || 0;
            if (q === n) { m++; } else {
                l.push(j(m));
                n = q;
                m = 1;
            }
        }
        if (m) l.push(j(m));
        return k(o + l.join(''));
    };

    function j(l) {
        var m = l.toString(2),
            n = '0'.repeat(m.length - 1);
        return n + m;
    }

    function k(l) {
        var m = (l + '00000').match(/[01]{6}/g),
            n = '';
        for (var o = 0; o < m.length; o++)
            n += h[parseInt(m[o], 2)];
        return n;
    }
    f.exports = i;
}), null);