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);