Deobfuscate Silverpop's %%RECIPIENTID%%

gistfile1.js

function deobfuscate_sp_recipientid(a) {
    var b = a.length,
        c = a.substring(b -
            1);
    a = a.substring(0, b - 2);
    switch (parseInt(c, void 0)) {
        case 1:
            a += "*";
            break;
        case 2:
            a += "**"
    }
    c = [];
    for (b = 0; 25 >= b; b++) c[b] = String.fromCharCode(65 + b);
    for (b = 0; 25 >= b; b++) c[b + 26] = String.fromCharCode(97 + b);
    for (b = 0; 9 >= b; b++) c[b + 52] = String.fromCharCode(48 + b);
    c[62] = "_";
    c[63] = "-";
    for (var b = [], d = 0; 256 > d; d++) b[d] = -1;
    for (d = 0; 64 > d; d++) b[c[d].charCodeAt(0)] = d;
    b[61] = -2;
    for (var c = Array(3 * (a.length / 4)), g = 0, e = 0, f = 0, k = 0, d = 0; d < a.length; d++) {
        var h = a.charAt(d),
            h = "." == h ? "\n" : "*" == h ? "\x3d" : a.charAt(d),
            h = h.charCodeAt(0),
            h = 255 >= h ?
            b[h] : -1;
        switch (h) {
            case -1:
                break;
            case -2:
                h = 0, k++;
            default:
                switch (g) {
                    case 0:
                        e = h;
                        g = 1;
                        break;
                    case 1:
                        e <<= 6;
                        e |= h;
                        g = 2;
                        break;
                    case 2:
                        e <<= 6;
                        e |= h;
                        g = 3;
                        break;
                    case 3:
                        e <<= 6, e |= h, c[f + 2] = e & 255, e >>>= 8, c[f + 1] = e & 255, e >>>= 8, c[f] = e & 255, f += 3, g = 0
                }
        }
    }
    if (0 != g) a = null;
    else
        for (f -= k, a = "", b = c.length != f ? f : c.length, d = 0; d < b; d++) a += String.fromCharCode(c[d]);
    return a
}