Caesar cipher

Range1 caesarCipher(Range1, Range2)(Range1 input, Range2 output, int shift)
    if (isInputRange!Range1 && isRandomAccessRange!Range2 && hasLength!Range2)
{
    auto rotAb = lowercase.dtext.dup; // rotated alphabet
    bool changeLength;

    shift %= lowercase.length.to!int; // bring the shift within the length of the alphabet

    if (output.empty)
        changeLength = true;

    if (shift < 0)
        bringToFront(rotAb[0 .. $ + shift], rotAb[$ + shift .. $]);
    else
        bringToFront(rotAb[0 .. shift], rotAb[shift .. $]);

    foreach (i, c; input)
    {
        if (changeLength)
            ++output.length;

        if (isAlpha(c))
            if (isUpper(c))
                output[i] = toUpper(rotAb[lowercase.countUntil(toLower(c))]).to!ubyte;
            else
                output[i] = rotAb[lowercase.countUntil(c)].to!ubyte;
        else
            output[i] = c;
    }

    return cast(Range1) output;
}