Coffeescript efficient base64 decode to typed array.

base64decode.coffee

decodingTable = new Uint8Array(256)
for c, i in 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    decodingTable[c.charCodeAt(0)] = i

base64decode = (string) ->
    last = string[string.length-2...string.length]
    if last == '=='
        remainder = 2
    else if last[1] == '='
        remainder = 1
    else
        remainder = 0

    filledLength = string.length - string.length % 4
    byteLength = (filledLength/4)*3 + remainder
    result = new Uint8Array(byteLength)

    for i in [0...filledLength] by 4
        c1 = decodingTable[string.charCodeAt(i)]
        c2 = decodingTable[string.charCodeAt(i+1)]
        c3 = decodingTable[string.charCodeAt(i+2)]
        c4 = decodingTable[string.charCodeAt(i+3)]
        idx = (i/4)*3
        result[idx] = (c1<<2) + (c2>>4)
        result[idx+1] = ((c2&15)<<4) + (c3>>2)
        result[idx+2] = ((c3&3)<<6) + c4

    idx = (i/4)*3
    if remainder > 0
        c1 = decodingTable[string.charCodeAt(i)]
        c2 = decodingTable[string.charCodeAt(i+1)]
        result[idx] = (c1<<2) + (c2>>4)
    if remainder > 1
        c3 = decodingTable[string.charCodeAt(i+2)]
        result[idx+1] = ((c2&15)<<4) + (c3>>2)

    return result