// About base64: https://en.wikipedia.org/wiki/Base64

/**

* Converts a base64 string to an array of bytes

* @param {string} b64 A base64 string

* @returns {ArrayBuffer} An ArrayBuffer representing the bytes encoded by the base64 string

*/

function base64ToBuffer (b64) {

// The base64 encoding uses the following set of characters to encode any binary data as text

const base64Table = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

// Find the index of char '=' first occurrence

const paddingIdx = b64.indexOf('=')

// Remove padding chars from base64 string, if there are any

const b64NoPadding = paddingIdx !== -1 ? b64.slice(0, paddingIdx) : b64

// Calculate the length of the result buffer

const bufferLength = Math.floor((b64NoPadding.length * 6) / 8)

// Create the result buffer

const result = new ArrayBuffer(bufferLength)

// Create an instance of Uint8Array, to write to the `result` buffer

const byteView = new Uint8Array(result)

// Loop through all chars in the base64 string, in increments of 4 chars, and in increments of 3 bytes

for (let i = 0, j = 0; i < b64NoPadding.length; i += 4, j += 3) {

// Get the index of the next 4 base64 chars

const b64Char1 = base64Table.indexOf(b64NoPadding[i])

const b64Char2 = base64Table.indexOf(b64NoPadding[i + 1])

let b64Char3 = base64Table.indexOf(b64NoPadding[i + 2])

let b64Char4 = base64Table.indexOf(b64NoPadding[i + 3])

// If base64 chars 3 and 4 don't exit, then set them to 0

if (b64Char3 === -1) b64Char3 = 0

if (b64Char4 === -1) b64Char4 = 0

// Calculate the next 3 bytes

const byte1 = (b64Char1 << 2) + ((b64Char2 & 48) >> 4)

const byte2 = ((b64Char2 & 15) << 4) + ((b64Char3 & 60) >> 2)

const byte3 = ((b64Char3 & 3) << 6) + b64Char4

byteView[j] = byte1

byteView[j + 1] = byte2

byteView[j + 2] = byte3

}

return result

}

export { base64ToBuffer }