/**

* Keyword shifted alphabet is a simple cipher using a translation table created with a help of a keyword.

* Keyword must be a word where each character can occur only once.

* To create the translation table, we write all the alphabet characters to the first.

* Second row start with the keyword, then we continue with the rest of the characters that are missing in alphabetical order.

*

* |A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z|

* |K|E|Y|W|O|R|D|A|B|C|F|G|H|I|J|L|M|N|P|Q|S|T|U|V|W|Z|

*

* Encryption is then just a matter of writing the matching (same index) letter from the second row instead of the first row:

* 'Hello world' -> 'Aoggj ujngw'

*

* Decryption is then just the reverse process of writing the matching (same index) letter from the first row instead of the second row

* 'Aogg ujngw' -> 'Hello world'

*

* Non alphabetical characters (space, exclamation mark, ...) are kept as they are

*/

const alphabet = [

'a',

'b',

'c',

'd',

'e',

'f',

'g',

'h',

'i',

'j',

'k',

'l',

'm',

'n',

'o',

'p',

'q',

'r',

's',

't',

'u',

'v',

'w',

'x',

'y',

'z'

]

function checkKeywordValidity(keyword) {

keyword.split('').forEach((char, index) => {

const rest = keyword.slice(0, index) + keyword.slice(index + 1)

if (rest.indexOf(char) !== -1) {

return false

}

})

return true

}

function getEncryptedAlphabet(keyword) {

const encryptedAlphabet = keyword.split('')

alphabet.forEach((char) => {

if (encryptedAlphabet.indexOf(char) === -1) {

encryptedAlphabet.push(char)

}

})

return encryptedAlphabet

}

function translate(sourceAlphabet, targetAlphabet, message) {

return message.split('').reduce((encryptedMessage, char) => {

const isUpperCase = char === char.toUpperCase()

const encryptedCharIndex = sourceAlphabet.indexOf(char.toLowerCase())

const encryptedChar =

encryptedCharIndex !== -1 ? targetAlphabet[encryptedCharIndex] : char

encryptedMessage += isUpperCase

? encryptedChar.toUpperCase()

: encryptedChar

return encryptedMessage

}, '')

}

function checkInputs(keyword, message) {

if (!keyword || !message) {

throw new Error('Both keyword and message must be specified')

}

if (!checkKeywordValidity(keyword)) {

throw new Error('Invalid keyword!')

}

}

function encrypt(keyword, message) {

checkInputs(keyword, message)

return translate(

alphabet,

getEncryptedAlphabet(keyword.toLowerCase()),

message

)

}

function decrypt(keyword, message) {

checkInputs(keyword, message)

return translate(

getEncryptedAlphabet(keyword.toLowerCase()),

alphabet,

message

)

}

export { encrypt, decrypt }

// encrypt('keyword', 'Hello world!') // Prints 'Aoggj ujngw!'

// decrypt('keyword', 'Aoggj ujngw!') // Prints 'Hello world!