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// Package caesar is the shift cipher

// description: Caesar cipher

// details : Caesar cipher is a type of substitution cipher in which each letter in the plaintext is shifted a certain number of places down the alphabet.

// time complexity: O(n)

// space complexity: O(n)

// ref: https://en.wikipedia.org/wiki/Caesar_cipher

package caesar

// Encrypt encrypts by right shift of "key" each character of "input"

func Encrypt(input string, key int) string {

// if key is negative value,

// updates "key" the number which congruents to "key" modulo 26

key8 := byte(key%26+26) % 26

var outputBuffer []byte

// b is a byte, which is the equivalent of uint8.

for _, b := range []byte(input) {

newByte := b

if 'A' <= b && b <= 'Z' {

outputBuffer = append(outputBuffer, 'A'+(newByte-'A'+key8)%26)

} else if 'a' <= b && b <= 'z' {

outputBuffer = append(outputBuffer, 'a'+(newByte-'a'+key8)%26)

} else {

outputBuffer = append(outputBuffer, newByte)

}

}

return string(outputBuffer)

}

// Decrypt decrypts by left shift of "key" each character of "input"

func Decrypt(input string, key int) string {

// left shift of "key" is same as right shift of 26-"key"

return Encrypt(input, 26-key)

}