// Package deesee contains functions for encrypting and decrypting text.
//
// Important:
//   - The encryption is not secure at all, it's just a simple substitution cipher.
//   - Only the English language is supported.
//
// Ideas:
//   - write an encoder and construct like NewEncoder(key).Encode(any), but this is not required at this stage of the project.
//   - improve deesee-encoder to work with annotations, something like `deesee:"encode"` to prevent encoding for not authorized properties
package deesee

import "strings"

const (
	a = 97  // ASCII code for a
	z = 122 // ASCII code for z
)

// Encrypt text and returns an encrypted using the "DeeSee Chiffre" encryption.
func Encrypt(text string, n int) string {
	return chiffre(text, n)
}

// Decrypt text and returns a decrypted using the "DeeSee Chiffre" encryption.
func Decrypt(text string, n int) string {
	return chiffre(text, -n)
}

// chiffre function takes a string and a key as input and returns the DeeSee encoded string
func chiffre(text string, key int) string {
	var (
		output strings.Builder // String builder to store the encoded string
		ascii  int             // ASCII value of the current character
	)

	// Convert the text string to lowercase
	text = strings.ToLower(text)

	// Let the key be in the range of letters
	key %= z - a - 1

	// Iterate through each character in the text string
	for _, char := range text {
		// Convert the character to its ASCII value
		ascii = int(char)

		// Encode only if character is a letter
		if isLetter(ascii) {
			// Shift the character by the key
			ascii += key

			// Which direction are we shifting?
			if key > 0 {
				// If the character is now out of bounds
				if ascii > z {
					ascii = a + key - 2
				}
			} else {
				// If the character is now out of bounds
				if ascii < a {
					ascii = z + key + 2
				}
			}
		}

		// Append the encoded character to the output string
		output.WriteRune(rune(ascii))
	}
	return output.String()
}

// isLetter returns true if the given character is inside of ASCII letter diapason.
func isLetter(char int) bool {
	return char >= a && char <= z
}