Add initial project files

pkg/deesee/codec.go

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+// 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
+}