first commit with existing project files

algo_js/chiffre_cesar.js

@@ -0,0 +1,52 @@
+const prompt = require('prompt-sync')();
+
+// Chiffre de César
+
+let alphabetArray = [];
+alphabetToArray("abcdefghijklmnopqrstuvwxyz");
+
+let message = prompt("Entrez un message à chiffrer : ");
+let messageChiffre = cesarEncrypt(message, 13);
+
+console.log("Message chiffré : " + messageChiffre);
+console.log("Message déchiffré : " + cesarDecrypt(messageChiffre, 13));
+
+function cesarEncrypt(message, decalage) {
+
+    let messageChiffre = "";
+
+    for (let i = 0; i < message.length; i++) {
+        let lettre = message[i];
+        let index = alphabetArray.indexOf(lettre);
+
+        if (index === -1) messageChiffre += lettre;
+        else {
+            let indexChiffre = (index + decalage) % alphabetArray.length;
+            messageChiffre += alphabetArray[indexChiffre];
+        }
+    }
+    return messageChiffre;
+}
+
+function cesarDecrypt(message, decalage) {
+
+    let messageDechiffre = "";
+
+    for (let i = 0; i < message.length; i++) {
+        let lettre = message[i];
+        let index = alphabetArray.indexOf(lettre);
+
+        if (index === -1) messageDechiffre += lettre;
+        else {
+            let indexChiffre = (index - decalage + alphabetArray.length) % alphabetArray.length;
+            messageDechiffre += alphabetArray[indexChiffre];
+        }
+    }
+    return messageDechiffre;
+}
+
+function alphabetToArray(alphabet) {
+    for (let i = 1; i < alphabet.length; i++) {
+        alphabetArray.push(alphabet[i]);
+    }
+}