Completed the Playfair Cipher challenge

javascript/solution.js

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+class PlayfairCipher {
+  constructor(key) {
+    this.matrix = this.generateMatrix(key);
+  }
+
+  // Function to generate the 5x5 matrix
+  generateMatrix(key) {
+    key = key.toUpperCase().replace(/J/g, 'I');
+    const alphabet = 'ABCDEFGHIKLMNOPQRSTUVWXYZ';
+    const keyMatrix = [];
+    const usedChars = new Set();
+
+    // Add key characters to the matrix
+    for (let char of key) {
+      if (!usedChars.has(char)) {
+        keyMatrix.push(char);
+        usedChars.add(char);
+      }
+    }
+
+    // Add remaining alphabet characters to the matrix
+    for (let char of alphabet) {
+      if (!usedChars.has(char)) {
+        keyMatrix.push(char);
+        usedChars.add(char);
+      }
+    }
+
+    // Create 5x5 matrix
+    const matrix = [];
+    while (keyMatrix.length) {
+      matrix.push(keyMatrix.splice(0, 5));
+    }
+
+    return matrix;
+  }
+
+  // Function to preprocess the input text
+  preprocessInput(text) {
+    text = text.toUpperCase().replace(/J/g, 'I').replace(/[^A-Z]/g, '');
+    const digraphs = [];
+
+    for (let i = 0; i < text.length; i += 2) {
+      let pair = text[i];
+      if (i + 1 < text.length) {
+        if (text[i] === text[i + 1]) {
+          pair += 'X';
+        } else {
+          pair += text[i + 1];
+        }
+      } else {
+        pair += 'X';
+      }
+      digraphs.push(pair);
+    }
+
+    return digraphs;
+  }
+
+  // Function to find the position of a character in the matrix
+  findPosition(char) {
+    for (let row = 0; row < 5; row++) {
+      for (let col = 0; col < 5; col++) {
+        if (this.matrix[row][col] === char) {
+          return { row, col };
+        }
+      }
+    }
+  }
+
+  // Function to decrypt a single digraph
+  decryptPair(pair) {
+    const pos1 = this.findPosition(pair[0]);
+    const pos2 = this.findPosition(pair[1]);
+    let decryptedPair = '';
+
+    if (pos1.row === pos2.row) {
+      // Same row: shift left
+      decryptedPair += this.matrix[pos1.row][(pos1.col + 4) % 5];
+      decryptedPair += this.matrix[pos2.row][(pos2.col + 4) % 5];
+    } else if (pos1.col === pos2.col) {
+      // Same column: shift up
+      decryptedPair += this.matrix[(pos1.row + 4) % 5][pos1.col];
+      decryptedPair += this.matrix[(pos2.row + 4) % 5][pos2.col];
+    } else {
+      // Rectangle swap
+      decryptedPair += this.matrix[pos1.row][pos2.col];
+      decryptedPair += this.matrix[pos2.row][pos1.col];
+    }
+
+    return decryptedPair;
+  }
+
+  // Main decryption function
+  decrypt(ciphertext) {
+    const digraphs = this.preprocessInput(ciphertext);
+    let plaintext = '';
+
+    for (let digraph of digraphs) {
+      plaintext += this.decryptPair(digraph);
+    }
+
+    // Remove padding 'X' and ensure uppercase output without spaces or special characters
+    plaintext = plaintext.replace(/X/g, '');
+    plaintext = plaintext.replace(/[^A-Z]/g, '');
+
+    return plaintext.toUpperCase();
+  }
+}
+
+// Example usage
+const key = "SUPERSPY";
+const ciphertext = "IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV";
+
+const playfair = new PlayfairCipher(key);
+const plaintext = playfair.decrypt(ciphertext);
+
+console.log("Decrypted message:", plaintext);