JavaScript字符串编解码器：常见问题与优化实践

for (l = 0; l < dstring.length / 5; l++) {
  currentdt = "";
  for (i = 0; i < 5; i++) {
    currentdt = currentdt + dstring.charAt(l * i);
  }
  decodedtext = decodedtext + find(currentdt)
}

for (let l = 0; l < dstring.length / 5; l++) {
  currentdt = "";
  for (let i = 0; i < 5; i++) {
    // 正确的索引计算方式：(当前块的起始位置) + (块内偏移)
    currentdt = currentdt + dstring.charAt((l * 5) + i);
  }
  decodedtext = decodedtext + find(currentdt);
}

alphabet = 'abcdefghijklmnopqrstuvwxyz'.split('');

if (letter.charCodeAt(0) === 32) {
  letternum = 26;
} else {
  letternum = letter.charCodeAt(0) - 97
}
encodedtext = encodedtext + encoded[letternum];

// 在索引 26 处添加空格字符
const alphabet = 'abcdefghijklmnopqrstuvwxyz '; // 注意末尾的空格
// 无需 .split()，字符串本身就可以通过索引访问字符

for (l = 0; l < dstring.length / 5; l++) { // l 未声明
  // ...
  for (i = 0; i < 5; i++) { // i 未声明
    // ...
  }
}

function encode() {
  estring = prompt("enter a string to encode, can only include non capital letters A-Z or SPACE");
  for (let i = 0; i < estring.length; i++) { // 使用 let 声明 i
    let letter = estring.charAt(i); // 局部变量也应使用 let
    // ...
  }
  alert(encodedtext);
}

function decode() {
  decodedtext = "";
  dstring = prompt("enter a string to decode", "sdlfjpijgnpjnsedsfnn");
  for (let l = 0; l < dstring.length / 5; l++) { // 使用 let 声明 l
    currentdt = "";
    for (let i = 0; i < 5; i++) { // 使用 let 声明 i
      currentdt = currentdt + dstring.charAt((l * 5) + i);
    }
    decodedtext = decodedtext + find(currentdt);
  }
  alert(decodedtext);
}

let dstring = "";
let estring = "";
let decodedtext = "";
let encodedtext = "";

const encoded = [
  "lsdjf", "flwek", "sdlfj", "pjnse", "dsfnn", "dsnfe", "sjndv", "sdfew", "sdfjn", "sfnkh",
  "ådpfk", "qpwkd", "sorif", "efjnf", "pijgn", "sijfb", "feuwh", "renfv", "soduh", "osdnv",
  "ksdjn", "tonrn", "frjng", "fjenw", "fnkrj", "vrejl", "fgwei"
];
// 修正：将 alphabet 定义为 const，并添加空格字符
const alphabet = 'abcdefghijklmnopqrstuvwxyz ';

function find(input) {
  for (let i = 0; i < encoded.length; i++) {
    if (input === encoded[i]) {
      return alphabet[i];
    }
  }
  // 如果未找到匹配，可以考虑返回一个特定值或抛出错误
  return undefined; // 或者 '?'
}

function encode() {
  encodedtext = ""; // 每次编码前清空
  estring = prompt("enter a string to encode, can only include non capital letters A-Z or SPACE");
  if (!estring) return; // 处理用户取消输入的情况
  estring = estring.toLowerCase(); // 统一转换为小写处理
  for (let i = 0; i < estring.length; i++) {
    let letter = estring.charAt(i);
    let letternum;
    if (letter === ' ') { // 直接判断字符是否为 ' '
      letternum = 26;
    } else if (letter >= 'a' && letter <= 'z') { // 确保是小写字母
      letternum = letter.charCodeAt(0) - 97;
    } else {
      // 处理无法编码的字符，例如跳过或替换
      console.warn(`Warning: Character '${letter}' cannot be encoded.`);
      continue; // 跳过当前字符
    }
    encodedtext = encodedtext + encoded[letternum];
  }
  alert(encodedtext);
}

function decode() {
  decodedtext = ""; // 每次解码前清空
  dstring = prompt("enter a string to decode", "sdlfjpijgnpjnsedsfnn");
  if (!dstring) return; // 处理用户取消输入的情况

  // 确保输入字符串长度是编码单元长度的倍数
  if (dstring.length % 5 !== 0) {
      alert("Error: The input string length is not a multiple of 5, which is required for decoding.");
      return;
  }

  for (let l = 0; l < dstring.length / 5; l++) {
    let currentdt = ""; // 将 currentdt 声明为局部变量
    for (let i = 0; i < 5; i++) {
      currentdt = currentdt + dstring.charAt((l * 5) + i); // 修正索引逻辑
    }
    const decodedChar = find(currentdt);
    if (decodedChar === undefined) {
        console.warn(`Warning: Could not decode chunk '${currentdt}'.`);
        decodedtext += '?'; // 用问号或其他标记表示无法解码的字符
    } else {
        decodedtext += decodedChar;
    }
  }
  alert(decodedtext);
}

// 示例调用 (可以根据需要选择调用 encode 或 decode)
// encode();
// decode();