const s = "hello";

// Index like an array
console.log(s[0]); // 'h'
console.log(s[4]); // 'o'

// Classic for-loop iteration
for (let i = 0; i < s.length; i++) {
  process(s[i]);
}

// for...of iterates Unicode code points (safer for emoji)
for (const ch of s) {
  process(ch);
}

// When you need to mutate: convert to array, edit, join back
const chars = s.split('');
chars[0] = 'H';
console.log(chars.join('')); // 'Hello'

const s = "hello";

// Does NOT work — assignment is silently ignored
s[0] = 'H';
console.log(s); // still 'hello'

// Correct: build a new string
const result = 'H' + s.slice(1);
console.log(result); // 'Hello'

// ❌ Performance trap — O(n²) in a loop
let bad = '';
for (let i = 0; i < 100_000; i++) {
  bad += 'x'; // copies the entire string every iteration
}

// ✅ Correct — O(n) with array buffer
const parts = [];
for (let i = 0; i < 100_000; i++) {
  parts.push('x');
}
const good = parts.join(''); // single allocation

const s = "  Hello, World!  ";

// Trim whitespace, change case
console.log(s.trim().toLowerCase()); // 'hello, world!'

// slice — end index is exclusive, negatives count from end
console.log(s.trim().slice(0, 5));   // 'Hello'
console.log(s.trim().slice(-6, -1)); // 'World'

// indexOf / includes
console.log(s.indexOf('World')); // 9
console.log(s.indexOf('xyz'));   // -1
console.log(s.includes('Hello')); // true

// split / join
console.log('a,b,c'.split(','));  // ['a', 'b', 'c']
console.log('hello'.split(''));   // ['h','e','l','l','o']

// replace
console.log('aabbcc'.replace('b', 'X'));    // 'aaXbcc'  (first only)
console.log('aabbcc'.replaceAll('b', 'X')); // 'aaXXcc'  (all)

// padStart — useful for zero-padding numbers
console.log(String(7).padStart(3, '0')); // '007'

// Option 1 — Map (works for any character set)
function charFreq(s) {
  const freq = new Map();
  for (const ch of s) {
    freq.set(ch, (freq.get(ch) ?? 0) + 1);
  }
  return freq;
}

// Option 2 — Plain object (slightly faster due to V8 optimisations)
function charFreqObj(s) {
  const freq = {};
  for (const ch of s) {
    freq[ch] = (freq[ch] || 0) + 1;
  }
  return freq;
}

// Option 3 — Fixed array (fastest; only for lowercase a-z)
function charFreqArray(s) {
  const freq = new Array(26).fill(0);
  const a = 'a'.charCodeAt(0);
  for (const ch of s) {
    freq[ch.charCodeAt(0) - a]++;
  }
  return freq;
}

// Classic use case: anagram check
function isAnagram(s, t) {
  if (s.length !== t.length) return false;
  const freq = new Array(26).fill(0);
  const a = 'a'.charCodeAt(0);
  for (let i = 0; i < s.length; i++) {
    freq[s.charCodeAt(i) - a]++;
    freq[t.charCodeAt(i) - a]--;
  }
  return freq.every(n => n === 0);
}

console.log(isAnagram('listen', 'silent')); // true
console.log(isAnagram('hello',  'world'));  // false

// charCodeAt / fromCharCode
console.log('A'.charCodeAt(0));        // 65
console.log('a'.charCodeAt(0));        // 97  (65 + 32)
console.log(String.fromCharCode(65));  // 'A'

// 0-based alphabet index
const idx = ch => ch.charCodeAt(0) - 'a'.charCodeAt(0);
console.log(idx('a')); // 0
console.log(idx('z')); // 25

// Emoji / surrogate pair trap
const emoji = '😀';
console.log(emoji.length);       // 2  ← code units, not characters!
console.log([...emoji].length);  // 1  ← spread uses code points (correct)

// Safe iteration for any Unicode text
for (const ch of '😀Hi') {
  console.log(ch); // '😀', 'H', 'i'
}

// Toggle case using the 32-bit gap
function toggleCase(ch) {
  const code = ch.charCodeAt(0);
  if (code >= 65 && code <= 90)  return String.fromCharCode(code + 32); // upper→lower
  if (code >= 97 && code <= 122) return String.fromCharCode(code - 32); // lower→upper
  return ch;
}