wordle/server/src/feedback.ts

/**
 * Compute Wordle feedback colors for a guess against the target word.
 *
 * Algorithm (Feedback Scoring Rule):
 * 1. Mark exact-position matches (green/'g'), decrement remaining letter count
 * 2. For remaining positions, mark as yellow/'y' if letter still has remaining count
 * 3. All other positions are gray/'x'
 *
 * Returns a 5-element array of 'g' | 'y' | 'x'.
 */
export function computeFeedback(target: string, guess: string): string[] {
  if (target.length !== 5 || guess.length !== 5) {
    throw new Error('Target and guess must be exactly 5 letters');
  }

  const targetUpper = target.toLowerCase();
  const guessUpper = guess.toLowerCase();

  // Count letters in target
  const targetCounts: Record<string, number> = {};
  for (const ch of targetUpper) {
    targetCounts[ch] = (targetCounts[ch] ?? 0) + 1;
  }

  const result: string[] = Array(5).fill('x');

  // Pass 1: greens (exact position matches)
  for (let i = 0; i < 5; i++) {
    if (guessUpper[i] === targetUpper[i]) {
      result[i] = 'g';
      targetCounts[guessUpper[i]]--;
    }
  }

  // Pass 2: yellows (letter present but wrong position, remaining count)
  for (let i = 0; i < 5; i++) {
    if (result[i] === 'g') continue; // already matched
    const ch = guessUpper[i];
    if ((targetCounts[ch] ?? 0) > 0) {
      result[i] = 'y';
      targetCounts[ch]--;
    }
  }

  return result;
}