palindrome permutations

0-README.md

This uses generator functions to generate permutations on demand.

The typescript is the original, the js version is just the compiled typescript.

1-permutations.ts

function* permGenerator<T>(items: T[]): Generator<T[]>  {
  const indices = items.map((_, idx) => idx);

  const reformArray = (idxs: number[]): T[] => idxs.map(idx => items[idx]);

  function* permute(stack: number[]): Generator<T[]> {
    if (stack.length === indices.length) {
      yield reformArray(stack);
    }

    for (let i = 0; i < indices.length; i++) {
      const idx = indices[i];
      if (!stack.includes(idx)) {
        stack.push(idx);
        yield* permute(stack);
        stack.pop();
      }
    }
  }

  yield* permute([]);
}

function findPalindrome(str: string): string | null {
  const gen = permGenerator(str.toLowerCase().split(''));

  for (const perm of gen) {
    const reverse = [...perm].reverse().join('');
    const forward = perm.join('');
    if (forward === reverse) {
      return forward;
    }
  }
  return null;
}

console.log(findPalindrome('tactcoa'));
console.log(findPalindrome('acecarr'));

2-permutations.js

function* permGenerator(items) {
    const indices = items.map((_, idx)=>idx
    );
    const reformArray = (idxs)=>idxs.map((idx)=>items[idx]
        )
    ;
    function* permute(stack) {
        if (stack.length === indices.length) {
            yield reformArray(stack);
        }
        for(let i = 0; i < indices.length; i++){
            const idx = indices[i];
            if (!stack.includes(idx)) {
                stack.push(idx);
                yield* permute(stack);
                stack.pop();
            }
        }
    }
    yield* permute([]);
}
function findPalindrome(str) {
    const gen = permGenerator(str.toLowerCase().split(''));
    for (const perm of gen){
        const reverse = [
            ...perm
        ].reverse().join('');
        const forward = perm.join('');
        if (forward === reverse) {
            return forward;
        }
    }
    return null;
}
console.log(findPalindrome('tactcoa'));
console.log(findPalindrome('acecarr'));