const IsNanError = TypeError('resulted in NaN');
const FailedToConvergeError = Error('failed to converge');
const InvalidInputsError = Error('invalid inputs');

export default async function GoalSeek({
  fn,
  fnParams,
  percentTolerance,
  customToleranceFn,
  maxIterations,
  maxStep,
  goal,
  independentVariableIdx,
}) {
  if (typeof customToleranceFn !== 'function') {
    if (!percentTolerance) {
      throw InvalidInputsError;
    }
  }
  let g;
  let y;
  let y1;
  let oldGuess;
  let newGuess;
  let res;
  const absoluteTolerance = ((percentTolerance || 0) / 100) * goal;
  // iterate through the guesses
  for (let i = 0; i < maxIterations; i++) {
    // define the root of the function as the error
    res = await fn(...fnParams);
    y = res - goal;
    if (isNaN(y)) throw IsNanError;
    // was our initial guess a good one?
    if (typeof customToleranceFn !== 'function') {
      if (Math.abs(y) <= Math.abs(absoluteTolerance)) return fnParams[independentVariableIdx];
    } else {
      if (customToleranceFn(res)) return fnParams[independentVariableIdx];
    }
    // set the new guess, correcting for maxStep
    oldGuess = fnParams[independentVariableIdx];
    newGuess = oldGuess + y;
    if (Math.abs(newGuess - oldGuess) > maxStep) {
      if (newGuess > oldGuess) {
        newGuess = oldGuess + maxStep;
      } else {
        newGuess = oldGuess - maxStep;
      }
    }
    fnParams[independentVariableIdx] = newGuess;
    // re-run the fn with the new guess
    y1 = (await fn(...fnParams)) - goal;
    if (isNaN(y1)) throw IsNanError;
    // calculate the error
    g = (y1 - y) / y;
    if (g === 0) g = 0.0001;
    // set the new guess based on the error, correcting for maxStep
    newGuess = oldGuess - y / g;
    if (maxStep && Math.abs(newGuess - oldGuess) > maxStep) {
      if (newGuess > oldGuess) {
        newGuess = oldGuess + maxStep;
      } else {
        newGuess = oldGuess - maxStep;
      }
    }
    fnParams[independentVariableIdx] = newGuess;
  }
  // done with iterations, and we failed to converge
  throw FailedToConvergeError;
}

// const fn = (x, y) => x / y;
// const fnParams = [2037375, 15897178];
// const customToleranceFn = (x) => {
//   return x < 1;
// };

// try {
//   const result = goalSeek({
//     fn,
//     fnParams,
//     customToleranceFn,
//     maxIterations: 1000,
//     maxStep: 0.01,
//     goal: 0.15,
//     independentVariableIdx: 0,
//   });

//   console.log(`result: ${result}`);
// } catch (e) {
//   console.log("error", e);
// }