function reportException(e) {
  const error = e instanceof Error ? e : new Error(e.toString());
  console.log(JSON.stringify(error.stack));
  showToast(`An error has occurred!\n\n${error}\n\n${error.stack}`);
}

/***********************************************************************
 * Actual FSRS5 algorithm                                              *
 ***********************************************************************/

const W = [
  0.40255, 1.18385, 3.173, 15.69105, 7.1949, 0.5345, 1.4604, 0.0046, 1.54575, 0.1192, 1.01925,
  1.9395, 0.11, 0.29605, 2.2698, 0.2315, 2.9898, 0.51655, 0.6621,
];

const F = 19.0 / 81.0;
const C = -0.5;

const Grade = {
  forgot: 1.0,
  hard: 2.0,
  good: 3.0,
  easy: 4.0
};

function retrievability(t, s) {
  return Math.pow(1.0 + F * (t / s), C);
}

function interval(r_d, s) {
  return (s / F) * (Math.pow(r_d, 1.0 / C) - 1.0);
}

function s_0(g) {
  switch (g) {
  case Grade.forgot: return W[0];
  case Grade.hard: return W[1];
  case Grade.good: return W[2];
  case Grade.easy: return W[3];
  }
}

function s_success(d, s, r, g) {
  const t_d = 11.0 - d;
  const t_s = Math.pow(s, -W[9]);
  const t_r = Math.exp(W[10] * (1.0 - r)) - 1.0;
  const h = g === Grade.hard ? W[15] : 1.0;
  const b = g === Grade.easy ? W[16] : 1.0;
  const c = Math.exp(W[8]);
  const alpha = 1.0 + t_d * t_s * t_r * h * b * c;
  return s * alpha;
}

function s_fail(d, s, r) {
  const d_f = Math.pow(d, -W[12]);
  const s_f = Math.pow(s + 1.0, W[13]) - 1.0;
  const r_f = Math.exp(W[14] * (1.0 - r));
  const c_f = W[11];
  const result = d_f * s_f * r_f * c_f;
  return Math.min(result, s);
}

function stability(d, s, r, g) {
  return g === Grade.forgot ? s_fail(d, s, r) : s_success(d, s, r, g);
}

function clamp_d(d) {
  return Math.min(10.0, Math.max(1.0, d));
}

function d_0(g) {
  return clamp_d(W[4] - Math.exp(W[5] * (g - 1.0)) + 1.0);
}

function difficulty(d, g) {
  return clamp_d(W[7] * d_0(Grade.easy) + (1.0 - W[7]) * dp(d, g));
}

function dp(d, g) {
  return d + delta_d(g) * ((10.0 - d) / 9.0);
}

function delta_d(g) {
  return -W[6] * (g - 3.0);
}

function sim(grades) {
  let t = 0.0;
  const r_d = 0.9;
  const steps = [];

  // Initial review
  if (grades.length === 0) {
    throw new Error("Grades cannot be empty");
  }

  let gradesCopy = [...grades];
  let g = gradesCopy.shift(); // Remove the first grade
  let s = s_0(g);
  let d = d_0(g);
  let i = Math.max(Math.round(interval(r_d, s)), 1.0);

  steps.push({ t, s, d, i });

  // n-th review
  for (let g of gradesCopy) {
    t += i;
    const r = retrievability(i, s);
    s = stability(d, s, r, g);
    d = difficulty(d, g);
    i = Math.max(Math.round(interval(r_d, s)), 1.0);
    steps.push({ t, s, d, i });
  }

  return steps;
}

/***********************************************************************
 * State file processing                                               *
 ***********************************************************************/

function statusString() {
  const now = Date.now();
  const elapsedSeconds = Math.floor((now - window.started) / 1000);
  const minutes = Math.floor(elapsedSeconds / 60);
  const seconds = elapsedSeconds % 60;
  const pad = (n) => n < 10 ? '0' + n : n;
  return `Card ${window.index + 1} of ${window.cards.length} scheduled for review, ${window.allCards.length} total
Current session: ${pad(minutes)}:${pad(seconds)}`;
}

function loadCard(card) {
  window.card.revealed = false;
  window.card.front.innerHTML = card.front;
  window.card.front.style.display = "block";
  window.card.back.innerHTML = card.back;
  window.card.back.style.display = "none";
  window.buttons.style.display = "none";
}

function flipCard() {
  if (window.index >= window.cards.length) {
    return;
  }

  window.card.revealed = true;

  // Reveal the "back".
  if (window.card.front.style.display !== "block") {
    window.card.front.style.display = "block";
    window.card.back.style.display = "none";
  } else {
    window.card.front.style.display = "none";
    window.card.back.style.display = "block";
  }

  // Reveal the buttons.
  window.buttons.style.display = "flex";
}

function endReview() {
  clearInterval(window.updateInterval);
  window.statusInfo.innerHTML = "Session complete!";
  window.flashcard.style.display = "none";
  window.buttons.style.display = "none";
  updateAllCards();
  console.log(window.allCards);
}

function rateCard(difficulty) {
  if (!window.card.revealed) {
    return;
  }

  const card = window.cards[window.index];

  const r_d = 0.9;
  const s = card.scheduler.s;
  const d = card.scheduler.d;
  const i = card.scheduler.i;
  const g = card.flagged ? Grade.forgot : Grade[difficulty];

  let s_n = undefined, d_n = undefined, i_n = undefined;

  if (s == null || d == null || i == null) {
    s_n = s_0(g);
    d_n = d_0(g);
  } else {
    const r = retrievability(i, s);
    s_n = stability(d, s, r, g);
    d_n = difficulty(d, g);
  }
  i_n = Math.max(Math.round(interval(r_d, s)), 1.0);

  let scheduled = today();
  scheduled.setDate(scheduled.getDate() + i_n);

  if (difficulty === "forgot") {
    // Shift forgotten card to end.
    window.cards.push(card);
    window.cards.splice(window.index, 1);
    loadCard(window.cards[window.index]);
  } else {
    // Unflag and update scheduler parameters.
    delete card.flagged;
    card.scheduler.s = s_n;
    card.scheduler.d = d_n;
    card.scheduler.i = i_n;
    card.scheduled = scheduled;

    if (++window.index < window.cards.length) {
      loadCard(window.cards[window.index]);
    } else {
      endReview();
    }
  }
}

// Update `window.allCards` with new scheduler parameters.
function updateAllCards() {
  for (let i = 0; i < window.allCards.length; i++) {
    const updated = window.cards.find((card) => card.id == window.allCards[i].id);
    if (updated) {
      window.allCards[i] = updated;
    }
  }
}

function today() {
  let today = new Date();
  today.setHours(0, 0, 0, 0);
  return today;
}

function checkCards(cards) {
  for (let i = 0; i < cards.length; i++) {
    const card = cards[i];
    if (!card.id) {
      throw new Error("malformed card: missing id")
    } else if (!card.front) {
      throw new Error(`malformed card ${card.id}: missing front`)
    } else if (!card.back) {
      throw new Error(`malformed card ${card.id}: missing bacak`)
    }
    if (!card.scheduled) {
      card.scheduled = today().toJSON();
    }
    if (!card.scheduler) {
      card.scheduler = {
        s: null,
        d: null,
        i: null
      }
    }
  }
}

function loadCards(data) {
  checkCards(data);

  document.getElementById('flashcard').style.display = "block";

  window.allCards = data;
  window.index = 0;
  window.started = Date.now();
  window.statusInfo = document.getElementById('status-info');
  window.buttons = document.getElementById('button-container');
  window.card = {
    front: document.getElementById('flashcard-front'),
    back: document.getElementById('flashcard-back')
  };

  document.getElementById('rating-button-forgot')
    .addEventListener('click', () => rateCard('forgot'));
  document.getElementById('rating-button-hard')
    .addEventListener('click', () => rateCard('hard'));
  document.getElementById('rating-button-good')
    .addEventListener('click', () => rateCard('good'));
  document.getElementById('rating-button-easy')
    .addEventListener('click', () => rateCard('easy'));

  let today = new Date();
  // Granularity only at the day level.
  today.setHours(0, 0, 0, 0);
  window.cards = data.filter((card) => {
    return (new Date(card.scheduled)) <= today;
  });

  loadCard(window.cards[0]);

  window.updateInterval = setInterval(() => {
    window.statusInfo.innerText = statusString()
  }, 1000);
}

/***********************************************************************
 * Status display                                                      *
 ***********************************************************************/

function showToast(message, duration = 3000) {
  const toast = document.getElementById("toast");
  toast.innerHTML = "<pre>" + message + "</pre>";

  // Show the toast
  toast.classList.add("show");

  // Hide the toast after specified time
  setTimeout(() => {
    toast.classList.remove("show");
  }, duration);
}

window.addEventListener("load", (event) => {

  document.getElementById('cards-input').addEventListener('change', function(event) {
    const file = event.target.files[0];
    if (!file) return;

    const reader = new FileReader();
    reader.onload = function(e) {
      try {
        const content = e.target.result;
        loadCards(JSON.parse(content));
      } catch (e) {
        reportException(e);
      }
    };

    reader.onerror = function(error) {
      document.getElementById('result').textContent = "Error reading file: " + error;
      reportException(error);
    };

    reader.readAsText(file); // Read the file as text
  });

});

window.addEventListener('keydown', (event) => {
  // console.log(event.code);
  if (event.code === 'Space') {
    flipCard();
  } else if (event.code === 'Digit1' || event.code === "Numpad1") {
    rateCard('forgot')
  } else if (event.code === 'Digit2' || event.code === "Numpad2") {
    rateCard('easy')
  } else if (event.code === 'Digit3' || event.code === "Numpad3") {
    rateCard('good')
  } else if (event.code === 'Digit4' || event.code === "Numpad4") {
    rateCard('hard')
  }
});