let currentDay = 1;

    /*
      Safely obtain localStorage.

      Accessing window.localStorage can itself throw a SecurityError
      when the document runs inside a sandbox without allow-same-origin.
    */
    function getSafeStorage() {
      try {
        const storage = window.localStorage;

        /*
          Test whether reading and writing are actually permitted.
          Some browsers expose localStorage but block its methods.
        */
        const testKey = "__superMathStorageTest__";

        storage.setItem(testKey, "1");
        storage.removeItem(testKey);

        return storage;
      } catch (error) {
        console.warn(
          "Persistent storage is unavailable. " +
          "Completed days will reset when the page is reloaded.",
          error
        );

        return null;
      }
    }

    const safeStorage = getSafeStorage();

    function loadCompletedDays() {
      if (!safeStorage) {
        return [];
      }

      try {
        const savedValue =
          safeStorage.getItem("completedMathDays");

        if (!savedValue) {
          return [];
        }

        const parsedValue = JSON.parse(savedValue);

        if (!Array.isArray(parsedValue)) {
          return [];
        }

        /*
          Remove invalid values, duplicates, and day numbers
          outside the challenge.
        */
        return Array.from(
          new Set(
            parsedValue.filter(function(dayNumber) {
              return (
                Number.isInteger(dayNumber) &&
                dayNumber >= 1 &&
                dayNumber <= challenge.length
              );
            })
          )
        ).sort(function(a, b) {
          return a - b;
        });
      } catch (error) {
        console.warn(
          "The saved math progress could not be loaded.",
          error
        );

        return [];
      }
    }

    function saveCompletedDays() {
      if (!safeStorage) {
        /*
          The completedDays array still works in memory.
          Progress simply will not survive a page reload.
        */
        return;
      }

      try {
        safeStorage.setItem(
          "completedMathDays",
          JSON.stringify(completedDays)
        );
      } catch (error) {
        console.warn(
          "The math progress could not be saved.",
          error
        );
      }
    }

    let completedDays = loadCompletedDays();

    const dayTitle =
      document.getElementById("dayTitle");

    const missionMessage =
      document.getElementById("missionMessage");

    const starCount =
      document.getElementById("starCount");

    const progressBar =
      document.getElementById("progressBar");

    const progressContainer =
      document.getElementById("progressContainer");

    const divisionList =
      document.getElementById("divisionList");

    const multiplicationList =
      document.getElementById("multiplicationList");

    const algebraList =
      document.getElementById("algebraList");

    const wordList =
      document.getElementById("wordList");

    const encouragement =
      document.getElementById("encouragement");

    const completeButton =
      document.getElementById("completeButton");

    const previousButton =
      document.getElementById("previousButton");

    const nextButton =
      document.getElementById("nextButton");

    const daySelector =
      document.getElementById("daySelector");

    const answerLock =
      document.getElementById("answerLock");

    const answerForm =
      document.getElementById("answerForm");

    const passwordInput =
      document.getElementById("passwordInput");

    const passwordMessage =
      document.getElementById("passwordMessage");

    const answerContent =
      document.getElementById("answerContent");

    const divisionAnswers =
      document.getElementById("divisionAnswers");

    const multiplicationAnswers =
      document.getElementById("multiplicationAnswers");

    const algebraAnswers =
      document.getElementById("algebraAnswers");

    const wordAnswers =
      document.getElementById("wordAnswers");

    const hideAnswersButton =
      document.getElementById("hideAnswersButton");

    function createList(items, element) {
      element.innerHTML = "";

      items.forEach(function(item) {
        const listItem = document.createElement("li");

        listItem.textContent = item;

        element.appendChild(listItem);
      });
    }

    function createAlgebraList(items) {
      algebraList.innerHTML = "";

      items.forEach(function(item) {
        const listItem = document.createElement("li");

        const typeLabel = document.createElement("span");

        typeLabel.className = "algebra-type";
        typeLabel.textContent = item.type;

        const questionText = document.createElement("span");

        questionText.textContent = item.question;

        listItem.appendChild(typeLabel);
        listItem.appendChild(questionText);

        algebraList.appendChild(listItem);
      });
    }

    function calculateDivision(question) {
      const numbers = question
        .split("÷")
        .map(function(part) {
          return Number(part.trim());
        });

      const dividend = numbers[0];
      const divisor = numbers[1];

      const quotient = Math.floor(dividend / divisor);
      const remainder = dividend % divisor;

      return (
        question +
        " = " +
        quotient +
        " R" +
        remainder
      );
    }

    function calculateMultiplication(question) {
      const numbers = question
        .split("×")
        .map(function(part) {
          return Number(part.trim());
        });

      const product = numbers[0] * numbers[1];

      return (
        question +
        " = " +
        product.toLocaleString("en-US")
      );
    }

    function renderAnswers() {
      const day = challenge[currentDay - 1];

      createList(
        day.division.map(calculateDivision),
        divisionAnswers
      );

      createList(
        day.multiplication.map(calculateMultiplication),
        multiplicationAnswers
      );

      createList(
        day.algebra.map(function(item) {
          return (
            item.question +
            "\n\n" +
            item.answer
          );
        }),
        algebraAnswers
      );

      createList(
        day.wordAnswers,
        wordAnswers
      );
    }

    function lockAnswers() {
      answerContent.hidden = true;
      answerLock.hidden = false;

      passwordInput.value = "";
      passwordMessage.textContent = "";
    }

    function unlockAnswers() {
      renderAnswers();

      answerLock.hidden = true;
      answerContent.hidden = false;

      passwordMessage.textContent = "";
    }

    function updateStarDisplay() {
      starCount.textContent =
        String(completedDays.length);

      if (completedDays.includes(currentDay)) {
        completeButton.textContent =
          "⭐ Mission completed! Star earned!";

        completeButton.classList.add("completed");
      } else {
        completeButton.textContent =
          "⭐ I finished today's mission!";

        completeButton.classList.remove("completed");
      }
    }

    function createCelebration() {
      const symbols = [
        "⭐",
        "🌟",
        "🎉",
        "🚀",
        "🌈",
        "🏆"
      ];

      for (let index = 0; index < 24; index++) {
        const piece = document.createElement("div");

        piece.className = "celebration";

        piece.textContent =
          symbols[
            Math.floor(
              Math.random() * symbols.length
            )
          ];

        piece.style.left =
          Math.random() * 95 + "vw";

        piece.style.top =
          55 + Math.random() * 35 + "vh";

        piece.style.fontSize =
          22 + Math.random() * 24 + "px";

        piece.style.animationDelay =
          Math.random() * 0.4 + "s";

        document.body.appendChild(piece);

        setTimeout(function() {
          piece.remove();
        }, 2000);
      }
    }

    function completeCurrentDay() {
      if (!completedDays.includes(currentDay)) {
        completedDays.push(currentDay);

        completedDays.sort(function(a, b) {
          return a - b;
        });

        /*
          This safely saves progress if storage is available.
          It does not throw an error when the page is sandboxed.
        */
        saveCompletedDays();

        createCelebration();
      }

      updateStarDisplay();
    }

    function displayDay(dayNumber) {
      currentDay = Math.min(
        challenge.length,
        Math.max(1, dayNumber)
      );

      const day = challenge[currentDay - 1];

      dayTitle.textContent =
        "Day " + currentDay + " Mission";

      missionMessage.textContent =
        missionMessages[currentDay - 1];

      progressBar.style.width =
        (
          (currentDay / challenge.length) *
          100
        ) + "%";

      progressContainer.setAttribute(
        "aria-valuenow",
        String(currentDay)
      );

      createList(
        day.division,
        divisionList
      );

      createList(
        day.multiplication,
        multiplicationList
      );

      createAlgebraList(day.algebra);

      createList(
        day.words,
        wordList
      );

      encouragement.textContent =
        encouragements[currentDay - 1];

      previousButton.disabled =
        currentDay === 1;

      nextButton.disabled =
        currentDay === challenge.length;

      daySelector.value =
        String(currentDay);

      document.title =
        "Day " +
        currentDay +
        " – Super Math Adventure";

      updateStarDisplay();
      lockAnswers();

      /*
        Scrolling can also be restricted in some embedded environments,
        so use a fallback if smooth scrolling is unavailable.
      */
      try {
        window.scrollTo({
          top: 0,
          behavior: "smooth"
        });
      } catch (error) {
        try {
          window.scrollTo(0, 0);
        } catch (ignoredError) {
          // The sandbox does not permit scrolling.
        }
      }
    }

    /*
      Add all 15 days to the day selector.
    */
    for (
      let dayNumber = 1;
      dayNumber <= challenge.length;
      dayNumber++
    ) {
      const option =
        document.createElement("option");

      option.value = String(dayNumber);

      option.textContent =
        "Day " +
        dayNumber +
        " of " +
        challenge.length;

      daySelector.appendChild(option);
    }

    previousButton.addEventListener(
      "click",
      function() {
        displayDay(currentDay - 1);
      }
    );

    nextButton.addEventListener(
      "click",
      function() {
        displayDay(currentDay + 1);
      }
    );

    daySelector.addEventListener(
      "change",
      function() {
        displayDay(
          Number(daySelector.value)
        );
      }
    );

    completeButton.addEventListener(
      "click",
      completeCurrentDay
    );

    answerForm.addEventListener(
      "submit",
      function(event) {
        event.preventDefault();

        if (
          passwordInput.value ===
          PARENT_PASSWORD
        ) {
          unlockAnswers();
        } else {
          passwordMessage.textContent =
            "That password is not correct. " +
            "Please try again.";

          passwordInput.value = "";
          passwordInput.focus();
        }
      }
    );

    hideAnswersButton.addEventListener(
      "click",
      function() {
        lockAnswers();
      }
    );

    displayDay(1);
  </script>
</body>
</html>