// Check first whether the letter is a single character.
    // If letter is a single character, count how many times a letter has occurred in a given sentence then return count.
    // If letter is invalid, return undefined.

    function countLetter(letter, sentence) {
    if (letter.length !== 1) {
        return undefined; // Return undefined if letter is not a single character
    }

    const cleanedLetter = letter.toLowerCase(); // Convert letter to lowercase for case insensitivity
    const cleanedSentence = sentence.toLowerCase(); // Convert sentence to lowercase for case insensitivity
    let count = 0;

    for (const char of cleanedSentence) {
        if (char === cleanedLetter) {
            count++;
        }
    }

    return count;
}







function isIsogram(text) {
    // An isogram is a word where there are no repeating letters.
    // The function should disregard text casing before doing anything else.
    // If the function finds a repeating letter, return false. Otherwise, return true.
    
    // Convert the text to lowercase for case insensitivity
    function isIsogram(text) {
    const cleanedText = text.toLowerCase(); // Convert text to lowercase for case insensitivity
    const uniqueLetters = new Set();

    for (const letter of cleanedText) {
        if (letter !== ' ') {
            if (uniqueLetters.has(letter)) {
                return false; // Found a repeating letter
            } else {
                uniqueLetters.add(letter);
            }
        }
    }

    return true; // No repeating letters found
}



function purchase(age, price) {
    // Return undefined for people aged below 13.
    // Return the discounted price (rounded off) for students aged 13 to 21 and senior citizens. (20% discount)
    // Return the rounded off price for people aged 22 to 64.
    // The returned value should be a string.
    
   function purchase(age, price) {
    if (age < 13) {
        return undefined;
    } else if (age >= 13 && age <= 21 || age >= 65) {
        // Apply 20% discount for students aged 13 to 21 and senior citizens
        const discountedPrice = Math.round(price * 0.8);
        return discountedPrice.toString();
    } else {
        // For people aged 22 to 64, return the rounded off price
        return Math.round(price).toString();
    }
}


function findHotCategories(items) {
    // Find categories that has no more stocks.
    // The hot categories must be unique; no repeating categories.

    // The passed items array from the test are the following:
    { id: 'tltry001', name: 'soap', stocks: 14, category: 'toiletries' }
    { id: 'tltry002', name: 'shampoo', stocks: 8, category: 'toiletries' }
    { id: 'tltry003', name: 'tissues', stocks: 0, category: 'toiletries' }
    { id: 'gdgt001', name: 'phone', stocks: 0, category: 'gadgets' }
    { id: 'gdgt002', name: 'monitor', stocks: 0, category: 'gadgets' }

    // The expected output after processing the items array is ['toiletries', 'gadgets'].
    // Only putting return ['toiletries', 'gadgets'] will not be counted as a passing test during manual checking of codes.
    // Use a Set to keep track of unique categories
    const uniqueCategories = new Set();

    // Iterate through the items
    for (const item of items) {
        if (item.stocks === 0) {
            uniqueCategories.add(item.category);
        }
    }

    // Convert the Set to an array and return it
    return Array.from(uniqueCategories);
}

}

function findFlyingVoters(candidateA, candidateB) {
    // Find voters who voted for both candidate A and candidate B.

    // The passed values from the test are the following:
    candidateA: ['LIWf1l', 'V2hjZH', 'rDmZns', 'PvaRBI', 'i7Xw6C', 'NPhm2m']
    candidateB: ['kcUtuu', 'LLeUTl', 'r04Zsl', '84EqYo', 'V2hjZH', 'LIWf1l']

    The expected output after processing the candidates array is ['LIWf1l', 'V2hjZH'].
    Only putting return ['LIWf1l', 'V2hjZH'] will not be counted as a passing test during manual checking of codes.
    
}

module.exports = {
    countLetter,
    isIsogram,
    purchase,
    findHotCategories,
    findFlyingVoters
};