Decode Ways - JS Solution

Solution code

const CURRENT_INDEX = 'current_index', CURRENT_INDEX_COLOR = 'gold';

function visMainFunction(s) {
    initClassProperties();

    if (s[0] === '0') {
        notification(`❌ <b>Invalid input:</b> Cannot start with <code>'0'</code>. Returning <b>0</b>.`);
        return 0;
    }

    colorSchemaAndExplanationNotifications();
    
    startBatch();
    const sVis = VisArray.from([...s]).options({ label: 's' });

    let prevWays = 1;
    let lastWays = 1;

    makeVisVariable(prevWays).options({ label: 'prevWays' }).createVis();
    makeVisVariable(lastWays).options({ label: 'lastWays' }).createVis();

    let currentIndex = 1;
    makeVisVariable(currentIndex).registerAsIndexPointer(sVis, CURRENT_INDEX);
    endBatch();

    notification(`
        ⚙️ <b>Initialization before iteration</b>:
        <ul>
            <li><b>prevWays = 1</b>: Represents the number of ways to decode up to the <b>character before the last evaluated one</b> — initially, the empty prefix has one valid decoding.</li>
            <li><b>lastWays = 1</b>: Represents the number of ways to decode up to the <b>last evaluated character</b> — the first digit (already checked to be non-zero) forms one valid decoding.</li>
        </ul>
        ▶️ We’ll now iterate through the string starting from index <b>1</b> (the second character).<br/><br/>
        At each step, we’ll determine how many new decoding combinations can be formed 
        based on the validity of the <b>${makeColoredSpan("current digit", CURRENT_INDEX_COLOR)}</b> and the pair formed with the previous one.
        We then update our running counts accordingly to reflect all valid ways so far.
    `);

    while (currentIndex < s.length) {
        startPartialBatch();

        let newWays = 0;
        makeVisVariable(newWays).options({ label: 'newWays' }).createVis();
        
        const singleDigit = s[currentIndex];
        const twoDigit = s[currentIndex - 1] + s[currentIndex];

        notification(`📌 Analyzing single digit <b>${makeColoredSpan('"' + singleDigit + '"', CURRENT_INDEX_COLOR)}</b> at index <b>${makeColoredSpan(currentIndex, CURRENT_INDEX_COLOR)}</b> and two-digit pair <b>"${twoDigit}"</b> from indices <b>${currentIndex - 1}</b> and <b>${makeColoredSpan(currentIndex, CURRENT_INDEX_COLOR)}</b>.`);

        // Single-digit decode check
        if (singleDigit !== '0') {
            newWays += lastWays;
            notification(`✅ <b>Single-digit decode is valid</b>:
              <ul>
                <li>Digit <b>${makeColoredSpan('"' + singleDigit + '"', CURRENT_INDEX_COLOR)}</b> represents letters (1–9), meaning each valid decoding up to the <b>last evaluated character</b> (whose count is stored in <b>lastWays = ${lastWays}</b>) can be extended by this digit.</li>
                <li>Therefore, we increment <b>newWays</b> by <b>lastWays = ${lastWays}</b>, resulting in <b>newWays = ${newWays}</b>.</li>
              </ul>`);
        } else {
            notification(`⚠️ <b>Single-digit decode invalid</b>:
              <ul>
                <li>Digit <b>${makeColoredSpan('"0"', CURRENT_INDEX_COLOR)}</b> cannot decode to any letter alone.</li>
                <li>Hence, it does not contribute to <b>newWays</b>.</li>
              </ul>`);
        }

        // Two-digit decode check
        if (twoDigit >= "10" && twoDigit <= "26") {
            newWays += prevWays;
            notification(`✅ <b>Two-digit decode is valid</b>:
              <ul>
                <li>Pair <b>"${twoDigit}"</b> can represent letters (10–26), meaning each valid decoding up to the <b>character before the last evaluated one</b> (whose count is stored in <b>prevWays = ${prevWays}</b>) can be extended by this pair.</li>
                <li>Therefore, we increment <b>newWays</b> by <b>prevWays = ${prevWays}</b>, resulting in <b>newWays = ${newWays}</b>.</li>
              </ul>`);
        } else {
            notification(`⚠️ <b>Two-digit decode invalid</b>:
              <ul>
                <li>Pair <b>"${twoDigit}"</b> is not between 10 and 26.</li>
                <li>Hence, it does not contribute to <b>newWays</b>.</li>
              </ul>`);
        }

        notification(`📌 <b>Updating decoding counts</b>:
          <ul>
            <li>Shift variables forward:</li>
            <ul>
              <li><b>prevWays</b> ← previous <b>lastWays (${lastWays})</b></li>
              <li><b>lastWays</b> ← new calculated <b>newWays (${newWays})</b></li>
            </ul>
          </ul>`);

        prevWays = lastWays;
        lastWays = newWays;

        currentIndex += 1;
        endBatch();
    }

    notification(`🏁 <b>Finished!</b> The total number of ways to decode <b>"${s}"</b> is stored in <b>lastWays = ${lastWays}</b>.`);
    return lastWays;
}

function colorSchemaAndExplanationNotifications() {
  explanationNotification();

  notification(`
    🎨 <b>Color Schema</b>:
    <ul>
      <li><span style="background-color:${CURRENT_INDEX_COLOR}; padding:2px 6px; border-radius:4px;">&nbsp;</span>
        — highlights the <b>current index/character</b> being evaluated.</li>
    </ul>
    These colors help distinguish which character is currently under evaluation 
    as we iterate through the string and update decoding counts.
  `);
}

function initClassProperties() {
    setClassProperties(CURRENT_INDEX, {
        backgroundColor: CURRENT_INDEX_COLOR
    });
}