CameraExposureApp/app.js

const isoAEl = document.getElementById("isoA");
const fAEl = document.getElementById("fA");
const tAEl = document.getElementById("tA");

const isoBEl = document.getElementById("isoB");
const fBEl = document.getElementById("fB");
const tBEl = document.getElementById("tB");

const resultEl = document.getElementById("result");

document.getElementById("calcBtn").addEventListener("click", calculate);
document.getElementById("resetBtn").addEventListener("click", reset);

document.addEventListener("keydown", (e) => {
  if (e.key === "Enter") calculate();
});

function parseShutter(value) {
  const raw = String(value ?? "").trim();
  if (!raw) return NaN;

  if (raw.includes("/")) {
    const parts = raw.split("/");
    if (parts.length !== 2) return NaN;
    const num = parseFloat(parts[0]);
    const den = parseFloat(parts[1]);
    if (!isFinite(num) || !isFinite(den) || den === 0) return NaN;
    return num / den;
  }

  const v = parseFloat(raw);
  if (!isFinite(v)) return NaN;
  return v;
}

function parsePositiveOrEmpty(value, parser = parseFloat) {
  const raw = String(value ?? "").trim();
  if (raw === "") return { empty: true, value: null };
  const parsed = parser(raw);
  if (!isFinite(parsed) || parsed <= 0) return { empty: false, value: NaN };
  return { empty: false, value: parsed };
}

function prettyShutter(seconds) {
  if (!(seconds > 0) || !isFinite(seconds)) return "Invalid";

  // exact decimal seconds (up to 8 decimals, trimmed)
  const exactSeconds = seconds.toFixed(8).replace(/0+$/, "").replace(/\.$/, "");

  if (seconds < 1) {
    // exact reciprocal (not snapped to common shutter values)
    const reciprocal = 1 / seconds;
    const reciprocalStr = reciprocal
      .toFixed(6)
      .replace(/0+$/, "")
      .replace(/\.$/, "");

    return `1/${reciprocalStr} s (${exactSeconds} s)`;
  }

  return `${exactSeconds} s`;
}

function formatAperture(n) {
  if (!(n > 0) || !isFinite(n)) return "Invalid";
  return `f/${n.toFixed(2).replace(/0+$/, "").replace(/\.$/, "")}`;
}

function formatISO(iso) {
  if (!(iso > 0) || !isFinite(iso)) return "Invalid";
  return `ISO ${Math.round(iso)}`;
}

function calculate() {
  // Reference (must all be present)
  const isoA = parseFloat(isoAEl.value);
  const fA = parseFloat(fAEl.value);
  const tA = parseShutter(tAEl.value);

  if (
    !(isoA > 0) ||
    !(fA > 0) ||
    !(tA > 0) ||
    !isFinite(isoA) ||
    !isFinite(fA) ||
    !isFinite(tA)
  ) {
    resultEl.innerHTML = `<span class="error">Camera A must have valid positive ISO, f-stop, and shutter speed.</span>`;
    return;
  }

  // Camera B: exactly one missing
  const isoBParsed = parsePositiveOrEmpty(isoBEl.value, parseFloat);
  const fBParsed = parsePositiveOrEmpty(fBEl.value, parseFloat);
  const tBParsed = parsePositiveOrEmpty(tBEl.value, parseShutter);

  const fields = [
    { key: "iso", ...isoBParsed },
    { key: "f", ...fBParsed },
    { key: "t", ...tBParsed },
  ];

  const emptyFields = fields.filter((f) => f.empty);
  if (emptyFields.length !== 1) {
    resultEl.innerHTML = `<span class="error">For Camera B, leave exactly one field empty (the one to calculate).</span>`;
    return;
  }

  if (fields.some((f) => !f.empty && (!isFinite(f.value) || f.value <= 0))) {
    resultEl.innerHTML = `<span class="error">Camera B has invalid values. Use positive numbers, and shutter like 1/250 or 0.004.</span>`;
    return;
  }

  const missing = emptyFields[0].key;
  let isoB = isoBParsed.value;
  let fB = fBParsed.value;
  let tB = tBParsed.value;

  // From:
  // tB = tA * (fB^2 / fA^2) * (isoA / isoB)
  // Rearranged for each missing variable:
  if (missing === "t") {
    tB = tA * ((fB * fB) / (fA * fA)) * (isoA / isoB);
    if (!(tB > 0) || !isFinite(tB)) {
      resultEl.innerHTML = `<span class="error">Could not compute shutter speed from given values.</span>`;
      return;
    }
    tBEl.value = tB.toPrecision(6);

    resultEl.innerHTML = `
      <div class="ok">Calculated shutter speed for Camera B: <span class="mono">${prettyShutter(tB)}</span></div>
      <div style="margin-top:8px;">B now: <span class="mono">${formatISO(isoB)}, ${formatAperture(fB)}, ${prettyShutter(tB)}</span></div>
    `;
    return;
  }

  if (missing === "iso") {
    // isoB = isoA * tA * fB^2 / ( tB * fA^2 )
    isoB = (isoA * tA * (fB * fB)) / (tB * (fA * fA));
    if (!(isoB > 0) || !isFinite(isoB)) {
      resultEl.innerHTML = `<span class="error">Could not compute ISO from given values.</span>`;
      return;
    }
    isoBEl.value = Math.round(isoB).toString();

    resultEl.innerHTML = `
      <div class="ok">Calculated ISO for Camera B: <span class="mono">${formatISO(isoB)}</span></div>
      <div style="margin-top:8px;">B now: <span class="mono">${formatISO(isoB)}, ${formatAperture(fB)}, ${prettyShutter(tB)}</span></div>
    `;
    return;
  }

  if (missing === "f") {
    // fB^2 = (tB * fA^2 * isoB) / (tA * isoA)
    // fB = sqrt(...)
    const inside = (tB * (fA * fA) * isoB) / (tA * isoA);
    fB = Math.sqrt(inside);
    if (!(fB > 0) || !isFinite(fB)) {
      resultEl.innerHTML = `<span class="error">Could not compute aperture from given values.</span>`;
      return;
    }
    fBEl.value = fB.toFixed(2);

    resultEl.innerHTML = `
      <div class="ok">Calculated aperture for Camera B: <span class="mono">${formatAperture(fB)}</span></div>
      <div style="margin-top:8px;">B now: <span class="mono">${formatISO(isoB)}, ${formatAperture(fB)}, ${prettyShutter(tB)}</span></div>
    `;
  }
}

function reset() {
  isoAEl.value = "100";
  fAEl.value = "2.0";
  tAEl.value = "1/1000";

  isoBEl.value = "";
  fBEl.value = "";
  tBEl.value = "";

  resultEl.textContent = "Result will appear here.";
}