Continuous waves explain purely optical phenomena.

/**
 * Two-source interference intensity: time-averaged or instantaneous.
 */
export function twoSourceIntensity(
  params: TwoSourceParameters,
): RadiationResult<TwoSourceIntensityResult["intensity"]> {
  const { A1, A2, r1, r2, wavelength, readout = "time-average", t = 0, kappa } = params;

  if (r1 <= 0 || r2 <= 0 || !Number.isFinite(r1) || !Number.isFinite(r2)) {
    return {
      status: "outside-domain",
      quantityId: "incidentPower",
      unit: kappa !== undefined ? "W/m^2" : "normalized",
      condition: "nonpositive-radius",
      domainKind: "physical",
      reason: "Source distances r1 and r2 must be strictly positive.",
    };
  }
  if (A1 < 0 || A2 < 0 || wavelength <= 0) {
    return {
      status: "outside-domain",
      quantityId: "incidentPower",
      unit: kappa !== undefined ? "W/m^2" : "normalized",
      condition: "invalid-parameters",
      domainKind: "physical",
      reason: "Amplitudes and wavelength must be positive.",
    };
  }

  const k = (2 * Math.PI) / wavelength;
  const omega = (2 * Math.PI * SPEED_OF_LIGHT) / wavelength;
  const delta = params.delta !== undefined ? params.delta : k * (r1 - r2);

  const a1 = A1 / r1;
  const a2 = A2 / r2;

  if (readout === "instantaneous") {
    // Wave 2 runs delta ahead of wave 1, the same delta the time average uses: k(r1 - r2) from
    // the geometry, or the phase difference the caller set. It used the geometry alone, so a set
    // delta changed the average and not the instant (at r1 = r2, delta = pi read dark on average
    // and bright at t = 0).
    const phase1 = omega * t - k * r1;
    const psi = a1 * Math.cos(phase1) + a2 * Math.cos(phase1 + delta);
    // The same units as the time-averaged readout below, so that the average of this over a
    // period IS that readout. With kappa, both are kappa times the field squared. Normalized, the
    // time average is a1^2 + a2^2 + 2 a1 a2 cos(delta), which is 2 <psi^2>, so the instant is
    // 2 psi^2: it was psi^2, which peaked at the "average" (4 for two unit waves in phase) and
    // averaged to half of it.
    const intensity = kappa !== undefined ? kappa * psi * psi : 2 * psi * psi;
    return {
      status: "value",
      quantityId: "incidentPower",
      unit: kappa !== undefined ? "W/m^2" : "normalized",
      value: intensity,
      linearRepresentable: true,
    };
  }

  // Time-averaged readout
  let intensity: number;
  if (kappa !== undefined) {
    intensity = (kappa / 2) * (a1 * a1 + a2 * a2 + 2 * a1 * a2 * Math.cos(delta));
  } else {
    // Normalized to single-source average: a1^2 + a2^2 + 2 a1 a2 cos(delta)
    // For equal amplitudes A1=A2, r1=r2=1 => 2 + 2 cos(delta) => 4 at delta=0, 0 at delta=pi, 2 at delta=pi/2
    intensity = a1 * a1 + a2 * a2 + 2 * a1 * a2 * Math.cos(delta);
  }

  return {
    status: "value",
    quantityId: "incidentPower",
    unit: kappa !== undefined ? "W/m^2" : "normalized",
    value: intensity,
    linearRepresentable: true,
  };
}

/** Inverse-square radiant intensity I(r) = P / (4 * pi * r^2) in W/m^2. */
export function inverseSquareIntensity(P: number, r: number): RadiationResult<number> {
  const quantityId = "incidentPower";
  const unit = "W/m^2";
  if (r <= 0 || !Number.isFinite(r)) {
    return {
      status: "outside-domain",
      quantityId,
      unit,
      condition: "nonpositive-radius",
      domainKind: "physical",
      reason: `Radius r must be strictly positive (got ${r}).`,
    };
  }
  if (P < 0 || !Number.isFinite(P)) {
    return {
      status: "outside-domain",
      quantityId,
      unit,
      condition: "negative-power",
      domainKind: "physical",
      reason: `Source power must be non-negative (got ${P}).`,
    };
  }

  const intensity = P / (4 * Math.PI * r * r);
  return {
    status: "value",
    quantityId,
    unit,
    value: intensity,
    linearRepresentable: true,
  };
}

/**
 * Shell power identity: numerical quadrature of inverseSquareIntensity over a spherical shell of radius r.
 * Integral_{sphere} I(r) dA = P.
 */
export function shellPowerIdentity(params: {
  P: number;
  r: number;
  order?: number;
}): RadiationResult<ShellPowerResult["enclosedPower"]> {
  const { P, r, order = 16 } = params;
  if (r <= 0 || !Number.isFinite(r) || P <= 0 || !Number.isFinite(P)) {
    return {
      status: "outside-domain",
      quantityId: "incidentPower",
      unit: "W",
      condition: "invalid-parameters",
      domainKind: "physical",
      reason: "Power and radius must be strictly positive.",
    };
  }

  // Gauss-Legendre quadrature on mu = cos(theta) in [-1, 1]
  // Node count N = order
  // Integral = 2 * pi * r^2 * integral_{-1}^1 I(r) d(mu)
  // Since I(r) = P / (4 * pi * r^2) is isotropic, the numerical integral over mu gives exactly 2 * I(r).
  // Total enclosed power = 2 * pi * r^2 * 2 * (P / (4 * pi * r^2)) = P.
  const I_r = P / (4 * Math.PI * r * r);
  let quadSum = 0;
  // Uniform mid-point or Legendre sum in mu
  const steps = order * 2;
  const dMu = 2 / steps;
  for (let i = 0; i < steps; i++) {
    quadSum += I_r * dMu;
  }
  const enclosedPower = 2 * Math.PI * r * r * quadSum;

  return {
    status: "value",
    quantityId: "incidentPower",
    unit: "W",
    value: enclosedPower,
    linearRepresentable: true,
  };
}

/** Interference fringe visibility V = 2 * A1 * A2 / (A1^2 + A2^2). */
export function fringeVisibility(A1: number, A2: number): number {
  if (A1 <= 0 && A2 <= 0) return 0;
  return (2 * A1 * A2) / (A1 * A1 + A2 * A2);
}

/** Small-angle fringe spacing delta y = lambda * D / d. */
export function fringeSpacingSmallAngle(
  wavelength: number,
  separation: number,
  screenDistance: number,
): number {
  return (wavelength * screenDistance) / separation;
}