// Highlight roll-off, both ends of the pipeline, as one soft knee:
//
//   L' = L                                , L <  T
//   L' = T + (L - T) / (1 + 2 S (L - T))  , L >= T
//
// The develop draws it on the sensor's own levels (T = 0.7, S = 1 / (2 (1 - T)),
// which puts the asymptote on 1.0) so the two stops the sensor holds above its
// white level are COMPRESSED into the frame instead of being thrown away by the
// old fade-to-white; the tone pass draws the same curve in linear light on the
// value the develop and the camera match left, where -HL is the knob (T = 0.5,
// S = |hl|). Before this, a blown sky left the develop on exactly 1.0 in all
// three channels and HL had a flat white to pull on: measured on DSC03453.ARW,
// where the camera's own preview is clipped the develop's luma was 253.4 with a
// standard deviation of 2.4, against 251.2 / 10.0 through the knee.
//
// Both are SkSL, so the shape is pinned on the source; the curve itself is
// checked as arithmetic, with the constants the source is asserted to carry.
//
//   node scripts/highlight-knee-check.mjs
import assert from 'node:assert/strict';
import { readFileSync } from 'node:fs';

const develop = readFileSync(new URL('../src/engine/rawDevelop.ts', import.meta.url), 'utf8');
const tone = readFileSync(new URL('../shared/utils/toneShader.ts', import.meta.url), 'utf8');

// The develop: knee on the sensor's max channel, the channel ratios kept, so the
// hue and the saturation of a blown area survive the pull-down.
const dev = develop.match(/if \(mx > 0\.7\) \{[\s\S]*?\n  \}/)?.[0];
assert.ok(dev, 'the develop knee is gone — a blown sky is flat 1.0 again');
assert.match(dev, /float over = mx - 0\.7;/);
assert.match(dev, /rgb \*= \(0\.7 \+ over \/ \(1\.0 \+ over \* 3\.3333\)\) \/ mx;/);
assert.doesNotMatch(develop, /mix\(rgb \/ mx, float3\(1\.0\)/, 'the fade-to-white is back');

// The tone pass: the knee runs in LINEAR light and before the luma is read, the
// lift keeps its headroom weight — and the recovery must not also ride the
// additive term, which would darken the white the knee protects.
assert.match(tone, /if \(hl < 0\.0\) \{/);
assert.match(tone, /vec3 lin = toLinear\(rgb\);/);
assert.match(tone, /float l0 = dot\(lin, vec3\(0\.2126, 0\.7152, 0\.0722\)\);/);
assert.match(tone, /lin \*= \(0\.5 \+ over \/ \(1\.0 \+ S \* over \* 2\.0\)\) \/ l0;/);
assert.match(tone, /rgb = clamp\(toEncoded\(lin\), 0\.0, 1\.0\);/);
assert.match(tone, /float o = t \+ max\(hl, 0\.0\) \* hlMask \* \(1\.0 - t\) \+ sh \* 0\.34 \* shMask;/);
// The transfer pair has to be the accurate one, or the knee is drawn in a space
// that is not linear at all.
assert.match(tone, /return mix\(c \/ 12\.92, pow\(\(c \+ 0\.055\) \/ 1\.055, vec3\(2\.4\)\), step\(vec3\(0\.04045\), c\)\);/);

// The arithmetic. T = 0.7 / S = 1 / (2 (1 - T)) is the develop's pair (S is what
// puts the asymptote on 1.0: T + 1/(2S) = 1); T = 0.5 with S = 1 is the top of
// the tone pass's knob.
const knee = (l, T, S) => (l < T ? l : T + (l - T) / (1 + 2 * S * (l - T)));

for (const [T, S] of [
  [0.7, 1 / (2 * (1 - 0.7))],
  [0.5, 0.25],
  [0.5, 1],
]) {
  // Below the knee the frame is untouched, and the curve is continuous and C1 at
  // T — slope 1 on both sides — so there is no seam for a later pass to mask.
  assert.equal(knee(T - 0.2, T, S), T - 0.2);
  assert.equal(knee(T, T, S), T);
  const slope = (x) => (knee(x + 1e-6, T, S) - knee(x, T, S)) / 1e-6;
  assert.ok(Math.abs(slope(T) - 1) < 1e-3, `seam at T=${T}: slope ${slope(T)}`);
  // Monotone, and never a brightening: a recovery slider that lifted a highlight
  // would be a lift in disguise, and an inverted pair of pixels is a visible edge.
  let prev = -Infinity;
  for (let l = 0; l <= 2; l += 1 / 512) {
    assert.ok(slope(l) > 0, `inverted at ${l} (T=${T}, S=${S})`);
    assert.ok(knee(l, T, S) <= l + 1e-9, `brightened ${l} -> ${knee(l, T, S)}`);
    assert.ok(knee(l, T, S) >= prev);
    prev = knee(l, T, S);
  }
  // The asymptote: everything the sensor held above the knee lands under it.
  // The develop's pair puts it exactly on 1.0, the tone pass's S = 1 on 0.75.
  assert.ok(Math.abs(knee(1e6, T, S) - (T + 1 / (2 * S))) < 1e-4);
}
assert.ok(Math.abs(0.7 + 1 / (2 * (1 / (2 * (1 - 0.7)))) - 1) < 1e-9, 'the develop plateau left 1.0');
// ...and the same pair in the encoded domain, which is the domain the develop
// hands over: mx = 1.0 (the white level) lands on 237, the sensor's own plateau
// (1.93, see the gain above) on 248 — a ramp of a dozen code values where the
// old fade-to-white left nothing above 250 at all.
const enc = (x) => (x <= 0.0031308 ? x * 12.92 : 1.055 * x ** (1 / 2.4) - 0.055);
const DEV_S = 1 / (2 * (1 - 0.7));
assert.equal(Math.round(enc(knee(1.0, 0.7, DEV_S)) * 255), 237);
assert.equal(Math.round(enc(knee(1.93, 0.7, DEV_S)) * 255), 248);

console.log('highlight-knee-check ok');