Faster roof scan and heat pass with identical results (t21)

- The roof is rescanned only in area columns that changed since the last scan: a quiet Huge
  world goes from 4.60 to 1.88 ms per scan (every 20 ticks).
- The heat pass skips react() for solids and powders below their first reaction threshold:
  Huge world digging 2.54 -> 2.34 ms/tick, quiet 1.39 -> 1.18.
- Every suite's output is identical apart from timing lines; seeds make the same worlds.
- test/t_sim_optimizations.js checks the dirty roof against a full scan and the heat loop
  against the reference loop byte for byte.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/sim/cells.js

@@ -186,6 +186,7 @@ function simMove() {
       for (let k = 0; k < CW; k++) {
         const cx = ltr ? k : CW - 1 - k;
         if (!awake[rowBase + cx]) continue;
+        roofDirty[cx] = 1;
         const x0 = cx << 5, x1 = Math.min(W, x0 + 32);
         // alternate rows scan left to right and right to left, so nothing drifts one way
         const dx = ltr ? 1 : -1, xEnd = ltr ? x1 : x0 - 1;

src/sim/heat.js

@@ -7,6 +7,20 @@ function airflowAt(i) { const a = airS[i]; return a !== 0 && ((curAir() - a) & 0
 const POROUS = new Uint8Array(NMAT); POROUS[ASH] = 1; POROUS[SLAG] = 1; POROUS[COAL] = 1; POROUS[CHARCOAL] = 1;
 const SOIL = new Uint8Array(NMAT); SOIL[DIRT] = 1; SOIL[GRASS] = 1; SOIL[LDIRT] = 1;
 const HOTLIQ = new Uint8Array(NMAT); for (const m of [MAGMA, MCU, MFE, MST, MGL, MAG, MAU, MBZ, MBS, MSLAG]) HOTLIQ[m] = 1;
+// Cold solids and powders cannot react below their first applicable threshold.
+const REACT_MIN = new Float64Array(NMAT).fill(Infinity);
+for (let m = 0; m < NMAT; m++) {
+  if (PH[m] === PH_LIQUID || PH[m] === PH_GAS) REACT_MIN[m] = -Infinity;
+  else {
+    if (IGN[m] !== 0) REACT_MIN[m] = Math.min(REACT_MIN[m], IGN[m]);
+    if (MELT[m] !== 0) REACT_MIN[m] = Math.min(REACT_MIN[m], MELT[m]);
+  }
+}
+REACT_MIN[CLAY] = Math.min(REACT_MIN[CLAY], 880);
+REACT_MIN[LCLAY] = Math.min(REACT_MIN[LCLAY], 880);
+REACT_MIN[WOOD] = Math.min(REACT_MIN[WOOD], 380);
+REACT_MIN[LWOOD] = Math.min(REACT_MIN[LWOOD], 380);
+REACT_MIN[GRASS] = Math.min(REACT_MIN[GRASS], 260);
 // Alloys: a molten additive grain turns neighbouring molten copper into the alloy,
 // and is itself used up after about this many (the ratio: bronze ~1:7 tin, brass ~1:2 zinc).
 const ALLOY = { [MSN]: { into: MBZ, per: 7 }, [MZN]: { into: MBS, per: 2 } };
@@ -51,6 +65,7 @@ function heatPass() {
 function heatChunk(c, sc) {
   const cx = c % CW, cy = (c / CW) | 0;
   const x0 = cx << 5, y0 = cy << 5, x1 = Math.min(W, x0 + 32), y1 = Math.min(H, y0 + 32);
+  roofDirty[cx] = 1;
   const leftCold = cx > 0 && !hot[c - 1], upCold = cy > 0 && !hot[c - CW];
   let maxDev = 0, burning = false;
   for (let y = y0; y < y1; y++) {
@@ -80,7 +95,11 @@ function heatChunk(c, sc) {
       } else rel = RELAX[m];
       t += (amb - t) * rel * sc;
       temp[i] = t;
-      if (m !== EMPTY) { react(i, x, y, m); m = mat[i]; t = temp[i]; }
+      if (m !== EMPTY) {
+        // Test the stored Float32 temperature, just as react() reads it.
+        if ((flags[i] & F_BURN) || !(temp[i] < REACT_MIN[m])) react(i, x, y, m);
+        m = mat[i]; t = temp[i];
+      }
       const dv = t > amb ? t - amb : amb - t;
       if (dv > maxDev) maxDev = dv;
       if (flags[i] & F_BURN) burning = true;

src/sim/ledger.js

@@ -35,7 +35,18 @@ function checkLedger() {
 
 // Topmost non-air cell of each column: air above it can shed heat to the sky.
 function updateRoof() {
-  for (let x = 0; x < W; x++) {
+  scanRoof(0, W);
+  roofDirty.fill(0);
+}
+function updateDirtyRoof() {
+  for (let cx = 0; cx < CW; cx++) {
+    if (!roofDirty[cx]) continue;
+    scanRoof(cx << 5, Math.min(W, (cx + 1) << 5));
+    roofDirty[cx] = 0;
+  }
+}
+function scanRoof(x0, x1) {
+  for (let x = x0; x < x1; x++) {
     let y = 0;
     for (; y < H; y++) { const ph = PH[mat[y * W + x]]; if (ph === PH_SOLID || ph === PH_POWDER || ph === PH_LIQUID) break; }
     roofY[x] = y;

src/sim/tick.js

@@ -3,7 +3,7 @@
 function simTick() {
   tick++;
   tickStamp = (tickStamp % 255) + 1;
-  if (tick % 20 === 1) updateRoof();
+  if (tick % 20 === 1) updateDirtyRoof();
   weatherTick();
   machinesTick();
   pipesTick();

src/sim/world.js

@@ -13,7 +13,7 @@ let ents = []; // loose objects (bars)
 let bgm; // background layer: trees. Liquids, powders and gases pass in front of it.
 let flyers = []; // grains in flight, blown out of the bot
 let ghostBlocks = new Map(), ghostMolds = [];
-let awake, hot, hotLvl, powderArea;
+let awake, hot, hotLvl, powderArea, roofDirty;
 let tick = 0, tickStamp = 1;
 let machines = [null], molds = [], nets = [null], pcells = new Set(), powerDirty = true;
 let beltL = [], beltR = [], tubeLists = [[], [], [], [], []];
@@ -50,6 +50,7 @@ function alloc(w, h) {
   surfY = new Int16Array(w); roofY = new Int16Array(w); dirtY = new Int16Array(w); graniteY = new Int16Array(w); basaltY = new Int16Array(w); magmaY = new Int16Array(w);
   AMB = new Float32Array(h);
   awake = new Uint8Array(NCH); hot = new Uint8Array(NCH); hotLvl = new Uint8Array(NCH);
+  roofDirty = new Uint8Array(CW).fill(1);
   powderArea = new Uint8Array(NCH).fill(1); // unknown areas get one visit before being ruled out
   machines = [null]; molds = []; nets = [null]; pcells = new Set(); powerDirty = true;
   beltL = []; beltR = []; tubeLists = [[], [], [], [], []]; brokenMachines = [];
@@ -85,6 +86,7 @@ function fillNoise() {
 // ---------- Waking ----------
 function wakeXY(x, y) {
   const cx = x >> 5, cy = y >> 5, c = cy * CW + cx;
+  roofDirty[cx] = 1;
   awake[c] = 3;
   const lx = x & 31, ly = y & 31;
   if (lx === 0) { if (cx > 0) awake[c - 1] = 3; } else if (lx === 31 && cx < CW - 1) awake[c + 1] = 3;
@@ -92,6 +94,7 @@ function wakeXY(x, y) {
 }
 function wakeHeatXY(x, y) {
   const c = (y >> 5) * CW + (x >> 5);
+  roofDirty[x >> 5] = 1;
   hot[c] = 6; hotLvl[c] = 2;
 }
 function wakeIdx(i) { wakeXY(i % W, (i / W) | 0); }