availableZ candidate rejection for 3 and 4 layer

Author: seveibar

lib/computeRegionCost.ts

@@ -3,8 +3,9 @@ const viaSizeSq = viaSize ** 2
 const traceWidth = 0.1
 const IMPOSSIBLE_SINGLE_LAYER_INTERSECTION_COST = 10
 
-const isKnownSingleLayerMask = (regionAvailableZMask: number) =>
-  regionAvailableZMask === 1 || regionAvailableZMask === 2
+export const isKnownSingleLayerMask = (regionAvailableZMask: number) =>
+  regionAvailableZMask > 0 &&
+  (regionAvailableZMask & (regionAvailableZMask - 1)) === 0
 
 export const computeRegionCost = (
   regionWidth: number,

lib/core.ts

@@ -1,7 +1,7 @@
 import { BaseSolver } from "@tscircuit/solver-utils"
 import type { GraphicsObject } from "graphics-debug"
 import { convertToSerializedHyperGraph } from "./compat/convertToSerializedHyperGraph"
-import { computeRegionCost } from "./computeRegionCost"
+import { computeRegionCost, isKnownSingleLayerMask } from "./computeRegionCost"
 import { countNewIntersectionsWithValues } from "./countNewIntersections"
 import { MinHeap } from "./MinHeap"
 import { shuffle } from "./shuffle"
@@ -513,7 +513,7 @@ export class TinyHyperGraphSolver extends BaseSolver {
 
   isKnownSingleLayerRegion(regionId: RegionId): boolean {
     const regionAvailableZMask = this.topology.regionAvailableZMask?.[regionId] ?? 0
-    return regionAvailableZMask === 1 || regionAvailableZMask === 2
+    return isKnownSingleLayerMask(regionAvailableZMask)
   }
 
   populateSegmentGeometryScratch(

tests/solver/single-layer-region-cost.test.ts

@@ -4,6 +4,10 @@ import {
   TinyHyperGraphSolver,
   type TinyHyperGraphTopology,
 } from "lib/index"
+import {
+  computeRegionCost,
+  isKnownSingleLayerMask,
+} from "lib/computeRegionCost"
 
 const createTopology = (
   regionAvailableZMask: number,
@@ -64,9 +68,55 @@ const getCrossingCost = (regionAvailableZMask: number, portZ: number) => {
 test("same-layer crossings in known single-layer regions are rejected as candidates", () => {
   const topLayerCrossingCost = getCrossingCost(1 << 0, 0)
   const bottomLayerCrossingCost = getCrossingCost(1 << 1, 1)
+  const innerLayer2CrossingCost = getCrossingCost(1 << 2, 2)
+  const innerLayer3CrossingCost = getCrossingCost(1 << 3, 3)
   const multiLayerCrossingCost = getCrossingCost((1 << 0) | (1 << 1), 0)
 
   expect(topLayerCrossingCost).toBe(Number.POSITIVE_INFINITY)
   expect(bottomLayerCrossingCost).toBe(Number.POSITIVE_INFINITY)
+  expect(innerLayer2CrossingCost).toBe(Number.POSITIVE_INFINITY)
+  expect(innerLayer3CrossingCost).toBe(Number.POSITIVE_INFINITY)
   expect(multiLayerCrossingCost).toBeLessThan(0.1)
 })
+
+test("single-bit availableZ masks are all treated as known single-layer regions", () => {
+  expect(isKnownSingleLayerMask(1 << 0)).toBe(true)
+  expect(isKnownSingleLayerMask(1 << 1)).toBe(true)
+  expect(isKnownSingleLayerMask(1 << 2)).toBe(true)
+  expect(isKnownSingleLayerMask(1 << 3)).toBe(true)
+  expect(isKnownSingleLayerMask((1 << 0) | (1 << 1))).toBe(false)
+  expect(isKnownSingleLayerMask(0)).toBe(false)
+})
+
+test("same-layer crossings incur the impossible single-layer penalty for higher routed layers too", () => {
+  const width = 3
+  const height = 3
+  const sameLayerIntersections = 1
+  const crossLayerIntersections = 0
+  const entryExitChanges = 0
+  const traceCount = 2
+
+  expect(
+    computeRegionCost(
+      width,
+      height,
+      sameLayerIntersections,
+      crossLayerIntersections,
+      entryExitChanges,
+      traceCount,
+      1 << 2,
+    ),
+  ).toBeGreaterThan(10)
+
+  expect(
+    computeRegionCost(
+      width,
+      height,
+      sameLayerIntersections,
+      crossLayerIntersections,
+      entryExitChanges,
+      traceCount,
+      1 << 3,
+    ),
+  ).toBeGreaterThan(10)
+})