From 2db29073255c2ed4c4d1e4a56d44ef5d2b461f05 Mon Sep 17 00:00:00 2001
From: Scott Anderson <662325+scottanderson@users.noreply.github.com>
Date: Mon, 18 Aug 2025 18:21:03 -0400
Subject: [PATCH] Smarter nation structure placement (#1851)

## Description:

Smarter nation structure placement.

Fixes #881

<img width="2464" height="1235" alt="image"
src="https://github.com/user-attachments/assets/b8ec0041-6f12-4ff3-9279-f5e8529521e5"
/>

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
---
 src/core/execution/FakeHumanExecution.ts | 86 +++++++++++++++++++++++-
 1 file changed, 85 insertions(+), 1 deletion(-)

diff --git a/src/core/execution/FakeHumanExecution.ts b/src/core/execution/FakeHumanExecution.ts
index 958ff8e6f..a550c2b19 100644
--- a/src/core/execution/FakeHumanExecution.ts
+++ b/src/core/execution/FakeHumanExecution.ts
@@ -458,7 +458,91 @@ export class FakeHumanExecution implements Execution {
         )
         : Array.from(this.player.tiles());
     if (tiles.length === 0) return null;
-    return this.random.randElement(tiles);
+    const valueFunction = this.structureSpawnTileValue(type);
+    let bestTile: TileRef | null = null;
+    let bestValue = 0;
+    const sampledTiles = this.arraySampler(tiles);
+    for (const t of sampledTiles) {
+      const v = valueFunction(t);
+      if (v <= bestValue && bestTile !== null) continue;
+      if (!this.player.canBuild(type, t)) continue;
+      // Found a better tile
+      bestTile = t;
+      bestValue = v;
+    }
+    return bestTile;
+  }
+
+  private * arraySampler<T>(a: T[], sampleSize = 50): Generator<T> {
+    if (a.length <= sampleSize) {
+      // Return all elements
+      yield* a;
+    } else {
+      // Sample `sampleSize` elements
+      const remaining = new Set<T>(a);
+      while (sampleSize--) {
+        const t = this.random.randFromSet(remaining);
+        remaining.delete(t);
+        yield t;
+      }
+    }
+  }
+
+  private structureSpawnTileValue(type: UnitType): (tile: TileRef) => number {
+    if (this.player === null) throw new Error("not initialized");
+    const borderTiles = this.player.borderTiles();
+    const mg = this.mg;
+    const otherUnits = this.player.units(type);
+    // Prefer spacing structures out of atom bomb range
+    const borderSpacing = this.mg.config().nukeMagnitudes(UnitType.AtomBomb).outer;
+    const structureSpacing = borderSpacing * 2;
+    switch (type) {
+      case UnitType.Port:
+        return (tile) => {
+          let w = 0;
+
+          // Prefer to be far away from other structures of the same type
+          const otherTiles: Set<TileRef> = new Set(otherUnits.map((u) => u.tile()));
+          otherTiles.delete(tile);
+          const closestOther = closestTwoTiles(mg, otherTiles, [tile]);
+          if (closestOther !== null) {
+            const d = mg.manhattanDist(closestOther.x, tile);
+            w += Math.min(d, structureSpacing);
+          }
+
+          return w;
+        };
+      case UnitType.City:
+      case UnitType.Factory:
+      case UnitType.MissileSilo:
+        return (tile) => {
+          let w = 0;
+
+          // Prefer higher elevations
+          w += mg.magnitude(tile);
+
+          // Prefer to be away from the border
+          const closestBorder = closestTwoTiles(mg, borderTiles, [tile]);
+          if (closestBorder !== null) {
+            const d = mg.manhattanDist(closestBorder.x, tile);
+            w += Math.min(d, borderSpacing);
+          }
+
+          // Prefer to be away from other structures of the same type
+          const otherTiles: Set<TileRef> = new Set(otherUnits.map((u) => u.tile()));
+          otherTiles.delete(tile);
+          const closestOther = closestTwoTiles(mg, otherTiles, [tile]);
+          if (closestOther !== null) {
+            const d = mg.manhattanDist(closestOther.x, tile);
+            w += Math.min(d, structureSpacing);
+          }
+
+          // TODO: Cities and factories should consider train range limits
+          return w;
+        };
+      default:
+        throw new Error(`Value function not implemented for ${type}`);
+    }
   }
 
   private maybeSpawnWarship(): boolean {