🔨 rename package compute -> solver

packages/solver/getPathTo.ts

@@ -0,0 +1,66 @@
+import { isInsideLarge, getColor, isInside, isEmpty } from "@snk/types/grid";
+import { around4 } from "@snk/types/point";
+import {
+  getHeadX,
+  getHeadY,
+  nextSnake,
+  snakeEquals,
+  snakeWillSelfCollide,
+} from "@snk/types/snake";
+import { sortPush } from "./utils/sortPush";
+import type { Snake } from "@snk/types/snake";
+import type { Grid } from "@snk/types/grid";
+
+type M = { parent: M | null; snake: Snake; w: number; h: number; f: number };
+
+/**
+ * starting from snake0, get to the cell x,y
+ * return the snake chain (reversed)
+ */
+export const getPathTo = (grid: Grid, snake0: Snake, x: number, y: number) => {
+  const openList: M[] = [{ snake: snake0, w: 0 } as any];
+  const closeList: Snake[] = [];
+
+  while (openList.length) {
+    const c = openList.shift()!;
+
+    const cx = getHeadX(c.snake);
+    const cy = getHeadY(c.snake);
+
+    for (let i = 0; i < around4.length; i++) {
+      const { x: dx, y: dy } = around4[i];
+
+      const nx = cx + dx;
+      const ny = cy + dy;
+
+      if (nx === x && ny === y) {
+        // unwrap
+        const path = [nextSnake(c.snake, dx, dy)];
+        let e: M["parent"] = c;
+        while (e.parent) {
+          path.push(e.snake);
+          e = e.parent;
+        }
+        return path;
+      }
+
+      if (
+        isInsideLarge(grid, 2, nx, ny) &&
+        !snakeWillSelfCollide(c.snake, dx, dy) &&
+        (!isInside(grid, nx, ny) || isEmpty(getColor(grid, nx, ny)))
+      ) {
+        const nsnake = nextSnake(c.snake, dx, dy);
+
+        if (!closeList.some((s) => snakeEquals(nsnake, s))) {
+          const w = c.w + 1;
+          const h = Math.abs(nx - x) + Math.abs(ny - y);
+          const f = w + h;
+          const o = { snake: nsnake, parent: c, w, h, f };
+
+          sortPush(openList, o, (a, b) => a.f - b.f);
+          closeList.push(nsnake);
+        }
+      }
+    }
+  }
+};