Add USE_WORKER_THREADS environment var

Also implement currently unused solveSync and generateSync methods,
useful for testing/debugging.

Add static get() to SudokuMath which returns cached SudokuMath
instances.

src/graphql/sudoku.ts

@@ -1,5 +1,5 @@
 import { gql } from "../mods.js";
-import { generate, GenerateArguments } from "../sudoku/index.js";
+import { Sudoku, GenerateArguments, SolveArguments } from "../sudoku/index.js";
 
 export const typeDefs = gql`
   """
@@ -22,16 +22,22 @@ export const typeDefs = gql`
   type Query {
     "Generates a new sudoku."
     generate(regionWidth: Int = 3, regionHeight: Int = 3, clues: Int!): Sudoku!
+
+    "Solves the given sudoku."
+    solve(regionWidth: Int = 3, regionHeight: Int = 3, cells: [Int!]!): Sudoku!
   }
 `;
 
+interface SudokuFuncs {
+  solve(args: SolveArguments): Promise<Sudoku> | Sudoku;
+  generate(args: GenerateArguments): Promise<Sudoku> | Sudoku;
+}
+
 export const resolvers = {
   Query: {
-    generate: (
-      obj: any,
-      { regionWidth, regionHeight, clues }: GenerateArguments
-    ) => {
-      return generate(regionWidth, regionHeight, clues);
-    },
+    generate: (obj: any, args: GenerateArguments, ctx: SudokuFuncs) =>
+      ctx.generate(args),
+    solve: (obj: any, args: SolveArguments, ctx: SudokuFuncs) =>
+      ctx.solve(args),
   },
 };

src/main.ts

@@ -1,6 +1,13 @@
 import { Application, bodyParser } from "./mods.js";
 import { applyGraphQL } from "./graphql.js";
 import { typeDefs, resolvers } from "./graphql/index.js";
+import {
+  initializeWorkers,
+  solve,
+  generate,
+  solveSync,
+  generateSync,
+} from "./sudoku/index.js";
 import stoppable from "stoppable";
 
 import cors from "@koa/cors";
@@ -31,10 +38,25 @@ async function main() {
     );
   });
 
+  let sudokuFuncs: any;
+  if (process.env.USE_WORKER_THREADS) {
+    initializeWorkers();
+    sudokuFuncs = {
+      solve,
+      generate,
+    };
+  } else {
+    sudokuFuncs = {
+      solve: solveSync,
+      generate: generateSync,
+    };
+  }
+
   applyGraphQL({
     app,
     typeDefs: typeDefs,
     resolvers: resolvers,
+    context: () => sudokuFuncs,
   });
 
   runtime.server = stoppable(

src/sudoku/dlx.ts

@@ -114,10 +114,10 @@ export class DLX {
 
     let row = c as DNode;
     while ((row = row.down) !== c) {
-      // traverse DOWN the rows this column contained
+      // traverse down the rows this column
       let col = row;
       while ((col = col.right) !== row) {
-        // traverse the columns of this row (to the RIGHT)
+        // traverse the columns of this row to the right
 
         // remove this node from its column, and shrink its column's size
         this.updates++;

src/sudoku/index.ts

@@ -1,7 +1,9 @@
 import { spawn, Thread, Worker } from "threads";
+import type { ModuleThread } from "threads";
+import { SudokuMath } from "./math.js";
+import { prettyPrint } from "./util.js";
 
 import WORKERS from "physical-cpu-count";
-import { prettyPrint } from "./util.js";
 
 const TIMEOUT = 20000;
 
@@ -13,6 +15,12 @@ export type GenerateArguments = {
   clues: number;
 };
 
+export type SolveArguments = {
+  regionWidth: number;
+  regionHeight: number;
+  cells: Cell[];
+};
+
 export type Sudoku = {
   regionWidth: number;
   regionHeight: number;
@@ -20,19 +28,41 @@ export type Sudoku = {
   cells: Cell[];
 };
 
-function getWorker() {
-  return spawn(new Worker("./worker"));
+type SudokuWorker = {
+  generate: (
+    regionWidth: number,
+    regionHeight: number,
+    clues: number
+  ) => Promise<number[]>;
+  solve: (
+    regionWidth: number,
+    regionHeight: number,
+    cells: number[]
+  ) => Promise<[boolean, number[]]>;
+};
+
+const available: ModuleThread<SudokuWorker>[] = [];
+
+function spawnWorker() {
+  spawn<SudokuWorker>(new Worker("./worker")).then((worker) =>
+    available.push(worker)
+  );
 }
 
-const available: any = [];
-
-function initialize() {
+export function initializeWorkers() {
   console.log(`Starting ${WORKERS} worker threads`);
   for (let n = 0; n < WORKERS; n++) {
-    getWorker().then((worker) => available.push(worker));
+    spawnWorker();
   }
 }
-initialize();
+
+function pickWorker() {
+  const proxy = available.pop();
+  if (!proxy) {
+    throw new Error("No workers available right now. Please try again.");
+  }
+  return proxy;
+}
 
 /**
  * Awaits a promise with a timeout.
@@ -52,27 +82,33 @@ function withTimeout<T>(promise: Promise<T>, ms: number, cb: () => any) {
   }).finally(() => clearTimeout(timeout!));
 }
 
-export async function generate(
-  regionWidth: number,
-  regionHeight: number,
-  clues: number
-): Promise<Sudoku> {
-  const proxy = available.pop();
-  if (!proxy) {
-    throw new Error("No workers available right now. Please try again.");
-  }
-
-  const puzzle = await withTimeout<number[]>(
-    proxy.generate(regionWidth, regionHeight, clues),
-    TIMEOUT,
-    () => {
-      Thread.terminate(proxy);
-      getWorker().then((worker) => available.push(worker));
-      return new Error("Timed out. Try reducing the number of clues.");
+function workerTaskWithTimeout(
+  task: (worker: ModuleThread) => Promise<any>,
+  timeout: number
+) {
+  const worker = pickWorker();
+  let timedOut = false;
+  return withTimeout(task(worker), timeout, () => {
+    timedOut = true;
+    Thread.terminate(worker);
+    spawnWorker();
+    return new Error("Timed out.");
+  }).finally(() => {
+    if (!timedOut) {
+      available.push(worker);
     }
-  );
+  });
+}
 
-  available.push(proxy);
+export async function generate({
+  regionWidth,
+  regionHeight,
+  clues,
+}: GenerateArguments): Promise<Sudoku> {
+  const puzzle = await workerTaskWithTimeout(
+    (worker) => worker.generate(regionWidth, regionHeight, clues),
+    TIMEOUT
+  );
   prettyPrint(regionWidth, regionHeight, puzzle);
   return {
     regionWidth,
@@ -81,3 +117,67 @@ export async function generate(
     cells: puzzle,
   };
 }
+
+export function generateSync({
+  regionWidth,
+  regionHeight,
+  clues,
+}: GenerateArguments): Sudoku {
+  const math = SudokuMath.get(regionWidth, regionHeight);
+  const cells = math.generate(clues, 1, TIMEOUT);
+  prettyPrint(regionWidth, regionHeight, cells);
+  return {
+    regionWidth,
+    regionHeight,
+    size: (regionWidth * regionHeight) ** 2,
+    cells,
+  };
+}
+
+export async function solve({
+  regionWidth,
+  regionHeight,
+  cells,
+}: SolveArguments): Promise<Sudoku> {
+  const size = (regionWidth * regionHeight) ** 2;
+  if (size !== cells.length) {
+    throw new Error(
+      "The given region dimensions do not align with the number of cells."
+    );
+  }
+  const [solved, result] = await workerTaskWithTimeout(
+    (proxy) => proxy.solve(regionWidth, regionHeight, cells),
+    TIMEOUT
+  );
+  if (!solved) {
+    throw new Error("The given puzzle has no solution.");
+  }
+  return {
+    regionWidth,
+    regionHeight,
+    size,
+    cells: result,
+  };
+}
+
+export function solveSync({
+  regionWidth,
+  regionHeight,
+  cells,
+}: SolveArguments): Sudoku {
+  const size = (regionWidth * regionHeight) ** 2;
+  if (size !== cells.length) {
+    throw new Error(
+      "The given region dimensions do not align with the number of cells."
+    );
+  }
+  if (!SudokuMath.get(regionWidth, regionHeight).solve(cells)) {
+    throw new Error("The given puzzle has no solution.");
+  }
+  return {
+    regionWidth,
+    regionHeight,
+    size,
+    cells,
+  };
+}

src/sudoku/math.ts

@@ -141,27 +141,28 @@ export class SudokuMath {
     return [...firstRow, ...Array(this.values2 - this.values).fill(0)];
   }
 
-  generateComplete() {
+  generateComplete(): [number[], number] {
     const result = this._baseBoard();
     const [header] = this.getDLXHeader(result, true);
-
-    const callback = (solution: DNode[]) => {
+    const dlx = new DLX(header, (solution: DNode[]) => {
       solution.forEach((node) => {
         const meta: NodeMeta = node.meta;
         result[meta.index] = meta.value;
       });
-      // return the first solution
+      // stop after the first solution
       return true;
-    };
-
-    const dlx = new DLX(header, callback);
+    });
 
     dlx.search();
-    return result;
+    return [result, dlx.updates];
   }
 
   generate(clues: number, attempts = Infinity, totalTime = Infinity) {
-    const completed = this.generateComplete();
+    if (clues === 0) {
+      return Array(this.values2).fill(0);
+    }
+
+    let [completed, updates] = this.generateComplete();
 
     const [header, dlxRows] = this.getDLXHeader(); // complete header - no candidates removed
 
@@ -176,61 +177,93 @@ export class SudokuMath {
       candidates[meta.index][meta.value - 1] = node;
     });
 
-    // board positions which have been removed
-    const removed = new Set<number>();
-    const masked: DNode[] = [];
-
     const hasOneSolution = () => {
       solutions = 0;
       dlx.search();
+      updates += dlx.updates;
       return solutions === 1;
     };
 
-    const mask = () => {
-      // mask all DLX rows which are nullified by existing values
-      for (let n = 0; n < this.values2; n++) {
-        if (removed.has(n)) {
-          continue;
-        }
-        const nodes = candidates[n];
-        nodes.forEach((node) => {
-          if (node.meta.value !== completed[n]) {
-            masked.push(node);
-            maskRow(node);
-          }
-        });
-      }
-    };
-
-    const unmask = () => {
-      // unmask all DLX rows
-      while (masked.length > 0) {
-        unmaskRow(masked.pop()!);
-      }
-    };
-
     const start = Date.now();
     const elapsed = () => Date.now() - start;
 
+    // masked rows in the order they were masked
+    const masked: DNode[] = [];
+
+    const maskAtIdx = (idx: number) => {
+      const nodes = candidates[idx];
+      nodes.forEach((node) => {
+        if (node.meta.value !== completed[idx]) {
+          masked.push(node);
+          maskRow(node);
+        }
+      });
+    };
+
+    const removed = new Set<number>();
     const removeable = Array.from(this.indexes);
+
+    const maskUpto = (n: number) => {
+      for (let j = 0; j < n; j++) {
+        // process up to what we've handled
+        const idx = removeable[j];
+        if (removed.has(idx)) {
+          // if we've removed this cell, do not mask it
+          continue;
+        }
+        // otherwise, we had given up it; mask it leaving the original value
+        // we won't try this cell again
+        maskAtIdx(idx);
+      }
+
+      for (let j = this.values2 - 1; j >= n; j--) {
+        // for all those we haven't handled, mask leaving the original values
+        // for us to to attempt to unmask one cell at a time
+        maskAtIdx(removeable[j]);
+      }
+    };
+
+    const unmaskAll = () => {
+      let node;
+      while ((node = masked.pop())) {
+        unmaskRow(node);
+      }
+    };
+
+    const unmaskNextCell = () => {
+      for (let j = 0; j < this.values - 1; j++) {
+        unmaskRow(masked.pop()!);
+      }
+    };
+
+    let best = 0;
     const attempt = () => {
-      // attempt remove cells until 'clues' cells remain
+      // attempt to remove cells until 'clues' cells remain
+      attempts--;
+      removed.clear();
       shuffle(removeable);
-      for (let n = 0; n < this.values2; n++) {
+      maskUpto(0);
+
+      for (let i = 0; i < this.values2; i++) {
         if (elapsed() > totalTime || this.values2 - removed.size == clues) {
           break;
         }
-
-        let toRemove = removeable[n];
-        removed.add(toRemove);
-        mask();
+        unmaskNextCell();
 
         if (!hasOneSolution()) {
-          removed.delete(toRemove);
+          // failed attempt. prepare for next
+          unmaskAll();
+          maskUpto(i + 1);
+          continue;
         }
 
-        unmask();
+        removed.add(removeable[i]);
       }
+      if (removed.size > best) {
+        best = removed.size;
+      }
+
+      unmaskAll();
     };
 
     while (
@@ -240,8 +273,6 @@ export class SudokuMath {
     ) {
       // try to reach the clue goal up to `attempts` times or as long as
       // elapsed time is less than `totalTime`
-      attempts--;
-      removed.clear();
       attempt();
     }
 
@@ -251,16 +282,28 @@ export class SudokuMath {
     return completed;
   }
 
-  solve(existing: Cell[]): void {
-    const [header] = this.getDLXHeader(existing);
+  solve(puzzle: Cell[]) {
+    const [header] = this.getDLXHeader(puzzle);
+    let solved = false;
     const callback = (solution: DNode[]) => {
       solution.forEach((node) => {
         const meta: NodeMeta = node.meta;
-        existing[meta.index] = meta.value;
+        puzzle[meta.index] = meta.value;
       });
+      solved = true;
       // return the first solution
       return true;
     };
     new DLX(header, callback).search();
+    return solved;
+  }
+
+  static maths: { [key: string]: SudokuMath } = {};
+  static get(regionWidth: number, regionHeight: number) {
+    const key = `${regionWidth}:${regionHeight}`;
+    return (
+      SudokuMath.maths[key] ??
+      (SudokuMath.maths[key] = new SudokuMath(regionWidth, regionHeight))
+    );
   }
 }

src/sudoku/worker.js

@@ -1,16 +1,12 @@
 import { SudokuMath } from "./math.js";
 import { expose } from "threads/worker";
 
-const maths = {};
-
 expose({
   generate(regionWidth, regionHeight, clues) {
-    const math =
-      maths[`${regionWidth}:${regionHeight}`] ||
-      (maths[`${regionWidth}:${regionHeight}`] = new SudokuMath(
-        regionWidth,
-        regionHeight
-      ));
-    return math.generate(clues);
+    return SudokuMath.get(regionWidth, regionHeight).generate(clues);
+  },
+  solve(regionWidth, regionHeight, cells) {
+    const result = SudokuMath.get(regionWidth, regionHeight).solve(cells);
+    return [result, cells];
   },
 });