1 files changed, 205 insertions, 0 deletions
diff --git a/llparse-builder/src/loop-checker/index.ts b/llparse-builder/src/loop-checker/index.ts
new file mode 100644
index 0000000..5751955
--- /dev/null
+++ b/llparse-builder/src/loop-checker/index.ts
@@ -0,0 +1,205 @@
+import * as assert from 'assert';
+import * as debugAPI from 'debug';
+
+import { Node } from '../node';
+import { Reachability } from '../reachability';
+import { Lattice } from './lattice';
+
+const debug = debugAPI('llparse-builder:loop-checker');
+
+const EMPTY_VALUE = new Lattice('empty');
+const ANY_VALUE = new Lattice('any');
+
+/**
+ * This class implements a loop checker pass. The goal of this pass is to verify
+ * that the graph doesn't contain infinite loops.
+ */
+export class LoopChecker {
+  private readonly lattice: Map<Node, Lattice> = new Map();
+
+  // Just a cache of terminated keys
+  private readonly terminatedCache: Map<Node, Lattice> = new Map();
+
+  /**
+   * Run loop checker pass on a graph starting from `root`.
+   *
+   * Throws on failure.
+   *
+   * @param root  Graph root node
+   */
+  public check(root: Node): void {
+    const r = new Reachability();
+
+    const nodes = r.build(root);
+
+    for (const node of nodes) {
+      debug('checking loops starting from %j', node.name);
+
+      // Set initial lattice value for all nodes
+      this.clear(nodes);
+
+      // Mark root as reachable with any value
+      this.lattice.set(node, ANY_VALUE);
+
+      // Raise lattice values
+      let changed: Set<Node> = new Set([ root ]);
+      while (changed.size !== 0) {
+        if (debug.enabled) {
+          debug('changed %j', Array.from(changed).map((other) => other.name));
+        }
+
+        const next: Set<Node> = new Set();
+        for (const changedNode of changed) {
+          this.propagate(changedNode, next);
+        }
+        changed = next;
+      }
+
+      debug('lattice stabilized');
+
+      // Visit nodes and walk through reachable edges to detect loops
+      this.visit(node, []);
+    }
+  }
+
+  private clear(nodes: ReadonlyArray<Node>): void {
+    for (const node of nodes) {
+      this.lattice.set(node, EMPTY_VALUE);
+    }
+  }
+
+  private propagate(node: Node, changed: Set<Node>): void {
+    let value: Lattice = this.lattice.get(node)!;
+    debug('propagate(%j), initial value %j', node.name, value);
+
+    // Terminate values that are consumed by `match`/`select`
+    const terminated = this.terminate(node, value, changed);
+    if (!terminated.isEqual(EMPTY_VALUE)) {
+      debug('node %j terminates %j', node.name, terminated);
+      value = value.subtract(terminated);
+      if (value.isEqual(EMPTY_VALUE)) {
+        return;
+      }
+    }
+
+    const keysByTarget: Map<Node, Lattice> = new Map();
+    // Propagate value through `.peek()`/`.otherwise()` edges
+    for (const edge of node.getAllEdges()) {
+      if (!edge.noAdvance) {
+        continue;
+      }
+
+      let targetValue: Lattice;
+      if (keysByTarget.has(edge.node)) {
+        targetValue = keysByTarget.get(edge.node)!;
+      } else {
+        targetValue = this.lattice.get(edge.node)!;
+      }
+
+      // `otherwise` or `Invoke`'s edges
+      if (edge.key === undefined || typeof edge.key === 'number') {
+        targetValue = targetValue.union(value);
+      } else {
+        // `.peek()`
+        const edgeValue = new Lattice([ edge.key[0] ]).intersect(value);
+        if (edgeValue.isEqual(EMPTY_VALUE)) {
+          continue;
+        }
+
+        targetValue = targetValue.union(edgeValue);
+      }
+
+      keysByTarget.set(edge.node, targetValue);
+    }
+
+    for (const [ child, childValue ] of keysByTarget) {
+      debug('node %j propagates %j to %j', node.name, childValue,
+        child.name);
+      this.update(child, childValue, changed);
+    }
+  }
+
+  private update(node: Node, newValue: Lattice, changed: Set<Node>): boolean {
+    const value = this.lattice.get(node)!;
+    if (newValue.isEqual(value)) {
+      return false;
+    }
+
+    this.lattice.set(node, newValue);
+    changed.add(node);
+    return true;
+  }
+
+  private terminate(node: Node, value: Lattice, changed: Set<Node>): Lattice {
+    if (this.terminatedCache.has(node)) {
+      return this.terminatedCache.get(node)!;
+    }
+
+    const terminated: number[] = [];
+    for (const edge of node.getAllEdges()) {
+      if (edge.noAdvance) {
+        continue;
+      }
+
+      // Ignore `otherwise` and `Invoke`'s edges
+      if (edge.key === undefined || typeof edge.key === 'number') {
+        continue;
+      }
+
+      terminated.push(edge.key[0]);
+    }
+
+    const result = new Lattice(terminated);
+    this.terminatedCache.set(node, result);
+    return result;
+  }
+
+  private visit(node: Node, path: ReadonlyArray<Node>): void {
+    let value = this.lattice.get(node)!;
+    debug('enter %j, value is %j', node.name, value);
+
+    const terminated = this.terminatedCache.has(node) ?
+      this.terminatedCache.get(node)! : EMPTY_VALUE;
+    if (!terminated.isEqual(EMPTY_VALUE)) {
+      debug('subtract terminated %j', terminated);
+      value = value.subtract(terminated);
+      if (value.isEqual(EMPTY_VALUE)) {
+        debug('terminated everything');
+        return;
+      }
+    }
+
+    for (const edge of node.getAllEdges()) {
+      if (!edge.noAdvance) {
+        continue;
+      }
+
+      let edgeValue = value;
+
+      // `otherwise` or `Invoke`'s edges
+      if (edge.key === undefined || typeof edge.key === 'number') {
+        // nothing to do
+      // `.peek()`
+      } else {
+        edgeValue = edgeValue.intersect(new Lattice([ edge.key[0] ]));
+      }
+
+      // Ignore unreachable edges
+      if (edgeValue.isEqual(EMPTY_VALUE)) {
+        continue;
+      }
+      if (path.indexOf(edge.node) !== -1) {
+        if (path.length === 0) {
+          throw new Error(
+            `Detected loop in "${edge.node.name}" through "${edge.node.name}"`);
+        }
+        throw new Error(
+          `Detected loop in "${edge.node.name}" through chain ` +
+            `${path.map((parent) => '"' + parent.name + '"').join(' -> ')}`);
+      }
+      this.visit(edge.node, path.concat(edge.node));
+    }
+
+    debug('leave %j', node.name);
+  }
+}