feat: System priority

app/ecs-systems/index.js

@@ -1,3 +1,6 @@
+import System from '@/ecs/system.js';
 import gather from '@/util/gather.js';
 
-export default gather(import.meta.glob('./*.js', {eager: true, import: 'default'}));
+const gathered = gather(import.meta.glob('./*.js', {eager: true, import: 'default'}));
+
+export default System.sort(gathered);

app/ecs/system.js

@@ -1,3 +1,4 @@
+import Digraph from '@/util/digraph.js';
 
 import Query from './query.js';
 
@@ -54,6 +55,12 @@ export default class System {
     this.ecs.insertMany(components);
   }
 
+  static get priority() {
+    return {
+      phase: 'normal',
+    }
+  }
+
   static queries() {
     return {};
   }
@@ -76,6 +83,38 @@ export default class System {
     return this.queries[query].select();
   }
 
+  static sort(Systems) {
+    const phases = {
+      'pre': new Digraph(),
+      'normal': new Digraph(),
+      'post': new Digraph(),
+    };
+    for (const systemName in Systems) {
+      const {priority} = Systems[systemName];
+      const phase = phases[priority.phase || 'normal'];
+      phase.ensureTail(systemName);
+      if (priority.before) {
+        for (const before of Array.isArray(priority.before) ? priority.before : [priority.before]) {
+          phase.addDependency(before, systemName);
+        }
+      }
+      if (priority.after) {
+        for (const after of Array.isArray(priority.after) ? priority.after : [priority.after]) {
+          phase.addDependency(systemName, after);
+        }
+      }
+    }
+    const sorted = [
+      ...phases['pre'].sort(),
+      ...phases['normal'].sort(),
+      ...phases['post'].sort(),
+    ];
+    return Object.fromEntries(
+      Object.entries(Systems)
+        .toSorted(([l], [r]) => sorted.indexOf(l) - sorted.indexOf(r)),
+    );
+  }
+
   tickDestruction() {
     this.deindex(this.destroying);
     this.destroying = [];

app/util/digraph.js

@@ -0,0 +1,87 @@
+export default class Digraph {
+
+  arcs = new Map();
+
+  addDependency(head, tail) {
+    this.ensureTail(head);
+    this.ensureTail(tail).add(head);
+  }
+
+  detectCycles() {
+    const cycles = [];
+    const visited = new Set();
+    const walking = new Set();
+    const walk = (vertex) => {
+      if (!visited.has(vertex)) {
+        visited.add(vertex);
+        walking.add(vertex);
+        const it = this.neighbors(vertex);
+        for (let current = it.next(); true !== current.done; current = it.next()) {
+          const {value: neighbor} = current;
+          if (!visited.has(neighbor)) {
+            walk(neighbor);
+          }
+          else if (walking.has(neighbor)) {
+            cycles.push([vertex, neighbor]);
+          }
+        }
+      }
+      walking.delete(vertex);
+    };
+    const {tails} = this;
+    for (let current = tails.next(); true !== current.done; current = tails.next()) {
+      walk(current.value);
+    }
+    return cycles;
+  }
+
+  ensureTail(tail) {
+    if (!this.arcs.has(tail)) {
+      this.arcs.set(tail, new Set());
+    }
+    return this.arcs.get(tail);
+  }
+
+  neighbors(vertex) {
+    return this.arcs.get(vertex).values();
+  }
+
+  sort() {
+    const visited = new Set();
+    const scores = new Map();
+    const walk = (vertex, score) => {
+      visited.add(vertex);
+      const neighbors = this.neighbors(vertex);
+      for (let current = neighbors.next(); true !== current.done; current = neighbors.next()) {
+        const {value: neighbor} = current;
+        if (!visited.has(neighbor)) {
+          score = walk(neighbor, score);
+        }
+      }
+      scores.set(vertex, score);
+      return score - 1;
+    };
+    let score = this.arcs.size - 1;
+    const {tails} = this;
+    for (let current = tails.next(); true !== current.done; current = tails.next()) {
+      const {value: vertex} = current;
+      if (!visited.has(vertex)) {
+        score = walk(vertex, score);
+      }
+    }
+    return Array.from(scores.entries())
+      .sort(([, l], [, r]) => l - r)
+      .map(([vertex]) => vertex);
+  }
+
+  removeDependency(head, tail) {
+    if (this.arcs.has(tail)) {
+      this.arcs.get(tail).delete(head);
+    }
+  }
+
+  get tails() {
+    return this.arcs.keys();
+  }
+
+}