3 files changed, 66 insertions, 9 deletions
diff --git a/src/simulation/validity.js b/src/simulation/validity.js
new file mode 100644
index 0000000..de4acd1
--- /dev/null
+++ b/src/simulation/validity.js
@@ -0,0 +1,14 @@
+export const validity = [
+  // prevent agents from moving onto immutables
+  (world, proposal) => (proposal.agent_changes || []).reduce(
+    (acc, change) => {
+      const {x, y} = change;
+      if (x !== undefined && y !== undefined && world.lattice[y][x].type === 'immutable') {
+        return false;
+      } else {
+        return acc;
+      }
+    },
+    true,
+  ),
+];
diff --git a/src/simulation/validity.test.js b/src/simulation/validity.test.js
new file mode 100644
index 0000000..ba9e684
--- /dev/null
+++ b/src/simulation/validity.test.js
@@ -0,0 +1,43 @@
+'use strict';
+
+import { world_update } from '../world/world.js';
+import { validity } from './validity.js';
+
+test("agents are not allowed to move into immutables", () => {
+  const actions = [{
+    size: 1, propose: (world, agent, head) => {
+      return [{
+        agent_changes: [{
+          agent_id: agent.id,
+          x: agent.x + 1, y: agent.y,
+        }],
+      }];
+    },
+  }];
+
+  const agent = {
+    id: 1,
+    net: { compute: () => [[1], null] },
+    state: null,
+    x: 0, y: 0,
+    flags: {},
+  };
+
+  const lattice = [[{ type: 'empty', flags: {} }, { type: 'immutable', flags: {} }]];
+  
+  const world = {
+    lattice,
+    lattice_rules: { empty: ()=>{}, immutable: ()=>{} },
+    agents: [agent],
+    senses: [],
+    actions,
+    validity,
+  };
+
+  expect(world_update(world).agents[0]).toEqual(agent);
+  world.validity = [];
+  expect(world_update(world).agents[0]).toEqual({
+    ...agent,
+    x: 1, y: 0,
+  });
+});
diff --git a/src/world/world.js b/src/world/world.js
index e1ba0cf..4bab4d3 100644
--- a/src/world/world.js
+++ b/src/world/world.js
@@ -19,21 +19,21 @@ export function world_update(world, postprocess=[]) {
   const intermediate_lattice = lattice_apply(world.lattice, lattice_props);
 
   const decisions = world.agents
-    .map(a => agent_decide(world, agent, world.senses, world.actions))
+    .map(a => agent_decide(world, a, world.senses, world.actions))
     .reduce(
       ([agents, props], [agent, prop]) => [[...agents, agent], [...props, prop]],
       [[], []]
     );
   const intermediate_agents = decisions[0];
-  const agent_props = decisions[1]
-    .flat()
-    .reduce((acc, prop) => proposal_merge(acc, prop), [])
-    .filter(prop => lattice_valid(intermediate_lattice, prop))
+  const agent_props = world.validity.reduce(
+    (acc, rule) => acc.filter(prop => rule({...world, lattice: intermediate_lattice}, prop)),
+    decisions[1]
+      .flat()
+      .reduce((acc, prop) => proposal_merge(acc, prop), [])
+      .filter(prop => lattice_valid(intermediate_lattice, prop))
+  );
 
-  const lattice = lattice_apply(intermediate_lattice, world.validity.reduce(
-    (acc, rule) => acc.filter(rule),
-    agent_props
-  ));
+  const lattice = lattice_apply(intermediate_lattice, agent_props);
   const agents = intermediate_agents.map(a => agent_apply(a, agent_props));
 
   const new_world = {...world, lattice, agents};