3 files changed, 105 insertions, 0 deletions
diff --git a/src/world/cell.js b/src/world/cell.js
new file mode 100644
index 0000000..5783d72
--- /dev/null
+++ b/src/world/cell.js
@@ -0,0 +1,41 @@
+'use strict';
+
+export function cells_update(cells, update_rules) {
+  return cells
+    .map((row, y) => row.map((cell, x) => [x, y, cell.type]))
+    .flat()
+    .reduce((acc, [x, y, type]) => [...acc, update_rules[type](cells, x, y)], [])
+    .filter(x => x !== undefined)
+}
+
+
+export function cells_valid(cells, proposal) {
+  if (!proposal.world_updates) { return true; }
+  return proposal.world_updates.reduce(
+    (acc, update) => {
+      const valid = 
+        (update.x >= 0 && update.x < cells[0].length) &&
+        (update.y >= 0 && update.y < cells.length) &&
+        (cells[update.y][update.x].type == update.from)
+        return valid && acc;
+    },
+    true
+  );
+}
+
+
+export function cells_apply(cells, proposals) {
+  return proposals.reduce(
+    (acc, prop) => {
+      const change = (prop.world_updates || []).reduce(
+        (acc_, update) => {
+          acc_[update.y][update.x].type = update.to;
+          return acc_
+        },
+        [...acc]
+      );
+      return change;
+    },
+    [...cells]
+  );
+}
diff --git a/src/world/cell.test.js b/src/world/cell.test.js
new file mode 100644
index 0000000..a3749e5
--- /dev/null
+++ b/src/world/cell.test.js
@@ -0,0 +1,56 @@
+'use strict';
+
+import { cells_update, cells_valid, cells_apply } from './cell.js';
+
+
+test("growth update rule", () => {
+  const cells = [[ { type: 'empty' }, { type: 'empty' }, { type: 'plant' } ]];
+  const update_rules = {
+    plant: () => {},
+    empty: (cells, x, y) => {
+      if (cells[y][x+1].type === 'plant') {
+        return { world_updates: [{ x, y, from: 'empty', to: 'plant' }] };
+      }
+    },
+  };
+
+  expect(cells_update(cells, update_rules)).toEqual([
+    { world_updates: [{ x: 1, y: 0, from: 'empty', to: 'plant' }] },
+  ]);
+
+  cells[0][1] = { type: 'plant' };
+
+  expect(cells_update(cells, update_rules)).toEqual([
+    { world_updates: [{ x: 0, y: 0, from: 'empty', to: 'plant' }] },
+  ]);
+
+  cells[0][0] = { type: 'plant' };
+
+  expect(cells_update(cells, update_rules)).toEqual([]);
+});
+
+
+test("growth update rule applied", () => {
+  const cells = [[ { type: 'empty' }, { type: 'empty' }, { type: 'plant' } ]];
+  expect(cells_apply(cells, [{ world_updates:[{ x: 1, y: 0, from: 'empty', to: 'plant' }]}])).toEqual([
+    [ { type: 'empty' }, { type: 'plant' }, { type: 'plant' } ],
+  ]);
+
+  expect(cells_apply(cells, [
+    { world_updates: [{ x: 2, y: 0, from: 'plant', to: 'empty' } ]},
+    { world_updates: [{ x: 1, y: 0, from: 'empty', to: 'plant' } ]},
+    { world_updates: [{ x: 0, y: 0, from: 'empty', to: 'plant' } ]},
+  ])).toEqual([
+    [ { type: 'plant' }, { type: 'plant' }, { type: 'empty' } ],
+  ]);
+});
+
+
+test("check proposals agains cells for validity", () => {
+  const cells = [[ { type: 'empty' }, { type: 'empty' }, { type: 'plant' } ]];
+  expect(cells_valid(cells, { world_updates: [{ x: -1, y: 0, from: 'blah', to: 'blah' }] })).toBe(false);
+  expect(cells_valid(cells, { world_updates: [{ x: 0, y: 0, from: 'blah', to: 'blah' }] })).toBe(false);
+  expect(cells_valid(cells, { world_updates: [{ x: 0, y: 0, from: 'empty', to: 'blah' }] })).toBe(true);
+  expect(cells_valid(cells, { world_updates: [{ x: 2, y: 0, from: 'empty', to: 'blah' }] })).toBe(false);
+  expect(cells_valid(cells, { world_updates: [{ x: 2, y: 1, from: 'empty', to: 'blah' }] })).toBe(false);
+});
diff --git a/src/world/proposal.js b/src/world/proposal.js
index d1f2eb0..775466c 100644
--- a/src/world/proposal.js
+++ b/src/world/proposal.js
@@ -10,6 +10,14 @@ import { pairs, deepEqual } from '../util.js';
  * }
  */
 
+
+/* cell structure:
+ * {
+ *   type: string
+ *   flags: object
+ * }
+ */
+
 /* action structure:
  * {
  *   name: string,