4 files changed, 39 insertions, 2 deletions
diff --git a/src/engine/liberty.js b/src/engine/liberty.js
new file mode 100644
index 0000000..372057e
--- /dev/null
+++ b/src/engine/liberty.js
@@ -0,0 +1,24 @@
+import { board } from "../board.js"
+import { Vector2 } from "../engine/vector2.js"
+
+const SURROUNDING = [
+    new Vector2(-1, 0),
+    new Vector2(0, 1),
+    new Vector2(0, -1),
+    new Vector2(1, 0),
+]
+
+export function hasLiberties({
+    position,
+    team
+}) {
+    return Array.from(SURROUNDING)
+	.map(v => Vector2.sum(v, position))
+	.filter(z => 0 <= z.x && z.x < board.size &&
+	    0 <= z.y && z.y < board.size
+	)
+	.some(z => board.stones[z.x][z.y] == undefined ?
+	    true :
+	    board.stones[z.x][z.y].team == team
+	)
+}
diff --git a/src/engine/moves.js b/src/engine/moves.js
index f820f97..26158a0 100644
--- a/src/engine/moves.js
+++ b/src/engine/moves.js
@@ -1,11 +1,17 @@
 import { board } from "../board.js"
+import { hasLiberties } from "./liberty.js"
 
 export function isMoveLegal({
     position,
-    turn = "none"
+    team = "none"
 }) {
     if (board.stones[position.y][position.x] != undefined) {
 	return false
     }
+
+    if (!hasLiberties({position: position, team: team})) {
+	return false
+    }
+
     return true
 }
diff --git a/src/engine/turns.js b/src/engine/turns.js
index 6a5e6fb..ee5b9e2 100644
--- a/src/engine/turns.js
+++ b/src/engine/turns.js
@@ -1,5 +1,5 @@
 import { board } from "../board.js"
-import { Vector2 } from "../engine/visual/vector2.js"
+import { Vector2 } from "../engine/vector2.js"
 import { Link } from "../link.js"
 import { PLAYER } from "../constants.js"
 import { isMoveLegal } from "../engine/moves.js"
diff --git a/src/engine/visual/vector2.js b/src/engine/vector2.js
index a3a1bc2..6c2a2a2 100644
--- a/src/engine/visual/vector2.js
+++ b/src/engine/vector2.js
@@ -3,4 +3,11 @@ export class Vector2 {
         this.x = x
         this.y = y
     }
+
+    static sum(a, b) {
+	return new Vector2(
+	    a.x + b.x,
+	    a.y + b.y,
+	)
+    }
 }