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diff --git a/lib/Neighbors.rakumod b/lib/Neighbors.rakumod
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+unit module Neighbors;
+
+# neighbors returns the neighbors of given index. Neighbors are cached
+# in @neighbors array. This way we don't have to compute them
+# everytime neighbors subroutine is called for the same position.
+sub neighbors (
+    @puzzle, Int $y, Int $x --> List
+) is export {
+    # @directions is holding a list of directions we can move in. It's
+    # used later for neighbors subroutine.
+    state List @directions = (
+        # $y, $x
+        ( +1, +0 ), # bottom
+        ( -1, +0 ), # top
+        ( +0, +1 ), # left
+        ( +0, -1 ), # right
+    );
+
+    # @neighbors holds the neighbors of given position.
+    state Array @neighbors;
+
+    if @puzzle[$y][$x] {
+
+        # If we've already computed the neighbors then no need to do
+        # it again.
+        unless @neighbors[$y][$x] {
+            my Int $pos-x;
+            my Int $pos-y;
+
+            # Starting from the intital position of $y, $x we move to
+            # each direction according to the values specified in
+            # @directions array. In this case we're just trying to
+            # move in 4 directions (top, bottom, left & right).
+            DIRECTION: for @directions -> $direction {
+                $pos-y = $y + $direction[0];
+                $pos-x = $x + $direction[1];
+
+                # If movement in this direction is out of puzzle grid
+                # boundary then move on to next direction.
+                next DIRECTION unless @puzzle[$pos-y][$pos-x];
+
+                # If neighbors exist in this direction then add them
+                # to @neighbors[$y][$x] array.
+                push @neighbors[$y][$x], [$pos-y, $pos-x];
+            }
+        }
+    } else {
+        # If it's out of boundary then return no neighbor.
+        @neighbors[$y][$x] = [];
+    }
+
+    return @neighbors[$y][$x];
+}