2 files changed, 37 insertions, 2 deletions
diff --git a/src/common.hpp b/src/common.hpp
index e745ece..d35d1bc 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -13,6 +13,26 @@ struct Pt {
     Pt operator-(Pt o) const { return { x - o.x, y - o.y }; }
     Pt operator*(int k) const { return { x * k, y * k }; }
     Pt operator/(int k) const { return { x / k, y / k }; }
+    Pt& operator+=(Pt o) {
+        x += o.x;
+        y += o.y;
+        return *this;
+    }
+    Pt& operator-=(Pt o) {
+        x -= o.x;
+        y -= o.y;
+        return *this;
+    }
+    Pt& operator*=(Pt o) {
+        x *= o.x;
+        y *= o.y;
+        return *this;
+    }
+    Pt& operator/=(Pt o) {
+        x /= o.x;
+        y /= o.y;
+        return *this;
+    }
     bool operator==(const Pt&) const = default;
 };
 
@@ -25,8 +45,6 @@ inline Pt pt_min(Pt a, Pt b) {
 inline Pt pt_max(Pt a, Pt b) {
     return Pt{ MMAX(a.x, b.x), MMAX(a.y, b.y) };
 }
-#undef MMIN
-#undef MMAX
 
 struct Rect {
     Pt bl, tr;
@@ -46,3 +64,13 @@ inline Rect rect_union(Rect a, Rect b) {
 inline Rect rect_union(Rect a, Pt b) {
     return Rect(pt_min(a.bl, b), pt_max(a.tr, b));
 }
+inline Rect rect_intersect(Rect a, Rect b) {
+    // https://stackoverflow.com/a/19754915
+    int x0 = MMAX(a.bl.x, b.bl.x);
+    int y0 = MMAX(a.bl.y, b.bl.y);
+    int x1 = MMIN(a.tr.x, b.tr.x);
+    int y1 = MMIN(a.tr.y, b.tr.y);
+    return Rect(x0, y0, x1, y1);
+}
+#undef MMIN
+#undef MMAX
diff --git a/src/sandbox.cpp b/src/sandbox.cpp
index c6d0294..2c09d77 100644
--- a/src/sandbox.cpp
+++ b/src/sandbox.cpp
@@ -102,17 +102,24 @@ void Sandbox::simulate_step() {
     dirty_writeto = {};
     const auto [x0, y0] = dirty_curr.bl;
     const auto [x1, y1] = dirty_curr.tr;
+
     // Clear update bit for this cycle
     for (_y = y0; _y <= y1; ++_y) {
         for (_x = x0; _x <= x1; ++_x) {
             gs(_x, _y).updated = false;
         }
     }
+
+    // Update
     for (_y = y0; _y <= y1; ++_y) {
         for (_x = x0; _x <= x1; ++_x) {
             simulate_sand_tile(*this, _x, _y);
         }
     }
+    dirty_writeto.bl -= Pt(1, 1);
+    dirty_writeto.tr += Pt(1, 1);
+    dirty_writeto = rect_intersect(dirty_writeto, Rect(0, 0, width - 1, height - 1));
+
     ++ncycle;
 }