faster 10

2019/10.cpp

@@ -52,21 +52,34 @@ auto angle(Coord c) -> Rational<std::int64_t> {
                            mk(4, c.y,  c.x);
 }
 
+auto ClearView(Grid const& grid, Coord src, Coord dst) -> bool {
+  auto vec = dst - src;
+  auto steps = std::gcd(vec.x, vec.y);
+  Coord delta {vec.x / steps, vec.y / steps};
+  for (auto cursor = src+delta; cursor != dst; cursor+=delta) {
+    if (grid[cursor] == '#') {
+      return false;
+    }
+  }
+  return true;
+}
+
 auto Part1(Grid const& grid) {
   std::size_t best = 0;
   Coord base {};
   grid.each([&](Coord src, char s) {
     if ('#' == s) {
-      std::set<Rational<std::int64_t>> angles;
+      std::size_t visible = 0;
       grid.each([&](Coord dst, char d){
         if ('#' == d && src != dst) {
-          auto delta = dst - src;
-          angles.insert(angle(dst - src));
+          if (ClearView(grid, src, dst)) {
+            visible++;
+          }
         }
       });
-      auto n = angles.size();
-      if (n > best) {
-        best = n;
+      
+      if (visible > best) {
+        best = visible;
         base = src;
       }
     }