Merge pull request #88 from rsk0315/improve/floor_sum

Improve floor_sum

Author: yosupo06

atcoder/math.hpp

@@ -82,18 +82,17 @@ std::pair<long long, long long> crt(const std::vector<long long>& r,
 long long floor_sum(long long n, long long m, long long a, long long b) {
     long long ans = 0;
     if (a >= m) {
-        ans += (n - 1) * n * (a / m) / 2;
+        ans += (n - 1) * n / 2 * (a / m);
         a %= m;
     }
     if (b >= m) {
         ans += n * (b / m);
         b %= m;
     }
 
-    long long y_max = (a * n + b) / m, x_max = (y_max * m - b);
-    if (y_max == 0) return ans;
-    ans += (n - (x_max + a - 1) / a) * y_max;
-    ans += floor_sum(y_max, a, m, (a - x_max % a) % a);
+    long long y_max = a * n + b;
+    if (y_max < m) return ans;
+    ans += floor_sum(y_max / m, a, m, y_max % m);
     return ans;
 }