Improve floor_sum

atcoder/ac-library#88

src/math.rs

@@ -188,21 +188,19 @@ pub fn crt(r: &[i64], m: &[i64]) -> (i64, i64) {
 pub fn floor_sum(n: i64, m: i64, mut a: i64, mut b: i64) -> i64 {
     let mut 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;
     }
 
-    let y_max = (a * n + b) / m;
-    let x_max = y_max * m - b;
-    if y_max == 0 {
+    let y_max = a * n + b;
+    if y_max < m {
         return ans;
     }
-    ans += (n - (x_max + a - 1) / a) * y_max;
-    ans += floor_sum(y_max, a, m, (a - x_max % a) % a);
+    ans += floor_sum(y_max / m, a, m, y_max % m);
     ans
 }