Binomial coefficient test

Author: adamant-pwn

cp-algo/math/combinatorics.hpp

@@ -1,10 +1,11 @@
 #ifndef CP_ALGO_MATH_COMBINATORICS_HPP
 #define CP_ALGO_MATH_COMBINATORICS_HPP
 #include "common.hpp"
+#include <cassert>
 namespace cp_algo::math {
     template<typename T>
     T binom_large(T n, int r) {
-        assert(r < math::maxn);
+        assert(r < maxn);
         T ans = 1;
         for(int i = 0; i < r; i++) {
             ans = ans * T(n - i) * small_inv<T>(i + 1);
@@ -15,10 +16,10 @@ namespace cp_algo::math {
     T binom(int n, int r) {
         if(r < 0 || r > n) {
             return T(0);
-        } else if(n > math::maxn) {
-            return binom_large(n, r);
+        } else if(n >= maxn) {
+            return binom_large(T(n), r);
         } else {
-            return math::fact<T>(n) * math::rfact<T>(r) * math::rfact<T>(n-r);
+            return fact<T>(n) * rfact<T>(r) * rfact<T>(n - r);
         }
     }
 }

cp-algo/math/common.hpp

@@ -29,6 +29,8 @@ namespace cp_algo::math {
     T bpow(T const& x, int64_t n) {
         return bpow(x, n, T(1));
     }
+    // fact/rfact/small_inv are caching
+    // Beware of usage with dynamic mod
     template<typename T>
     T fact(int n) {
         static std::vector<T> F(maxn);
@@ -42,13 +44,15 @@ namespace cp_algo::math {
         }
         return F[n];
     }
+    // Only works for modint types
     template<typename T>
     T rfact(int n) {
         static std::vector<T> F(maxn);
         static bool init = false;
         if(!init) {
-            F[maxn - 1] = T(1) / fact<T>(maxn - 1);
-            for(int i = maxn - 2; i >= 0; i--) {
+            int t = std::min<int64_t>(T::mod(), maxn) - 1;
+            F[t] = T(1) / fact<T>(t);
+            for(int i = t - 1; i >= 0; i--) {
                 F[i] = F[i + 1] * T(i + 1);
             }
             init = true;

verify/combi/binom.test.cpp

@@ -0,0 +1,33 @@
+// @brief Binomial Coefficient (Prime Mod)
+#define PROBLEM "https://judge.yosupo.jp/problem/binomial_coefficient_prime_mod"
+#pragma GCC optimize("Ofast,unroll-loops")
+#pragma GCC target("avx2,tune=native")
+#define CP_ALGO_MAXN 1e7
+#include "cp-algo/math/modint.hpp"
+#include "cp-algo/math/combinatorics.hpp"
+#include <bits/stdc++.h>
+
+using namespace std;
+using namespace cp_algo;
+using namespace math;
+using base = dynamic_modint;
+
+void solve() {
+    int n, r;
+    cin >> n >> r;
+    cout << binom<base>(n, r) << "\n";
+}
+
+signed main() {
+    //freopen("input.txt", "r", stdin);
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+    int t = 1;
+    cin >> t;
+    int m;
+    cin >> m;
+    base::switch_mod(m);
+    while(t--) {
+        solve();
+    }
+}