fix binomial0 & add test

2 files changed, 22 insertions, 1 deletions

diff --git a/math/binomial0.cpp b/math/binomial0.cpp
index d9af917..f37aea5 100644
--- a/math/binomial0.cpp
+++ b/math/binomial0.cpp
@@ -10,5 +10,5 @@ void precalc() {
 
 ll calc_binom(ll n, ll k) {
 	if (n < 0 || n < k || k < 0) return 0;
-	return (fac[n] * fac[n-k] % mod) * fac[k] % mod;
+	return (fac[n] * inv[n-k] % mod) * inv[k] % mod;
 }
diff --git a/math/test/binomial0.cpp b/math/test/binomial0.cpp
new file mode 100644
index 0000000..d6c3a03
--- /dev/null
+++ b/math/test/binomial0.cpp
@@ -0,0 +1,21 @@
+constexpr ll mod = 1'000'000'007;
+
+#include "../shortModInv.cpp"
+#include "../binomial0.cpp"
+
+int main() {
+	precalc();
+	assert(calc_binom(5, -1) == 0);
+	assert(calc_binom(5, 0) == 1);
+	assert(calc_binom(5, 1) == 5);
+	assert(calc_binom(5, 2) == 10);
+	assert(calc_binom(5, 3) == 10);
+	assert(calc_binom(5, 4) == 5);
+	assert(calc_binom(5, 5) == 1);
+	assert(calc_binom(5, 6) == 0);
+	assert(calc_binom(0, 0) == 1);
+	assert(calc_binom(-1, 0) == 0);
+	assert(calc_binom(-1, -1) == 0);
+	assert(calc_binom(-1, -2) == 0);
+	assert(calc_binom(100'000, 50'000) == 149033233);
+}