Added all functions inside a class + added more asserts

math/ncr_modulo_p.cpp

@@ -8,13 +8,35 @@
 #include <iostream>
 #include <vector>
 
-/** Finds the value of x, y such that a*x + b*y = gcd(a,b)
+
+/** Class which contains all methods required for calculating nCr mod p
+ *
+ */
+class NCRModuloP {
+private:
+    std::vector<int64_t> fac;
+    int64_t p;
+public:
+    /** Constructor which precomputes the values of n! % mod from n=0 to size
+     *  and stores them in vector 'fac'
+     *  @params[in] the numbers 'size', 'mod'
+    */
+    NCRModuloP(int64_t size, int64_t mod){
+        p = mod;
+        fac = std::vector<int64_t>(size);
+        fac[0] = 1;
+        for (int i = 1; i <= size; i++) {
+            fac[i] = (fac[i - 1] * i) % p;
+        }
+    }
+
+    /** Finds the value of x, y such that a*x + b*y = gcd(a,b)
      *
      * @params[in] the numbers 'a', 'b' and address of 'x' and 'y' from above
      * equation
      * @returns the gcd of a and b
      */
-int64_t gcdExtended(int64_t a, int64_t b, int64_t *x, int64_t *y) {
+    int64_t gcdExtended(int64_t a, int64_t b, int64_t *x, int64_t *y) {
         if (a == 0) {
             *x = 0, *y = 1;
             return b;
@@ -26,14 +48,14 @@ int64_t gcdExtended(int64_t a, int64_t b, int64_t *x, int64_t *y) {
         *x = y1 - (b / a) * x1;
         *y = x1;
         return gcd;
-}
+    }
 
-/** Find modular inverse of a with m i.e. a number x such that (a*x)%m = 1
+    /** Find modular inverse of a with m i.e. a number x such that (a*x)%m = 1
      *
      * @params[in] the numbers 'a' and 'm' from above equation
      * @returns the modular inverse of a
      */
-int64_t modInverse(int64_t a, int64_t m) {
+    int64_t modInverse(int64_t a, int64_t m) {
         int64_t x = 0, y = 0;
         int64_t g = gcdExtended(a, m, &x, &y);
         if (g != 1) {  // modular inverse doesn't exist
@@ -43,16 +65,14 @@ int64_t modInverse(int64_t a, int64_t m) {
             int64_t res = (x % m + m) % m;
             return res;
         }
-}
+    }
 
-std::vector<int64_t> fac;
+    /** Find nCr % p
-
-/** Find nCr % p
      *
      * @params[in] the numbers 'n', 'r' and 'p'
      * @returns the value nCr % p
      */
-int64_t ncr(int64_t n, int64_t r, int64_t p) {
+    int64_t ncr(int64_t n, int64_t r, int64_t p) {
         // Base cases
         if (r > n) {
             return 0;
@@ -73,8 +93,19 @@ int64_t ncr(int64_t n, int64_t r, int64_t p) {
             return -1;
         }
         return (fac[n] * denominator) % p;
+    }
+};
+
+void tests(NCRModuloP ncrObj) {
+    // (52323 C 26161) % (1e9 + 7) = 224944353
+    assert(ncrObj.ncr(52323, 26161, 1000000007) == 224944353);
+    // 6 C 2 = 30, 30%5 = 0
+    assert(ncrObj.ncr(6,2,5) == 0);
+    // 7C3 = 35, 35 % 29 = 8
+    assert(ncrObj.ncr(7,3,29) == 6);
 }
 
+
 /**
  * @brief Main function
  * @returns 0 on exit
@@ -82,19 +113,12 @@ int64_t ncr(int64_t n, int64_t r, int64_t p) {
 int main() {
     // populate the fac array
     const int64_t size = 1e6 + 1;
-    fac = std::vector<int64_t>(size);
-    fac[0] = 1;
     const int64_t p = 1e9 + 7;
-    for (int i = 1; i <= size; i++) {
+    NCRModuloP ncrObj = NCRModuloP(size, p);
-        fac[i] = (fac[i - 1] * i) % p;
-    }
-
     // test 6Ci for i=0 to 7
     for (int i = 0; i <= 7; i++) {
-        std::cout << 6 << "C" << i << " = " << ncr(6, i, p) << "\n";
+        std::cout << 6 << "C" << i << " = " << ncrObj.ncr(6, i, p) << "\n";
     }
-
+    tests(ncrObj);
-    // (52323 C 26161) % (1e9 + 7) = 224944353
+    std::cout << "Assertions passed\n";
-    assert(ncr(52323, 26161, p) == 224944353);
-    std::cout << "Assertion passed, (52323 C 26161) % (1e9 + 7) = 224944353\n";
 }