docs, tests, fix: fit fibonacci_fast to contributing guidelines (#2893)

* Update fibonacci_fast.cpp

Modify the fibonacci_fast.cpp to fit the project style requirements.

* Update fibonacci_fast.cpp to meet the revision suggestions.

* Update fibonacci_fast.cpp to try to pass some CI test

* Update fibonacci_fast.cpp

* Update fibonacci_fast.cpp to meet the coding style requirements

* Update fibonacci_fast.cpp

* Update fibonacci_fast.cpp

---------

Co-authored-by: realstealthninja <[email protected]>

Author: setbit123

math/fibonacci_fast.cpp

@@ -1,57 +1,178 @@
 /**
  * @file
- * @brief Faster computation of Fibonacci series
+ * @brief Faster computation of Fibonacci series.
  *
+ * @details
  * An efficient way to calculate nth fibonacci number faster and simpler than
- * \f$O(n\log n)\f$ method of matrix exponentiation This works by using both
- * recursion and dynamic programming. as 93rd fibonacci exceeds 19 digits, which
+ * \f$O(n\log n)\f$ method of matrix exponentiation. This works by using both
+ * recursion and dynamic programming. As 93rd fibonacci exceeds 19 digits, which
  * cannot be stored in a single long long variable, we can only use it till 92nd
  * fibonacci we can use it for 10000th fibonacci etc, if we implement
  * bigintegers. This algorithm works with the fact that nth fibonacci can easily
- * found if we have already found n/2th or (n+1)/2th fibonacci It is a property
+ * found if we have already found \f$n/2\f$th or \f$(n+1)/2\f$th fibonacci. It is a property
  * of fibonacci similar to matrix exponentiation.
  *
- * \author [Krishna Vedala](https://github.com/kvedala)
+ * @author [Krishna Vedala](https://github.com/kvedala)
  * @see fibonacci_large.cpp, fibonacci.cpp, string_fibonacci.cpp
  */
-
-#include <cinttypes>
-#include <cstdio>
-#include <iostream>
+#include <cinttypes>  /// for uint64_t
+#include <cstdio>     /// for standard IO
+#include <iostream>   /// for IO operations
+#include <cassert>    /// for assert
+#include <string>     /// for std::to_string
+#include <stdexcept>  /// for std::invalid_argument
 
 /**
- * maximum number that can be computed - The result after 93 cannot be stored
- * in a `uint64_t` data type.
+ * @brief Maximum Fibonacci number that can be computed
+ *
+ * @details
+ * The result after 93 cannot be stored in a `uint64_t` data type.
  */
+constexpr uint64_t MAX = 93;
 
-#define MAX 93
-
-/** Algorithm */
+/**
+ * @brief Function to compute the nth Fibonacci number
+ * @param n The index of the Fibonacci number to compute
+ * @return uint64_t The nth Fibonacci number
+ */
 uint64_t fib(uint64_t n) {
-    static uint64_t f1 = 1,
-                    f2 = 1;  // using static keyword will retain the values of
-                             // f1 and f2 for the next function call.
+    // Using static keyword will retain the values of
+    // f1 and f2 for the next function call.
+    static uint64_t f1 = 1, f2 = 1;
 
-    if (n <= 2)
+    if (n <= 2) {
         return f2;
-    if (n >= 93) {
-        std::cerr
-            << "Cannot compute for n>93 due to limit of 64-bit integers\n";
+    } if (n >= MAX) {
+        throw std::invalid_argument("Cannot compute for n>=" + std::to_string(MAX) +
+                                    " due to limit of 64-bit integers");
         return 0;
     }
-
-    uint64_t temp = f2;  // we do not need temp to be static
+    
+    // We do not need temp to be static.
+    uint64_t temp = f2;  
     f2 += f1;
     f1 = temp;
 
     return f2;
 }
 
-/** Main function */
-int main() {
-    // Main Function
-    for (uint64_t i = 1; i < 93; i++) {
-        std::cout << i << " th fibonacci number is " << fib(i) << std::endl;
+/**
+ * @brief Function to test the Fibonacci computation
+ * @returns void
+ */
+static void test() {
+    // Test for valid Fibonacci numbers
+    assert(fib(1) == 1);
+    assert(fib(2) == 1);
+    assert(fib(3) == 2);
+    assert(fib(4) == 3);
+    assert(fib(5) == 5);
+    assert(fib(6) == 8);
+    assert(fib(7) == 13);
+    assert(fib(8) == 21);
+    assert(fib(9) == 34);
+    assert(fib(10) == 55);
+    assert(fib(11) == 89);
+    assert(fib(12) == 144);
+    assert(fib(13) == 233);
+    assert(fib(14) == 377);
+    assert(fib(15) == 610);
+    assert(fib(16) == 987);
+    assert(fib(17) == 1597);
+    assert(fib(18) == 2584);
+    assert(fib(19) == 4181);
+    assert(fib(20) == 6765);
+    assert(fib(21) == 10946);
+    assert(fib(22) == 17711);
+    assert(fib(23) == 28657);
+    assert(fib(24) == 46368);
+    assert(fib(25) == 75025);
+    assert(fib(26) == 121393);
+    assert(fib(27) == 196418);
+    assert(fib(28) == 317811);
+    assert(fib(29) == 514229);
+    assert(fib(30) == 832040);
+    assert(fib(31) == 1346269);
+    assert(fib(32) == 2178309);
+    assert(fib(33) == 3524578);
+    assert(fib(34) == 5702887);
+    assert(fib(35) == 9227465);
+    assert(fib(36) == 14930352);
+    assert(fib(37) == 24157817);
+    assert(fib(38) == 39088169);
+    assert(fib(39) == 63245986);
+    assert(fib(40) == 102334155);
+    assert(fib(41) == 165580141);
+    assert(fib(42) == 267914296);
+    assert(fib(43) == 433494437);
+    assert(fib(44) == 701408733);
+    assert(fib(45) == 1134903170);
+    assert(fib(46) == 1836311903);
+    assert(fib(47) == 2971215073);
+    assert(fib(48) == 4807526976);
+    assert(fib(49) == 7778742049);
+    assert(fib(50) == 12586269025);
+    assert(fib(51) == 20365011074);
+    assert(fib(52) == 32951280099);
+    assert(fib(53) == 53316291173);
+    assert(fib(54) == 86267571272);
+    assert(fib(55) == 139583862445);
+    assert(fib(56) == 225851433717);
+    assert(fib(57) == 365435296162);
+    assert(fib(58) == 591286729879);
+    assert(fib(59) == 956722026041);
+    assert(fib(60) == 1548008755920);
+    assert(fib(61) == 2504730781961);
+    assert(fib(62) == 4052739537881);
+    assert(fib(63) == 6557470319842);
+    assert(fib(64) == 10610209857723);
+    assert(fib(65) == 17167680177565);
+    assert(fib(66) == 27777890035288);
+    assert(fib(67) == 44945570212853);
+    assert(fib(68) == 72723460248141);
+    assert(fib(69) == 117669030460994);
+    assert(fib(70) == 190392490709135);
+    assert(fib(71) == 308061521170129);
+    assert(fib(72) == 498454011879264);
+    assert(fib(73) == 806515533049393);
+    assert(fib(74) == 1304969544928657);
+    assert(fib(75) == 2111485077978050);
+    assert(fib(76) == 3416454622906707);
+    assert(fib(77) == 5527939700884757);
+    assert(fib(78) == 8944394323791464);
+    assert(fib(79) == 14472334024676221);
+    assert(fib(80) == 23416728348467685);
+    assert(fib(81) == 37889062373143906);
+    assert(fib(82) == 61305790721611591);
+    assert(fib(83) == 99194853094755497);
+    assert(fib(84) == 160500643816367088);
+    assert(fib(85) == 259695496911122585);
+    assert(fib(86) == 420196140727489673);
+    assert(fib(87) == 679891637638612258);
+    assert(fib(88) == 1100087778366101931);
+    assert(fib(89) == 1779979416004714189);
+    assert(fib(90) == 2880067194370816120);
+    assert(fib(91) == 4660046610375530309);
+    assert(fib(92) == 7540113804746346429);
+
+    // Test for invalid Fibonacci numbers
+    try {
+        fib(MAX + 1);
+        assert(false && "Expected an invalid_argument exception to be thrown");
+    } catch (const std::invalid_argument& e) {
+        const std::string expected_message = "Cannot compute for n>=" + std::to_string(MAX) +
+                                             " due to limit of 64-bit integers";
+        assert(e.what() == expected_message);
     }
+
+    std::cout << "All Fibonacci tests have successfully passed!\n";
+}
+
+/**
+ * @brief Main Function
+ * @returns 0 on exit
+ */
+int main() {
+    test();  // run self-test implementations
     return 0;
 }