Fast Doubling Fibonacci in C++

#include <stdio.h>
#include <vector>
#include <algorithm>
#include <stdexcept>
#include <cstdint>
#include <limits>
// Fast Doubling Fibonacci in C++
//
// PURPOSE
// Fast doubling computes Fibonacci numbers using identities for doubling an index, reducing
// the number of recurrence steps logarithmically.
//
// METHOD
// Read n bits from most significant to least. Derive F(2k) and F(2k+1) from F(k),F(k+1),
// then advance one extra position for a set bit.
//
// COMPLEXITY
// O(log n) arithmetic operations, O(1) auxiliary space under bounded integer arithmetic.
//
// CONTRACT
// 0 <= n <= 45 so all intermediates fit signed 32-bit. Returns F(n), with F(0)=0.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// C++17: const vector references for inputs, mutable references for outputs, RAII workspace,
// standard lower_bound for LIS. Invalid inputs throw invalid_argument; overflow throws
// overflow_error; allocation failure propagates bad_alloc.
//
// EXAMPLE
// Expected return: 0.
// Run: g++ -std=c++17 -O2 example.cpp -o example && ./example
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

static int alg_min(int a, int b) { return a < b ? a : b; }
static int alg_max(int a, int b) { return a > b ? a : b; }
static void alg_require(bool valid) { if (!valid) throw std::invalid_argument("invalid algorithm input"); }
static int alg_checked(int64_t v) { if (v < INT32_MIN || v > INT32_MAX) throw std::overflow_error("32-bit result overflow"); return (int)v; }
static int alg_add(int a,int b) { return alg_checked((int64_t)a+b); }
static int alg_sub(int a,int b) { return alg_checked((int64_t)a-b); }
static int alg_mul(int a,int b) { return alg_checked((int64_t)a*b); }
static size_t alg_size(int n) { alg_require(n>=0 && n<=4000000); return (size_t)n; }

int fast_fibonacci(int n) {
    alg_require(((n >= 0) && (n <= 1000000)));
    alg_require(((n >= 0) && (n <= 45)));
    int a = 0;
    int b = 1;
    int c = 0;
    int d = 0;
    int bit = 1;
    while ((bit <= (n / 2))) {
        bit = alg_mul(bit, 2);
    }
    while ((bit > 0)) {
        c = alg_mul(a, alg_sub(alg_mul(2, b), a));
        d = alg_add(alg_mul(a, a), alg_mul(b, b));
        if ((((n / bit) % 2) == 0)) {
            a = c;
            b = d;
        } else {
            a = d;
            b = alg_add(c, d);
        }
        bit = (bit / 2);
    }
    return a;
}

int main(void) {
    int n = 0;
    int result = fast_fibonacci(n);
    printf("%d\n", result);
    return 0;
}