Euler Totient by Factorization in C++

#include <stdio.h>
#include <vector>
#include <algorithm>
#include <stdexcept>
#include <cstdint>
#include <limits>
// Euler Totient by Factorization in C++
//
// PURPOSE
// Euler totient counts positive integers up to n that are coprime with n.
//
// METHOD
// For each distinct prime divisor p, remove all factors p from a working value and replace
// result with result-result/p. Handle any remaining prime factor.
//
// COMPLEXITY
// O(sqrt(n)) trial-division time and O(1) space.
//
// CONTRACT
// n >= 1. phi(1)=1.
// 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: 6.
// 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 totient(int n) {
    alg_require((n >= 1));
    int result = n;
    int p = 2;
    while ((p <= (n / p))) {
        if (((n % p) == 0)) {
            while (((n % p) == 0)) {
                n = (n / p);
            }
            result = alg_sub(result, (result / p));
        }
        p = alg_add(p, 1);
    }
    if ((n > 1)) {
        result = alg_sub(result, (result / n));
    }
    return result;
}

int main(void) {
    int n = 9;
    int result = totient(n);
    printf("%d\n", result);
    return 0;
}