2進数を10進数に変換

#include <fstream>
#include <iostream>
#include <vector>
#include <numeric>
#include <algorithm>

#define D 3
#define K (1 << D) /* K = 2^D */
#define MASK (K - 1) /* MASK = K - 1 */


struct windows {
    struct window {
        int one;
        int length;
    } window[sizeof(unsigned long) * CHAR_BIT];
    int length;
};

int get_msb(unsigned long a)
{
    int n = 0;

    while (a > 0) {
        a >>= 1;
        n++;
    }

    return n;
}

void decompose_bits(unsigned long n, struct windows* windows)
{
    int one, j;
    int i = get_msb(n) - 1;

    windows->length = 0;

    while (i >= 0) {
        one = (n >> i) & 1;

        for (j = 1; j < D && i >= j && one == ((n >> (i - j)) & 1); j++) {
        }

        windows->window[windows->length].one = one;
        windows->window[windows->length].length = j;
        windows->length++;
        i -= j;
    }
}

unsigned long sliding_window(unsigned long a, unsigned long n)
{
    unsigned long x = 1, aa = a * a;
    int i, j;
    unsigned long table[D + 1];
    struct windows windows;

    table[0] = a;

    for (i = 1; i < D + 1; i++) {
        table[i] = aa * table[i - 1];
    }

    decompose_bits(n, &windows);

    for (i = 0; i < windows.length; i++) {
        for (j = 0; j < windows.window[i].length; j++) {
            x *= x;
        }

        if (windows.window[i].one) {
            j = (1 << (windows.window[i].length - 1)) - 1;
            x *= table[j];
        }
    }

    return x;
}

int main()
{
    int num[] = { 1,0,1,0,1,1,0,1,1, 1, 0, 1 , 1 };
    std::vector<int> v, calc;

    // 配列を逆順にコピー
    std::reverse_copy(std::begin(num), std::end(num), std::back_inserter(v));

    // 配列の要素数を取得
    int max_size = (int)v.size();

    for (int i = 0; i < max_size; i++) {
        // 2のiのべき乗を計算
        calc.push_back(num[i] * sliding_window(2, i));
    }

    // 合計を計算
    int sum = std::reduce(calc.begin(), calc.end());

    // 結果を表示
    std::cout << "2進数の\t0b";
    for (auto e : v) {
        std::cout << e;
    }
    std::cout << "\tは10進数に直すと";
    std::cout << sum << "です" << std::endl;
}