Task: Hilbert curve
C: #include <stdio.h>

#define N 32
#define K 3
#define MAX N * K

typedef struct { int x; int y; } point;

void rot(int n, point *p, int rx, int ry) {
    int t;
    if (!ry) {
        if (rx == 1) {
            p->x = n - 1 - p->x;
            p->y = n - 1 - p->y;
        }
        t = p->x;
        p->x = p->y;
        p->y = t;
    }
}

void d2pt(int n, int d, point *p) {
    int s = 1, t = d, rx, ry;
    p->x = 0;
    p->y = 0;
    while (s < n) {
        rx = 1 & (t / 2);
        ry = 1 & (t ^ rx);
        rot(s, p, rx, ry);
        p->x += s * rx;
        p->y += s * ry;
        t /= 4;
        s *= 2;
    }
}

int main() {
    int d, x, y, cx, cy, px, py;
    char pts[MAX][MAX];
    point curr, prev;
    for (x = 0; x < MAX; ++x)
        for (y = 0; y < MAX; ++y) pts[x][y] = ' ';
    prev.x = prev.y = 0;
    pts[0][0] = '.';
    for (d = 1; d < N * N; ++d) {
        d2pt(N, d, &curr);
        cx = curr.x * K;
        cy = curr.y * K;
        px = prev.x * K;
        py = prev.y * K;
        pts[cx][cy] = '.';
        if (cx == px ) {
            if (py < cy)
                for (y = py + 1; y < cy; ++y) pts[cx][y] = '|';
            else
                for (y = cy + 1; y < py; ++y) pts[cx][y] = '|';
        }
        else {
            if (px < cx)
                for (x = px + 1; x < cx; ++x) pts[x][cy] = '_';
            else
                for (x = cx + 1; x < px; ++x) pts[x][cy] = '_';
        }
        prev = curr;
    }
    for (x = 0; x < MAX; ++x) {
        for (y = 0; y < MAX; ++y) printf("%c", pts[y][x]);
        printf("\n");
    }
    return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>

struct Point {
    int x, y;

    
    void rot(int n, bool rx, bool ry) {
        if (!ry) {
            if (rx) {
                x = (n - 1) - x;
                y = (n - 1) - y;
            }
            std::swap(x, y);
        }
    }
};

Point fromD(int n, int d) {
    Point p = { 0, 0 };
    bool rx, ry;
    int t = d;
    for (int s = 1; s < n; s <<= 1) {
        rx = ((t & 2) != 0);
        ry = (((t ^ (rx ? 1 : 0)) & 1) != 0);
        p.rot(s, rx, ry);
        p.x += (rx ? s : 0);
        p.y += (ry ? s : 0);
        t >>= 2;
    }
    return p;
}

std::vector<Point> getPointsForCurve(int n) {
    std::vector<Point> points;
    for (int d = 0; d < n * n; ++d) {
        points.push_back(fromD(n, d));
    }
    return points;
}

std::vector<std::string> drawCurve(const std::vector<Point> &points, int n) {
    auto canvas = new char *[n];
    for (size_t i = 0; i < n; i++) {
        canvas[i] = new char[n * 3 - 2];
        std::memset(canvas[i], ' ', n * 3 - 2);
    }

    for (int i = 1; i < points.size(); i++) {
        auto lastPoint = points[i - 1];
        auto curPoint = points[i];
        int deltaX = curPoint.x - lastPoint.x;
        int deltaY = curPoint.y - lastPoint.y;
        if (deltaX == 0) {
            
            int row = std::max(curPoint.y, lastPoint.y);
            int col = curPoint.x * 3;
            canvas[row][col] = '|';
        } else {
            
            int row = curPoint.y;
            int col = std::min(curPoint.x, lastPoint.x) * 3 + 1;
            canvas[row][col] = '_';
            canvas[row][col + 1] = '_';
        }
    }

    std::vector<std::string> lines;
    for (size_t i = 0; i < n; i++) {
        std::string temp;
        temp.assign(canvas[i], n * 3 - 2);
        lines.push_back(temp);
    }
    return lines;
}

int main() {
    for (int order = 1; order < 6; order++) {
        int n = 1 << order;
        auto points = getPointsForCurve(n);
        std::cout << "Hilbert curve, order=" << order << '\n';
        auto lines = drawCurve(points, n);
        for (auto &line : lines) {
            std::cout << line << '\n';
        }
        std::cout << '\n';
    }
    return 0;
}