Task: Hilbert curve
C#: using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;

namespace HilbertCurve {
    class Program {
        static void Swap<T>(ref T a, ref T b) {
            var c = a;
            a = b;
            b = c;
        }

        struct Point {
            public int x, y;

            public Point(int x, int y) {
                this.x = x;
                this.y = y;
            }

            
            public void Rot(int n, bool rx, bool ry) {
                if (!ry) {
                    if (rx) {
                        x = (n - 1) - x;
                        y = (n - 1) - y;
                    }
                    Swap(ref x, ref y);
                }
            }

            public override string ToString() {
                return string.Format("({0}, {1})", x, y);
            }
        }

        static Point FromD(int n, int d) {
            var p = new Point(0, 0);
            int t = d;

            for (int s = 1; s < n; s <<= 1) {
                var rx = (t & 2) != 0;
                var 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;
        }

        static List<Point> GetPointsForCurve(int n) {
            var points = new List<Point>();
            int d = 0;
            while (d < n * n) {
                points.Add(FromD(n, d));
                d += 1;
            }
            return points;
        }

        static List<string> DrawCurve(List<Point> points, int n) {
            var canvas = new char[n, n * 3 - 2];
            for (int i = 0; i < canvas.GetLength(0); i++) {
                for (int j = 0; j < canvas.GetLength(1); j++) {
                    canvas[i, j] = ' ';
                }
            }

            for (int i = 1; i < points.Count; i++) {
                var lastPoint = points[i - 1];
                var curPoint = points[i];
                var deltaX = curPoint.x - lastPoint.x;
                var deltaY = curPoint.y - lastPoint.y;
                if (deltaX == 0) {
                    Debug.Assert(deltaY != 0, "Duplicate point");
                    
                    int row = Math.Max(curPoint.y, lastPoint.y);
                    int col = curPoint.x * 3;
                    canvas[row, col] = '|';
                } else {
                    Debug.Assert(deltaY == 0, "Duplicate point");
                    
                    var row = curPoint.y;
                    var col = Math.Min(curPoint.x, lastPoint.x) * 3 + 1;
                    canvas[row, col] = '_';
                    canvas[row, col + 1] = '_';
                }
            }

            var lines = new List<string>();
            for (int i = 0; i < canvas.GetLength(0); i++) {
                var sb = new StringBuilder();
                for (int j = 0; j < canvas.GetLength(1); j++) {
                    sb.Append(canvas[i, j]);
                }
                lines.Add(sb.ToString());
            }
            return lines;
        }

        static void Main() {
            for (int order = 1; order <= 5; order++) {
                var n = 1 << order;
                var points = GetPointsForCurve(n);
                Console.WriteLine("Hilbert curve, order={0}", order);
                var lines = DrawCurve(points, n);
                foreach (var line in lines) {
                    Console.WriteLine(line);
                }
                Console.WriteLine();
            }
        }
    }
}

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;
}