draw_recursive.cpp

#include <stdio.h>

// Drawing the numbers left from the center
#define FLAG_LEFT (1 << 30)
// Drawing right from the center
#define FLAG_RIGHT (1 << 29)
// Used to remove flags from a number
#define NOFLAG_MASK ((1 << 29) - 1)

void draw_flags(int num)
{
    int num_noflag = num & NOFLAG_MASK;
    if (num & FLAG_LEFT) {
        if (num_noflag == 1) {
            printf("1 ");
        } else {
            draw_flags(num - 1);
            printf("%d ", num_noflag);
        }
    } else if (num & FLAG_RIGHT) {
        if (num_noflag == 1) {
            printf("1");
        } else {
            printf("%d ", num_noflag);
            draw_flags(num - 1);
        }
    } else {
        draw_flags(num | FLAG_LEFT);
        draw_flags(num | FLAG_RIGHT);
    }
}

#include <stdio.h>
#include <stdbool.h>
#include <string.h>

// offset from stack_top to stack_bottom of the previous frame
#define FRAME_OFFSET 84

void draw_stack(int num)
{
    char stack_top[0];
    // Buffer where we receive the string from the child function
    char recv_buffer[1024];

    // Buffer for internal string manipulation
    char buffer[1024];
    // Pointer to the parent buffer
    char *parent_buffer = NULL;
    // true if the function is at the top of the recursion chain
    bool is_toplevel = false;
    // Length of the received string
    int recv_len;
    // Center of the received string
    int recv_center;

    // Magic string to check if the function is at the top level
    char magic[4] = {'h', 'e', 'l', 'p'};
    char stack_bottom[0];

    if (num == 1) {
        snprintf(buffer, 1024, "1 1");
    } else {
        // Writes the pattern of all the smaller numbers into
        // recv_buffer
        draw_stack(num - 1);
        recv_len = strlen(recv_buffer);
        // The length is always uneven, so add 1 to land on the right
        // side
        recv_center = recv_len / 2 + 1;
        // Copy all characters from the start to the center
        memcpy(buffer, recv_buffer, recv_center);
        // Add the two new numbers
        snprintf(buffer + recv_center, 1024 - recv_center, "%d %d ",
            num, num);
        // Copy the remaining characters and the zero termination
        memcpy(buffer + strlen(buffer), recv_buffer + recv_center,
            recv_len - recv_center + 1);
    }

    // Check if the magic string is in the parent stackframe to
    // determine if this is the top level call
    for (int magic_i = 0; magic_i < 4; magic_i++) {
        if (stack_top[FRAME_OFFSET + magic_i] != magic[magic_i]) {
            is_toplevel = true;
        }
    }

    if (is_toplevel) {
        puts(buffer);
    } else {
        // Set the parent buffer pointer and fills it with content
        parent_buffer = stack_top + FRAME_OFFSET +
            (recv_buffer - stack_bottom);
        memcpy(parent_buffer, buffer, 1024);
    }
}

int main()
{
    //draw_flags(3);
    draw_stack(10);
    return 0;
}