/* piano.c --- version 2026-09-29 --- public domain
 *
 * https://sedcore.eu.org/piano.c
 *
 * a small puzzle
 * inspired by https://sauropods.win/@futurebird/117328806582315024
 *
 * the goal is to put the big square initially at top-left to bottom-left
 * (can also be whatever else you want, be free!)
 *
 * was written on a debian system, the program should work on any
 * posix/X11/cairo system
 *
 * how to build/run:
 *   as root:
 *     apt install build-essential libx11-dev libcairo2-dev
 *   then as non-root:
 *     gcc -Wall -o piano piano.c -lX11 -lcairo
 *   then:
 *     ./piano
 *
 * how to play:
 *   move the mouse pointer around
 *   when on a piece, use left-right-up-down keys of your keyboard to move
 *   the piece (the piece will, obviously, move only if it can legally move)
 *
 * notes:
 *   - ugly colors? look for cairo_set_source_rgb and change values
 *   - cairo is overkill, sorry
 *   - it could be possible to use click&move to get rid of keyboard use
 *     i'm too lazy to implement this as of now
 *   - a javascript version? no, please... behave
 *   - why call it 'piano'? because the inspiring source has a picture
 *     where the big square is labelled 'piano'
 *
 * (this comment is way too long)
 */

#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/cursorfont.h>
#include <sys/select.h>
#include <cairo/cairo.h>
#include <cairo/cairo-xlib.h>
#include <string.h>

#define WIDTH 400
#define HEIGHT 500

char init_pos[5][4] = {
  { 'A', 'A', ' ', ' ' },
  { 'A', 'A', 'B', 'B' },
  { 'C', 'D', 'E', 'E' },
  { 'F', 'G', 'H', 'H' },
  { 'F', 'G', 'I', 'I' },
};

cairo_surface_t *cs;
cairo_surface_t *csout;

void expose(Display *d, Window w, Pixmap p)
{
  XCopyArea(d, p, w, DefaultGC(d, DefaultScreen(d)), 0,0,
            WIDTH, HEIGHT, 0,0);
}

void paint_cube(cairo_t *c, char target, char pos[5][4], char selected)
{
  int x_min = 10, x_max = -10;
  int y_min = 10, y_max = -10;

  /* find min/max for desired target piece */
  for (int i = 0; i < 5; i++) for (int j = 0; j < 4; j++) {
    if (pos[i][j] != target) continue;
    if (i < y_min) y_min = i;
    if (i > y_max) y_max = i;
    if (j < x_min) x_min = j;
    if (j > x_max) x_max = j;
  }

  x_max++;
  y_max++;

  int x_top_left_screen = (x_min * WIDTH) / 4 + 5;
  int x_bottom_right_screen = (x_max * WIDTH) / 4 - 5;
  int y_top_left_screen = (y_min * HEIGHT) / 5 + 5;
  int y_bottom_right_screen = (y_max * HEIGHT) / 5 - 5;

  if (target == selected)
    cairo_set_source_rgb(c, 0.9, 0.9, 0.7);
  else
    cairo_set_source_rgb(c, 0.7, 0.7, 0.3);
  cairo_rectangle(c, x_top_left_screen,
                     y_top_left_screen,
                     x_bottom_right_screen - x_top_left_screen + 1,
                     y_bottom_right_screen - y_top_left_screen + 1);
  cairo_fill(c);
}

void paint(char pos[5][4], char selected)
{
  cairo_t *c;

  c=cairo_create(cs);

  /* clear screen */
  cairo_rectangle(c, 0, 0, WIDTH, HEIGHT);
  cairo_set_source_rgb(c, 0.1, 0.1, 0.3);
  cairo_fill(c);

  /* draw stuff */
  for (char i = 'A'; i <= 'I'; i++) paint_cube(c, i, pos, selected);

  cairo_destroy(c);

  /* and put it all to the xpixmap */
  c=cairo_create(csout);
  cairo_set_source_surface(c, cs, 0, 0);
  cairo_rectangle(c, 0, 0, WIDTH, HEIGHT);
  cairo_paint(c);
  cairo_destroy(c);
}

char get_selected_from_mouse(char pos[5][4], int mouse_x, int mouse_y)
{
  if (mouse_x < 0 || mouse_y < 0) return -1;
  if (mouse_x >= WIDTH || mouse_y >= HEIGHT) return -1;
  int x = mouse_x * 4 / WIDTH;
  int y = mouse_y * 5 / HEIGHT;
  if (x >= 0 && x < 4
      && y >= 0 && y < 5) return pos[y][x];
  return -1;
}

int move(char pos[5][4], int x_delta, int y_delta, char selected)
{
  /* can we move? */
  for (int y = 0; y < 5; y++) for (int x = 0; x < 4; x++) {
    int target = pos[y][x];
    /* other objects won't move */
    if (target != selected) continue;
    int new_x = x + x_delta;
    int new_y = y + y_delta;
    /* target can move to empty (' ') or target only */
    if (new_x < 0 || new_x >= 4 || new_y < 0 || new_y >= 5) return 0;
    int destination = pos[new_y][new_x];
    if (!(destination == selected || destination == ' ')) return 0;
  }

  /* move */
  char new_pos[5][4];
  memset(new_pos, ' ', 5*4);
  for (int y = 0; y < 5; y++) for (int x = 0; x < 4; x++) {
    if (pos[y][x] == ' ') continue;
    if (pos[y][x] != selected) { new_pos[y][x] = pos[y][x]; continue; }
    int new_x = x + x_delta;
    int new_y = y + y_delta;
    new_pos[new_y][new_x] = selected;
  }

  memcpy(pos, new_pos, 5*4);

  return 1;
}

int main(void)
{
  Display *d;
  Window w;
  Pixmap p;
  fd_set r;
  int redraw;
  int repaint;

  int mouse_x = -1;
  int mouse_y = -1;
  char selected = -1;

  char pos[5][4]; memcpy(pos, init_pos, 5*4);

  cs = cairo_image_surface_create(CAIRO_FORMAT_ARGB32,
                                  WIDTH, HEIGHT);

  d = XOpenDisplay(0); if (!d) abort();
  w = XCreateSimpleWindow(d, DefaultRootWindow(d), 0, 0,
                          WIDTH, HEIGHT, 0, 0, 0);

  XSelectInput(d, w, KeyPressMask | ExposureMask | PointerMotionMask
                     | ButtonPressMask | EnterWindowMask | LeaveWindowMask);
  XMapWindow(d, w);

  p = XCreatePixmap(d, w, WIDTH,HEIGHT,
                    DefaultDepth(d, DefaultScreen(d)));
  csout = cairo_xlib_surface_create(d, p, DefaultVisual(d, DefaultScreen(d)),
                                    WIDTH, HEIGHT);

  /* get a smaller cursor */
  Cursor c = XCreateFontCursor(d, XC_plus);
  XDefineCursor(d, w, c);
  XFreeCursor(d, c);

  paint(pos, selected);
  redraw = 0;
  repaint = 0;

  while (1) {
    XEvent ev;
    while (XPending(d)) {
      XNextEvent(d, &ev);
      switch (ev.type) {
      case Expose: redraw = 1; break;
      case KeyPress: {
        char buf[65];
        KeySym ks;
        XLookupString(&ev.xkey, buf, 64, &ks, NULL);
        switch (ks) {
        case XK_Right: if (move(pos, 1, 0, selected)) repaint = 1; break;
        case XK_Left: if (move(pos, -1, 0, selected)) repaint = 1; break;
        case XK_Up: if (move(pos, 0, -1, selected)) repaint = 1; break;
        case XK_Down: if (move(pos, 0, 1, selected)) repaint = 1; break;
        default: break;
        }
        break;
      }
      case MotionNotify: {
        mouse_x = ev.xmotion.x;
        mouse_y = ev.xmotion.y;
        break;
      }
      case EnterNotify: {
        mouse_x = ev.xcrossing.x;
        mouse_y = ev.xcrossing.y;
        break;
      }
      case LeaveNotify: {
        mouse_x = mouse_y = -1;
        break;
      }
      default: break;
      }
    }
    char new_selected = get_selected_from_mouse(pos, mouse_x, mouse_y);
    if (new_selected != selected) {
      selected = new_selected;
      repaint = 1;
    }
    if (repaint) { paint(pos, selected); repaint = 0; redraw = 1; }
    if (redraw) { expose(d, w, p); redraw = 0; }
    FD_ZERO(&r);
    FD_SET(ConnectionNumber(d), &r);
    XFlush(d);
    if (select(ConnectionNumber(d)+1, &r, 0, 0, 0) == -1) abort();
  }

  XFreePixmap(d, p);
  XCloseDisplay(d);
  cairo_surface_destroy(cs);
  cairo_surface_destroy(csout);
  cairo_debug_reset_static_data();

  return 0;
}