/*
 * Copyright (c) Camden Dixie O'Brien
 * SPDX-License-Identifier: AGPL-3.0-only
 */

#define _POSIX_C_SOURCE 199309L

#include <X11/Xlib.h>
#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <time.h>

#define MAZE_SIZE 24

#define GRID_SIZE (2 * MAZE_SIZE - 1)
#define MARGIN 2
#define WALL_THICKNESS 1
#define WINDOW_SIZE (GRID_SIZE + 2 * (WALL_THICKNESS + MARGIN))

#define PX(x) ((x) << 3)

#define GOAL (GRID_SIZE - 1)

typedef enum { LEFT, RIGHT, UP, DOWN } dir_t;

typedef struct {
	int x, y;
} vec2_t;

typedef bool (*coord_pred_t)(vec2_t c, vec2_t im);
typedef bool (*visit_fn_t)(vec2_t c, vec2_t im);

typedef struct {
	bool is_path : 1;
	bool visited : 1;
} cell_t;

static const struct timespec pause = { .tv_nsec = 5000000 };

static const vec2_t steps[] = {
	[LEFT] = { .x = -2, .y = 0 },
	[RIGHT] = { .x = 2, .y = 0 },
	[UP] = { .x = 0, .y = -2 },
	[DOWN] = { .x = 0, .y = 2 },
};

static Display *dpy;
static Window window;
static GC ctx;
static cell_t maze[GRID_SIZE][GRID_SIZE];
static int bg_col, wall_col, visited_col;

static void draw_walls(void)
{
	XSetForeground(dpy, ctx, wall_col);

	XFillRectangle(
	    dpy, window, ctx, PX(MARGIN), PX(MARGIN), PX(GRID_SIZE + 2),
	    PX(WALL_THICKNESS));
	XFillRectangle(
	    dpy, window, ctx, PX(MARGIN),
	    PX(MARGIN + WALL_THICKNESS + GRID_SIZE), PX(GRID_SIZE + 2),
	    PX(WALL_THICKNESS));
	XFillRectangle(
	    dpy, window, ctx, PX(MARGIN), PX(MARGIN + WALL_THICKNESS),
	    PX(WALL_THICKNESS), PX(GRID_SIZE));
	XFillRectangle(
	    dpy, window, ctx, PX(MARGIN + WALL_THICKNESS + GRID_SIZE),
	    PX(MARGIN + WALL_THICKNESS), PX(WALL_THICKNESS), PX(GRID_SIZE - 1));

	XFlush(dpy);
}

static void draw_maze(void)
{
	const int margin_px = PX(MARGIN + WALL_THICKNESS);
	XClearArea(
	    dpy, window, margin_px, margin_px, PX(GRID_SIZE), PX(GRID_SIZE),
	    false);

	for (int x = 0; x < GRID_SIZE; ++x) {
		for (int y = 0; y < GRID_SIZE; ++y) {
			if (!maze[x][y].is_path)
				XSetForeground(dpy, ctx, wall_col);
			else if (maze[x][y].visited)
				XSetForeground(dpy, ctx, visited_col);
			else
				continue;
			const int left = margin_px + PX(x);
			const int top = margin_px + PX(y);
			XFillRectangle(dpy, window, ctx, left, top, PX(1), PX(1));
		}
	}

	XFlush(dpy);
}

static bool
random_walk(coord_pred_t should_visit, visit_fn_t visit_fn, vec2_t start)
{
	draw_maze();
	nanosleep(&pause, NULL);

	dir_t visit[] = { LEFT, RIGHT, UP, DOWN };
	for (int i = 3; i > 0; --i) {
		const int r = rand() % (i + 1);
		const dir_t tmp = visit[r];
		visit[r] = visit[i];
		visit[i] = tmp;
	}

	vec2_t next, im;
	for (int i = 0; i < 4; ++i) {
		next.x = start.x + steps[visit[i]].x;
		next.y = start.y + steps[visit[i]].y;
		im.x = (start.x + next.x) / 2;
		im.y = (start.y + next.y) / 2;

		const bool x_in_bounds = next.x >= 0 && next.x < GRID_SIZE;
		const bool y_in_bounds = next.y >= 0 && next.y < GRID_SIZE;
		if (x_in_bounds && y_in_bounds && should_visit(next, im)) {
			if (visit_fn(next, im)
			    || random_walk(should_visit, visit_fn, next))
				return true;
		}
	}

	return false;
}

static bool is_wall(vec2_t c, vec2_t im)
{
	(void)im;
	return !maze[c.x][c.y].is_path;
}

static bool generation_visit(vec2_t c, vec2_t im)
{
	maze[c.x][c.y].is_path = true;
	maze[im.x][im.y].is_path = true;
	return false;
}

static bool accessible_and_unvisited(vec2_t c, vec2_t im)
{
	return maze[im.x][im.y].is_path && !maze[c.x][c.y].visited;
}

static bool solve_visit(vec2_t c, vec2_t im)
{
	maze[c.x][c.y].visited = true;
	maze[im.x][im.y].visited = true;
	return GOAL == c.x && GOAL == c.y;
}

int main(void)
{
	// Seed random number generation from time
	struct timeval tv;
	gettimeofday(&tv, NULL);
	srand(tv.tv_usec);

	XEvent evt;
	dpy = XOpenDisplay(NULL);
	assert(dpy);

	// Create window and configure graphics context
	wall_col = BlackPixel(dpy, DefaultScreen(dpy));
	bg_col = WhitePixel(dpy, DefaultScreen(dpy));
	window = XCreateSimpleWindow(
	    dpy, DefaultRootWindow(dpy), 0, 0, PX(WINDOW_SIZE), PX(WINDOW_SIZE),
	    0, bg_col, bg_col);
	Atom del = XInternAtom(dpy, "WM_DELETE_WINDOW", false);
	XSetWMProtocols(dpy, window, &del, 1);
	ctx = DefaultGC(dpy, DefaultScreen(dpy));

	// Create colormap and allocate colour for visited cells
	Colormap cm = XCreateColormap(
	    dpy, window, DefaultVisual(dpy, DefaultScreen(dpy)), AllocNone);
	XColor xcol = { .red = 55555, .green = 10000, .blue = 10000 };
	XAllocColor(dpy, cm, &xcol);
	visited_col = xcol.pixel;

	// Map window
	XSelectInput(dpy, window, StructureNotifyMask);
	XMapWindow(dpy, window);
	do
		XNextEvent(dpy, &evt);
	while (MapNotify != evt.type);

	draw_walls();

	// Generate
	memset(&maze, 0, sizeof(maze));
	const vec2_t gen_start = { GOAL, GOAL };
	maze[GOAL][GOAL].is_path = true;
	random_walk(is_wall, generation_visit, gen_start);

	// Solve
	const vec2_t solve_start = { 0, 0 };
	maze[0][0].visited = true;
	random_walk(accessible_and_unvisited, solve_visit, solve_start);

	// Draw exit path
	draw_maze();
	const int x = PX(MARGIN + GRID_SIZE + 1), y = PX(MARGIN + GRID_SIZE);
	XSetForeground(dpy, ctx, visited_col);
	XFillRectangle(dpy, window, ctx, x, y, PX(1), PX(1));
	XFlush(dpy);

	// Wait for window exit
	bool is_del = false;
	do {
		XNextEvent(dpy, &evt);
		if (ClientMessage == evt.type)
			is_del = (unsigned long)evt.xclient.data.l[0] == del;
	} while (!is_del);

	XCloseDisplay(dpy);
	return EXIT_SUCCESS;
}