/*
 * 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>
#include <unistd.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)
#define MAX_PATH_LENGTH (MAZE_SIZE * MAZE_SIZE)

#define STACK_SIZE (MAZE_SIZE * MAZE_SIZE)

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 gen_pause = { .tv_nsec = 5000000 };
static const struct timespec solve_pause = { .tv_nsec = 10000000 };

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 clear_path(vec2_t p)
{
	const int margin_px = PX(MARGIN + WALL_THICKNESS);
	const int left = margin_px + PX(p.x);
	const int top = margin_px + PX(p.y);
	XFillRectangle(dpy, window, ctx, left, top, PX(1), PX(1));
	XClearArea(dpy, window, left, top, PX(1), PX(1), false);
	XFlush(dpy);
}

static void draw_visited(vec2_t p)
{
	const int margin_px = PX(MARGIN + WALL_THICKNESS);
	const int left = margin_px + PX(p.x);
	const int top = margin_px + PX(p.y);
	XFillRectangle(dpy, window, ctx, left, top, PX(1), PX(1));
	XFlush(dpy);
}

static bool in_bounds(vec2_t p)
{
	const bool valid_x = p.x >= 0 && p.x < GRID_SIZE;
	const bool valid_y = p.y >= 0 && p.y < GRID_SIZE;
	return valid_x && valid_y;
}

static bool finished_gen(vec2_t p)
{
	vec2_t n;
	for (int i = 0; i < 4; ++i) {
		n.x = p.x + steps[i].x;
		n.y = p.y + steps[i].y;
		if (in_bounds(n) && !maze[n.x][n.y].is_path)
			return false;
	}
	return true;
}

static void generate(vec2_t p)
{
	vec2_t stack[STACK_SIZE], *sp = stack;
	do {
		if (finished_gen(p)) {
			p = *(--sp);
			continue;
		}

		vec2_t n;
		do {
			dir_t d = rand() % 4;
			n.x = p.x + steps[d].x;
			n.y = p.y + steps[d].y;
		} while (!in_bounds(n) || maze[n.x][n.y].is_path);

		const vec2_t im = {
			.x = (p.x + n.x) / 2,
			.y = (p.y + n.y) / 2,
		};
		maze[im.x][im.y].is_path = maze[n.x][n.y].is_path = true;
		clear_path(im);
		clear_path(n);

		*sp++ = p;
		p = n;

		nanosleep(&gen_pause, NULL);
	} while (sp != stack);
}

static void solve(vec2_t p, vec2_t *sp, vec2_t **end)
{
	*sp++ = p;
	while (1) {
		if (GOAL == p.x && GOAL == p.y) {
			*sp++ = p;
			*end = sp;
			return;
		}

		vec2_t n, im;
		bool got_n = false;
		for (int i = 0; i < 4; ++i) {
			n.x = p.x + steps[i].x;
			n.y = p.y + steps[i].y;
			if (!in_bounds(n))
				continue;

			im.x = (p.x + n.x) / 2;
			im.y = (p.y + n.y) / 2;
			if (maze[im.x][im.y].is_path && !maze[n.x][n.y].visited) {
				got_n = true;
				break;
			}
		}

		if (!got_n) {
			p = *(--sp);
			continue;
		}

		maze[im.x][im.y].visited = maze[n.x][n.y].visited = true;

		*sp++ = p;
		p = n;
	}
}

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);

	while (1) {
		// Draw black box for walls
		XClearWindow(dpy, window);
		XSetForeground(dpy, ctx, wall_col);
		XFillRectangle(
		    dpy, window, ctx, PX(MARGIN), PX(MARGIN), PX(GRID_SIZE + 2),
		    PX(GRID_SIZE + 2));
		const vec2_t exit = { GOAL + 1, GOAL };
		clear_path(exit);
		XFlush(dpy);

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

		// Solve
		const vec2_t solve_start = { 0, 0 };
		vec2_t path[MAX_PATH_LENGTH], *path_end;
		maze[0][0].visited = true;
		solve(solve_start, path, &path_end);

		sleep(1);

		// Draw solution path
		XSetForeground(dpy, ctx, visited_col);
		const vec2_t *prev = &solve_start;
		for (const vec2_t *p = path; p < path_end; ++p) {
			const vec2_t im = {
				.x = (prev->x + p->x) / 2,
				.y = (prev->y + p->y) / 2,
			};
			draw_visited(*p);
			draw_visited(im);
			nanosleep(&solve_pause, NULL);
			prev = p;
		}
		draw_visited(exit);

		sleep(1);
	}

	// 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;
}