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

#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <assert.h>
#include <ctype.h>
#include <libxml/parser.h>
#include <math.h>
#include <stdbool.h>
#include <string.h>

#define SCALE 4

#define TILESIZE 16
#define VIEWWIDTH (16 * TILESIZE)
#define VIEWHEIGHT (12 * TILESIZE)

#define MAX_PATH_LEN 128

#define MAP_ASSET "/map.tmx"
#define MAPNLAYERS 2
#define MAPWIDTH 64
#define MAPHEIGHT 64
#define MAPSHIFTX 32
#define MAPSHIFTY 32
#define MAPMINX (TILESIZE * (-32))
#define MAPMAXX (TILESIZE * 32)
#define MAPMINY (TILESIZE * (-32))
#define MAPMAXY (TILESIZE * 32)

#define TSASSET "/overworld.png"
#define TSCOLS 40

#define PIDLE_ASSET "/player/idle.png"
#define PWALK_ASSET "/player/walk.png"
#define PWIDTH 48
#define PHEIGHT 64
#define PANIMLEN 8
#define PFBWIDTH 20
#define PFBHEIGHT 16
#define PFBOFFX -10
#define PFBOFFY 2

#define WALKSPEED 72 // pixels per second

#define BASEANIMPERIOD 100

#define NELEMS(a) (sizeof(a) / sizeof(a[0]))

typedef enum {
	SPRITE_DIR_DOWN = 0,
	SPRITE_DIR_LEFT_DOWN = 1,
	SPRITE_DIR_LEFT_UP = 2,
	SPRITE_DIR_UP = 3,
	SPRITE_DIR_RIGHT_UP = 4,
	SPRITE_DIR_RIGHT_DOWN = 5,
} sprite_dir_t;

typedef struct {
	bool left, right, up, down;
} input_state_t;

typedef struct {
	int x, y;
} ivec_t;

typedef struct {
	double x, y;
} dvec_t;

typedef struct {
	dvec_t off, ext;
} dbox_t;

typedef struct {
	int svar, animlen;
	double speed;
	dvec_t pos, dir;
	ivec_t animstep, svarstep;
	dbox_t fbox;
	SDL_Texture *tex;
	SDL_Rect src;
} entity_t;

static SDL_Window *window;
static SDL_Renderer *renderer;
static unsigned map[MAPNLAYERS][MAPWIDTH][MAPHEIGHT];
static SDL_Texture *tstex, *pidle, *pwalk;
static input_state_t input;
static dvec_t vpos = { -128, -96 };
static const unsigned impassable[] = {
	284, 485, 486, 525, 527, 566, 567, 731,
	768, 770, 771, 804, 805, 806, 808, 845,
};

static inline double mag(dvec_t v)
{
	return sqrt(v.x * v.x + v.y * v.y);
}

static inline double dot(dvec_t v, dvec_t u)
{
	return v.x * u.x + v.y * u.y;
}

static inline unsigned tileat(int layeridx, double x, double y)
{
	const unsigned row = (unsigned)floor(x / TILESIZE) + MAPSHIFTX;
	const unsigned col = (unsigned)floor(y / TILESIZE) + MAPSHIFTY;
	if (row >= MAPWIDTH || col >= MAPHEIGHT)
		return 0;
	else
		return map[layeridx][row][col];
}

static inline bool tilepassable(int x, int y)
{
	for (unsigned l = 0; l < MAPNLAYERS; ++l) {
		const unsigned id = tileat(l, x, y);
		for (unsigned i = 0; i < NELEMS(impassable); ++i) {
			if (impassable[i] == id)
				return false;
		}
	}
	return true;
}

static void maploadlayer(xmlNodePtr layernode, int layeridx)
{
	xmlNodePtr node = layernode->xmlChildrenNode;
	while (NULL != node
	       && xmlStrcmp(node->name, (const xmlChar *)"data") != 0)
		node = node->next;
	assert(NULL != node);

	// Iterate through chunks and populate map array.
	xmlNodePtr chunk_contents;
	for (node = node->xmlChildrenNode; NULL != node; node = node->next) {
		if (0 != xmlStrcmp(node->name, (const xmlChar *)"chunk"))
			continue;

		xmlChar *x_attr, *y_attr;
		x_attr = xmlGetProp(node, (const xmlChar *)"x");
		y_attr = xmlGetProp(node, (const xmlChar *)"y");
		const int chunk_x = atoi((const char *)x_attr);
		const int chunk_y = atoi((const char *)y_attr);

		chunk_contents = node->xmlChildrenNode;
		assert(
		    0 == xmlStrcmp(chunk_contents->name, (const xmlChar *)"text"));
		int x = chunk_x + MAPSHIFTX, y = chunk_y + MAPSHIFTY;
		xmlChar buf[10];
		const xmlChar *p, *q;
		q = chunk_contents->content;
		while (isspace((const char)*q))
			++q;
		p = q;
		while ('\0' != *q) {
			switch (*q) {
			case (xmlChar)',':
				memset(buf, 0, sizeof(buf));
				memcpy(buf, p, sizeof(xmlChar) * (q - p));
				assert(x < MAPWIDTH && y < MAPHEIGHT);
				map[layeridx][x][y] = (unsigned)atoi((const char *)buf);
				++x;
				p = ++q;
				break;
			case (xmlChar)'\n':
				if (x < MAPWIDTH) {
					memset(buf, 0, sizeof(buf));
					memcpy(buf, p, sizeof(xmlChar) * (q - p));
					assert(y < MAPHEIGHT);
					map[layeridx][x][y] = (unsigned)atoi((const char *)buf);
				}
				x = chunk_x + MAPSHIFTX;
				++y;
				p = ++q;
				break;
			default:
				++q;
				break;
			}
		}
	}
}

static void mapload(const char *path)
{
	xmlDocPtr doc;
	xmlNodePtr node;
	doc = xmlParseFile(path);
	assert(NULL != doc);
	node = xmlDocGetRootElement(doc);
	assert(0 == xmlStrcmp(node->name, (const xmlChar *)"map"));
	node = node->xmlChildrenNode;
	for (int l = 0; l < MAPNLAYERS; ++l) {
		do
			node = node->next;
		while (NULL != node
		       && xmlStrcmp(node->name, (const xmlChar *)"layer") != 0);
		assert(NULL != node);
		maploadlayer(node, l);
	}
}

static void mapdraw(void)
{
	const int startx = TILESIZE * floor(vpos.x / TILESIZE);
	const int starty = TILESIZE * floor(vpos.y / TILESIZE);
	for (int l = 0; l < MAPNLAYERS; ++l) {
		for (int y = starty; y < vpos.y + VIEWHEIGHT; y += TILESIZE) {
			for (int x = startx; x < vpos.x + VIEWWIDTH; x += TILESIZE) {
				const unsigned tileid = tileat(l, x, y);
				if (0 == tileid)
					continue;
				const SDL_Rect src = {
					.x = TILESIZE * ((tileid - 1) % TSCOLS),
					.y = TILESIZE * ((tileid - 1) / TSCOLS),
					.w = TILESIZE,
					.h = TILESIZE,
				};
				const SDL_Rect dest = {
					.x = SCALE * (x - vpos.x),
					.y = SCALE * (y - vpos.y),
					.w = SCALE * TILESIZE,
					.h = SCALE * TILESIZE,
				};
				SDL_RenderCopy(renderer, tstex, &src, &dest);
			}
		}
	}
}

static void entityupdate(entity_t *e, double dt)
{
	if (0 == e->speed)
		return;

	// Update sprite variant
	if (e->dir.y >= 0) {
		if (e->dir.x > 0)
			e->svar = SPRITE_DIR_RIGHT_DOWN;
		else if (e->dir.x < 0)
			e->svar = SPRITE_DIR_LEFT_DOWN;
		else
			e->svar = SPRITE_DIR_DOWN;
	} else if (e->dir.y < 0) {
		if (e->dir.x > 0)
			e->svar = SPRITE_DIR_RIGHT_UP;
		else if (e->dir.x < 0)
			e->svar = SPRITE_DIR_LEFT_UP;
		else
			e->svar = SPRITE_DIR_UP;
	}

	// Apply velocity (handling map collisions)
	const double nextx = e->pos.x + dt * e->speed * e->dir.x;
	const double nexty = e->pos.y + dt * e->speed * e->dir.y;
	const dvec_t pfb
	    = { .x = nextx + e->fbox.off.x, .y = nexty + e->fbox.off.y };
	bool valid = true;
	valid &= tilepassable(pfb.x, pfb.y);
	valid &= tilepassable(pfb.x + e->fbox.ext.x, pfb.y);
	valid &= tilepassable(pfb.x, pfb.y + e->fbox.ext.y);
	valid &= tilepassable(pfb.x + e->fbox.ext.x, pfb.y + e->fbox.ext.y);
	if (valid) {
		e->pos.x = nextx;
		e->pos.y = nexty;
	}
}

static void entitydraw(entity_t *e, uint64_t t)
{
	const unsigned frame = (t / BASEANIMPERIOD) % e->animlen;
	e->src.x = e->animstep.x * frame + e->svarstep.x * e->svar;
	e->src.y = e->animstep.y * frame + e->svarstep.y * e->svar;
	SDL_Rect dest = {
		.x = SCALE * (int)(e->pos.x - vpos.x - e->src.w / 2),
		.y = SCALE * (int)(e->pos.y - vpos.y - e->src.h / 2),
		.w = SCALE * e->src.w,
		.h = SCALE * e->src.h,
	};
	SDL_RenderCopy(renderer, e->tex, &e->src, &dest);
}

int main(int argc, char *argv[])
{
	char path[MAX_PATH_LEN];

	if (2 != argc) {
		fprintf(stderr, "Usage: %s ASSETS-DIR\n", argv[0]);
		return 1;
	}

	// Set up SDL window
	int err = SDL_Init(SDL_INIT_VIDEO);
	assert(0 == err);
	window = SDL_CreateWindow(
	    "2D Game", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
	    SCALE * VIEWWIDTH, SCALE * VIEWHEIGHT, 0);
	assert(NULL != window);
	renderer = SDL_CreateRenderer(window, -1, 0);
	assert(NULL != renderer);

	// Load map
	assert(strlen(argv[1]) + strlen(MAP_ASSET) < MAX_PATH_LEN);
	strcpy(path, argv[1]);
	strcat(path, MAP_ASSET);
	mapload(path);

	// Load tileset
	assert(strlen(argv[1]) + strlen(TSASSET) < MAX_PATH_LEN);
	strcpy(path, argv[1]);
	strcat(path, TSASSET);
	tstex = IMG_LoadTexture(renderer, path);
	assert(NULL != tstex);

	// Load player spritesheets
	assert(strlen(argv[1]) + strlen(PIDLE_ASSET) < MAX_PATH_LEN);
	strcpy(path, argv[1]);
	strcat(path, PIDLE_ASSET);
	pidle = IMG_LoadTexture(renderer, path);
	assert(NULL != pidle);
	assert(strlen(argv[1]) + strlen(PWALK_ASSET) < MAX_PATH_LEN);
	strcpy(path, argv[1]);
	strcat(path, PWALK_ASSET);
	pwalk = IMG_LoadTexture(renderer, path);
	assert(NULL != pwalk);

	// Initialize player
	entity_t p = {
		.svar = SPRITE_DIR_DOWN,
		.animlen = PANIMLEN,
		.tex = pidle,
		.animstep = { .x = PWIDTH },
		.svarstep = { .y = PHEIGHT },
		.fbox = {
			.off = { .x = PFBOFFX, .y = PFBOFFY },
			.ext = { .x = PFBWIDTH, .y = PFBHEIGHT },
		},
		.src = { .w = PWIDTH, .h = PHEIGHT },
	};

	SDL_Event event;
	uint64_t prevt = SDL_GetTicks64();
	while (1) {
		// Calculate dt
		const uint64_t t = SDL_GetTicks64();
		const uint64_t dt_ms = t - prevt;
		const double dt = dt_ms / 1000.0;
		prevt = t;

		// Handle events
		SDL_PollEvent(&event);
		switch (event.type) {
		case SDL_QUIT:
			goto quit;

		case SDL_KEYDOWN:
			switch (event.key.keysym.sym) {
			case SDLK_LEFT:
				input.left = true;
				break;
			case SDLK_RIGHT:
				input.right = true;
				break;
			case SDLK_UP:
				input.up = true;
				break;
			case SDLK_DOWN:
				input.down = true;
				break;
			default:
				break;
			}
			break;

		case SDL_KEYUP:
			switch (event.key.keysym.sym) {
			case SDLK_LEFT:
				input.left = false;
				break;
			case SDLK_RIGHT:
				input.right = false;
				break;
			case SDLK_UP:
				input.up = false;
				break;
			case SDLK_DOWN:
				input.down = false;
				break;
			default:
				break;
			}
			break;

		default:
			break;
		}

		// Calculate player velocity and update player
		p.dir.x = (input.left ? -1 : 0) + (input.right ? 1 : 0);
		p.dir.y = (input.up ? -1 : 0) + (input.down ? 1 : 0);
		const double dmag = mag(p.dir);
		if (dmag != 0) {
			p.dir.x /= dmag;
			p.dir.y /= dmag;
			p.tex = pwalk;
			p.speed = WALKSPEED;
		} else {
			p.tex = pidle;
			p.speed = 0;
		}
		entityupdate(&p, dt);

		// Update view
		const dvec_t pvdisp = {
			.x = p.pos.x - (vpos.x + VIEWWIDTH / 2),
			.y = p.pos.y - (vpos.y + VIEWHEIGHT / 2),
		};
		if (mag(pvdisp) > 72 && dot(pvdisp, p.dir) > 0) {
			const double nextx = vpos.x + dt * p.speed * p.dir.x;
			const double nexty = vpos.y + dt * p.speed * p.dir.y;
			const bool validx = nextx >= MAPMINX && nextx < MAPMAXX;
			const bool validy = nexty >= MAPMINY && nexty < MAPMAXY;
			if (validx && validy) {
				vpos.x = nextx;
				vpos.y = nexty;
			}
		}

		SDL_RenderClear(renderer);
		mapdraw();
		entitydraw(&p, t);
		SDL_RenderPresent(renderer);
	}

quit:
	SDL_DestroyRenderer(renderer);
	SDL_DestroyWindow(window);
	SDL_Quit();

	return 0;
}