#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <SDL2/SDL.h>

#define DEBUG 0
#define TDISP 0

typedef uint8_t		byte;
typedef uint16_t 	word;

typedef struct {
	byte memory[0x1000];
	byte screen[64][32];
	byte registers[0x10];
	word stack[24];
	byte input[0x10];

	word address_I;
	word program_counter;
	byte stack_pointer;
} emulator_state_s;

byte chip8_fontset[80] =
{ 
  0xF0, 0x90, 0x90, 0x90, 0xF0, // 0
  0x20, 0x60, 0x20, 0x20, 0x70, // 1
  0xF0, 0x10, 0xF0, 0x80, 0xF0, // 2
  0xF0, 0x10, 0xF0, 0x10, 0xF0, // 3
  0x90, 0x90, 0xF0, 0x10, 0x10, // 4
  0xF0, 0x80, 0xF0, 0x10, 0xF0, // 5
  0xF0, 0x80, 0xF0, 0x90, 0xF0, // 6
  0xF0, 0x10, 0x20, 0x40, 0x40, // 7
  0xF0, 0x90, 0xF0, 0x90, 0xF0, // 8
  0xF0, 0x90, 0xF0, 0x10, 0xF0, // 9
  0xF0, 0x90, 0xF0, 0x90, 0x90, // A
  0xE0, 0x90, 0xE0, 0x90, 0xE0, // B
  0xF0, 0x80, 0x80, 0x80, 0xF0, // C
  0xE0, 0x90, 0x90, 0x90, 0xE0, // D
  0xF0, 0x80, 0xF0, 0x80, 0xF0, // E
  0xF0, 0x80, 0xF0, 0x80, 0x80  // F
};

byte keymap[0x10] = { 	SDL_SCANCODE_X, SDL_SCANCODE_1, SDL_SCANCODE_2, SDL_SCANCODE_3, SDL_SCANCODE_Q, 
						SDL_SCANCODE_W, SDL_SCANCODE_E, SDL_SCANCODE_A, SDL_SCANCODE_S, SDL_SCANCODE_D, 
						SDL_SCANCODE_Z, SDL_SCANCODE_C, SDL_SCANCODE_4, SDL_SCANCODE_R, SDL_SCANCODE_F, SDL_SCANCODE_V };

int main(int argc, char **argv) {
	SDL_Window *window = NULL;
	SDL_Renderer *renderer = NULL;
	SDL_Init(SDL_INIT_VIDEO);

	window = SDL_CreateWindow("Chip8 Emulator", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 64 * 8, 32 * 8, SDL_WINDOW_SHOWN);
	renderer = SDL_CreateRenderer(window, 0, SDL_RENDERER_ACCELERATED);
	if (renderer == NULL) printf( "Renderer could not be created! SDL Error: %s\n", SDL_GetError() ); 
	SDL_SetRenderDrawColor(renderer, 0xFF, 0xFF, 0xFF, 0xFF);

	srand(time(NULL));
	
	emulator_state_s state = { 0 }; // initialize everything to 0
	state.program_counter = 0x200; // chip8 programs start at 0x200 address space
	memcpy(state.memory, chip8_fontset, 80);
	
	FILE *f = fopen(argv[1], "rb");
	fread(&state.memory[0x200], 0x1000 - 0x200, 1, f); // see above comment
	fclose(f);

	SDL_Event e;

	int delay = 0;
	int sound = 0;
	int frame = 0;
	int press = 0;
	int wait = 0;
	int store = 0;

	while (1) {
		if (DEBUG) printf("0x%x ", state.program_counter);
	
		if (frame == 10)
		{
			frame = 0;
			if (delay > 0) --delay;
			if (sound > 0) --sound;
			memset(state.input, 0, 0x10);
			press = 0;
		}

		// opcodes are 16 bits

		SDL_PollEvent(&e);
		if (e.type == SDL_QUIT) break;

		// SDL_PumpEvents();
		const byte *keystate = SDL_GetKeyboardState(NULL);

		for (int i = 0; i < 0x10; ++i) {
			if (keystate[keymap[i]]){
				state.input[i] = 1;
				press = 1;
			}

		}

		if (wait && !press) continue;
		if (wait && press) 
		{
			for (int i = 0; i < 0x10; ++i) {
				if (state.input[i] != 0)
				{
					state.registers[store] = i;
					break;
				}
			}
			wait = 0;
		}

		word opcode = (state.memory[state.program_counter++] << 8) | (state.memory[state.program_counter++]);

		switch (opcode & 0xF000) 
		{
			case 0x0000: // screen clear or function return 
				if (opcode == 0x00E0){
					memset(state.screen, 0, 64 * 32);
					if (DEBUG) printf("Clearing screen ");
				}
				
				else if (opcode == 0x00EE) {
					state.program_counter = state.stack[--state.stack_pointer];
					if (DEBUG) printf("Returning to %x ", state.stack[state.stack_pointer]);
				}

				break;
			
			case 0x1000: // goto statement
				state.program_counter = opcode & 0x0FFF;
				if (DEBUG) printf("Jumping to %x ", state.program_counter);

				break;

			case 0x2000: // CALL
				state.stack[state.stack_pointer++] = state.program_counter;
				state.program_counter = opcode & 0x0FFF;
				if (DEBUG) printf("Calling subroutine at %x (returning to %x)", state.program_counter, state.stack[state.stack_pointer - 1]);

				break;

			case 0x3000:; // skip if reg 0x0F00 is equal to constant 0x00FF 
				int reg = (opcode & 0x0F00) >> 8;
				if (DEBUG) printf("Comparing (==) register %i (%x) to %x ", reg, state.registers[reg], (byte) opcode);
				if (state.registers[reg] == (byte) opcode){
					state.program_counter += 2;
					if (DEBUG) printf("and incrementing program_counter ");
				}

				break;

			case 0x4000: // skip if not equal
				reg = (opcode & 0x0F00) >> 8;
				if (DEBUG) printf("Comparing (!=) register %i (%x) to %x ", reg, state.registers[reg], (byte) opcode);
				if (state.registers[reg] != (byte) opcode){
					state.program_counter += 2;
					if (DEBUG) printf("and incrementing program_counter ");
				}

				break;

			case 0x5000:; // skip if reg 0x0F00 is equal to 0x00F0
				int first = (opcode & 0x0F00) >> 8;
				int second = (opcode & 0x00F0) >> 4;
				if (DEBUG) printf("Comparing (==) register %i (%x) to %i (%x) ", first, state.registers[first], second, state.registers[second]);
				if (state.registers[first] == state.registers[second]){
					state.program_counter += 2;
					if (DEBUG) printf("and incrementing program_counter ");
				}

				break;

			case 0x6000: // set reg to constant
				reg = (opcode & 0x0F00) >> 8;
				state.registers[reg] = (byte) opcode;
				if (DEBUG) printf("Assigning %hx to %i", state.registers[reg], reg);

				break;

			case 0x7000: // add constant to reg
				reg = (opcode & 0x0F00) >> 8;
				state.registers[reg] += (byte) opcode;
				if (DEBUG) printf("Add %hi to reg %i (%hi)", opcode & 0x00FF, reg, state.registers[reg]);
				
				break;

			case 0x8000:; // math/bitop operators
				first = (opcode & 0x0F00) >> 8; // all use XY regs
				second = (opcode & 0x00F0) >> 4;

				switch (opcode & 0x000F) 
				{
					case 0: // copy 1 register to another
						state.registers[first] = state.registers[second];
						if (DEBUG) printf("Copied register %i to %i (%hx)", first, second, state.registers[first]);
						break;

					case 1: // X = X | Y
						state.registers[first] |= state.registers[second];
						if (DEBUG) printf("OR register %i and %i (%hx)", first, second, state.registers[first]);
						break;

					case 2: // X = X & Y
						state.registers[first] &= state.registers[second];
						if (DEBUG) printf("AND register %i and %i (%hx)", first, second, state.registers[first]);
						break;

					case 3: // X = X ^ Y
						state.registers[first] ^= state.registers[second];
						if (DEBUG) printf("XOR register %i to %i (%hx)", first, second, state.registers[first]);
						break;

					case 4:; // X = X + Y, VF = X + 5 > 255
						unsigned int add = state.registers[first] + state.registers[second];
						if (add > 255) state.registers[0xF] = 1;
						else state.registers[0xF] = 0;
						state.registers[first] = (byte) add;
						if (DEBUG) printf("ADD register %i to %i (%hx) ", second, first, state.registers[first]);
						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
						break;

					case 5: // X = X - Y, VF = X - Y < 0
						state.registers[first] -= state.registers[second];
						if (state.registers[first] > state.registers[second]) state.registers[0xF] = 0;
						else state.registers[0xF] = 1;
						if (DEBUG) printf("SUB register %i from %i (%hx) ", second, first, state.registers[first]);
						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
						break;

					case 6: // set least significant bit of X in VF, bitshift right
						state.registers[0xF] = state.registers[first] & 0b00000001;
						state.registers[first] /= 2;
						if (DEBUG) printf("RS register %i (%hx) ", first, state.registers[first]);
						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
						break;

					case 7: // X = Y - X, VF = Y - X < 0
						state.registers[first] = state.registers[second] - state.registers[first];
						if (state.registers[second] > state.registers[first]) state.registers[0xF] = 1;
						else state.registers[0xF] = 0;
						if (DEBUG) printf("SUB %i from %i (%hx) ", first, second, state.registers[first]);
						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
						break;

					case 0xE:; // set msot significant bit of X in VF, bitshift left
						state.registers[0xF] = (state.registers[first] & 0b10000000) >> 7;
						state.registers[first] *= 2;
						if (DEBUG) printf("LS register %i (%hx) ", first, state.registers[first]);
						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
						break;

					default: break;

				} break;

			case 0x9000: // skip if two regs are not equal
				first = (opcode & 0x0F00) >> 8;
				second = (opcode & 0x00F0) >> 4;
				if (DEBUG) printf("Comparing (!=) register %i (%hx) to %i (%hx) ", first, state.registers[first], second, state.registers[second]);
				if (state.registers[first] != state.registers[second]) {
					state.program_counter += 2;
					if (DEBUG) printf("and incrementing program_counter");
				}
				break;

			case 0xA000: // assign I address
				state.address_I = opcode & 0x0FFF;
				if (DEBUG) printf("Assigning %x to I address ", opcode & 0x0FFF);

				break;

			case 0xB000: // jump to 0x0FFF + V0
				state.program_counter = (opcode & 0x0FFF) + state.registers[0x0];
				if (DEBUG) printf("Jumping to %x + %hx (%x)", opcode & 0x0FFF, state.registers[0], (opcode & 0x0FFF) + state.registers[0x0]);

				break;

			case 0xC000: // random generator
				reg = (opcode & 0x0F00) >> 8;
				state.registers[reg] = (rand() % 255) & ((byte) opcode);
				if (DEBUG) printf("Storing random in %i (%x)\n", reg, state.registers[reg]);

				break;

			case 0xD000: // draw sprite
				first = (opcode & 0x0F00) >> 8;
				second = (opcode & 0x00F0) >> 4;

				int x = state.registers[first];
				int y = state.registers[second];
				int h = (opcode & 0x000F);

				for (int line = 0; line < h; ++line)
				{
					byte data = state.memory[state.address_I + line];
					for (int xpos = 0; xpos < 8; ++xpos)
					{
						int ypos = y + line;
						byte mask = 1 << 7 - xpos;
						if (ypos > 31) ypos -= 32;
						if (!(data & mask)) continue;
						if (state.screen[x + xpos][ypos]){
							state.registers[0xF] = 1;
						}
						state.screen[x + xpos][ypos] ^= 1;
					}
				}

				if (DEBUG) printf("Drawing");
				if (!TDISP) break;
				printf("\n");
				for (int y = 0; y < 32; ++y) {
					for (int x = 0; x < 64; ++x) {
						printf(state.screen[x][y] ? "O" : " ");
					}
					printf("\n");
				}

				break;

			case 0xE000: // input handling
				reg = (opcode & 0x0F00) >> 8;
				switch ((byte) opcode)
				{
					case 0x9E: // skip if button pressed
						if (DEBUG) printf("Testing if %i is pressed", state.registers[reg]);
						if (state.input[state.registers[reg]]) state.program_counter += 2;
						break;

					case 0xA1: // skip if button *not* pressed
						if (DEBUG) printf("Testing if %i is not pressed", state.registers[reg]);
						if (!state.input[state.registers[reg]]) state.program_counter += 2;
						break;
				}
				break;

			case 0xF000: // misc (timer, input, sound, memory)
				reg = (opcode & 0x0F00) >> 8;
				switch ((byte) opcode)
				{
					case 0x07:
						if (DEBUG) printf("Copying delay timer to reg %i (%hx)", reg, state.registers[reg]);
						state.registers[reg] = delay;
						break;

					case 0x0A:
						wait = 1;
						store = reg;
						break;

					case 0x15:
						delay = state.registers[reg];
						break;

					case 0x18:
						sound = state.registers[reg];
						break;

					case 0x1E:
						state.address_I += state.registers[reg];
						if (DEBUG) printf("Adding reg %i (%hx) to I (%x)", reg, state.registers[reg], state.address_I);
						break;

					case 0x29:
						state.address_I = state.registers[reg] * 5;
						break;

					case 0x33:;
						int value = state.registers[reg];
						state.memory[state.address_I] = value / 100;
						state.memory[state.address_I + 1] = (value / 10) % 10;
						state.memory[state.address_I + 2] = value % 10;
						break;

					case 0x55:
						for (int i = 0; i <= reg; ++i)
							state.memory[state.address_I + i] = state.registers[i];
						break;

					case 0x65:
						for (int i = 0; i <= reg + 1; ++i)
							state.registers[i] = state.memory[state.address_I + i];
						break;

					default:
						break;
				}
				break;
			default: 
				if (DEBUG) printf("%x", opcode);
				break;
		}
		if (DEBUG) printf("\n");

		for (int x = 0; x < 64; ++x) {
			for (int y = 0; y < 32; ++y)
			{
				SDL_Rect pixel = { .x = (x * 8), .y = (y * 8), .w = 8, .h = 8};
				if (state.screen[x][y])
				{
					SDL_SetRenderDrawColor(renderer, 0xff, 0x69, 0xb4, 0xFF);
					SDL_RenderFillRect(renderer, &pixel);
				}
				else 
				{
					SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0xFF);
					SDL_RenderFillRect(renderer, &pixel);
				}
			}
		}

		SDL_RenderPresent(renderer);

		SDL_Delay(1);
		++frame;
		// SDL_DestroyWindow(window);
		// SDL_Quit();
	}
}