/*
3ByteBadVM: a bad virtual machine using 3-byte instructions
x1phosura 2021

VM specifications

TODO: document VM stuff here once I write it in Markdown

use assert()s to make sure pc, sp, etc... are valid
*/

#include <assert.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef TRACE
	// readline is  L I T
	#include <readline/readline.h>
	#include <readline/history.h>
#endif

#include "vm.h"


#ifdef TRACE    // TRACE_VARS
struct TRACE_T trace_state;

char *trace_bad_cmd = "Unrecognized command.\n";
char *trace_cmd_help = "Supported commands are:\n"
           "s:               step single instruction\n"
           "c:               continue code execution until breakpoint or end\n"
           "b <addr>:        set breakpoint at given memory address\n"
           "pb:              print current breakpoints\n"
           "pr:              print register contents\n"
           "ps:              print stack contents\n"
           "pm <addr> <num>: print <num> bytes of memory starting at <addr>\n"
           "pz:              print zero page (1st 256 bytes)\n"
           "h:               print VM debugger command help message\n"
           "q:               quit VM debugger\n";
#endif          // TRACE_VARS


static inline void push(struct CPU *cpu, uint8_t val)
{
	cpu->stack[cpu->sp] = val;
	if (cpu->sp < STACK_LIM-1)
		++cpu->sp;
}


static inline uint8_t pop(struct CPU *cpu)
{
	uint8_t val;
	if (cpu->sp > 0) {
		val = cpu->stack[cpu->sp-1];
		--cpu->sp;
	} else
		return 0;
	return val;
}


/* vm_do_instruction:
 */
uint16_t vm_do_instruction(struct CPU *cpu, uint8_t *mem, uint8_t instr[3])
{
	uint16_t pc = cpu->pc;
	uint16_t sp = cpu->sp;
	uint8_t val = 0;
	uint8_t operands[2];
	uint8_t opcode = instr[0];
	operands[0] = instr[1];
	operands[1] = instr[2];
	// vvv- this format needed if operands treated as single 16-bit value
	uint16_t operand = ((uint16_t)instr[2] * 256) + instr[1]; 

	#ifdef TRACE
		printf("0x%04x:   ", pc);
		print_op_decoded(instr, true);
	#endif

	/* Note: the below code will likely be very unsafe. */
	switch (opcode) {
	case HALT:
		cpu->state = STOPPED;
		break;
	case PUSH:
		push(cpu, mem[operand]);
		pc += 3;
		break;
	case POP:
		mem[operand] = pop(cpu);
		pc += 3;
		break;
	case PUSHI:
		push(cpu, operands[0]);
		pc += 3;
		break;
	case LDLR: // TODO: fix!! This is broken
		// read little-endian
		cpu->lr = (uint16_t)(mem[operand+1]); // set MSB
		cpu->lr = cpu->lr << 8;               // set MSB
		cpu->lr += (uint16_t)(mem[operand]);  // set LSB
		pc += 3;
		break;
	case STLR:
		// write little-endian
		mem[operand]   = (uint8_t)((cpu->lr) & 0x00ff);      // set LSB
		mem[operand+1] = (uint8_t)((cpu->lr >> 8) & 0x00ff); // set MSB
		pc += 3;
		break;
	case SETI:   // peek ToS, write to memory
		mem[operand] = cpu->stack[sp-1];
		pc += 3;
		break;
	case DUP:
		val = cpu->stack[sp-1];
		push(cpu, val);
		pc += 3;
		break;
	case ADD:
		val = pop(cpu);
		val += pop(cpu);
		push(cpu, val);
		pc += 3;
		break;
	case SUB:
		val = pop(cpu);
		val = ~val;
		++val;
		val += pop(cpu);
		push(cpu, val);  // subtract 2nd from top by top of stack
		pc += 3;
		break;
	case XOR:
		val = pop(cpu);
		val ^= pop(cpu);
		push(cpu, val);
		pc += 3;
		break;
	case CALL:
		cpu->lr = pc+3;
		return operand;
		break;
	case RET:
		return cpu->lr;
		break;
	case JMP:
		return operand;
		break;
	case BEQ:
		if (sp > 1)      // if 2 or more items on stack
			if (sp > 0 && cpu->stack[sp-1] == cpu->stack[sp-2])
				return operand;
		pc += 3;
		break;
	case BNQ:
		if (sp > 1)      // if 2 or more items on stack
			if (sp > 0 && cpu->stack[sp-1] != cpu->stack[sp-2])
				return operand;
		pc += 3;
		break;
	case NOP:
		pc += 3;
		break;
	default:
		/* EVIL idea: have illegal instructions act as NOPs! */
		pc += 3;
	}

	return pc;
}


static inline void vm_fetch_instruction(uint16_t pc, uint8_t *mem,
                                                  uint8_t instr[3])
{
	assert(pc < RAM_SIZE - 3);
	instr[0] = mem[pc];
	instr[1] = mem[pc+1];
	instr[2] = mem[pc+2];                // finish instruction fetch
}


#ifdef TRACE    // TRACE_FUNCS
void print_op_decoded(uint8_t i[3], bool pargs)
{
	char fmt[] = " 0x%02x 0x%02x";
	uint8_t opcode = i[0];

	switch(opcode) {
	case HALT:  printf("HALT"); break;
	case PUSH:  printf("PUSH"); if (pargs) printf(fmt, i[1], i[2]); break;
	case POP:   printf("POP"); if (pargs) printf(fmt, i[1], i[2]); break;
	case PUSHI: printf("PUSHI"); if (pargs) printf(fmt, i[1], i[2]); break;
	case LDLR:  printf("LDLR"); if (pargs) printf(fmt, i[1], i[2]); break;
	case STLR:  printf("STLR"); if (pargs) printf(fmt, i[1], i[2]); break;
	case SETI:  printf("SETI"); if (pargs) printf(fmt, i[1], i[2]); break;
	case DUP:   printf("DUP"); break;
	case ADD:   printf("ADD"); break;
	case SUB:   printf("SUB"); break;
	case XOR:   printf("XOR"); break;
	case CALL:  printf("CALL"); if (pargs) printf(fmt, i[1], i[2]); break;
	case RET:   printf("RET"); break;
	case JMP:   printf("JMP"); if (pargs) printf(fmt, i[1], i[2]); break;
	case BEQ:   printf("BEQ"); if (pargs) printf(fmt, i[1], i[2]); break;
	case BNQ:   printf("BNQ"); if (pargs) printf(fmt, i[1], i[2]); break;
	case NOP:   printf("NOP"); break;
	default:    printf("0x%02x 0x%02x 0x%02x", i[0], i[1], i[2]);
	}

	putchar('\n');
}


void print_vm_registers(struct CPU *cpu)
{
	uint16_t pc = cpu->pc;
	uint16_t lr = cpu->lr;
	uint16_t sp = cpu->sp;
	//uint16_t a = cpu->a;
	printf("pc = %#x, lr = %#x, sp = %#x\n", pc, lr, sp);
}


void print_vm_stack(struct CPU *cpu)
{
	uint16_t sp = cpu->sp;

	for (uint8_t i = 0; i < sp; ++i) {
		if (i % 16 == 0)
			putchar('\n');
		printf("%02x ", cpu->stack[i]);
	}
	putchar('\n');
	for (int8_t j = 0; j < sp % 17; ++j)
		printf("   ");
	printf("^sp = 0x%02x\n", sp);
}


void print_vm_memory(uint8_t *mem, uint16_t start_addr, uint16_t num_bytes)
{
	for (uint16_t i = 0; i < num_bytes; ++i) {
		if (i % 16 == 0)
			printf("\n0x%04x: ", start_addr+i);
		printf("%02x ", mem[start_addr + i]);
	}
	putchar('\n');
}


void vm_trace(struct CPU *cpu, uint8_t *mem, struct TRACE_T *tstate)
{
	char *command;
	//char command[24];
	int mem_start = 0;
	int num_bytes = 0;
	rl_bind_key('\t', rl_insert); // make readline treat tabs and tabs

	if (cpu->pc == tstate->breakpoint) {
		printf("Breakpoint reached!\n");
		tstate->mode = STEP;
	}

	while(tstate->mode == STEP) {
		if ((command = readline("trace> ")) == NULL) {
			printf("Error: readline returned NULL. Aborting...\n");
			exit(0);
		}
		// Note: technically, command should eventually be freed, but
		// it's not actually that important here, and the OS will
		// reclaim heap space when the program finishes anyway.
		if (strlen(command) > 0)
			add_history(command);

		// yeah, yeah, I KNOW a switch-case would be better for this
		if (command[0] == 's') {
			break;
		} else if (command[0] == 'b') {
			int addr = 0;
			if (sscanf(command+1, "%i", &addr) == EOF)
				printf("%s\n%s", trace_bad_cmd, trace_cmd_help);
			tstate->breakpoint = (uint16_t)addr;
			printf("Set breakpoint at address 0x%2x\n", addr);
		} else if (command[0] == 'p') {
			switch (command[1]) {
			case 'b':
				printf("Current breakpoint at 0x%2x\n",
				                           tstate->breakpoint);
				break;
			case 'r':
				print_vm_registers(cpu);
				break;
			case 's':
				print_vm_stack(cpu);
				break;
			case 'z':
				print_vm_memory(mem, 0, 0x100);
				break;
			case 'm':
				if (sscanf(command+2,"%i %i", &mem_start,
				                            &num_bytes) == EOF)
					; // So far, sscanf always returns EOF
					// (idk why). TODO: fix eventually
				printf("Dumping %d bytes of memory starting "
				       "at 0x%04x\n", num_bytes, mem_start);
				print_vm_memory(mem, (uint16_t)mem_start,
						     (uint16_t)num_bytes);
				break;
			default:
				printf("%s\n%s", trace_bad_cmd, trace_cmd_help);
			}
		} else if (command[0] == 'd' && command[1] == 'b') {
			printf("Deleted breakpoint at 0x%x\n",
			                                   tstate->breakpoint);
			// vvv - disable breakpoint by not point to instr start
			tstate->breakpoint = 0x01;

		} else if (command[0] == 'c') {
			tstate->mode = CONT;
		} else if (command[0] == 'h') {
			printf("%s", trace_cmd_help);
		} else if (command[0] == 'q') {
			printf("Exiting debugger and virtual machine...\n");
			free(command);
			exit(0);
		} else {
			printf("%s\n%s", trace_bad_cmd, trace_cmd_help);
		}
	}
	// Idea: maybe return value to exit debugger?
}
#endif          // TRACE_FUNCS


void vm_run(struct CPU *cpu, uint8_t *mem)
{
	uint16_t pc = cpu->pc;
	uint8_t curr_instr[3];             // current instruction
	cpu->state = RUNNING;
	#ifdef TRACE
		trace_state.breakpoint = 0xffff;
		trace_state.mode = STEP;
	#endif

	while (cpu->state == RUNNING) {
		#ifdef TRACE
			vm_trace(cpu, mem, &trace_state);
		#endif

		vm_fetch_instruction(pc, mem, curr_instr);
		pc = vm_do_instruction(cpu, mem, curr_instr);
		cpu->pc = pc;
	}

	// eventually, maybe make it return some kind of CPU status code
}


/* vm_init(): initialize CPU/memory starting states
 */
void vm_init(struct CPU *cpu, uint8_t *mem, uint8_t *memimage, uint16_t imgsize)
{
        uint8_t placeholder_memimage[] = {65, 66, 66, 67, 68, 68, 69, 70, 70,0};

	cpu->pc = 0x100;       // default: start code execution after zero page
	cpu->lr = 0;
	cpu->sp = 0;
	//cpu->a = 0;
	cpu->state = STOPPED;
	memset(cpu->stack, 0, 64);

	if (memimage == NULL) {
		imgsize = sizeof(placeholder_memimage);
		memimage = (uint8_t *)placeholder_memimage;
	}
	for (uint16_t i = 0; i < imgsize; ++i)
		mem[i] = memimage[i];
}