use endian;
use io;
use os;
use fs;
use fmt;

type stack_overflow = !void;
type stack_underflow = !void;

type cpu = struct {
	sp: u8,
	rp: u8,
	pc: u16,
	mem: []u8,
	halted: bool,
};

fn cpu_new() cpu = cpu {
	sp = 0,
	rp = 0,
	pc = 0x0200,
	mem = alloc([0...], 65536): []u8,
	halted = false,
};

fn rmem(mem: []u8, addr: u16) u16 =
	endian::begetu16(mem[addr..addr+2]);
fn wmem(mem: []u8, addr: u16, w: u16) void =
	endian::beputu16(mem[addr..addr+2], w);

// stacks grow up, for a change
fn push(cpu: *cpu, val: u16) void = {
	wmem(cpu.mem, 0 + cpu.sp, val);
	cpu.sp += 2;
};
fn pop(cpu: *cpu) u16 = {
	cpu.sp -= 2;
	return rmem(cpu.mem, 0 + cpu.sp);
};
fn peek(cpu: *cpu, n: u16 = 0) u16 =
	rmem(cpu.mem, 0 + cpu.sp - 2 + n);


fn rpush(cpu: *cpu, val: u16) void = {
	wmem(cpu.mem, 0x100 + cpu.rp:u16, val);
	cpu.rp += 2;
};
fn rpop(cpu: *cpu) u16 = {
	cpu.rp -= 2;
	return rmem(cpu.mem, 0x100 + cpu.rp:u16);
};



def MASK_CALL: u16 = 0b1000000000000000;
def MASK_LIT : u16 = 0b0110000000000000;
def MASK_JMP : u16 = 0b0100000000000000;
fn maskeq(x: u16, m: u16) bool =
	// still must be called in the right order
	(x&m) == m;

fn do_word(cpu: *cpu, word: u16) void = {
	if (maskeq(word, MASK_CALL)) {
		const addr = word & 0b0111111111111111;
		const addr = addr << 1;
		do_call(cpu, addr);
	} else if (maskeq(word, MASK_LIT)) {
		const lval = word & 0b0001111111111111;
		push(cpu, lval);
	} else if (maskeq(word, MASK_JMP)) {
		const is_cond = (word & 0b0001000000000000) != 0;
		const rel: u16 = word & 0b0000111111111111;
		const rel: i16 = rel:i16 - 2048i16;
		const target: i16 = rel + cpu.pc:i16 - 2i16;
		const target: u16 = target:u16;
		if (is_cond)
			do_0branch(cpu, target)
		else
			do_jmp(cpu, target);
	} else {
		const opc1: u8 = ((word & 0b0011111110000000) >> 7):u8;
		const opc2: u8 = (word & 0b0000000001111111):u8;
		do_opc(cpu, opc1);
		do_opc(cpu, opc2);
	};
};

fn do_call(cpu: *cpu, addr: u16) void = {
	rpush(cpu, cpu.pc);
	cpu.pc = addr;
};

fn do_jmp(cpu: *cpu, addr: u16) void = {
	cpu.pc = addr;
};

fn do_0branch(cpu: *cpu, addr: u16) void = {
	const val = pop(cpu);
	if (val == 0) {
		cpu.pc = addr;
	};
};

fn mainloop(cpu: *cpu) void = {
	for (! cpu.halted) {
		const word = rmem(cpu.mem, cpu.pc);
		cpu.pc += 2;
		do_word(cpu, word);
	};
};

export fn main() void = {
	const cpu = cpu_new();
	const fp = match (os::open("sys0.r1b")) {
	case let i: io::file => yield i;
	case let e: fs::error => fmt::fatalf("MISSING BOOT SECTOR: {}",
		fs::strerror(e));
	};

	match (io::readall(fp, cpu.mem[0x0200..0x0300])) {
	case let e: io::error => fmt::fatalf("BOOT SECTOR READ ERROR: {}", io::strerror(e));
	case => yield;
	};

	mainloop(&cpu);
};