open Printf

(** http://bgb.bircd.org/pandocs.htm#cpuregistersandflags
    http://gameboy.mongenel.com/dmg/lesson1.html *)

type registers = {
  mutable a : char; (* accumulator *)
  mutable f : char; (* flags *)
  mutable b : char;
  mutable c : char;
  mutable d : char;
  mutable e : char;
  mutable h : char;
  mutable l : char;

  mutable sp : int; (* stack pointer *)
  mutable pc : int; (* program counter *)
}

(* type flags = {
  z  : bool; (* zero *)
  n  : bool; (* substraction *)
  h  : bool; (* half-carry *)
  cy : bool; (* carry *)
} *)

type t = {
  reg : registers;
  mutable cycles : int;
}

(** http://bgb.bircd.org/pandocs.htm#powerupsequence *)
let init_registers =
  {
    a = '\x00';
    f = '\x00';
    b = '\x00';
    c = '\x00';
    d = '\x00';
    e = '\x00';
    h = '\x00';
    l = '\x00';
    sp = 0xFFFE;
    pc = 0x0100;
  }

let init_cpu =
  { reg = init_registers; cycles = 0 }

let inc_pc cpu count =
  cpu.reg.pc <- cpu.reg.pc + count

let inc_cycles cpu count =
  cpu.cycles <- cpu.cycles + count

let merge_bytes low high =
  (int_of_char high) * 256 + (int_of_char low)

let split_2B x =
  let low = x mod 256 |> char_of_int in
  let high = x / 256 |> char_of_int in
  (high, low)

let read_2B b addr =
  let low = Bytes.get b addr in
  let high = Bytes.get b (addr + 1) in
  merge_bytes low high

let inc_BC cpu =
  let v = merge_bytes cpu.reg.c cpu.reg.b in
  let low, high = split_2B (v + 1) in
  cpu.reg.c <- low;
  cpu.reg.b <- high

(** http://imrannazar.com/GameBoy-Z80-Opcode-Map *)
let run cpu (cartridge: Cartridge.t) =
  let data = cartridge.full_rom in
  let n = Bytes.get data (cpu.reg.pc + 1) in
  let nn = Bytes.get data (cpu.reg.pc + 2) in
(*  Hexa.print_slice cartridge.full_rom cpu.reg.pc (cpu.reg.pc + 7); *)
  match Bytes.get data cpu.reg.pc with
  | '\x00' -> printf "  NOP\n";
              inc_pc cpu 1; inc_cycles cpu 4

  | '\x03' -> printf "  INC \tBC\n";
              inc_BC cpu;
              inc_pc cpu 1; inc_cycles cpu 8

  | '\x3E' -> printf "  LD \tA, 0x%02X\n" (int_of_char n);
              cpu.reg.a <- n;
              inc_pc cpu 2; inc_cycles cpu 8

  | '\xAF' -> printf "  XOR \tA, A\n";
              let int_A = int_of_char cpu.reg.a in
              cpu.reg.a <- char_of_int @@ int_A lxor int_A;
              inc_pc cpu 1; inc_cycles cpu 4

  | '\xC3' -> let addr = read_2B data (cpu.reg.pc + 1) in
              printf "  JP \t0x%04X\n" addr;
              cpu.reg.pc <- addr; inc_cycles cpu 16

  | '\xE0' -> printf "  LDH \t(0xFF%02X), A\n" (int_of_char n);
              (* fixme *)
              inc_pc cpu 2; inc_cycles cpu 12

  | '\xF0' -> printf "  LDH \tA, (0xFF%02X)\n" (int_of_char n);
              (* fixme *)
              inc_pc cpu 2; inc_cycles cpu 12

  | '\xF3' -> printf "  DI\n";
              (* fixme *)
              inc_pc cpu 1; inc_cycles cpu 4

  | _ as x -> eprintf "opcode %02X\n" (int_of_char x); failwith "Unimplemented opcode."