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package j1

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io/ioutil"
)

type J1 struct {
	dsp    uint16       // 5 bit Data stack pointer
	st0    uint16       // 5 bit Return stack pointer
	pc     uint16       // 13 bit
	rsp    uint16       // 5 bit
	dstack [0x20]uint16 // Data stack
	rstack [0x20]uint16 // Return stack
	memory [0x8000]uint16
}

func (vm *J1) String() string {
	s := fmt.Sprintf("\tPC %0.4X\n", vm.pc)
	s += fmt.Sprintf("\tD %v %0.4X %0.4X\n", vm.dsp, vm.dstack[:vm.dsp], vm.st0)
	s += fmt.Sprintf("\tR %v %0.4X\n", vm.rsp, vm.rstack[:vm.rsp])
	return s
}

func (vm *J1) LoadBytes(data []byte) error {
	size := len(data) >> 1
	if size > len(vm.memory) {
		return fmt.Errorf("too big")
	}
	return binary.Read(bytes.NewReader(data), binary.BigEndian, vm.memory[:size])
}

func (vm *J1) LoadFile(fname string) error {
	data, err := ioutil.ReadFile(fname)
	if err != nil {
		return err
	}
	return vm.LoadBytes(data)
}

func (vm *J1) Eval() {
	for {
		ins := Decode(vm.memory[vm.pc])
		if ins == Jump(0) {
			break
		}
		vm.eval(ins)
	}
}

func (vm *J1) eval(ins Instruction) {
	next := vm.pc + 1
	switch v := ins.(type) {
	case Lit:
		vm.st0 = uint16(v)
		vm.dstack[vm.dsp] = vm.st0
		vm.dsp += 1
	case Jump:
		next = uint16(v)
	case Cond:
		if vm.st0 == 0 {
			next = uint16(v)
		}
		vm.st0 = vm.dstack[vm.dsp]
		vm.dsp -= 1
	case Call:
		vm.rstack[vm.rsp] = next
		vm.rsp += 1
		next = uint16(v)
	case ALU:
		vm.st0 = vm.newST0(v)
		if v.NtoAtT {
			vm.memory[vm.st0] = vm.dstack[vm.dsp]
		}
		if v.TtoR {
			vm.rstack[vm.rsp] = vm.st0
		}
		if v.TtoN {
			vm.dstack[vm.dsp] = vm.st0
		}
		vm.dsp = uint16(int8(vm.dsp) + v.Ddir)
		vm.rsp = uint16(int8(vm.rsp) + v.Rdir)
		if v.RtoPC {
			next = vm.rstack[vm.rsp]
		}
	}
	vm.pc = next
	fmt.Println(ins)
	fmt.Println(vm)
}

func (vm *J1) newST0(v ALU) uint16 {
	T, N, R := vm.st0, vm.dstack[vm.dsp], vm.rstack[vm.rsp]
	switch v.Opcode {
	case 0: // T
		return T
	case 1: // N
		return N
	case 2: // T+N
		return T + N
	case 3: // T&N
		return T & N
	case 4: // T|N
		return T | N
	case 5: // T^N
		return T ^ N
	case 6: // ~T
		return ^T
	case 7: // N==T
		if N == T {
			return 1
		}
		return 0
	case 8: // N<T
		if int16(N) < int16(T) {
			return 1
		}
		return 0
	case 9: // N>>T
		return N >> (T & 0xf)
	case 10: // T-1
		return T - 1
	case 11: // R (rT)
		return R
	case 12: // [T]
		return vm.memory[T]
	case 13: // N<<T
		return N << (T & 0xf)
	case 14: // depth (dsp)
		return (vm.rsp << 8) | vm.dsp
	case 15: // Nu<T
		if N < T {
			return 1
		}
		return 0
	}
	return 0
}