package vm1

import (
	"fmt"     // for tracing only
	"reflect" // for optional CallX only
	"strconv" // for tracing only
)

// Byte-code instruction set.
const (
	// instruction effect on stack: values consumed -- values produced
	Nop       = iota // --
	Add              // n1 n2 -- sum ; sum = n1+n2
	Assign           // val -- ; mem[$1] = val
	Call             // f [a1 .. ai] -- [r1 .. rj] ; r1, ... = prog[f](a1, ...)
	CallX            // f [a1 .. ai] -- [r1 .. rj] ; r1, ... = mem[f](a1, ...)
	Dup              // addr -- value ; value = mem[addr]
	Fdup             // addr -- value ; value = mem[addr]
	Exit             // -- ;
	Jump             // -- ; ip += $1
	JumpTrue         // cond -- ; if cond { ip += $1 }
	JumpFalse        // cond -- ; if cond { ip += $1 }
	Lower            // n1 n2 -- cond ; cond = n1 < n2
	Loweri           // n1 -- cond ; cond = n1 < $1
	Pop              // v --
	Push             // -- v
	Return           // [r1 .. ri] -- ; exit frame: sp = fp, fp = pop
	Sub              // n1 n2 -- diff ; diff = n1 - n2
	Subi             // n1 -- diff ; diff = n1 - $1
)

var strop = [...]string{ // for VM tracing.
	Nop:       "Nop",
	Add:       "Add",
	Assign:    "Assign",
	Call:      "Call",
	CallX:     "CallX",
	Dup:       "Dup",
	Fdup:      "Fdup",
	Exit:      "Exit",
	Jump:      "Jump",
	JumpTrue:  "JumpTrue",
	JumpFalse: "JumpFalse",
	Lower:     "Lower",
	Loweri:    "Loweri",
	Pop:       "Pop",
	Push:      "Push",
	Return:    "Return",
	Sub:       "Sub",
	Subi:      "Subi",
}

// Machine represents a virtual machine.
type Machine struct {
	code   [][]int64 // code to execute
	mem    []any     // memory, as a stack
	ip, fp int       // instruction and frame pointer
	// flags  uint      // to set debug mode, restrict CallX, etc...
}

// Run runs a program.
func (m *Machine) Run() {
	code, mem, ip, fp, sp := m.code, m.mem, m.ip, m.fp, 0

	defer func() { m.mem, m.ip, m.fp = mem, ip, fp }()

	trace := func() {
		var op2, op3 string
		c := code[ip]
		if l := len(c); l > 2 {
			op2 = strconv.Itoa(int(c[2]))
			if l > 3 {
				op3 = strconv.Itoa(int(c[3]))
			}
		}
		fmt.Printf("ip:%-4d sp:%-4d fp:%-4d op:[%-9s %-4s %-4s] mem:%v\n", ip, sp, fp, strop[c[1]], op2, op3, mem)
	}
	_ = trace

	for {
		sp = len(mem) // stack pointer
		trace()
		switch op := code[ip]; op[1] {
		case Add:
			mem[sp-2] = mem[sp-2].(int) + mem[sp-1].(int)
			mem = mem[:sp-1]
		case Assign:
			mem[op[2]] = mem[sp-1]
			mem = mem[:sp-1]
		case Call:
			mem = append(mem, ip+1, fp)
			fp = sp + 2
			ip += int(op[2])
			continue
		case CallX: // Should be made optional.
			l := int(op[2])
			in := make([]reflect.Value, l)
			for i := range in {
				in[l-1-i] = reflect.ValueOf(mem[sp-1-i])
			}
			f := reflect.ValueOf(mem[sp-l-1])
			mem = mem[:sp-l-1]
			for _, v := range f.Call(in) {
				mem = append(mem, v.Interface())
			}
		case Dup:
			mem = append(mem, mem[int(op[2])])
		case Exit:
			return
		case Fdup:
			mem = append(mem, mem[int(op[2])+fp-1])
		case Jump:
			ip += int(op[2])
			continue
		case JumpTrue:
			cond := mem[sp-1].(bool)
			mem = mem[:sp-1]
			if cond {
				ip += int(op[2])
				continue
			}
		case JumpFalse:
			cond := mem[sp-1].(bool)
			mem = mem[:sp-1]
			if !cond {
				ip += int(op[2])
				continue
			}
		case Lower:
			mem[sp-2] = mem[sp-2].(int) < mem[sp-1].(int)
			mem = mem[:sp-1]
		case Loweri:
			mem[sp-1] = mem[sp-1].(int) < int(op[2])
		case Pop:
			mem = mem[:sp-1]
		case Push:
			mem = append(mem, int(op[2]))
		case Return:
			ip = mem[fp-2].(int)
			ofp := fp
			fp = mem[fp-1].(int)
			mem = append(mem[:ofp-int(op[2])-int(op[3])-1], mem[sp-int(op[2]):]...)
			continue
		case Sub:
			mem[sp-2] = mem[sp-2].(int) - mem[sp-1].(int)
			mem = mem[:sp-1]
		case Subi:
			mem[sp-1] = mem[sp-1].(int) - int(op[2])
		}
		ip++
	}
}

func (m *Machine) PushCode(code [][]int64) (p int) {
	p = len(m.code)
	m.code = append(m.code, code...)
	return
}

func (m *Machine) SetIP(ip int)       { m.ip = ip }
func (m *Machine) Push(v any) (l int) { l = len(m.mem); m.mem = append(m.mem, v); return }
func (m *Machine) Pop() (v any)       { l := len(m.mem) - 1; v = m.mem[l]; m.mem = m.mem[:l]; return }

// Disassemble returns the code as a readable string.
func Disassemble(code [][]int64) (asm string) {
	for _, op := range code {
		switch len(op) {
		case 2:
			asm += strop[op[1]] + "\n"
		case 3:
			asm += fmt.Sprintf("%s %d\n", strop[op[1]], op[2])
		case 4:
			asm += fmt.Sprintf("%s %d %d\n", strop[op[1]], op[2], op[3])
		}
	}
	return asm
}