// AVM ISA v1 — Go Port (Strict Functional Execution)
package avm

import "errors"

const Q16Scale = 65536

func q16Mul(a, b int32) int32 {
	return int32((int64(a) * int64(b)) / Q16Scale)
}
func q16Div(a, b int32) (int32, error) {
	if b == 0 { return 0, errors.New("div by zero") }
	return int32((int64(a) * Q16Scale) / b), nil
}

// ── Value ─────────────────────────────────────────
type ValType int
const (
	ValQ0 ValType = iota
	ValQ16
	ValBool
)
type AvmVal struct { Ty ValType; Raw int32 }

func Q16(x int32) AvmVal { return AvmVal{ValQ16, x} }
func Bool(x bool) AvmVal {
	if x { return AvmVal{ValBool, 1} }
	return AvmVal{ValBool, 0}
}

// ── Primitives ─────────────────────────────────────
type Prim int
const (
	AddQ16 Prim = iota; SubQ16; MulQ16; DivQ16; LtQ16; EqQ16; And; Or; Not
)

func execPrim(p Prim, a, b AvmVal) (AvmVal, error) {
	switch p {
	case AddQ16: return Q16(a.Raw + b.Raw), nil
	case SubQ16: return Q16(a.Raw - b.Raw), nil
	case MulQ16: return Q16(q16Mul(a.Raw, b.Raw)), nil
	case DivQ16:
		r, e := q16Div(a.Raw, b.Raw)
		return Q16(r), e
	case LtQ16: return Bool(a.Raw < b.Raw), nil
	case EqQ16: return Bool(a.Raw == b.Raw), nil
	case And: return Bool(a.Raw != 0 && b.Raw != 0), nil
	case Or: return Bool(a.Raw != 0 || b.Raw != 0), nil
	case Not: return Bool(a.Raw == 0), nil
	}
	return AvmVal{}, errors.New("unknown prim")
}

// ── Instruction ────────────────────────────────────
type InstrOp int
const (
	PushQ16 InstrOp = iota; PushBool; Pop; Dup; Swap; Load; Store; Jump; JumpIf; PrimOp; Halt
)
type Instr struct { Op InstrOp; Arg int32; Arg2 bool }

func Push(x int32) Instr { return Instr{PushQ16, x, false} }
func PushB(x bool) Instr { return Instr{PushBool, boolTo32(x), true} }
func Pop() Instr         { return Instr{Pop, 0, false} }
func Dup() Instr         { return Instr{Dup, 0, false} }
func Swap() Instr        { return Instr{Swap, 0, false} }
func Load(i int) Instr   { return Instr{Load, int32(i), false} }
func Store(i int) Instr  { return Instr{Store, int32(i), false} }
func Jump(t int) Instr   { return Instr{Jump, int32(t), false} }
func JumpIf(t int) Instr { return Instr{JumpIf, int32(t), false} }
func PrimOp(p Prim) Instr{ return Instr{PrimOp, int32(p), false} }
func Halt() Instr        { return Instr{Halt, 0, false} }

func boolTo32(x bool) int32 { if x { return 1 }; return 0 }

// ── State ──────────────────────────────────────────
type State struct {
	Pc int
	Stack []AvmVal
	Locals []*AvmVal
	Halted bool
}
func NewState(n int) State {
	return State{0, []AvmVal{}, make([]*AvmVal, n), false}
}

// ── Step ───────────────────────────────────────────
func Step(s State, prog []Instr) (State, error) {
	if s.Halted { return s, errors.New("halted") }
	if s.Pc < 0 || s.Pc >= len(prog) {
		s.Halted = true; return s, nil
	}

	instr := prog[s.Pc]
	ns := State{s.Pc + 1, append([]AvmVal{}, s.Stack...),
		append([]*AvmVal{}, s.Locals...), false}
	st := &ns.Stack

	switch instr.Op {
	case PushQ16: *st = append(*st, Q16(instr.Arg))
	case PushBool: *st = append(*st, Bool(instr.Arg != 0))
	case Pop: *st = (*st)[:len(*st)-1]
	case Dup: *st = append(*st, (*st)[len(*st)-1])
	case Swap:
		(*st)[len(*st)-2], (*st)[len(*st)-1] = (*st)[len(*st)-1], (*st)[len(*st)-2]
	case Load:
		if ns.Locals[instr.Arg] == nil { return s, errors.New("missing local") }
		*st = append(*st, *ns.Locals[instr.Arg])
	case Store:
		ns.Locals[instr.Arg] = &(*st)[len(*st)-1]
		*st = (*st)[:len(*st)-1]
	case Jump: ns.Pc = int(instr.Arg)
	case JumpIf:
		v := (*st)[len(*st)-1]; *st = (*st)[:len(*st)-1]
		if v.Raw != 0 { ns.Pc = int(instr.Arg) }
	case PrimOp: {
		p := Prim(instr.Arg)
		arity := 2; if p == Not { arity = 1 }
		var b AvmVal
		if arity == 2 { b = (*st)[len(*st)-1]; *st = (*st)[:len(*st)-1] }
		a := (*st)[len(*st)-1]; *st = (*st)[:len(*st)-1]
		r, e := execPrim(p, a, b)
		if e != nil { return s, e }
		*st = append(*st, r)
	}
	case Halt: ns.Halted = true
	}
	return ns, nil
}

func Run(init State, prog []Instr, fuel int) (State, error) {
	s := init
	for i := 0; i < fuel && !s.Halted; i++ {
		var e error
		s, e = Step(s, prog)
		if e != nil { return s, e }
	}
	return s, nil
}