// Package forge implements sort of a stack-based interpreter.
package forge

import (
	"git.sr.ht/~cco/go-scopes/common/ptr"
	"git.sr.ht/~cco/go-scopes/common/stack"
	"git.sr.ht/~cco/go-scopes/common/voc"
)

type fitem interface{}
type FItem = fitem

type fptr = ptr.Ptr[fitem]
type FPtr = fptr

type fstack = stack.Stack[fitem]
type fvoc = voc.Vocabulary[fitem]

type Callable func(*forgeEnv, XT)

type forgeEnv struct {
	ds, rs fstack
	ip     fptr
	voc    *fvoc
}

type FE = *forgeEnv

func NewFE() *forgeEnv {
	return newFE(newVoc(nil))
}

func (f *forgeEnv) ChildFE() *forgeEnv {
	return newFE(newVoc(f.voc))
}

func newFE(voc *fvoc) *forgeEnv {
	return &forgeEnv{
		ds:  stack.NewStack[fitem](),
		rs:  stack.NewStack[fitem](),
		ip:  ptr.NewSlice[fitem](),
		voc: voc,
	}
}

var newVoc = voc.NewVoc[fitem]

var newScalar = ptr.NewScalar[fitem]
var newPtr = ptr.NewSlice[fitem]

// XT (execution token)

type XT interface {
	Name() string
	Fct() Callable
	Code() fptr
}

type name string

func (n name) Name() string {
	return string(n)
}

func Register(voc *fvoc, it XT) XT {
	voc.Register(it.Name(), it)
	return it
}

// gofunc: XT consisting only of a function written in Go

type gofunc struct {
	name
	fct Callable
}

func GoFunc(n string, fct Callable) XT {
	return &gofunc{name(n), fct}
}

func (it *gofunc) Fct() Callable {
	return it.fct
}

func (it *gofunc) Code() fptr {
	return nil
}

// fcode: with forge code

type fcode struct {
	name
	code fptr
}

func FCode(n string, c fptr) XT {
	return &fcode{name(n), c}
}

func (it *fcode) Fct() Callable {
	return doFCode
}

func (it *fcode) Code() fptr {
	return it.code
}

// forgeEnv methods

func (f *forgeEnv) Code(items ...fitem) fptr {
	code := newPtr(items...)
	// ... pre-process (compile) code ...
	return code
}

func (f *forgeEnv) Call(code fptr) *forgeEnv {
	f.rs.Push(f.ip)
	f.ip = code.Clone()
	for f.ip.Next() != nil {
		value := f.ip.Value()
		if xt, ok := value.(XT); ok {
			xt.Fct()(f, xt)
		} else {
			f.Push(value)
		}
	}
	f.ip = f.rs.Pop().(fptr)
	return f
}

func (f *forgeEnv) Exec(items ...fitem) *forgeEnv {
	return f.Call(f.Code(items...))
}

// core methods

func (f *forgeEnv) NewVar() fptr {
	return newScalar()
}

func (f *forgeEnv) Literal() fitem {
	return f.ip.Next().Value()
}

func (f *forgeEnv) Voc() *fvoc {
	return f.voc
}

func (f *forgeEnv) Push(it fitem) {
	f.ds.Push(it)
}

func (f *forgeEnv) Pop() fitem {
	return f.ds.Pop()
}

func (f *forgeEnv) Peek(d int) fitem {
	return f.ds.Peek(d)
}

// basic functions for executable items

func doFCode(f *forgeEnv, xt XT) {
	f.Call(xt.Code())
}