// Copyright 2014 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package vm

import (
	"sync"

	"github.com/holiman/uint256"
)

const stackLimit = 1024

var stackPool = sync.Pool{
	New: func() any {
		return &Stack{}
	},
}

// Stack is an object for basic stack operations. Items popped to the stack are
// expected to be changed and modified. stack does not take care of adding newly
// initialized objects.
type Stack struct {
	data [stackLimit]uint256.Int
	top  int
}

func newstack() *Stack {
	return stackPool.Get().(*Stack)
}

func returnStack(s *Stack) {
	s.top = 0
	stackPool.Put(s)
}

// Data returns the underlying uint256.Int array.
func (st *Stack) Data() []uint256.Int {
	return st.data[:st.top]
}

func (st *Stack) push(d *uint256.Int) {
	// NOTE push limit (1024) is checked in baseCheck
	st.data[st.top] = *d
	st.top++
}

func (st *Stack) pop() (ret uint256.Int) {
	st.top--
	ret = st.data[st.top]
	return
}

func (st *Stack) len() int {
	return st.top
}

func (st *Stack) swap(n int) {
	st.data[st.top-n-1], st.data[st.top-1] = st.data[st.top-1], st.data[st.top-n-1]
}

func (st *Stack) dup(n int) {
	st.data[st.top] = st.data[st.top-n]
	st.top++
}

func (st *Stack) peek() *uint256.Int {
	return &st.data[st.top-1]
}

// Back returns the n'th item in stack
func (st *Stack) Back(n int) *uint256.Int {
	return &st.data[st.top-n-1]
}