// Copyright 2023 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 internal

import (
	"errors"
	"fmt"

	"github.com/holiman/uint256"
)

const (
	memoryPadLimit = 1024 * 1024
)

// GetMemoryCopyPadded returns offset + size as a new slice.
// It zero-pads the slice if it extends beyond memory bounds.
func GetMemoryCopyPadded(m []byte, offset, size int64) ([]byte, error) {
	if offset < 0 || size < 0 {
		return nil, errors.New("offset or size must not be negative")
	}
	length := int64(len(m))
	if offset+size < length { // slice fully inside memory
		return memoryCopy(m, offset, size), nil
	}
	paddingNeeded := offset + size - length
	if paddingNeeded > memoryPadLimit {
		return nil, fmt.Errorf("reached limit for padding memory slice: %d", paddingNeeded)
	}
	cpy := make([]byte, size)
	if overlap := length - offset; overlap > 0 {
		copy(cpy, MemoryPtr(m, offset, overlap))
	}
	return cpy, nil
}

func memoryCopy(m []byte, offset, size int64) (cpy []byte) {
	if size == 0 {
		return nil
	}

	if len(m) > int(offset) {
		cpy = make([]byte, size)
		copy(cpy, m[offset:offset+size])

		return
	}

	return
}

// MemoryPtr returns a pointer to a slice of memory.
func MemoryPtr(m []byte, offset, size int64) []byte {
	if size == 0 {
		return nil
	}

	if len(m) > int(offset) {
		return m[offset : offset+size]
	}

	return nil
}

// StackBack returns the n'th item in stack
func StackBack(st []uint256.Int, n int) *uint256.Int {
	return &st[len(st)-n-1]
}