//
// Copyright © 2020 Vulcanize, Inc
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program 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 Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.

package iterator

import (
	"bytes"
	"math/bits"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/core/state"
	"github.com/ethereum/go-ethereum/trie"
)

// Simplified trie node iterator
type NodeIterator interface {
	Next(bool) bool
	Error() error
	Hash() common.Hash
	Path() []byte
	Leaf() bool
}

type prefixBoundIterator struct {
	current trie.NodeIterator
	endKey  []byte
}

func (it *prefixBoundIterator) Next(descend bool) bool {
	if it.endKey == nil {
		return it.current.Next(descend)
	}
	cmp := bytes.Compare(it.current.Path(), it.endKey) // stop before endKey
	if cmp >= 0 {
		return false
	}
	return it.current.Next(descend)
}

func (it *prefixBoundIterator) Error() error {
	return it.current.Error()
}
func (it *prefixBoundIterator) Hash() common.Hash {
	return it.current.Hash()
}
func (it *prefixBoundIterator) Path() []byte {
	return it.current.Path()
}
func (it *prefixBoundIterator) Leaf() bool {
	return it.current.Leaf()
}

// Iterator with an upper bound value (hex path prefix)
func NewPrefixBoundIterator(it trie.NodeIterator, to []byte) NodeIterator {
	return &prefixBoundIterator{current: it, endKey: to}
}

type prefixGenerator struct {
	current []byte
	step byte
	stepIndex uint
}

func newPrefixGenerator(nbins uint) prefixGenerator {
	if bits.OnesCount(nbins) != 1 {
		panic("nbins must be a power of 2")
	}
	// determine step dist. and path index at which to step
	var step byte
	var stepIndex uint
	for ; nbins != 0; stepIndex++ {
		divisor := byte(nbins & 0xf)
		if divisor != 0 {
			step = 0x10 / divisor
		}
		nbins = nbins >> 4
	}
	return prefixGenerator{
		current: make([]byte, stepIndex),
		step: step,
		stepIndex: stepIndex-1,
	}
}

func (gen *prefixGenerator) Value() []byte {
	return gen.current
}

func (gen *prefixGenerator) HasNext() bool {
	return gen.current[0] <= 0xf
}

func (gen *prefixGenerator) Next() {
	gen.current[gen.stepIndex] += gen.step
	overflow := false
	for ix := 0; ix < len(gen.current); ix++ {
		rix := len(gen.current)-1-ix // reverse
		if overflow {
			gen.current[rix]++
			overflow = false
		}
		if rix != 0 && gen.current[rix] > 0xf {
			gen.current[rix] = 0
			overflow = true
		}
	}
}

// Generates paths that cut the trie domain into "nbins" bins, w/ optional prefix
// eg. MakePaths([], 2) => [[0] [8]]
//	   MakePaths([4], 32) => [[4 0 0] [4 0 8] [4 1 0]... [4 f 8]]
func MakePaths(prefix []byte, nbins uint) [][]byte {
	var res [][]byte
	for it := newPrefixGenerator(nbins); it.HasNext(); it.Next() {
		next := make([]byte, len(prefix))
		copy(next, prefix)
		next = append(next, it.Value()...)
		res = append(res, next)
	}
	return res
}

// Apply a function to nbins subtries divided according to path prefix
func VisitSubtries(tree state.Trie, nbins uint, callback func(NodeIterator)) {
	prefixes := MakePaths(nil, nbins)
	// pre- and postpend nil to include root & tail
	prefixes = append(prefixes, nil)
	prefixes = append([][]byte{nil}, prefixes...)

	for i := 0; i < len(prefixes)-1; i++ {
		key := prefixes[i]
		if len(key)%2 != 0 {	// zero-pad for odd-length keys
			key = append(key, 0)
		}
		it := tree.NodeIterator(HexToKeyBytes(key))
		callback(NewPrefixBoundIterator(it, prefixes[i+1]))
	}
}