// Copyright 2019 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 snapshot

import (
	"bytes"
	"fmt"
	"sort"

	"github.com/ethereum/go-ethereum/common"
)

type weightedIterator struct {
	it       AccountIterator
	priority int
}

// fastAccountIterator is a more optimized multi-layer iterator which maintains a
// direct mapping of all iterators leading down to the bottom layer
type fastAccountIterator struct {
	iterators []*weightedIterator
	initiated bool
	fail      error
}

// newFastAccountIterator creates a new fastAccountIterator
func (dl *diffLayer) newFastAccountIterator() AccountIterator {
	f := &fastAccountIterator{
		initiated: false,
	}
	for i, it := range dl.iterators() {
		f.iterators = append(f.iterators, &weightedIterator{it, -i})
	}
	f.Seek(common.Hash{})
	return f
}

// Len returns the number of active iterators
func (fi *fastAccountIterator) Len() int {
	return len(fi.iterators)
}

// Less implements sort.Interface
func (fi *fastAccountIterator) Less(i, j int) bool {
	a := fi.iterators[i].it.Key()
	b := fi.iterators[j].it.Key()
	bDiff := bytes.Compare(a[:], b[:])
	if bDiff < 0 {
		return true
	}
	if bDiff > 0 {
		return false
	}
	// keys are equal, sort by iterator priority
	return fi.iterators[i].priority < fi.iterators[j].priority
}

// Swap implements sort.Interface
func (fi *fastAccountIterator) Swap(i, j int) {
	fi.iterators[i], fi.iterators[j] = fi.iterators[j], fi.iterators[i]
}

func (fi *fastAccountIterator) Seek(key common.Hash) {
	// We need to apply this across all iterators
	var seen = make(map[common.Hash]int)

	length := len(fi.iterators)
	for i := 0; i < len(fi.iterators); i++ {
		//for i, it := range fi.iterators {
		it := fi.iterators[i]
		it.it.Seek(key)
		for {
			if !it.it.Next() {
				// To be removed
				// swap it to the last position for now
				fi.iterators[i], fi.iterators[length-1] = fi.iterators[length-1], fi.iterators[i]
				length--
				break
			}
			v := it.it.Key()
			if other, exist := seen[v]; !exist {
				seen[v] = i
				break
			} else {
				// This whole else-block can be avoided, if we instead
				// do an inital priority-sort of the iterators. If we do that,
				// then we'll only wind up here if a lower-priority (preferred) iterator
				// has the same value, and then we will always just continue.
				// However, it costs an extra sort, so it's probably not better

				// One needs to be progressed, use priority to determine which
				if fi.iterators[other].priority < it.priority {
					// the 'it' should be progressed
					continue
				} else {
					// the 'other' should be progressed - swap them
					it = fi.iterators[other]
					fi.iterators[other], fi.iterators[i] = fi.iterators[i], fi.iterators[other]
					continue
				}
			}
		}
	}
	// Now remove those that were placed in the end
	fi.iterators = fi.iterators[:length]
	// The list is now totally unsorted, need to re-sort the entire list
	sort.Sort(fi)
	fi.initiated = false
}

// Next implements the Iterator interface. It returns false if no more elemnts
// can be retrieved (false == exhausted)
func (fi *fastAccountIterator) Next() bool {
	if len(fi.iterators) == 0 {
		return false
	}
	if !fi.initiated {
		// Don't forward first time -- we had to 'Next' once in order to
		// do the sorting already
		fi.initiated = true
		return true
	}
	return fi.innerNext(0)
}

// innerNext handles the next operation internally,
// and should be invoked when we know that two elements in the list may have
// the same value.
// For example, if the list becomes [2,3,5,5,8,9,10], then we should invoke
// innerNext(3), which will call Next on elem 3 (the second '5'). It will continue
// along the list and apply the same operation if needed
func (fi *fastAccountIterator) innerNext(pos int) bool {
	if !fi.iterators[pos].it.Next() {
		//Exhausted, remove this iterator
		fi.remove(pos)
		if len(fi.iterators) == 0 {
			return false
		}
		return true
	}
	if pos == len(fi.iterators)-1 {
		// Only one iterator left
		return true
	}
	// We next:ed the elem at 'pos'. Now we may have to re-sort that elem
	var (
		current, neighbour = fi.iterators[pos], fi.iterators[pos+1]
		val, neighbourVal  = current.it.Key(), neighbour.it.Key()
	)
	if diff := bytes.Compare(val[:], neighbourVal[:]); diff < 0 {
		// It is still in correct place
		return true
	} else if diff == 0 && current.priority < neighbour.priority {
		// So still in correct place, but we need to iterate on the neighbour
		fi.innerNext(pos + 1)
		return true
	}
	// At this point, the elem is in the wrong location, but the
	// remaining list is sorted. Find out where to move the elem
	iteratee := -1
	index := sort.Search(len(fi.iterators), func(n int) bool {
		if n < pos {
			// No need to search 'behind' us
			return false
		}
		if n == len(fi.iterators)-1 {
			// Can always place an elem last
			return true
		}
		neighbour := fi.iterators[n+1].it.Key()
		if diff := bytes.Compare(val[:], neighbour[:]); diff < 0 {
			return true
		} else if diff > 0 {
			return false
		}
		// The elem we're placing it next to has the same value,
		// so whichever winds up on n+1 will need further iteraton
		iteratee = n + 1
		if current.priority < fi.iterators[n+1].priority {
			// We can drop the iterator here
			return true
		}
		// We need to move it one step further
		return false
		// TODO benchmark which is best, this works too:
		//iteratee = n
		//return true
		// Doing so should finish the current search earlier
	})
	fi.move(pos, index)
	if iteratee != -1 {
		fi.innerNext(iteratee)
	}
	return true
}

// move moves an iterator to another position in the list
func (fi *fastAccountIterator) move(index, newpos int) {
	if newpos > len(fi.iterators)-1 {
		newpos = len(fi.iterators) - 1
	}
	var (
		elem   = fi.iterators[index]
		middle = fi.iterators[index+1 : newpos+1]
		suffix []*weightedIterator
	)
	if newpos < len(fi.iterators)-1 {
		suffix = fi.iterators[newpos+1:]
	}
	fi.iterators = append(fi.iterators[:index], middle...)
	fi.iterators = append(fi.iterators, elem)
	fi.iterators = append(fi.iterators, suffix...)
}

// remove drops an iterator from the list
func (fi *fastAccountIterator) remove(index int) {
	fi.iterators = append(fi.iterators[:index], fi.iterators[index+1:]...)
}

// Error returns any failure that occurred during iteration, which might have
// caused a premature iteration exit (e.g. snapshot stack becoming stale).
func (fi *fastAccountIterator) Error() error {
	return fi.fail
}

// Key returns the current key
func (fi *fastAccountIterator) Key() common.Hash {
	return fi.iterators[0].it.Key()
}

// Value returns the current key
func (fi *fastAccountIterator) Value() []byte {
	return fi.iterators[0].it.Value()
}

// Debug is a convencience helper during testing
func (fi *fastAccountIterator) Debug() {
	for _, it := range fi.iterators {
		fmt.Printf("[p=%v v=%v] ", it.priority, it.it.Key()[0])
	}
	fmt.Println()
}