package mmm

import (
	"bytes"
	"encoding/binary"
	"encoding/hex"
	"iter"
	"slices"
	"strconv"
	"strings"

	"fiatjaf.com/nostr"
	"github.com/PowerDNS/lmdb-go/lmdb"
)

type iterator struct {
	query query

	// iteration stuff
	cursor *lmdb.Cursor
	key    []byte
	posb   []byte
	err    error

	// this keeps track of last timestamp value pulled from this
	last uint32

	// if we shouldn't fetch more from this
	exhausted bool

	// results not yet emitted
	posbs      [][]byte
	timestamps []uint32
}

func (it *iterator) pull(n int, since uint32) {
	for range n {
		// in the beginning we already have a k and a v and an err from the cursor setup, so check and use these
		if it.err != nil {
			it.exhausted = true
			return
		}

		if len(it.key) != it.query.keySize || !bytes.HasPrefix(it.key, it.query.prefix) {
			// we reached the end of this prefix
			it.exhausted = true
			return
		}

		createdAt := binary.BigEndian.Uint32(it.key[len(it.key)-4:])
		if createdAt < since {
			it.exhausted = true
			return
		}

		// got a key
		it.posbs = append(it.posbs, it.posb)
		it.timestamps = append(it.timestamps, createdAt)
		it.last = createdAt

		// advance the cursor for the next call
		it.next()
	}

	return
}

// goes backwards
func (it *iterator) seek(key []byte) {
	if _, _, errsr := it.cursor.Get(key, nil, lmdb.SetRange); errsr != nil {
		if operr, ok := errsr.(*lmdb.OpError); !ok || operr.Errno != lmdb.NotFound {
			// in this case it's really an error
			panic(operr)
		} else {
			// we're at the end and we just want notes before this,
			// so we just need to set the cursor the last key, this is not a real error
			it.key, it.posb, it.err = it.cursor.Get(nil, nil, lmdb.Last)
		}
	} else {
		// move one back as the first step
		it.key, it.posb, it.err = it.cursor.Get(nil, nil, lmdb.Prev)
	}
}

// goes backwards
func (it *iterator) next() {
	// move one back (we'll look into k and v and err in the next iteration)
	it.key, it.posb, it.err = it.cursor.Get(nil, nil, lmdb.Prev)
}

type iterators []*iterator

// quickselect reorders the slice just enough to make the top k elements be arranged at the end
// i.e. [1, 700, 25, 312, 44, 28] with k=3 becomes something like [700, 312, 44, 1, 25, 28]
// in this case it's hardcoded to use the 'last' field of the iterator
// copied from https://github.com/chrislee87/go-quickselect
func (its iterators) quickselect(k int) {
	if len(its) == 0 || k >= len(its) {
		return
	}

	left, right := 0, len(its)-1
	for {
		// insertion sort for small ranges
		if right-left <= 20 {
			for i := left + 1; i <= right; i++ {
				for j := i; j > 0 && its[j].last > its[j-1].last; j-- {
					its[j], its[j-1] = its[j-1], its[j]
				}
			}
			return
		}

		// median-of-three to choose pivot
		pivotIndex := left + (right-left)/2
		if its[right].last > its[left].last {
			its[right], its[left] = its[left], its[right]
		}
		if its[pivotIndex].last > its[left].last {
			its[pivotIndex], its[left] = its[left], its[pivotIndex]
		}
		if its[right].last > its[pivotIndex].last {
			its[right], its[pivotIndex] = its[pivotIndex], its[right]
		}

		// partition
		its[left], its[pivotIndex] = its[pivotIndex], its[left]
		ll := left + 1
		rr := right
		for ll <= rr {
			for ll <= right && its[ll].last > its[left].last {
				ll++
			}
			for rr >= left && its[left].last > its[rr].last {
				rr--
			}
			if ll <= rr {
				its[ll], its[rr] = its[rr], its[ll]
				ll++
				rr--
			}
		}
		its[left], its[rr] = its[rr], its[left] // swap into right place
		pivotIndex = rr

		if k == pivotIndex {
			return
		}

		if k < pivotIndex {
			right = pivotIndex - 1
		} else {
			left = pivotIndex + 1
		}
	}
}

// return the highest 'last' value among the first k items in its
func (its iterators) threshold(k int) uint32 {
	highest := its[0].last
	for i := 1; i < k; i++ {
		if its[i].last > highest {
			highest = its[i].last
		}
	}
	return highest
}

type key struct {
	dbi lmdb.DBI
	key []byte
}

func (il *IndexingLayer) getIndexKeysForEvent(evt nostr.Event) iter.Seq[key] {
	return func(yield func(key) bool) {
		{
			// ~ by pubkey+date
			k := make([]byte, 8+4)
			copy(k[0:8], evt.PubKey[0:8])
			binary.BigEndian.PutUint32(k[8:8+4], uint32(evt.CreatedAt))
			if !yield(key{dbi: il.indexPubkey, key: k[0 : 8+4]}) {
				return
			}
		}

		{
			// ~ by kind+date
			k := make([]byte, 2+4)
			binary.BigEndian.PutUint16(k[0:2], uint16(evt.Kind))
			binary.BigEndian.PutUint32(k[2:2+4], uint32(evt.CreatedAt))
			if !yield(key{dbi: il.indexKind, key: k[0 : 2+4]}) {
				return
			}
		}

		{
			// ~ by pubkey+kind+date
			k := make([]byte, 8+2+4)
			copy(k[0:8], evt.PubKey[0:8])
			binary.BigEndian.PutUint16(k[8:8+2], uint16(evt.Kind))
			binary.BigEndian.PutUint32(k[8+2:8+2+4], uint32(evt.CreatedAt))
			if !yield(key{dbi: il.indexPubkeyKind, key: k[0 : 8+2+4]}) {
				return
			}
		}

		// ~ by tagvalue+date
		// ~ by p-tag+kind+date
		for i, tag := range evt.Tags {
			if len(tag) < 2 || len(tag[0]) != 1 || len(tag[1]) == 0 || len(tag[1]) > 100 {
				// not indexable
				continue
			}
			firstIndex := slices.IndexFunc(evt.Tags, func(t nostr.Tag) bool {
				return len(t) >= 2 && t[0] == tag[0] && t[1] == tag[1]
			})
			if firstIndex != i {
				// duplicate
				continue
			}

			// get key prefix (with full length) and offset where to write the created_at
			dbi, k, offset := il.getTagIndexPrefix(tag[0], tag[1])
			binary.BigEndian.PutUint32(k[offset:], uint32(evt.CreatedAt))
			if !yield(key{dbi: dbi, key: k}) {
				return
			}

			// now the p-tag+kind+date
			if dbi == il.indexTag32 && tag[0] == "p" {
				k := make([]byte, 8+2+4)
				hex.Decode(k[0:8], []byte(tag[1][0:8*2]))
				binary.BigEndian.PutUint16(k[8:8+2], uint16(evt.Kind))
				binary.BigEndian.PutUint32(k[8+2:8+2+4], uint32(evt.CreatedAt))
				dbi := il.indexPTagKind
				if !yield(key{dbi: dbi, key: k[0 : 8+2+4]}) {
					return
				}
			}
		}

		{
			// ~ by date only
			k := make([]byte, 4)
			binary.BigEndian.PutUint32(k[0:4], uint32(evt.CreatedAt))
			if !yield(key{dbi: il.indexCreatedAt, key: k[0:4]}) {
				return
			}
		}
	}
}

func (il *IndexingLayer) getTagIndexPrefix(tagName string, tagValue string) (lmdb.DBI, []byte, int) {
	var k []byte   // the key with full length for created_at and idx at the end, but not filled with these
	var offset int // the offset -- i.e. where the prefix ends and the created_at and idx would start
	var dbi lmdb.DBI

	letterPrefix := byte(int(tagName[0]) % 256)

	// if it's 32 bytes as hex, save it as bytes
	if len(tagValue) == 64 {
		// but we actually only use the first 8 bytes, with letter (tag name) prefix
		k = make([]byte, 1+8+4)
		if _, err := hex.Decode(k[1:1+8], []byte(tagValue[0:8*2])); err == nil {
			k[0] = letterPrefix
			offset = 1 + 8
			dbi = il.indexTag32
			return dbi, k[0 : 1+8+4], offset
		}
	}

	// if it looks like an "a" tag, index it in this special format, with letter (tag name) prefix
	spl := strings.Split(tagValue, ":")
	if len(spl) == 3 && len(spl[1]) == 64 {
		k = make([]byte, 1+2+8+30+4)
		if _, err := hex.Decode(k[1+2:1+2+8], []byte(spl[1][0:8*2])); err == nil {
			if kind, err := strconv.ParseUint(spl[0], 10, 16); err == nil {
				k[0] = letterPrefix
				k[1] = byte(kind >> 8)
				k[2] = byte(kind)
				// limit "d" identifier to 30 bytes (so we don't have to grow our byte slice)
				n := copy(k[1+2+8:1+2+8+30], spl[2])
				offset = 1 + 2 + 8 + n
				dbi = il.indexTagAddr
				return dbi, k[0 : offset+4], offset
			}
		}
	}

	// index whatever else as utf-8, but limit it to 40 bytes, with letter (tag name) prefix
	k = make([]byte, 1+40+4)
	k[0] = letterPrefix
	n := copy(k[1:1+40], tagValue)
	offset = 1 + n
	dbi = il.indexTag

	return dbi, k[0 : 1+n+4], offset
}
