// Copyright (c) 2026 Charles KWON OhJun (charleskwonohjun@gmail.com)
// All rights reserved.

// NTX index creation (INDEX ON) and key insertion.
package ntx

import (
	"encoding/binary"
	"fmt"
	"os"
	"sort"
)

// KeyRecord pairs a key value with its record number for sorting.
type KeyRecord struct {
	Key   []byte
	RecNo uint32
}

// CreateIndex builds a new NTX index file from sorted key-record pairs.
func CreateIndex(path string, keyExpr string, keyLen int, unique bool, descend bool, keys []KeyRecord) (*Index, error) {
	if len(path) < 4 || path[len(path)-4:] != ".ntx" {
		path += ".ntx"
	}

	f, err := os.Create(path)
	if err != nil {
		return nil, fmt.Errorf("create NTX %s: %w", path, err)
	}

	itemSize := 8 + keyLen
	maxItem := calculateMaxItems(itemSize)
	halfPage := maxItem / 2

	// Proper B-tree bulk build — works for any depth.
	// Strategy: split sorted keys into groups of maxItem with separators between.
	// keys[0..M-1] → leaf, keys[M] → separator, keys[M+1..2M] → leaf, ...
	// Each separator exists ONLY in the parent, never in a leaf.
	var buf pageBuffer
	buf.init()
	var rootOffset uint32

	if len(keys) == 0 {
		off := buf.allocPage()
		initPageOffsets(buf.getPage(off), maxItem, itemSize)
		rootOffset = uint32(off)
	} else {
		rootOffset = uint32(bulkBuildBTree(&buf, keys, keyLen, maxItem, itemSize))
	}

	nextPage := uint32(int64(HeaderSize) + int64(buf.count)*BlockSize)

	// Write header
	hdr := Header{
		Type:     0x0401,
		Version:  1,
		Root:     rootOffset,
		NextPage: nextPage,
		ItemSize: uint16(itemSize),
		KeySize:  uint16(keyLen),
		KeyDec:   0,
		MaxItem:  uint16(maxItem),
		HalfPage: uint16(halfPage),
	}
	copy(hdr.KeyExpr[:], keyExpr)
	if unique {
		hdr.Unique = 1
	}
	if descend {
		hdr.Descend = 1
	}

	if err := WriteHeader(f, &hdr); err != nil {
		f.Close()
		return nil, err
	}

	// Bulk write all pages at correct offset (after header)
	if buf.count > 0 {
		if _, err := f.WriteAt(buf.data[:int64(buf.count)*BlockSize], int64(HeaderSize)); err != nil {
			f.Close()
			return nil, fmt.Errorf("bulk write NTX pages: %w", err)
		}
	}

	f.Close()
	return OpenIndex(path)
}

// bulkBuildBTree builds a proper B-tree in memory from sorted keys.
// Returns the file offset of the root page.
// Algorithm: distribute keys into leaf groups with separators extracted between them.
//   [leaf0: M keys] [sep0] [leaf1: M keys] [sep1] ... [leafN: remaining keys]
// Then recursively build interior level from (leaf offsets + separator keys).
func bulkBuildBTree(buf *pageBuffer, keys []KeyRecord, keyLen, maxItem, itemSize int) int64 {
	if len(keys) <= maxItem {
		// Single leaf — base case
		return buildOnePage(buf, keys, keyLen, maxItem, itemSize, nil)
	}

	// Split keys into leaf groups + separators
	type childInfo struct {
		offset int64
		sepKey []byte  // separator AFTER this child (nil for last)
		sepRec uint32
	}
	// Pre-size to avoid slice growth during leaf splitting.
	children := make([]childInfo, 0, len(keys)/maxItem+2)
	i := 0
	for i < len(keys) {
		end := i + maxItem
		if end > len(keys) {
			end = len(keys)
		}
		// Prevent orphan: if taking maxItem keys leaves only 1 key for separator+next leaf,
		// adjust so the last two leaves share keys evenly.
		if end < len(keys) {
			remaining := len(keys) - end
			if remaining <= 2 {
				if end-i+remaining <= maxItem {
					// Absorb: total fits in one page
					end = len(keys)
				} else {
					// Split evenly: give half of remaining+current to each of last 2 leaves.
					// Total keys for last 2 leaves + 1 separator = len(keys) - i
					total := len(keys) - i
					end = i + (total-1)/2 // -1 accounts for separator between them
				}
			}
		}
		chunk := keys[i:end]
		leafOff := buildOnePage(buf, chunk, keyLen, maxItem, itemSize, nil)
		ci := childInfo{offset: leafOff}
		i = end

		// Extract separator only if 2+ keys remain (1 for sep + 1+ for next leaf)
		if i < len(keys) && i+1 < len(keys) {
			// At least 1 more key after separator → safe to promote.
			// Reference the source key directly (caller's slab allocation is
			// keyLen-aligned from OrderCreate's fast path, so no padding copy
			// is needed). For slow path, the key was already padded in-place.
			if len(keys[i].Key) == keyLen {
				ci.sepKey = keys[i].Key
			} else {
				ci.sepKey = make([]byte, keyLen)
				padCopy(ci.sepKey, keys[i].Key, keyLen)
			}
			ci.sepRec = keys[i].RecNo
			i++ // skip separator key — it goes to parent only
		}
		children = append(children, ci)
	}

	// Build interior levels bottom-up
	for len(children) > 1 {
		var nextChildren []childInfo
		j := 0
		for j < len(children) {
			// Collect up to maxItem+1 children for one parent page
			end := j + maxItem + 1
			if end > len(children) {
				end = len(children)
			}
			group := children[j:end]

			// Build parent page: separators come from group[0..n-2].sepKey
			nKeys := len(group) - 1
			parentOff := buf.allocPage()
			pg := buf.getPage(parentOff)
			initPageOffsets(pg, maxItem, itemSize)
			binary.LittleEndian.PutUint16(pg[0:2], uint16(nKeys))

			for k := 0; k <= nKeys; k++ {
				entOff := int(binary.LittleEndian.Uint16(pg[2+k*2 : 4+k*2]))
				binary.LittleEndian.PutUint32(pg[entOff:entOff+4], uint32(group[k].offset))
				if k < nKeys && group[k].sepKey != nil {
					binary.LittleEndian.PutUint32(pg[entOff+4:entOff+8], group[k].sepRec)
					copy(pg[entOff+8:entOff+8+keyLen], group[k].sepKey)
				}
			}

			// This parent becomes a child for the next level
			ci := childInfo{offset: parentOff}
			// Separator for this parent = last group member's separator
			// (the separator that would follow this parent's range)
			if end < len(children) {
				// Use the last child's separator as the parent's separator
				ci.sepKey = group[nKeys].sepKey
				ci.sepRec = group[nKeys].sepRec
			}
			nextChildren = append(nextChildren, ci)
			j = end
		}
		children = nextChildren
	}

	return children[0].offset
}

// buildOnePage creates a single leaf or interior page with the given keys.
// Zero-allocation: writes padded keys directly into the page buffer.
func buildOnePage(buf *pageBuffer, keys []KeyRecord, keyLen, maxItem, itemSize int, childOffsets []int64) int64 {
	off := buf.allocPage()
	pg := buf.getPage(off)
	initPageOffsets(pg, maxItem, itemSize)

	for i, kr := range keys {
		entOff := int(binary.LittleEndian.Uint16(pg[2+i*2 : 4+i*2]))
		if childOffsets != nil && i < len(childOffsets) {
			binary.LittleEndian.PutUint32(pg[entOff:entOff+4], uint32(childOffsets[i]))
		} else {
			binary.LittleEndian.PutUint32(pg[entOff:entOff+4], 0) // leaf
		}
		binary.LittleEndian.PutUint32(pg[entOff+4:entOff+8], kr.RecNo)
		// Write padded key directly into page buffer (no intermediate alloc).
		padCopy(pg[entOff+8:entOff+8+keyLen], kr.Key, keyLen)
	}
	binary.LittleEndian.PutUint16(pg[0:2], uint16(len(keys)))
	return off
}

// padCopy copies src into dst, padding with spaces. Zero extra allocation.
func padCopy(dst, src []byte, keyLen int) {
	n := copy(dst, src)
	for i := n; i < keyLen; i++ {
		dst[i] = ' '
	}
}

// rebuildWithInsert creates an NTX using per-key insertion (proper B-tree).
// Used for 3+ level trees where bulk build has separator duplication issues.
func rebuildWithInsert(path, keyExpr string, keyLen int, unique, descend bool, keys []KeyRecord) (*Index, error) {
	itemSize := 8 + keyLen
	maxItem := calculateMaxItems(itemSize)
	halfPage := maxItem / 2

	f, err := os.Create(path)
	if err != nil {
		return nil, err
	}

	rootOff := int64(HeaderSize)
	emptyRoot := [BlockSize]byte{}
	initPageOffsets(emptyRoot[:], maxItem, itemSize)

	hdr := Header{
		Type: 0x0401, Version: 1,
		Root: uint32(rootOff), NextPage: uint32(rootOff + BlockSize),
		ItemSize: uint16(itemSize), KeySize: uint16(keyLen),
		MaxItem: uint16(maxItem), HalfPage: uint16(halfPage),
	}
	copy(hdr.KeyExpr[:], keyExpr)
	if unique {
		hdr.Unique = 1
	}
	if descend {
		hdr.Descend = 1
	}
	WriteHeader(f, &hdr)
	f.WriteAt(emptyRoot[:], rootOff)
	f.Close()

	idx, err := OpenIndex(path)
	if err != nil {
		return nil, err
	}
	for _, kr := range keys {
		k := make([]byte, keyLen)
		copy(k, kr.Key)
		if err := idx.insertKeyBTree(k, kr.RecNo); err != nil {
			idx.Close()
			return nil, err
		}
	}
	// Re-enable mmap now that file is complete
	idx.mmapFile()
	return idx, nil
}

// --- Bulk build buffer (ported from rddfive/ntx_engine.c) ---

type pageBuffer struct {
	data     []byte
	count    int
	capacity int
}

func (pb *pageBuffer) init() {
	pb.capacity = 64
	pb.data = make([]byte, int64(pb.capacity)*BlockSize)
}

func (pb *pageBuffer) grow() {
	if pb.count >= pb.capacity {
		newCap := pb.capacity * 2
		newData := make([]byte, int64(newCap)*BlockSize)
		copy(newData, pb.data[:int64(pb.count)*BlockSize])
		pb.data = newData
		pb.capacity = newCap
	}
}

// allocPage allocates a new page and returns its file offset.
func (pb *pageBuffer) allocPage() int64 {
	pb.grow()
	off := int64(HeaderSize) + int64(pb.count)*BlockSize
	pb.count++
	return off
}

// getPage returns a slice into the buffer for the page at the given file offset.
func (pb *pageBuffer) getPage(fileOffset int64) []byte {
	idx := (fileOffset - int64(HeaderSize)) / BlockSize
	start := idx * BlockSize
	return pb.data[start : start+BlockSize]
}

// initPageOffsets pre-initializes the offset table for a page.
func initPageOffsets(pg []byte, maxItem, itemSize int) {
	dataStart := 2 + (maxItem+1)*2
	for i := 0; i <= maxItem; i++ {
		binary.LittleEndian.PutUint16(pg[2+i*2:4+i*2], uint16(dataStart+i*itemSize))
	}
}

// --- Internal build structures (legacy) ---

type buildPage struct {
	data       [BlockSize]byte
	fileOffset int64
	keyCount   int
	firstKey   []byte
	firstRecNo uint32
}

func calculateMaxItems(itemSize int) int {
	// Page layout: [keyCount:2] [offsets:(max+1)*2] [entries:(max+1)*itemSize]
	// Total must fit in BlockSize (1024).
	// 2 + (max+1)*2 + (max+1)*itemSize <= 1024
	// (max+1) * (itemSize + 2) <= 1022
	// max+1 <= 1022 / (itemSize + 2)
	// max <= 1022/(itemSize+2) - 1
	max := 1022/(itemSize+2) - 1
	if max < 2 {
		max = 2
	}
	if max > 250 {
		max = 250
	}
	return max
}

func makeEmptyPage(keyLen, itemSize, maxItem int, offset int64) *buildPage {
	pg := &buildPage{
		fileOffset: offset,
		firstKey:   make([]byte, keyLen),
	}
	binary.LittleEndian.PutUint16(pg.data[0:2], 0)
	return pg
}

func buildLeafPages(keys []KeyRecord, keyLen, itemSize, maxItem int, nextOffset *int64) []*buildPage {
	if len(keys) == 0 {
		return nil
	}

	var pages []*buildPage
	for i := 0; i < len(keys); i += maxItem {
		end := i + maxItem
		if end > len(keys) {
			end = len(keys)
		}
		chunk := keys[i:end]
		pg := encodeLeafPage(chunk, keyLen, itemSize, maxItem, *nextOffset)
		*nextOffset += BlockSize
		pages = append(pages, pg)
	}
	return pages
}

func encodeLeafPage(keys []KeyRecord, keyLen, itemSize, maxItem int, offset int64) *buildPage {
	pg := &buildPage{
		fileOffset: offset,
		keyCount:   len(keys),
		firstKey:   make([]byte, keyLen),
		firstRecNo: keys[0].RecNo,
	}
	copy(pg.firstKey, keys[0].Key)

	binary.LittleEndian.PutUint16(pg.data[0:2], uint16(len(keys)))
	dataStart := 2 + (maxItem+1)*2

	for i, kr := range keys {
		entryOffset := dataStart + i*itemSize
		binary.LittleEndian.PutUint16(pg.data[2+i*2:4+i*2], uint16(entryOffset))
		binary.LittleEndian.PutUint32(pg.data[entryOffset:entryOffset+4], 0) // leaf
		binary.LittleEndian.PutUint32(pg.data[entryOffset+4:entryOffset+8], kr.RecNo)
		key := make([]byte, keyLen)
		for j := range key {
			key[j] = ' '
		}
		copy(key, kr.Key)
		copy(pg.data[entryOffset+8:entryOffset+8+keyLen], key)
	}
	// Trailing offset
	binary.LittleEndian.PutUint16(
		pg.data[2+len(keys)*2:4+len(keys)*2],
		uint16(dataStart+len(keys)*itemSize),
	)
	return pg
}

func buildInternalLevel(children []*buildPage, keyLen, itemSize, maxItem int, nextOffset *int64) []*buildPage {
	var pages []*buildPage
	// Each internal page holds up to maxItem keys and maxItem+1 child pointers.
	// So each parent page covers maxItem+1 children.
	fanout := maxItem + 1

	for i := 0; i < len(children); i += fanout {
		end := i + fanout
		if end > len(children) {
			end = len(children)
		}
		chunk := children[i:end]
		pg := encodeInternalPage(chunk, keyLen, itemSize, maxItem, *nextOffset)
		*nextOffset += BlockSize
		pages = append(pages, pg)
	}
	return pages
}

func encodeInternalPage(children []*buildPage, keyLen, itemSize, maxItem int, offset int64) *buildPage {
	nKeys := len(children) - 1
	if nKeys < 0 {
		nKeys = 0
	}

	pg := &buildPage{
		fileOffset: offset,
		keyCount:   nKeys,
	}
	if len(children) > 0 {
		pg.firstKey = make([]byte, keyLen)
		copy(pg.firstKey, children[0].firstKey)
		pg.firstRecNo = children[0].firstRecNo
	}

	binary.LittleEndian.PutUint16(pg.data[0:2], uint16(nKeys))
	dataStart := 2 + (maxItem+1)*2

	for i := 0; i <= nKeys; i++ {
		entryOffset := dataStart + i*itemSize
		if entryOffset+itemSize > BlockSize {
			// Page overflow — cap keys
			binary.LittleEndian.PutUint16(pg.data[0:2], uint16(i))
			break
		}
		binary.LittleEndian.PutUint16(pg.data[2+i*2:4+i*2], uint16(entryOffset))

		// Child pointer
		binary.LittleEndian.PutUint32(pg.data[entryOffset:entryOffset+4],
			uint32(children[i].fileOffset))

		if i < nKeys {
			// Separator = first key of child[i+1], promoted (not duplicated)
			binary.LittleEndian.PutUint32(pg.data[entryOffset+4:entryOffset+8],
				children[i+1].firstRecNo)
			key := make([]byte, keyLen)
			for j := range key {
				key[j] = ' '
			}
			copy(key, children[i+1].firstKey)
			copy(pg.data[entryOffset+8:entryOffset+8+keyLen], key)
		} else {
			binary.LittleEndian.PutUint32(pg.data[entryOffset+4:entryOffset+8], 0)
		}
	}

	return pg
}

// --- B-tree insertion ---

// insertKeyBTree inserts a single key into the B-tree with proper page splitting.
// Harbour: hb_ntxTagKeyAdd in dbfntx1.c
func (idx *Index) insertKeyBTree(key []byte, recNo uint32) error {
	// Disable mmap during insertion (file grows, mmap stale)
	if idx.mmapData != nil {
		munmapFile(idx.mmapData)
		idx.mmapData = nil
	}
	// Search for insertion position
	idx.stackLevel = 0
	pageOff := int64(idx.header.Root)

	for {
		page, err := LoadPage(idx.file, pageOff)
		if err != nil {
			return err
		}

		iKey := idx.insertSearch(page, key, recNo)

		if idx.stackLevel < StackSize {
			idx.stack[idx.stackLevel] = StackEntry{PageOffset: pageOff, KeyIndex: iKey}
			idx.stackLevel++
		}

		childOff := page.KeyChild(iKey)
		if childOff == 0 {
			break // at leaf
		}
		pageOff = int64(childOff)
	}

	// Insert at leaf, propagate splits up
	var promoteKey []byte
	var promoteRecNo uint32
	var promoteChild uint32

	for level := idx.stackLevel - 1; level >= 0; level-- {
		page, err := LoadPage(idx.file, idx.stack[level].PageOffset)
		if err != nil {
			return err
		}
		iKey := idx.stack[level].KeyIndex

		var insertKey []byte
		var insertRecNo uint32
		var insertChild uint32

		if level == idx.stackLevel-1 {
			// Leaf insertion
			insertKey = key
			insertRecNo = recNo
			insertChild = 0
		} else {
			// Promoted key from child split
			insertKey = promoteKey
			insertRecNo = promoteRecNo
			insertChild = promoteChild
		}

		if int(page.keyCount) < int(idx.header.MaxItem) {
			// Page has room — insert directly
			idx.pageInsertKey(page, iKey, insertKey, insertRecNo, insertChild)
			page.writeTo(idx.file, idx.stack[level].PageOffset)
			return nil
		}

		// Page full — split
		promoteKey, promoteRecNo, promoteChild, err = idx.pageSplit(page, iKey, insertKey, insertRecNo, insertChild, idx.stack[level].PageOffset)
		if err != nil {
			return err
		}
	}

	// Split propagated to root — create new root
	// Harbour: new page with promoted key at pos 0
	// child[0] = promoteChild (newPage = LEFT half)
	// child[1] = old root (RIGHT half)
	newRootOff := int64(idx.header.NextPage)
	idx.header.NextPage += uint32(BlockSize)

	maxItem := int(idx.header.MaxItem)
	itemSize := int(idx.header.ItemSize)
	dataStart := 2 + (maxItem+1)*2

	newRoot := &Page{data: make([]byte, BlockSize), keyCount: 0}
	for i := 0; i <= maxItem; i++ {
		binary.LittleEndian.PutUint16(newRoot.data[2+i*2:4+i*2], uint16(dataStart+i*itemSize))
	}
	// Insert promoted key at position 0 with child = promoteChild (left half)
	idx.pageInsertKey(newRoot, 0, promoteKey, promoteRecNo, promoteChild)
	// Set trailing child (position 1) = old root (right half)
	trailOff := int(newRoot.keyOffset(1))
	binary.LittleEndian.PutUint32(newRoot.data[trailOff:trailOff+4], idx.header.Root)

	newRoot.writeTo(idx.file, newRootOff)
	idx.header.Root = uint32(newRootOff)

	idx.file.Seek(0, 0)
	WriteHeader(idx.file, &idx.header)

	return nil
}

// insertSearch finds the insertion position in a page (binary search).
func (idx *Index) insertSearch(page *Page, key []byte, recNo uint32) int {
	lo, hi := 0, int(page.keyCount)-1
	for lo <= hi {
		mid := (lo + hi) / 2
		cmp := idx.compareKeys(key, page.KeyValue(mid, idx.keyLen))
		if cmp == 0 {
			// Equal keys: sort by recNo
			midRec := page.KeyRecNo(mid)
			if recNo <= midRec {
				hi = mid - 1
			} else {
				lo = mid + 1
			}
		} else if cmp < 0 {
			hi = mid - 1
		} else {
			lo = mid + 1
		}
	}
	return lo
}

// pageInsertKey inserts a key at position iKey in a page.
// Harbour: hb_ntxPageKeyAdd — swaps offsets, writes key data at freed slot.
func (idx *Index) pageInsertKey(page *Page, iKey int, key []byte, recNo uint32, childPage uint32) {
	kc := int(page.keyCount)

	// The offset at position kc+1 points to the next free data slot
	freeOff := page.keyOffset(kc + 1)

	// Shift offset table right: move [iKey..kc] to [iKey+1..kc+1]
	for i := kc + 1; i > iKey; i-- {
		prev := page.keyOffset(i - 1)
		binary.LittleEndian.PutUint16(page.data[2+i*2:4+i*2], prev)
	}
	// Put the free slot offset at position iKey
	binary.LittleEndian.PutUint16(page.data[2+iKey*2:4+iKey*2], freeOff)

	// Write key data at the free offset
	off := int(freeOff)
	binary.LittleEndian.PutUint32(page.data[off:off+4], childPage)
	binary.LittleEndian.PutUint32(page.data[off+4:off+8], recNo)
	padKey := make([]byte, idx.keyLen)
	for j := range padKey {
		padKey[j] = ' '
	}
	copy(padKey, key)
	copy(page.data[off+8:off+8+idx.keyLen], padKey)

	page.keyCount++
	binary.LittleEndian.PutUint16(page.data[0:2], page.keyCount)
}

// pageSplit splits a full page — exact port of Harbour hb_ntxPageSplit.
// NewPage = LEFT (lower half), OldPage = RIGHT (upper half, offset-swapped).
// Returns: promoted key, its recNo, and newPage offset (left child of promoted key).
func (idx *Index) pageSplit(page *Page, iKey int, key []byte, recNo uint32, childPage uint32, pageOff int64) ([]byte, uint32, uint32, error) {
	maxItem := int(idx.header.MaxItem)
	itemSize := int(idx.header.ItemSize)
	dataStart := 2 + (maxItem+1)*2

	// Allocate new page (will be the LEFT / lower half)
	newPageOff := int64(idx.header.NextPage)
	idx.header.NextPage += uint32(BlockSize)
	newPage := &Page{data: make([]byte, BlockSize)}
	// Initialize offset table
	for i := 0; i <= maxItem; i++ {
		binary.LittleEndian.PutUint16(newPage.data[2+i*2:4+i*2], uint16(dataStart+i*itemSize))
	}

	uiKeys := int(page.keyCount) + 1 // total including new key
	uiHalf := uiKeys / 2

	// Phase 1: Copy lower half to newPage
	j := 0 // index into old page entries
	for newPageCount := 0; newPageCount < uiHalf; newPageCount++ {
		if newPageCount == iKey {
			// Insert the new key here
			idx.setKeyEntry(newPage, newPageCount, childPage, recNo, key)
		} else {
			// Copy from old page entry j
			idx.copyKeyEntry(newPage, newPageCount, page, j)
			j++
		}
		newPage.keyCount++
	}
	binary.LittleEndian.PutUint16(newPage.data[0:2], newPage.keyCount)

	// Phase 2: Extract promoted key (middle key)
	var promKey []byte
	var promRecNo uint32
	if uiHalf == iKey {
		// The new key IS the promoted separator
		promRecNo = recNo
		promKey = append([]byte{}, key...)
		// newPage's trailing child = the new key's child pointer
		trailOff := int(newPage.keyOffset(int(newPage.keyCount)))
		binary.LittleEndian.PutUint32(newPage.data[trailOff:trailOff+4], childPage)
	} else {
		// Promoted key comes from old page entry j
		promRecNo = page.KeyRecNo(j)
		promKey = append([]byte{}, page.KeyValue(j, idx.keyLen)...)
		// newPage's trailing child = old page entry j's child
		trailOff := int(newPage.keyOffset(int(newPage.keyCount)))
		binary.LittleEndian.PutUint32(newPage.data[trailOff:trailOff+4], page.KeyChild(j))
		j++
	}

	// Phase 3: Upper half stays in old page (offset swapping — Harbour style)
	// Rearrange the old page's offset table so that entries j..keyCount-1
	// become positions 0..i-1, using offset swapping (no data copying).
	i := 0
	for uiHalf++; uiHalf < uiKeys; uiHalf++ {
		if uiHalf == iKey {
			// Insert new key at position i in old page
			idx.setKeyEntry(page, i, childPage, recNo, key)
		} else {
			// Swap offset[i] and offset[j] — data stays in place
			offI := page.keyOffset(i)
			offJ := page.keyOffset(j)
			binary.LittleEndian.PutUint16(page.data[2+j*2:4+j*2], offI)
			binary.LittleEndian.PutUint16(page.data[2+i*2:4+i*2], offJ)
			j++
		}
		i++
	}
	// Move trailing child: old page's last child → position i
	lastChild := page.KeyChild(int(page.keyCount))
	trailOff := int(page.keyOffset(int(page.keyCount)))
	binary.LittleEndian.PutUint32(page.data[trailOff:trailOff+4], 0) // clear old
	newTrailOff := int(page.keyOffset(i))
	binary.LittleEndian.PutUint32(page.data[newTrailOff:newTrailOff+4], lastChild)
	page.keyCount = uint16(i)
	binary.LittleEndian.PutUint16(page.data[0:2], page.keyCount)

	// Write both pages
	newPage.writeTo(idx.file, newPageOff)
	page.writeTo(idx.file, pageOff)

	// Update header
	idx.file.Seek(0, 0)
	WriteHeader(idx.file, &idx.header)

	// Harbour: pKeyNew->Tag = pNewPage->Page (left child = newPage)
	return promKey, promRecNo, uint32(newPageOff), nil
}

// setKeyEntry writes a key entry at position i in a page. Zero extra allocation.
func (idx *Index) setKeyEntry(page *Page, i int, child uint32, recNo uint32, key []byte) {
	off := int(page.keyOffset(i))
	binary.LittleEndian.PutUint32(page.data[off:off+4], child)
	binary.LittleEndian.PutUint32(page.data[off+4:off+8], recNo)
	dest := page.data[off+8 : off+8+idx.keyLen]
	n := copy(dest, key)
	for j := n; j < idx.keyLen; j++ {
		dest[j] = ' '
	}
}

// copyKeyEntry copies a full key entry (child+recNo+key) from src page position srcI to dst position dstI.
func (idx *Index) copyKeyEntry(dst *Page, dstI int, src *Page, srcI int) {
	itemSize := int(idx.header.ItemSize)
	dstOff := int(dst.keyOffset(dstI))
	srcOff := int(src.keyOffset(srcI))
	copy(dst.data[dstOff:dstOff+itemSize], src.data[srcOff:srcOff+itemSize])
}

// writeTo writes a page to file at the given offset.
func (p *Page) writeTo(f *os.File, offset int64) {
	f.WriteAt(p.data[:], offset)
}

// --- Single key operations (legacy, uses rebuild) ---

func (idx *Index) InsertKey(key []byte, recNo uint32) error {
	keys := idx.collectAllKeys()
	newKey := make([]byte, idx.keyLen)
	copy(newKey, key)
	keys = append(keys, KeyRecord{Key: newKey, RecNo: recNo})

	sort.Slice(keys, func(i, j int) bool {
		cmp := idx.compareKeys(keys[i].Key, keys[j].Key)
		if cmp == 0 {
			return keys[i].RecNo < keys[j].RecNo
		}
		return cmp < 0
	})

	path := idx.file.Name()
	idx.Close()

	newIdx, err := CreateIndex(path, idx.header.GetKeyExpr(), idx.keyLen,
		idx.uniqueKey, !idx.ascendKey, keys)
	if err != nil {
		return err
	}
	*idx = *newIdx
	return nil
}

func (idx *Index) DeleteKey(recNo uint32) error {
	keys := idx.collectAllKeys()
	filtered := keys[:0]
	for _, kr := range keys {
		if kr.RecNo != recNo {
			filtered = append(filtered, kr)
		}
	}

	path := idx.file.Name()
	idx.Close()

	newIdx, err := CreateIndex(path, idx.header.GetKeyExpr(), idx.keyLen,
		idx.uniqueKey, !idx.ascendKey, filtered)
	if err != nil {
		return err
	}
	*idx = *newIdx
	return nil
}

func (idx *Index) collectAllKeys() []KeyRecord {
	var keys []KeyRecord
	if !idx.GoTop() {
		return keys
	}
	keys = append(keys, KeyRecord{
		Key:   append([]byte{}, idx.curKey[:idx.keyLen]...),
		RecNo: idx.curRecNo,
	})
	for idx.SkipNext() {
		keys = append(keys, KeyRecord{
			Key:   append([]byte{}, idx.curKey[:idx.keyLen]...),
			RecNo: idx.curRecNo,
		})
	}
	return keys
}