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

// CDX compound index creation.
// Builds Harbour-compatible CDX files with bit-packed leaf pages.

package cdx

import (
	"encoding/binary"
	"io"
	"math/bits"
	"os"
	"sort"
	"strings"

	"five/hbrdd/ntx"
)

// cdxTagMeta holds metadata for a tag during file assembly.
type cdxTagMeta struct {
	name      string
	headerOff int64
	rootPage  uint32
	keyExpr   string
	forExpr   string
	keyLen    int
	unique    bool
	desc      bool
}

// CreateOrAddTag creates a CDX file with a new tag, or appends a tag to
// an existing CDX file. The existing file's bytes are preserved verbatim;
// only the compound root directory is rebuilt to include the new tag.
//
// Harbour: ordCreate() → hb_cdxIndexCreateTag()
func CreateOrAddTag(path string, tagName, keyExpr, forExpr string,
	keyLen int, unique, descending bool, keys []ntx.KeyRecord) (*Index, error) {

	if !strings.HasSuffix(strings.ToLower(path), ".cdx") {
		path += ".cdx"
	}
	tagName = strings.ToUpper(tagName)
	if len(tagName) > MaxTagNameLen {
		tagName = tagName[:MaxTagNameLen]
	}

	// Read existing file contents (if any) and tag metadata
	var existingData []byte
	var existingTags []cdxTagMeta

	if fi, err := os.Stat(path); err == nil && fi.Size() > 0 {
		existing, err := OpenIndex(path)
		if err == nil {
			for _, t := range existing.Tags() {
				if strings.ToUpper(t.Name) == tagName {
					continue // will be replaced
				}
				existingTags = append(existingTags, cdxTagMeta{
					name:      t.Name,
					headerOff: t.HeaderOffset(),
					rootPage:  t.RootPtr(),
					keyExpr:   t.KeyExpr(),
					forExpr:   t.ForExpr(),
					keyLen:    t.KeyLen(),
				})
			}
			existing.Close()
		}
		// Read entire old file
		existingData, _ = os.ReadFile(path)
	}

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

	var appendOff int64 // where to start writing new data

	// Harbour CDX layout:
	//   0x0000: Compound root header (1024 bytes)
	//   0x0400: Compound directory leaf page (512 bytes) ← RootPtr points here
	//   0x0600: Tag 1 header (1024 bytes = 2 pages)
	//   Tag 1 B-tree pages...
	//   Tag 2 header...
	//   etc.
	// The compound leaf is always at offset HeaderLen (1024).
	// Tag headers start at HeaderLen + PageLen (1536).

	compoundLeafOff := int64(HeaderLen) // 1024 — fixed position

	if len(existingData) > 0 {
		// Write back existing data verbatim (preserves all old tag B-trees)
		f.Write(existingData)
		appendOff = int64(len(existingData))
		// Align to page boundary
		if appendOff%int64(PageLen) != 0 {
			appendOff = (appendOff/int64(PageLen) + 1) * int64(PageLen)
		}
	} else {
		// New file: skip compound root (1024) + compound leaf (512)
		appendOff = int64(HeaderLen) + int64(PageLen) // 1536
	}

	// Write the new tag's header (1024 bytes) + B-tree pages
	newTagHeaderOff := appendOff
	appendOff += int64(HeaderLen) // 1024 bytes for tag header

	// Build B-tree pages for the new tag
	var rootPageOff uint32
	if len(keys) == 0 {
		pg := make([]byte, PageLen)
		binary.LittleEndian.PutUint16(pg[0:2], NodeLeaf|NodeRoot)
		f.WriteAt(pg, appendOff)
		rootPageOff = uint32(appendOff)
		appendOff += PageLen
	} else {
		root, next := buildCDXBTree(f, keys, keyLen, appendOff)
		rootPageOff = uint32(root)
		appendOff = next
	}

	// Write the new tag header
	writeTagHeader(f, newTagHeaderOff, rootPageOff, keyExpr, forExpr,
		uint16(keyLen), unique, descending)

	newTag := cdxTagMeta{
		name: tagName, headerOff: newTagHeaderOff, rootPage: rootPageOff,
		keyExpr: keyExpr, forExpr: forExpr, keyLen: keyLen,
		unique: unique, desc: descending,
	}

	// Collect all tags (existing + new) in offset order (= creation order)
	allTags := append(existingTags, newTag)

	// Write compound directory leaf page at fixed offset 1024.
	// Harbour's compound B-tree stores entries in ALPHABETICAL order
	// (it's a B-tree keyed by tag name). Sort a copy for the leaf.
	sortedTags := make([]cdxTagMeta, len(allTags))
	copy(sortedTags, allTags)
	sort.Slice(sortedTags, func(i, j int) bool {
		return strings.ToUpper(sortedTags[i].name) < strings.ToUpper(sortedTags[j].name)
	})
	writeCompoundLeaf(f, compoundLeafOff, sortedTags)

	// Write compound root header at offset 0
	writeCompoundHeader(f, uint32(compoundLeafOff), len(allTags))

	f.Close()
	return OpenIndex(path)
}

// --- Tag header writing ---

func writeTagHeader(f *os.File, offset int64, rootPtr uint32,
	keyExpr, forExpr string, keySize uint16, unique, desc bool) {

	buf := make([]byte, HeaderLen)
	binary.LittleEndian.PutUint32(buf[0:4], rootPtr)
	// Counter = 0 initially (Harbour convention)
	binary.LittleEndian.PutUint16(buf[12:14], keySize)
	// Harbour sets TypeCompact | TypeCompound on data tags (0x60)
	opt := byte(TypeCompact | TypeCompound)
	if unique {
		opt |= TypeUnique
	}
	if forExpr != "" {
		opt |= TypeForFilter
	}
	buf[14] = opt
	buf[15] = 0x01
	binary.LittleEndian.PutUint16(buf[16:18], uint16(HeaderLen))
	binary.LittleEndian.PutUint16(buf[18:20], uint16(PageLen))
	if desc {
		binary.LittleEndian.PutUint16(buf[504:506], 1)
	}
	copy(buf[512:], []byte(keyExpr))
	if forExpr != "" {
		forOff := 512 + len(keyExpr) + 1
		if forOff+len(forExpr) < HeaderLen {
			copy(buf[forOff:], []byte(forExpr))
		}
	}
	f.WriteAt(buf, offset)
}

// --- Compound root ---

func writeCompoundHeader(f *os.File, rootPagePtr uint32, nTags int) {
	hdr := make([]byte, HeaderLen)
	binary.LittleEndian.PutUint32(hdr[0:4], rootPagePtr)
	// FreePtr = 0, Counter = 0 (Harbour convention for compound root)
	binary.LittleEndian.PutUint16(hdr[12:14], MaxTagNameLen)
	hdr[14] = TypeCompound | TypeStructure | TypeCompact // 0xE0
	hdr[15] = 0x01
	binary.LittleEndian.PutUint16(hdr[16:18], uint16(HeaderLen))
	binary.LittleEndian.PutUint16(hdr[18:20], uint16(PageLen))
	// Harbour writes "RCHB" signature at offset 20 in compound root
	copy(hdr[20:24], []byte("RCHB"))
	// IgnoreCase=1, Descending flags at offset 503-505 (Harbour convention)
	hdr[503] = 1  // IgnoreCase
	binary.LittleEndian.PutUint16(hdr[504:506], 1) // collation flag
	f.WriteAt(hdr, 0)
}

func writeCompoundLeaf(f *os.File, offset int64, tags []cdxTagMeta) {
	leaf := make([]byte, PageLen)
	nTags := len(tags)
	compKeyLen := MaxTagNameLen

	// Harbour uses fixed 16-bit recBits for compound leaf (offsets < 64KB)
	recBits := 16
	dupBits := bitsNeeded(uint32(compKeyLen)) // 4
	trlBits := bitsNeeded(uint32(compKeyLen)) // 4
	keyBytes := (recBits + dupBits + trlBits + 7) / 8 // 3

	binary.LittleEndian.PutUint16(leaf[0:2], NodeLeaf|NodeRoot)
	binary.LittleEndian.PutUint16(leaf[2:4], uint16(nTags))
	// Harbour sets LeftPtr/RightPtr to 0xFFFFFFFF for compound leaf
	binary.LittleEndian.PutUint32(leaf[4:8], 0xFFFFFFFF)
	binary.LittleEndian.PutUint32(leaf[8:12], 0xFFFFFFFF)
	binary.LittleEndian.PutUint32(leaf[14:18], (1<<uint(recBits))-1)
	leaf[18] = byte((1 << uint(dupBits)) - 1)
	leaf[19] = byte((1 << uint(trlBits)) - 1)
	leaf[20] = byte(recBits)
	leaf[21] = byte(dupBits)
	leaf[22] = byte(trlBits)
	leaf[23] = byte(keyBytes)

	prevKey := make([]byte, compKeyLen)
	for j := range prevKey {
		prevKey[j] = ' '
	}
	keyDataPos := PageLen

	for i, t := range tags {
		key := padTagName(t.name)
		dup := commonPrefix(key, prevKey, compKeyLen)
		trl := trailingSpaces(key, compKeyLen)
		newBytes := compKeyLen - dup - trl

		if newBytes > 0 {
			keyDataPos -= newBytes
			copy(leaf[keyDataPos:], key[dup:dup+newBytes])
		}

		recNo := uint32(t.headerOff)
		var val uint64
		val = uint64(trl)
		val <<= uint(dupBits)
		val |= uint64(dup)
		val <<= uint(recBits)
		val |= uint64(recNo)

		entryOff := ExtHeadSize + i*int(keyBytes)
		for j := 0; j < int(keyBytes); j++ {
			leaf[entryOff+j] = byte(val & 0xFF)
			val >>= 8
		}
		copy(prevKey, key)
	}

	freeSpc := keyDataPos - (ExtHeadSize + nTags*int(keyBytes))
	if freeSpc < 0 {
		freeSpc = 0
	}
	binary.LittleEndian.PutUint16(leaf[12:14], uint16(freeSpc))
	f.WriteAt(leaf, offset)
}

// --- B-tree builder ---

func buildCDXBTree(f *os.File, keys []ntx.KeyRecord, keyLen int,
	startOff int64) (rootOff int64, nextOff int64) {

	maxRecNo := uint32(0)
	for _, k := range keys {
		if k.RecNo > maxRecNo {
			maxRecNo = k.RecNo
		}
	}
	recBits := bitsNeeded(maxRecNo)
	dupBits := bitsNeeded(uint32(keyLen))
	trlBits := bitsNeeded(uint32(keyLen))
	keyBytesPerEntry := (recBits + dupBits + trlBits + 7) / 8

	maxKeysPerLeaf := (PageLen - ExtHeadSize) / (int(keyBytesPerEntry) + keyLen)
	if maxKeysPerLeaf < 2 {
		maxKeysPerLeaf = 2
	}
	intEntrySize := keyLen + 8
	maxKeysPerInt := (PageLen - 12) / intEntrySize
	if maxKeysPerInt < 2 {
		maxKeysPerInt = 2
	}

	curOff := startOff

	type leafInfo struct {
		pageOff int64
		lastKey []byte
		lastRec uint32
	}
	var leaves []leafInfo

	for i := 0; i < len(keys); {
		end := i + maxKeysPerLeaf
		if end > len(keys) {
			end = len(keys)
		}
		chunk := keys[i:end]
		pageOff := curOff
		writeCDXLeafPage(f, pageOff, chunk, keyLen, recBits, dupBits, trlBits, keyBytesPerEntry)
		curOff += PageLen
		leaves = append(leaves, leafInfo{
			pageOff: pageOff,
			lastKey: chunk[len(chunk)-1].Key,
			lastRec: chunk[len(chunk)-1].RecNo,
		})
		i = end
	}

	// Link leaf pages
	for i := range leaves {
		pg := make([]byte, PageLen)
		f.ReadAt(pg, leaves[i].pageOff)
		if i > 0 {
			binary.LittleEndian.PutUint32(pg[4:8], uint32(leaves[i-1].pageOff))
		}
		if i < len(leaves)-1 {
			binary.LittleEndian.PutUint32(pg[8:12], uint32(leaves[i+1].pageOff))
		}
		f.WriteAt(pg, leaves[i].pageOff)
	}

	if len(leaves) == 1 {
		pg := make([]byte, 2)
		f.ReadAt(pg, leaves[0].pageOff)
		attr := binary.LittleEndian.Uint16(pg)
		attr |= NodeRoot
		binary.LittleEndian.PutUint16(pg, attr)
		f.WriteAt(pg, leaves[0].pageOff)
		return leaves[0].pageOff, curOff
	}

	type childInfo struct {
		pageOff int64
		sepKey  []byte
		sepRec  uint32
	}
	children := make([]childInfo, len(leaves))
	for i, l := range leaves {
		children[i] = childInfo{pageOff: l.pageOff, sepKey: l.lastKey, sepRec: l.lastRec}
	}

	for len(children) > 1 {
		var nextLevel []childInfo
		for i := 0; i < len(children); {
			end := i + maxKeysPerInt + 1
			if end > len(children) {
				end = len(children)
			}
			group := children[i:end]
			nKeys := len(group) - 1

			pg := make([]byte, PageLen)
			attr := uint16(0)
			if len(children) <= maxKeysPerInt+1 {
				attr |= NodeRoot
			}
			binary.LittleEndian.PutUint16(pg[0:2], attr)
			binary.LittleEndian.PutUint16(pg[2:4], uint16(nKeys))
			binary.LittleEndian.PutUint32(pg[4:8], uint32(group[0].pageOff))

			off := 12
			for k := 0; k < nKeys; k++ {
				key := group[k].sepKey
				if len(key) < keyLen {
					padded := make([]byte, keyLen)
					copy(padded, key)
					for j := len(key); j < keyLen; j++ {
						padded[j] = ' '
					}
					key = padded
				}
				copy(pg[off:off+keyLen], key[:keyLen])
				off += keyLen
				binary.BigEndian.PutUint32(pg[off:off+4], group[k].sepRec)
				off += 4
				binary.BigEndian.PutUint32(pg[off:off+4], uint32(group[k+1].pageOff))
				off += 4
			}

			pageOff := curOff
			f.WriteAt(pg, pageOff)
			curOff += PageLen

			ci := childInfo{pageOff: pageOff}
			if end < len(children) {
				ci.sepKey = group[nKeys].sepKey
				ci.sepRec = group[nKeys].sepRec
			}
			nextLevel = append(nextLevel, ci)
			i = end
		}
		children = nextLevel
	}

	return children[0].pageOff, curOff
}

// --- Leaf page writer ---

func writeCDXLeafPage(f *os.File, offset int64, keys []ntx.KeyRecord,
	keyLen, recBits, dupBits, trlBits, keyBytesPerEntry int) {

	pg := make([]byte, PageLen)
	binary.LittleEndian.PutUint16(pg[0:2], NodeLeaf)
	binary.LittleEndian.PutUint16(pg[2:4], uint16(len(keys)))
	binary.LittleEndian.PutUint32(pg[14:18], (1<<uint(recBits))-1)
	pg[18] = byte((1 << uint(dupBits)) - 1)
	pg[19] = byte((1 << uint(trlBits)) - 1)
	pg[20] = byte(recBits)
	pg[21] = byte(dupBits)
	pg[22] = byte(trlBits)
	pg[23] = byte(keyBytesPerEntry)

	prevKey := make([]byte, keyLen)
	for j := range prevKey {
		prevKey[j] = ' '
	}
	keyDataPos := PageLen

	for i, kr := range keys {
		key := kr.Key
		if len(key) < keyLen {
			padded := make([]byte, keyLen)
			copy(padded, key)
			for j := len(key); j < keyLen; j++ {
				padded[j] = ' '
			}
			key = padded
		} else if len(key) > keyLen {
			key = key[:keyLen]
		}

		dup := commonPrefix(key, prevKey, keyLen)
		trl := trailingSpaces(key, keyLen)
		newBytes := keyLen - dup - trl

		if newBytes > 0 {
			keyDataPos -= newBytes
			copy(pg[keyDataPos:], key[dup:dup+newBytes])
		}

		var val uint64
		val = uint64(trl)
		val <<= uint(dupBits)
		val |= uint64(dup)
		val <<= uint(recBits)
		val |= uint64(kr.RecNo)

		entryOff := ExtHeadSize + i*keyBytesPerEntry
		for j := 0; j < keyBytesPerEntry; j++ {
			pg[entryOff+j] = byte(val & 0xFF)
			val >>= 8
		}
		copy(prevKey, key)
	}

	freeSpc := keyDataPos - (ExtHeadSize + len(keys)*keyBytesPerEntry)
	if freeSpc < 0 {
		freeSpc = 0
	}
	binary.LittleEndian.PutUint16(pg[12:14], uint16(freeSpc))
	f.WriteAt(pg, offset)
}

// --- Helpers ---

func bitsNeeded(maxVal uint32) int {
	if maxVal == 0 {
		return 1
	}
	return bits.Len32(maxVal)
}

func commonPrefix(a, b []byte, maxLen int) int {
	n := maxLen
	if len(a) < n {
		n = len(a)
	}
	if len(b) < n {
		n = len(b)
	}
	for i := 0; i < n; i++ {
		if a[i] != b[i] {
			return i
		}
	}
	return n
}

func trailingSpaces(key []byte, keyLen int) int {
	n := keyLen
	if len(key) < n {
		n = len(key)
	}
	count := 0
	for i := n - 1; i >= 0; i-- {
		if key[i] == ' ' {
			count++
		} else {
			break
		}
	}
	return count
}

func padTagName(name string) []byte {
	b := make([]byte, MaxTagNameLen)
	copy(b, []byte(strings.ToUpper(name)))
	for i := len(name); i < MaxTagNameLen; i++ {
		b[i] = ' '
	}
	return b
}

// Ensure io import is used (for potential future reads)
var _ = io.EOF