package pkg

import (
	"crypto/sha512"
	"encoding/binary"
	"errors"
	"io"
	"io/fs"
	"math"
	"os"
	"syscall"

	"hakurei.app/container/check"
)

// FlatEntry is a directory entry to be encoded for [Flatten].
type FlatEntry struct {
	Mode fs.FileMode // file mode bits
	Path string      // pathname of the file
	Data []byte      // file content or symlink destination
}

/*
| mode uint32 | path_sz uint32 |
|        data_sz uint64        |
|         path string          |
|         data []byte          |
*/

// wordSize is the boundary which binary segments are always aligned to.
const wordSize = 8

// alignSize returns the padded size for aligning sz.
func alignSize(sz int) int {
	return sz + (wordSize-(sz)%wordSize)%wordSize
}

// Encode encodes the entry for transmission or hashing.
func (ent *FlatEntry) Encode(w io.Writer) (n int, err error) {
	pPathSize := alignSize(len(ent.Path))
	if pPathSize > math.MaxUint32 {
		return 0, syscall.E2BIG
	}
	pDataSize := alignSize(len(ent.Data))

	payload := make([]byte, wordSize*2+pPathSize+pDataSize)
	binary.LittleEndian.PutUint32(payload, uint32(ent.Mode))
	binary.LittleEndian.PutUint32(payload[wordSize/2:], uint32(len(ent.Path)))
	binary.LittleEndian.PutUint64(payload[wordSize:], uint64(len(ent.Data)))
	copy(payload[wordSize*2:], ent.Path)
	copy(payload[wordSize*2+pPathSize:], ent.Data)
	return w.Write(payload)
}

// Decode decodes the entry from its representation produced by Encode.
func (ent *FlatEntry) Decode(r io.Reader) (n int, err error) {
	var nr int

	header := make([]byte, wordSize*2)
	nr, err = r.Read(header)
	n += nr
	if err != nil {
		if errors.Is(err, io.EOF) && n != 0 {
			err = io.ErrUnexpectedEOF
		}
		return
	}

	ent.Mode = fs.FileMode(binary.LittleEndian.Uint32(header))
	pathSize := int(binary.LittleEndian.Uint32(header[wordSize/2:]))
	pPathSize := alignSize(pathSize)
	dataSize := int(binary.LittleEndian.Uint64(header[wordSize:]))
	pDataSize := alignSize(dataSize)

	buf := make([]byte, pPathSize+pDataSize)
	nr, err = r.Read(buf)
	n += nr
	if err != nil {
		if errors.Is(err, io.EOF) {
			if nr != len(buf) {
				err = io.ErrUnexpectedEOF
				return
			}
		} else {
			return
		}
	}

	ent.Path = string(buf[:pathSize])
	if ent.Mode.IsDir() {
		ent.Data = nil
	} else {
		ent.Data = buf[pPathSize : pPathSize+dataSize]
	}
	return
}

// DirScanner provides an efficient interface for reading a stream of encoded
// [FlatEntry]. Successive calls to the Scan method will step through the
// entries in the stream.
type DirScanner struct {
	// Underlying reader to scan [FlatEntry] representations from.
	r io.Reader

	// First non-EOF I/O error, returned by the Err method.
	err error

	// Entry to store results in. Its address is returned by the Entry method
	// and is updated on every call to Scan.
	ent FlatEntry
}

// NewDirScanner returns the address of a new instance of [DirScanner] reading
// from r. The caller must no longer read from r after this function returns.
func NewDirScanner(r io.Reader) *DirScanner { return &DirScanner{r: r} }

// Err returns the first non-EOF I/O error.
func (s *DirScanner) Err() error {
	if errors.Is(s.err, io.EOF) {
		return nil
	}
	return s.err
}

// Entry returns the address to the [FlatEntry] value storing the last result.
func (s *DirScanner) Entry() *FlatEntry { return &s.ent }

// Scan advances to the next [FlatEntry].
func (s *DirScanner) Scan() bool {
	if s.err != nil {
		return false
	}

	var n int
	n, s.err = s.ent.Decode(s.r)
	if errors.Is(s.err, io.EOF) {
		return n != 0
	}
	return s.err == nil
}

// Flatten writes a deterministic representation of the contents of fsys to w.
// The resulting data can be hashed to produce a deterministic checksum for the
// directory.
func Flatten(fsys fs.FS, root string, w io.Writer) (n int, err error) {
	var nr int
	err = fs.WalkDir(fsys, root, func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}

		var fi fs.FileInfo
		fi, err = d.Info()
		if err != nil {
			return err
		}

		ent := FlatEntry{
			Path: path,
			Mode: fi.Mode(),
		}
		if ent.Mode.IsRegular() {
			if ent.Data, err = fs.ReadFile(fsys, path); err != nil {
				return err
			}
		} else if ent.Mode&fs.ModeSymlink != 0 {
			var newpath string
			if newpath, err = fs.ReadLink(fsys, path); err != nil {
				return err
			}
			ent.Data = []byte(newpath)
		}

		nr, err = ent.Encode(w)
		n += nr
		return err
	})
	return
}

// HashFS returns a checksum produced by hashing the result of [Flatten].
func HashFS(fsys fs.FS, root string) (Checksum, error) {
	h := sha512.New384()
	if _, err := Flatten(fsys, root, h); err != nil {
		return Checksum{}, err
	}
	return (Checksum)(h.Sum(nil)), nil
}

// HashDir returns a checksum produced by hashing the result of [Flatten].
func HashDir(pathname *check.Absolute) (Checksum, error) {
	return HashFS(os.DirFS(pathname.String()), ".")
}