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package afero
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import (
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	"io"
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	"os"
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	"path/filepath"
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	"syscall"
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)
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// The UnionFile implements the afero.File interface and will be returned
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// when reading a directory present at least in the overlay or opening a file
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// for writing.
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//
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// The calls to
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// Readdir() and Readdirnames() merge the file os.FileInfo / names from the
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// base and the overlay - for files present in both layers, only those
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// from the overlay will be used.
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//
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// When opening files for writing (Create() / OpenFile() with the right flags)
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// the operations will be done in both layers, starting with the overlay. A
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// successful read in the overlay will move the cursor position in the base layer
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// by the number of bytes read.
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type UnionFile struct {
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	Base   File
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	Layer  File
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	Merger DirsMerger
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	off    int
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	files  []os.FileInfo
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}
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func (f *UnionFile) Close() error {
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	// first close base, so we have a newer timestamp in the overlay. If we'd close
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	// the overlay first, we'd get a cacheStale the next time we access this file
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	// -> cache would be useless ;-)
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	if f.Base != nil {
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		f.Base.Close()
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	}
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	if f.Layer != nil {
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		return f.Layer.Close()
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	}
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	return BADFD
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}
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func (f *UnionFile) Read(s []byte) (int, error) {
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	if f.Layer != nil {
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		n, err := f.Layer.Read(s)
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		if (err == nil || err == io.EOF) && f.Base != nil {
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			// advance the file position also in the base file, the next
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			// call may be a write at this position (or a seek with SEEK_CUR)
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			if _, seekErr := f.Base.Seek(int64(n), io.SeekCurrent); seekErr != nil {
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				// only overwrite err in case the seek fails: we need to
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				// report an eventual io.EOF to the caller
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				err = seekErr
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			}
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		}
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		return n, err
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	}
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	if f.Base != nil {
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		return f.Base.Read(s)
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	}
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	return 0, BADFD
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}
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func (f *UnionFile) ReadAt(s []byte, o int64) (int, error) {
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	if f.Layer != nil {
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		n, err := f.Layer.ReadAt(s, o)
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		if (err == nil || err == io.EOF) && f.Base != nil {
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			_, err = f.Base.Seek(o+int64(n), io.SeekStart)
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		}
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		return n, err
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	}
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	if f.Base != nil {
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		return f.Base.ReadAt(s, o)
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	}
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	return 0, BADFD
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}
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func (f *UnionFile) Seek(o int64, w int) (pos int64, err error) {
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	if f.Layer != nil {
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		pos, err = f.Layer.Seek(o, w)
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		if (err == nil || err == io.EOF) && f.Base != nil {
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			_, err = f.Base.Seek(o, w)
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		}
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		return pos, err
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	}
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	if f.Base != nil {
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		return f.Base.Seek(o, w)
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	}
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	return 0, BADFD
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}
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func (f *UnionFile) Write(s []byte) (n int, err error) {
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	if f.Layer != nil {
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		n, err = f.Layer.Write(s)
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		if err == nil &&
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			f.Base != nil { // hmm, do we have fixed size files where a write may hit the EOF mark?
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			_, err = f.Base.Write(s)
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		}
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		return n, err
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	}
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	if f.Base != nil {
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		return f.Base.Write(s)
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	}
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	return 0, BADFD
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}
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func (f *UnionFile) WriteAt(s []byte, o int64) (n int, err error) {
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	if f.Layer != nil {
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		n, err = f.Layer.WriteAt(s, o)
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		if err == nil && f.Base != nil {
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			_, err = f.Base.WriteAt(s, o)
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		}
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		return n, err
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	}
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	if f.Base != nil {
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		return f.Base.WriteAt(s, o)
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	}
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	return 0, BADFD
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}
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func (f *UnionFile) Name() string {
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	if f.Layer != nil {
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		return f.Layer.Name()
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	}
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	return f.Base.Name()
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}
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// DirsMerger is how UnionFile weaves two directories together.
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// It takes the FileInfo slices from the layer and the base and returns a
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// single view.
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type DirsMerger func(lofi, bofi []os.FileInfo) ([]os.FileInfo, error)
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var defaultUnionMergeDirsFn = func(lofi, bofi []os.FileInfo) ([]os.FileInfo, error) {
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	files := make(map[string]os.FileInfo)
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	for _, fi := range lofi {
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		files[fi.Name()] = fi
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	}
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	for _, fi := range bofi {
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		if _, exists := files[fi.Name()]; !exists {
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			files[fi.Name()] = fi
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		}
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	}
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	rfi := make([]os.FileInfo, len(files))
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	i := 0
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	for _, fi := range files {
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		rfi[i] = fi
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		i++
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	}
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	return rfi, nil
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}
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// Readdir will weave the two directories together and
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// return a single view of the overlayed directories.
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// At the end of the directory view, the error is io.EOF if c > 0.
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func (f *UnionFile) Readdir(c int) (ofi []os.FileInfo, err error) {
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	merge := f.Merger
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	if merge == nil {
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		merge = defaultUnionMergeDirsFn
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	}
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	if f.off == 0 {
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		var lfi []os.FileInfo
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		if f.Layer != nil {
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			lfi, err = f.Layer.Readdir(-1)
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			if err != nil {
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				return nil, err
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			}
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		}
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		var bfi []os.FileInfo
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		if f.Base != nil {
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			bfi, err = f.Base.Readdir(-1)
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			if err != nil {
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				return nil, err
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			}
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		}
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		merged, err := merge(lfi, bfi)
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		if err != nil {
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			return nil, err
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		}
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		f.files = append(f.files, merged...)
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	}
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	files := f.files[f.off:]
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	if c <= 0 {
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		return files, nil
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	}
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	if len(files) == 0 {
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		return nil, io.EOF
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	}
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	if c > len(files) {
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		c = len(files)
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	}
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	defer func() { f.off += c }()
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	return files[:c], nil
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}
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func (f *UnionFile) Readdirnames(c int) ([]string, error) {
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	rfi, err := f.Readdir(c)
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	if err != nil {
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		return nil, err
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	}
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	var names []string
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	for _, fi := range rfi {
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		names = append(names, fi.Name())
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	}
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	return names, nil
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}
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func (f *UnionFile) Stat() (os.FileInfo, error) {
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	if f.Layer != nil {
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		return f.Layer.Stat()
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	}
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	if f.Base != nil {
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		return f.Base.Stat()
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	}
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	return nil, BADFD
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}
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func (f *UnionFile) Sync() (err error) {
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	if f.Layer != nil {
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		err = f.Layer.Sync()
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		if err == nil && f.Base != nil {
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			err = f.Base.Sync()
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		}
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		return err
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	}
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	if f.Base != nil {
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		return f.Base.Sync()
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	}
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	return BADFD
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}
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func (f *UnionFile) Truncate(s int64) (err error) {
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	if f.Layer != nil {
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		err = f.Layer.Truncate(s)
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		if err == nil && f.Base != nil {
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			err = f.Base.Truncate(s)
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		}
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		return err
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	}
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	if f.Base != nil {
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		return f.Base.Truncate(s)
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	}
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	return BADFD
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}
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func (f *UnionFile) WriteString(s string) (n int, err error) {
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	if f.Layer != nil {
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		n, err = f.Layer.WriteString(s)
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		if err == nil && f.Base != nil {
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			_, err = f.Base.WriteString(s)
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		}
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		return n, err
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	}
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	if f.Base != nil {
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		return f.Base.WriteString(s)
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	}
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	return 0, BADFD
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}
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func copyFile(base Fs, layer Fs, name string, bfh File) error {
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	// First make sure the directory exists
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	exists, err := Exists(layer, filepath.Dir(name))
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	if err != nil {
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		return err
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	}
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	if !exists {
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		err = layer.MkdirAll(filepath.Dir(name), 0o777) // FIXME?
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		if err != nil {
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			return err
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		}
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	}
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	// Create the file on the overlay
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	lfh, err := layer.Create(name)
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	if err != nil {
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		return err
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	}
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	n, err := io.Copy(lfh, bfh)
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	if err != nil {
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		// If anything fails, clean up the file
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		layer.Remove(name)
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		lfh.Close()
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		return err
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	}
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	bfi, err := bfh.Stat()
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	if err != nil || bfi.Size() != n {
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		layer.Remove(name)
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		lfh.Close()
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		return syscall.EIO
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	}
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	err = lfh.Close()
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	if err != nil {
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		layer.Remove(name)
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		lfh.Close()
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		return err
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	}
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	return layer.Chtimes(name, bfi.ModTime(), bfi.ModTime())
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}
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func copyToLayer(base Fs, layer Fs, name string) error {
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	bfh, err := base.Open(name)
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	if err != nil {
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		return err
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	}
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	defer bfh.Close()
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	return copyFile(base, layer, name, bfh)
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}
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func copyFileToLayer(base Fs, layer Fs, name string, flag int, perm os.FileMode) error {
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	bfh, err := base.OpenFile(name, flag, perm)
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	if err != nil {
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		return err
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	}
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	defer bfh.Close()
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	return copyFile(base, layer, name, bfh)
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}
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