// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package os

import (
	"errors"
	"io"
	"syscall"
)

func (file *File) readdir(n int) (fi []FileInfo, err error) {
	// If this file has no dirinfo, create one.
	if file.dirinfo == nil {
		file.dirinfo = new(dirInfo)
	}
	d := file.dirinfo
	size := n
	if size <= 0 {
		size = 100
		n = -1
	}
	result := make([]FileInfo, 0, size) // Empty with room to grow.
	for n != 0 {
		// Refill the buffer if necessary
		if d.bufp >= d.nbuf {
			d.bufp = 0
			var e error
			d.nbuf, e = file.Read(d.buf[:])
			if e != nil && e != io.EOF {
				return result, &PathError{"readdir", file.name, e}
			}
			if e == io.EOF {
				break
			}
			if d.nbuf < syscall.STATFIXLEN {
				return result, &PathError{"readdir", file.name, Eshortstat}
			}
		}

		// Get a record from buffer
		m, _ := gbit16(d.buf[d.bufp:])
		m += 2
		if m < syscall.STATFIXLEN {
			return result, &PathError{"readdir", file.name, Eshortstat}
		}
		dir, e := UnmarshalDir(d.buf[d.bufp : d.bufp+int(m)])
		if e != nil {
			return result, &PathError{"readdir", file.name, e}
		}
		result = append(result, fileInfoFromStat(dir))

		d.bufp += int(m)
		n--
	}

	if n >= 0 && len(result) == 0 {
		return result, io.EOF
	}
	return result, nil
}

func (file *File) readdirnames(n int) (names []string, err error) {
	fi, err := file.Readdir(n)
	names = make([]string, len(fi))
	for i := range fi {
		names[i] = fi[i].Name()
	}
	return
}

type Dir struct {
	// system-modified data
	Type uint16 // server type
	Dev  uint32 // server subtype
	// file data
	Qid    Qid    // unique id from server
	Mode   uint32 // permissions
	Atime  uint32 // last read time
	Mtime  uint32 // last write time
	Length uint64 // file length
	Name   string // last element of path
	Uid    string // owner name
	Gid    string // group name
	Muid   string // last modifier name
}

type Qid struct {
	Path uint64 // the file server's unique identification for the file
	Vers uint32 // version number for given Path
	Type uint8  // the type of the file (syscall.QTDIR for example)
}

var nullDir = Dir{
	^uint16(0),
	^uint32(0),
	Qid{^uint64(0), ^uint32(0), ^uint8(0)},
	^uint32(0),
	^uint32(0),
	^uint32(0),
	^uint64(0),
	"",
	"",
	"",
	"",
}

// Null assigns members of d with special "don't care" values indicating
// they should not be written by syscall.Wstat. 
func (d *Dir) Null() {
	*d = nullDir
}

// pdir appends a 9P Stat message based on the contents of Dir d to a byte slice b.
func pdir(b []byte, d *Dir) []byte {
	n := len(b)
	b = pbit16(b, 0) // length, filled in later	
	b = pbit16(b, d.Type)
	b = pbit32(b, d.Dev)
	b = pqid(b, d.Qid)
	b = pbit32(b, d.Mode)
	b = pbit32(b, d.Atime)
	b = pbit32(b, d.Mtime)
	b = pbit64(b, d.Length)
	b = pstring(b, d.Name)
	b = pstring(b, d.Uid)
	b = pstring(b, d.Gid)
	b = pstring(b, d.Muid)
	pbit16(b[0:n], uint16(len(b)-(n+2)))
	return b
}

// UnmarshalDir reads a 9P Stat message from a 9P protocol message stored in b,
// returning the corresponding Dir struct.
func UnmarshalDir(b []byte) (d *Dir, err error) {
	n := uint16(0)
	n, b = gbit16(b)

	if int(n) != len(b) {
		return nil, Ebadstat
	}

	d = new(Dir)
	d.Type, b = gbit16(b)
	d.Dev, b = gbit32(b)
	d.Qid, b = gqid(b)
	d.Mode, b = gbit32(b)
	d.Atime, b = gbit32(b)
	d.Mtime, b = gbit32(b)
	d.Length, b = gbit64(b)
	d.Name, b = gstring(b)
	d.Uid, b = gstring(b)
	d.Gid, b = gstring(b)
	d.Muid, b = gstring(b)

	if len(b) != 0 {
		return nil, Ebadstat
	}

	return d, nil
}

// gqid reads the qid part of a 9P Stat message from a 9P protocol message stored in b,
// returning the corresponding Qid struct and the remaining slice of b.
func gqid(b []byte) (Qid, []byte) {
	var q Qid
	q.Path, b = gbit64(b)
	q.Vers, b = gbit32(b)
	q.Type, b = gbit8(b)
	return q, b
}

// pqid appends a Qid struct q to a 9P message b.
func pqid(b []byte, q Qid) []byte {
	b = pbit64(b, q.Path)
	b = pbit32(b, q.Vers)
	b = pbit8(b, q.Type)
	return b
}

// gbit8 reads a byte-sized numeric value from a 9P protocol message stored in b,
// returning the value and the remaining slice of b.
func gbit8(b []byte) (uint8, []byte) {
	return uint8(b[0]), b[1:]
}

// gbit16 reads a 16-bit numeric value from a 9P protocol message stored in b,
// returning the value and the remaining slice of b.
func gbit16(b []byte) (uint16, []byte) {
	return uint16(b[0]) | uint16(b[1])<<8, b[2:]
}

// gbit32 reads a 32-bit numeric value from a 9P protocol message stored in b,
// returning the value and the remaining slice of b.
func gbit32(b []byte) (uint32, []byte) {
	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24, b[4:]
}

// gbit64 reads a 64-bit numeric value from a 9P protocol message stored in b,
// returning the value and the remaining slice of b.
func gbit64(b []byte) (uint64, []byte) {
	lo, b := gbit32(b)
	hi, b := gbit32(b)
	return uint64(hi)<<32 | uint64(lo), b
}

// gstring reads a string from a 9P protocol message stored in b,
// returning the value as a Go string and the remaining slice of b.
func gstring(b []byte) (string, []byte) {
	n, b := gbit16(b)
	return string(b[0:n]), b[n:]
}

// pbit8 appends a byte-sized numeric value x to a 9P message b.
func pbit8(b []byte, x uint8) []byte {
	n := len(b)
	if n+1 > cap(b) {
		nb := make([]byte, n, 100+2*cap(b))
		copy(nb, b)
		b = nb
	}
	b = b[0 : n+1]
	b[n] = x
	return b
}

// pbit16 appends a 16-bit numeric value x to a 9P message b.
func pbit16(b []byte, x uint16) []byte {
	n := len(b)
	if n+2 > cap(b) {
		nb := make([]byte, n, 100+2*cap(b))
		copy(nb, b)
		b = nb
	}
	b = b[0 : n+2]
	b[n] = byte(x)
	b[n+1] = byte(x >> 8)
	return b
}

// pbit32 appends a 32-bit numeric value x to a 9P message b.
func pbit32(b []byte, x uint32) []byte {
	n := len(b)
	if n+4 > cap(b) {
		nb := make([]byte, n, 100+2*cap(b))
		copy(nb, b)
		b = nb
	}
	b = b[0 : n+4]
	b[n] = byte(x)
	b[n+1] = byte(x >> 8)
	b[n+2] = byte(x >> 16)
	b[n+3] = byte(x >> 24)
	return b
}

// pbit64 appends a 64-bit numeric value x to a 9P message b.
func pbit64(b []byte, x uint64) []byte {
	b = pbit32(b, uint32(x))
	b = pbit32(b, uint32(x>>32))
	return b
}

// pstring appends a Go string s to a 9P message b.
func pstring(b []byte, s string) []byte {
	if len(s) >= 1<<16 {
		panic(errors.New("string too long"))
	}
	b = pbit16(b, uint16(len(s)))
	b = append(b, s...)
	return b
}