// Package fwd provides a buffered reader
// and writer. Each has methods that help improve
// the encoding/decoding performance of some binary
// protocols.
//
// The [Writer] and [Reader] type provide similar
// functionality to their counterparts in [bufio], plus
// a few extra utility methods that simplify read-ahead
// and write-ahead. I wrote this package to improve serialization
// performance for http://github.com/tinylib/msgp,
// where it provided about a 2x speedup over `bufio` for certain
// workloads. However, care must be taken to understand the semantics of the
// extra methods provided by this package, as they allow
// the user to access and manipulate the buffer memory
// directly.
//
// The extra methods for [Reader] are [Reader.Peek], [Reader.Skip]
// and [Reader.Next]. (*fwd.Reader).Peek, unlike (*bufio.Reader).Peek,
// will re-allocate the read buffer in order to accommodate arbitrarily
// large read-ahead. (*fwd.Reader).Skip skips the next 'n' bytes
// in the stream, and uses the [io.Seeker] interface if the underlying
// stream implements it. (*fwd.Reader).Next returns a slice pointing
// to the next 'n' bytes in the read buffer (like Reader.Peek), but also
// increments the read position. This allows users to process streams
// in arbitrary block sizes without having to manage appropriately-sized
// slices. Additionally, obviating the need to copy the data from the
// buffer to another location in memory can improve performance dramatically
// in CPU-bound applications.
//
// [Writer] only has one extra method, which is (*fwd.Writer).Next, which
// returns a slice pointing to the next 'n' bytes of the writer, and increments
// the write position by the length of the returned slice. This allows users
// to write directly to the end of the buffer.
package fwd

import (
	"io"
	"os"
)

const (
	// DefaultReaderSize is the default size of the read buffer
	DefaultReaderSize = 2048

	// minimum read buffer; straight from bufio
	minReaderSize = 16
)

// NewReader returns a new *Reader that reads from 'r'
func NewReader(r io.Reader) *Reader {
	return NewReaderSize(r, DefaultReaderSize)
}

// NewReaderSize returns a new *Reader that
// reads from 'r' and has a buffer size 'n'.
func NewReaderSize(r io.Reader, n int) *Reader {
	buf := make([]byte, 0, max(n, minReaderSize))
	return NewReaderBuf(r, buf)
}

// NewReaderBuf returns a new *Reader that
// reads from 'r' and uses 'buf' as a buffer.
// 'buf' is not used when has smaller capacity than 16,
// custom buffer is allocated instead.
func NewReaderBuf(r io.Reader, buf []byte) *Reader {
	if cap(buf) < minReaderSize {
		buf = make([]byte, 0, minReaderSize)
	}
	buf = buf[:0]
	rd := &Reader{
		r:    r,
		data: buf,
	}
	if s, ok := r.(io.Seeker); ok {
		rd.rs = s
	}
	return rd
}

// Reader is a buffered look-ahead reader
type Reader struct {
	r io.Reader // underlying reader

	// data[n:len(data)] is buffered data; data[len(data):cap(data)] is free buffer space
	data  []byte // data
	n     int    // read offset
	state error  // last read error

	// if the reader past to NewReader was
	// also an io.Seeker, this is non-nil
	rs io.Seeker
}

// Reset resets the underlying reader
// and the read buffer.
func (r *Reader) Reset(rd io.Reader) {
	r.r = rd
	r.data = r.data[0:0]
	r.n = 0
	r.state = nil
	if s, ok := rd.(io.Seeker); ok {
		r.rs = s
	} else {
		r.rs = nil
	}
}

// more() does one read on the underlying reader
func (r *Reader) more() {
	// move data backwards so that
	// the read offset is 0; this way
	// we can supply the maximum number of
	// bytes to the reader
	if r.n != 0 {
		if r.n < len(r.data) {
			r.data = r.data[:copy(r.data[0:], r.data[r.n:])]
		} else {
			r.data = r.data[:0]
		}
		r.n = 0
	}
	var a int
	a, r.state = r.r.Read(r.data[len(r.data):cap(r.data)])
	if a == 0 && r.state == nil {
		r.state = io.ErrNoProgress
		return
	} else if a > 0 && r.state == io.EOF {
		// discard the io.EOF if we read more than 0 bytes.
		// the next call to Read should return io.EOF again.
		r.state = nil
	} else if r.state != nil {
		return
	}
	r.data = r.data[:len(r.data)+a]
}

// pop error
func (r *Reader) err() (e error) {
	e, r.state = r.state, nil
	return
}

// pop error; EOF -> io.ErrUnexpectedEOF
func (r *Reader) noEOF() (e error) {
	e, r.state = r.state, nil
	if e == io.EOF {
		e = io.ErrUnexpectedEOF
	}
	return
}

// buffered bytes
func (r *Reader) buffered() int { return len(r.data) - r.n }

// Buffered returns the number of bytes currently in the buffer
func (r *Reader) Buffered() int { return len(r.data) - r.n }

// BufferSize returns the total size of the buffer
func (r *Reader) BufferSize() int { return cap(r.data) }

// Peek returns the next 'n' buffered bytes,
// reading from the underlying reader if necessary.
// It will only return a slice shorter than 'n' bytes
// if it also returns an error. Peek does not advance
// the reader. EOF errors are *not* returned as
// io.ErrUnexpectedEOF.
func (r *Reader) Peek(n int) ([]byte, error) {
	// in the degenerate case,
	// we may need to realloc
	// (the caller asked for more
	// bytes than the size of the buffer)
	if cap(r.data) < n {
		old := r.data[r.n:]
		r.data = make([]byte, n+r.buffered())
		r.data = r.data[:copy(r.data, old)]
		r.n = 0
	}

	// keep filling until
	// we hit an error or
	// read enough bytes
	for r.buffered() < n && r.state == nil {
		r.more()
	}

	// we must have hit an error
	if r.buffered() < n {
		return r.data[r.n:], r.err()
	}

	return r.data[r.n : r.n+n], nil
}

// discard(n) discards up to 'n' buffered bytes, and
// and returns the number of bytes discarded
func (r *Reader) discard(n int) int {
	inbuf := r.buffered()
	if inbuf <= n {
		r.n = 0
		r.data = r.data[:0]
		return inbuf
	}
	r.n += n
	return n
}

// Skip moves the reader forward 'n' bytes.
// Returns the number of bytes skipped and any
// errors encountered. It is analogous to Seek(n, 1).
// If the underlying reader implements io.Seeker, then
// that method will be used to skip forward.
//
// If the reader encounters
// an EOF before skipping 'n' bytes, it
// returns [io.ErrUnexpectedEOF]. If the
// underlying reader implements [io.Seeker], then
// those rules apply instead. (Many implementations
// will not return [io.EOF] until the next call
// to Read).
func (r *Reader) Skip(n int) (int, error) {
	if n < 0 {
		return 0, os.ErrInvalid
	}

	// discard some or all of the current buffer
	skipped := r.discard(n)

	// if we can Seek() through the remaining bytes, do that
	if n > skipped && r.rs != nil {
		nn, err := r.rs.Seek(int64(n-skipped), 1)
		return int(nn) + skipped, err
	}
	// otherwise, keep filling the buffer
	// and discarding it up to 'n'
	for skipped < n && r.state == nil {
		r.more()
		skipped += r.discard(n - skipped)
	}
	return skipped, r.noEOF()
}

// Next returns the next 'n' bytes in the stream.
// Unlike Peek, Next advances the reader position.
// The returned bytes point to the same
// data as the buffer, so the slice is
// only valid until the next reader method call.
// An EOF is considered an unexpected error.
// If an the returned slice is less than the
// length asked for, an error will be returned,
// and the reader position will not be incremented.
func (r *Reader) Next(n int) ([]byte, error) {
	// in case the buffer is too small
	if cap(r.data) < n {
		old := r.data[r.n:]
		r.data = make([]byte, n+r.buffered())
		r.data = r.data[:copy(r.data, old)]
		r.n = 0
	}

	// fill at least 'n' bytes
	for r.buffered() < n && r.state == nil {
		r.more()
	}

	if r.buffered() < n {
		return r.data[r.n:], r.noEOF()
	}
	out := r.data[r.n : r.n+n]
	r.n += n
	return out, nil
}

// Read implements [io.Reader].
func (r *Reader) Read(b []byte) (int, error) {
	// if we have data in the buffer, just
	// return that.
	if r.buffered() != 0 {
		x := copy(b, r.data[r.n:])
		r.n += x
		return x, nil
	}
	var n int
	// we have no buffered data; determine
	// whether or not to buffer or call
	// the underlying reader directly
	if len(b) >= cap(r.data) {
		n, r.state = r.r.Read(b)
	} else {
		r.more()
		n = copy(b, r.data)
		r.n = n
	}
	if n == 0 {
		return 0, r.err()
	}
	return n, nil
}

// ReadFull attempts to read len(b) bytes into
// 'b'. It returns the number of bytes read into
// 'b', and an error if it does not return len(b).
// EOF is considered an unexpected error.
func (r *Reader) ReadFull(b []byte) (int, error) {
	var n int  // read into b
	var nn int // scratch
	l := len(b)
	// either read buffered data,
	// or read directly for the underlying
	// buffer, or fetch more buffered data.
	for n < l && r.state == nil {
		if r.buffered() != 0 {
			nn = copy(b[n:], r.data[r.n:])
			n += nn
			r.n += nn
		} else if l-n > cap(r.data) {
			nn, r.state = r.r.Read(b[n:])
			n += nn
		} else {
			r.more()
		}
	}
	if n < l {
		return n, r.noEOF()
	}
	return n, nil
}

// ReadByte implements [io.ByteReader].
func (r *Reader) ReadByte() (byte, error) {
	for r.buffered() < 1 && r.state == nil {
		r.more()
	}
	if r.buffered() < 1 {
		return 0, r.err()
	}
	b := r.data[r.n]
	r.n++
	return b, nil
}

// WriteTo implements [io.WriterTo].
func (r *Reader) WriteTo(w io.Writer) (int64, error) {
	var (
		i   int64
		ii  int
		err error
	)
	// first, clear buffer
	if r.buffered() > 0 {
		ii, err = w.Write(r.data[r.n:])
		i += int64(ii)
		if err != nil {
			return i, err
		}
		r.data = r.data[0:0]
		r.n = 0
	}
	for r.state == nil {
		// here we just do
		// 1:1 reads and writes
		r.more()
		if r.buffered() > 0 {
			ii, err = w.Write(r.data)
			i += int64(ii)
			if err != nil {
				return i, err
			}
			r.data = r.data[0:0]
			r.n = 0
		}
	}
	if r.state != io.EOF {
		return i, r.err()
	}
	return i, nil
}

func max(a int, b int) int {
	if a < b {
		return b
	}
	return a
}