// Reader is the interface that wraps the basic Read method.
//
// Read reads up to len(p) bytes into p. It returns the number of bytes
// read (0 <= n <= len(p)) and any error encountered. Even if Read
// returns n < len(p), it may use all of p as scratch space during the call.
// If some data is available but not len(p) bytes, Read conventionally
// returns what is available instead of waiting for more.
//
// When Read encounters an error or end-of-file condition after
// successfully reading n > 0 bytes, it returns the number of
// bytes read. It may return the (non-nil) error from the same call
// or return the error (and n == 0) from a subsequent call.
// An instance of this general case is that a Reader returning
// a non-zero number of bytes at the end of the input stream may
// return either err == EOF or err == nil. The next Read should
// return 0, EOF.
//
// Callers should always process the n > 0 bytes returned before
// considering the error err. Doing so correctly handles I/O errors
// that happen after reading some bytes and also both of the
// allowed EOF behaviors.
//
// Implementations of Read are discouraged from returning a
// zero byte count with a nil error, except when len(p) == 0.
// Callers should treat a return of 0 and nil as indicating that
// nothing happened; in particular it does not indicate EOF.
//
// Implementations must not retain p.
type Reader interface {
	Read(p []byte) (n int, err error)
}

// LimitReader returns a Reader that reads from r
// but stops with EOF after n bytes.
// The underlying implementation is a *LimitedReader.
func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }

// A LimitedReader reads from R but limits the amount of
// data returned to just N bytes. Each call to Read
// updates N to reflect the new amount remaining.
// Read returns EOF when N <= 0 or when the underlying R returns EOF.
type LimitedReader struct {
	R Reader // underlying reader
	N int64  // max bytes remaining
}

func (l *LimitedReader) Read(p []byte) (n int, err error) {
    // Nが0以下の場合、0とEOF（error）を返す
	if l.N <= 0 {
		return 0, EOF
    }
    // pの大きさがl.Nより大きい場合は、pを0～Nまでに縮小する
	if int64(len(p)) > l.N {
		p = p[0:l.N]
    }
    // RにのReadメソッドにpを渡す
    n, err = l.R.Read(p)
    // 読み込んだバイト数だけNから引く
	l.N -= int64(n)
	return
}

// A Reader implements the io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,
// io.ByteScanner, and io.RuneScanner interfaces by reading
// from a string.
// The zero value for Reader operates like a Reader of an empty string.
type Reader struct {
	s        string
	i        int64 // current reading index
	prevRune int   // index of previous rune; or < 0
}

func (r *Reader) Read(b []byte) (n int, err error) {
	// 読み込んだインデックスが、stringの長さより大きいときにEOF(error)を返す
	if r.i >= int64(len(r.s)) {
		return 0, io.EOF
	}
	r.prevRune = -1
	// bにr.sのr.iから最後までをコピーする
	n = copy(b, r.s[r.i:])
	// r.iにコピーした要素数を加算する
	r.i += int64(n)
	return
}

// ReaderFrom is the interface that wraps the ReadFrom method.
//
// ReadFrom reads data from r until EOF or error.
// The return value n is the number of bytes read.
// Any error except io.EOF encountered during the read is also returned.
//
// The Copy function uses ReaderFrom if available.
type ReaderFrom interface {
	ReadFrom(r Reader) (n int64, err error)
}

// ReadFrom reads data from r until EOF and appends it to the buffer, growing
// the buffer as needed. The return value n is the number of bytes read. Any
// error except io.EOF encountered during the read is also returned. If the
// buffer becomes too large, ReadFrom will panic with ErrTooLarge.
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
	b.lastRead = opInvalid
	for {
		i := b.grow(MinRead)
		b.buf = b.buf[:i]
		m, e := r.Read(b.buf[i:cap(b.buf)])
		if m < 0 {
			panic(errNegativeRead)
		}

		b.buf = b.buf[:i+m]
		n += int64(m)
		if e == io.EOF {
			return n, nil // e is EOF, so return nil explicitly
		}
		if e != nil {
			return n, e
		}
	}
}