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// 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 strings

import (

        "errors"

        "io"

        "unicode/utf8"

)

// A Reader implements the io.Reader, io.ByteScanner, and

// io.RuneScanner interfaces by reading from a string.

type Reader struct {

        s        string

        i        int // current reading index

        prevRune int // index of previous rune; or < 0

}

// Len returns the number of bytes of the unread portion of the

// string.

func (r *Reader) Len() int {

        return len(r.s) - r.i

}

func (r *Reader) Read(b []byte) (n int, err error) {

        if r.i >= len(r.s) {

                return 0, io.EOF

        }

        n = copy(b, r.s[r.i:])

        r.i += n

        r.prevRune = -1

        return

}

func (r *Reader) ReadByte() (b byte, err error) {

        if r.i >= len(r.s) {

                return 0, io.EOF

        }

        b = r.s[r.i]

        r.i++

        r.prevRune = -1

        return

}

// UnreadByte moves the reading position back by one byte.

// It is an error to call UnreadByte if nothing has been

// read yet.

func (r *Reader) UnreadByte() error {

        if r.i <= 0 {

                return errors.New("strings.Reader: at beginning of string")

        }

        r.i--

        r.prevRune = -1

        return nil

}

// ReadRune reads and returns the next UTF-8-encoded

// Unicode code point from the buffer.

// If no bytes are available, the error returned is io.EOF.

// If the bytes are an erroneous UTF-8 encoding, it

// consumes one byte and returns U+FFFD, 1.

func (r *Reader) ReadRune() (ch rune, size int, err error) {

        if r.i >= len(r.s) {

                return 0, 0, io.EOF

        }

        r.prevRune = r.i

        if c := r.s[r.i]; c < utf8.RuneSelf {

                r.i++

                return rune(c), 1, nil

        }

        ch, size = utf8.DecodeRuneInString(r.s[r.i:])

        r.i += size

        return

}

// UnreadRune causes the next call to ReadRune to return the same rune

// as the previous call to ReadRune.

// The last method called on r must have been ReadRune.

func (r *Reader) UnreadRune() error {

        if r.prevRune < 0 {

                return errors.New("strings.Reader: previous operation was not ReadRune")

        }

        r.i = r.prevRune

        r.prevRune = -1

        return nil

}

// NewReader returns a new Reader reading from s.

// It is similar to bytes.NewBufferString but more efficient and read-only.

func NewReader(s string) *Reader { return &Reader{s, 0, -1} }