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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} }