PR c/85696

[official-gcc.git] / libgo / go / bytes / buffer.go

// 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 bytes

// Simple byte buffer for marshaling data.

import (
        "errors"
        "io"
        "unicode/utf8"
)

// A Buffer is a variable-sized buffer of bytes with Read and Write methods.
// The zero value for Buffer is an empty buffer ready to use.
type Buffer struct {
        buf       []byte   // contents are the bytes buf[off : len(buf)]
        off       int      // read at &buf[off], write at &buf[len(buf)]
        bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.
        lastRead  readOp   // last read operation, so that Unread* can work correctly.

        // FIXME: it would be advisable to align Buffer to cachelines to avoid false
        // sharing.
}

// The readOp constants describe the last action performed on
// the buffer, so that UnreadRune and UnreadByte can check for
// invalid usage. opReadRuneX constants are chosen such that
// converted to int they correspond to the rune size that was read.
type readOp int8

// Don't use iota for these, as the values need to correspond with the
// names and comments, which is easier to see when being explicit.
const (
        opRead      readOp = -1 // Any other read operation.
        opInvalid   readOp = 0  // Non-read operation.
        opReadRune1 readOp = 1  // Read rune of size 1.
        opReadRune2 readOp = 2  // Read rune of size 2.
        opReadRune3 readOp = 3  // Read rune of size 3.
        opReadRune4 readOp = 4  // Read rune of size 4.
)

// ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
var ErrTooLarge = errors.New("bytes.Buffer: too large")
var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")

const maxInt = int(^uint(0) >> 1)

// Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
// The slice is valid for use only until the next buffer modification (that is,
// only until the next call to a method like Read, Write, Reset, or Truncate).
// The slice aliases the buffer content at least until the next buffer modification,
// so immediate changes to the slice will affect the result of future reads.
func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }

// String returns the contents of the unread portion of the buffer
// as a string. If the Buffer is a nil pointer, it returns "<nil>".
//
// To build strings more efficiently, see the strings.Builder type.
func (b *Buffer) String() string {
        if b == nil {
                // Special case, useful in debugging.
                return "<nil>"
        }
        return string(b.buf[b.off:])
}

// empty returns whether the unread portion of the buffer is empty.
func (b *Buffer) empty() bool { return len(b.buf) <= b.off }

// Len returns the number of bytes of the unread portion of the buffer;
// b.Len() == len(b.Bytes()).
func (b *Buffer) Len() int { return len(b.buf) - b.off }

// Cap returns the capacity of the buffer's underlying byte slice, that is, the
// total space allocated for the buffer's data.
func (b *Buffer) Cap() int { return cap(b.buf) }

// Truncate discards all but the first n unread bytes from the buffer
// but continues to use the same allocated storage.
// It panics if n is negative or greater than the length of the buffer.
func (b *Buffer) Truncate(n int) {
        if n == 0 {
                b.Reset()
                return
        }
        b.lastRead = opInvalid
        if n < 0 || n > b.Len() {
                panic("bytes.Buffer: truncation out of range")
        }
        b.buf = b.buf[:b.off+n]
}

// Reset resets the buffer to be empty,
// but it retains the underlying storage for use by future writes.
// Reset is the same as Truncate(0).
func (b *Buffer) Reset() {
        b.buf = b.buf[:0]
        b.off = 0
        b.lastRead = opInvalid
}

// tryGrowByReslice is a inlineable version of grow for the fast-case where the
// internal buffer only needs to be resliced.
// It returns the index where bytes should be written and whether it succeeded.
func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
        if l := len(b.buf); n <= cap(b.buf)-l {
                b.buf = b.buf[:l+n]
                return l, true
        }
        return 0, false
}

// grow grows the buffer to guarantee space for n more bytes.
// It returns the index where bytes should be written.
// If the buffer can't grow it will panic with ErrTooLarge.
func (b *Buffer) grow(n int) int {
        m := b.Len()
        // If buffer is empty, reset to recover space.
        if m == 0 && b.off != 0 {
                b.Reset()
        }
        // Try to grow by means of a reslice.
        if i, ok := b.tryGrowByReslice(n); ok {
                return i
        }
        // Check if we can make use of bootstrap array.
        if b.buf == nil && n <= len(b.bootstrap) {
                b.buf = b.bootstrap[:n]
                return 0
        }
        c := cap(b.buf)
        if n <= c/2-m {
                // We can slide things down instead of allocating a new
                // slice. We only need m+n <= c to slide, but
                // we instead let capacity get twice as large so we
                // don't spend all our time copying.
                copy(b.buf, b.buf[b.off:])
        } else if c > maxInt-c-n {
                panic(ErrTooLarge)
        } else {
                // Not enough space anywhere, we need to allocate.
                buf := makeSlice(2*c + n)
                copy(buf, b.buf[b.off:])
                b.buf = buf
        }
        // Restore b.off and len(b.buf).
        b.off = 0
        b.buf = b.buf[:m+n]
        return m
}

// Grow grows the buffer's capacity, if necessary, to guarantee space for
// another n bytes. After Grow(n), at least n bytes can be written to the
// buffer without another allocation.
// If n is negative, Grow will panic.
// If the buffer can't grow it will panic with ErrTooLarge.
func (b *Buffer) Grow(n int) {
        if n < 0 {
                panic("bytes.Buffer.Grow: negative count")
        }
        m := b.grow(n)
        b.buf = b.buf[:m]
}

// Write appends the contents of p to the buffer, growing the buffer as
// needed. The return value n is the length of p; err is always nil. If the
// buffer becomes too large, Write will panic with ErrTooLarge.
func (b *Buffer) Write(p []byte) (n int, err error) {
        b.lastRead = opInvalid
        m, ok := b.tryGrowByReslice(len(p))
        if !ok {
                m = b.grow(len(p))
        }
        return copy(b.buf[m:], p), nil
}

// WriteString appends the contents of s to the buffer, growing the buffer as
// needed. The return value n is the length of s; err is always nil. If the
// buffer becomes too large, WriteString will panic with ErrTooLarge.
func (b *Buffer) WriteString(s string) (n int, err error) {
        b.lastRead = opInvalid
        m, ok := b.tryGrowByReslice(len(s))
        if !ok {
                m = b.grow(len(s))
        }
        return copy(b.buf[m:], s), nil
}

// MinRead is the minimum slice size passed to a Read call by
// Buffer.ReadFrom. As long as the Buffer has at least MinRead bytes beyond
// what is required to hold the contents of r, ReadFrom will not grow the
// underlying buffer.
const MinRead = 512

// 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)
                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
                }
        }
}

// makeSlice allocates a slice of size n. If the allocation fails, it panics
// with ErrTooLarge.
func makeSlice(n int) []byte {
        // If the make fails, give a known error.
        defer func() {
                if recover() != nil {
                        panic(ErrTooLarge)
                }
        }()
        return make([]byte, n)
}

// WriteTo writes data to w until the buffer is drained or an error occurs.
// The return value n is the number of bytes written; it always fits into an
// int, but it is int64 to match the io.WriterTo interface. Any error
// encountered during the write is also returned.
func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
        b.lastRead = opInvalid
        if nBytes := b.Len(); nBytes > 0 {
                m, e := w.Write(b.buf[b.off:])
                if m > nBytes {
                        panic("bytes.Buffer.WriteTo: invalid Write count")
                }
                b.off += m
                n = int64(m)
                if e != nil {
                        return n, e
                }
                // all bytes should have been written, by definition of
                // Write method in io.Writer
                if m != nBytes {
                        return n, io.ErrShortWrite
                }
        }
        // Buffer is now empty; reset.
        b.Reset()
        return n, nil
}

// WriteByte appends the byte c to the buffer, growing the buffer as needed.
// The returned error is always nil, but is included to match bufio.Writer's
// WriteByte. If the buffer becomes too large, WriteByte will panic with
// ErrTooLarge.
func (b *Buffer) WriteByte(c byte) error {
        b.lastRead = opInvalid
        m, ok := b.tryGrowByReslice(1)
        if !ok {
                m = b.grow(1)
        }
        b.buf[m] = c
        return nil
}

// WriteRune appends the UTF-8 encoding of Unicode code point r to the
// buffer, returning its length and an error, which is always nil but is
// included to match bufio.Writer's WriteRune. The buffer is grown as needed;
// if it becomes too large, WriteRune will panic with ErrTooLarge.
func (b *Buffer) WriteRune(r rune) (n int, err error) {
        if r < utf8.RuneSelf {
                b.WriteByte(byte(r))
                return 1, nil
        }
        b.lastRead = opInvalid
        m, ok := b.tryGrowByReslice(utf8.UTFMax)
        if !ok {
                m = b.grow(utf8.UTFMax)
        }
        n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
        b.buf = b.buf[:m+n]
        return n, nil
}

// Read reads the next len(p) bytes from the buffer or until the buffer
// is drained. The return value n is the number of bytes read. If the
// buffer has no data to return, err is io.EOF (unless len(p) is zero);
// otherwise it is nil.
func (b *Buffer) Read(p []byte) (n int, err error) {
        b.lastRead = opInvalid
        if b.empty() {
                // Buffer is empty, reset to recover space.
                b.Reset()
                if len(p) == 0 {
                        return 0, nil
                }
                return 0, io.EOF
        }
        n = copy(p, b.buf[b.off:])
        b.off += n
        if n > 0 {
                b.lastRead = opRead
        }
        return n, nil
}

// Next returns a slice containing the next n bytes from the buffer,
// advancing the buffer as if the bytes had been returned by Read.
// If there are fewer than n bytes in the buffer, Next returns the entire buffer.
// The slice is only valid until the next call to a read or write method.
func (b *Buffer) Next(n int) []byte {
        b.lastRead = opInvalid
        m := b.Len()
        if n > m {
                n = m
        }
        data := b.buf[b.off : b.off+n]
        b.off += n
        if n > 0 {
                b.lastRead = opRead
        }
        return data
}

// ReadByte reads and returns the next byte from the buffer.
// If no byte is available, it returns error io.EOF.
func (b *Buffer) ReadByte() (byte, error) {
        if b.empty() {
                // Buffer is empty, reset to recover space.
                b.Reset()
                return 0, io.EOF
        }
        c := b.buf[b.off]
        b.off++
        b.lastRead = opRead
        return c, 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 (b *Buffer) ReadRune() (r rune, size int, err error) {
        if b.empty() {
                // Buffer is empty, reset to recover space.
                b.Reset()
                return 0, 0, io.EOF
        }
        c := b.buf[b.off]
        if c < utf8.RuneSelf {
                b.off++
                b.lastRead = opReadRune1
                return rune(c), 1, nil
        }
        r, n := utf8.DecodeRune(b.buf[b.off:])
        b.off += n
        b.lastRead = readOp(n)
        return r, n, nil
}

// UnreadRune unreads the last rune returned by ReadRune.
// If the most recent read or write operation on the buffer was
// not a successful ReadRune, UnreadRune returns an error.  (In this regard
// it is stricter than UnreadByte, which will unread the last byte
// from any read operation.)
func (b *Buffer) UnreadRune() error {
        if b.lastRead <= opInvalid {
                return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
        }
        if b.off >= int(b.lastRead) {
                b.off -= int(b.lastRead)
        }
        b.lastRead = opInvalid
        return nil
}

// UnreadByte unreads the last byte returned by the most recent successful
// read operation that read at least one byte. If a write has happened since
// the last read, if the last read returned an error, or if the read read zero
// bytes, UnreadByte returns an error.
func (b *Buffer) UnreadByte() error {
        if b.lastRead == opInvalid {
                return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
        }
        b.lastRead = opInvalid
        if b.off > 0 {
                b.off--
        }
        return nil
}

// ReadBytes reads until the first occurrence of delim in the input,
// returning a slice containing the data up to and including the delimiter.
// If ReadBytes encounters an error before finding a delimiter,
// it returns the data read before the error and the error itself (often io.EOF).
// ReadBytes returns err != nil if and only if the returned data does not end in
// delim.
func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
        slice, err := b.readSlice(delim)
        // return a copy of slice. The buffer's backing array may
        // be overwritten by later calls.
        line = append(line, slice...)
        return line, err
}

// readSlice is like ReadBytes but returns a reference to internal buffer data.
func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
        i := IndexByte(b.buf[b.off:], delim)
        end := b.off + i + 1
        if i < 0 {
                end = len(b.buf)
                err = io.EOF
        }
        line = b.buf[b.off:end]
        b.off = end
        b.lastRead = opRead
        return line, err
}

// ReadString reads until the first occurrence of delim in the input,
// returning a string containing the data up to and including the delimiter.
// If ReadString encounters an error before finding a delimiter,
// it returns the data read before the error and the error itself (often io.EOF).
// ReadString returns err != nil if and only if the returned data does not end
// in delim.
func (b *Buffer) ReadString(delim byte) (line string, err error) {
        slice, err := b.readSlice(delim)
        return string(slice), err
}

// NewBuffer creates and initializes a new Buffer using buf as its
// initial contents. The new Buffer takes ownership of buf, and the
// caller should not use buf after this call. NewBuffer is intended to
// prepare a Buffer to read existing data. It can also be used to size
// the internal buffer for writing. To do that, buf should have the
// desired capacity but a length of zero.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }

// NewBufferString creates and initializes a new Buffer using string s as its
// initial contents. It is intended to prepare a buffer to read an existing
// string.
//
// In most cases, new(Buffer) (or just declaring a Buffer variable) is
// sufficient to initialize a Buffer.
func NewBufferString(s string) *Buffer {
        return &Buffer{buf: []byte(s)}
}