buffer.go

Documentation: golang.org/x/image/tiff

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package tiff
     6  
     7  import "io"
     8  
     9  // buffer buffers an io.Reader to satisfy io.ReaderAt.
    10  type buffer struct {
    11  	r   io.Reader
    12  	buf []byte
    13  }
    14  
    15  // fill reads data from b.r until the buffer contains at least end bytes.
    16  func (b *buffer) fill(end int) error {
    17  	m := len(b.buf)
    18  	if end > m {
    19  		if end > cap(b.buf) {
    20  			newcap := 1024
    21  			for newcap < end {
    22  				newcap *= 2
    23  			}
    24  			newbuf := make([]byte, end, newcap)
    25  			copy(newbuf, b.buf)
    26  			b.buf = newbuf
    27  		} else {
    28  			b.buf = b.buf[:end]
    29  		}
    30  		if n, err := io.ReadFull(b.r, b.buf[m:end]); err != nil {
    31  			end = m + n
    32  			b.buf = b.buf[:end]
    33  			return err
    34  		}
    35  	}
    36  	return nil
    37  }
    38  
    39  func (b *buffer) ReadAt(p []byte, off int64) (int, error) {
    40  	o := int(off)
    41  	end := o + len(p)
    42  	if int64(end) != off+int64(len(p)) {
    43  		return 0, io.ErrUnexpectedEOF
    44  	}
    45  
    46  	err := b.fill(end)
    47  	return copy(p, b.buf[o:end]), err
    48  }
    49  
    50  // Slice returns a slice of the underlying buffer. The slice contains
    51  // n bytes starting at offset off.
    52  func (b *buffer) Slice(off, n int) ([]byte, error) {
    53  	end := off + n
    54  	if err := b.fill(end); err != nil {
    55  		return nil, err
    56  	}
    57  	return b.buf[off:end], nil
    58  }
    59  
    60  // newReaderAt converts an io.Reader into an io.ReaderAt.
    61  func newReaderAt(r io.Reader) io.ReaderAt {
    62  	if ra, ok := r.(io.ReaderAt); ok {
    63  		return ra
    64  	}
    65  	return &buffer{
    66  		r:   r,
    67  		buf: make([]byte, 0, 1024),
    68  	}
    69  }
    70
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