github.com/sirkon/goproxy@v1.4.8/internal/buildid/rewrite.go (about)

     1  // Copyright 2017 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 buildid
     6  
     7  import (
     8  	"bytes"
     9  	"crypto/sha256"
    10  	"fmt"
    11  	"io"
    12  )
    13  
    14  // FindAndHash reads all of r and returns the offsets of occurrences of id.
    15  // While reading, findAndHash also computes and returns
    16  // a hash of the content of r, but with occurrences of id replaced by zeros.
    17  // FindAndHash reads bufSize bytes from r at a time.
    18  // If bufSize == 0, FindAndHash uses a reasonable default.
    19  func FindAndHash(r io.Reader, id string, bufSize int) (matches []int64, hash [32]byte, err error) {
    20  	if bufSize == 0 {
    21  		bufSize = 31 * 1024 // bufSize+little will likely fit in 32 kB
    22  	}
    23  	if len(id) > bufSize {
    24  		return nil, [32]byte{}, fmt.Errorf("buildid.FindAndHash: buffer too small")
    25  	}
    26  	zeros := make([]byte, len(id))
    27  	idBytes := []byte(id)
    28  
    29  	// The strategy is to read the file through buf, looking for id,
    30  	// but we need to worry about what happens if id is broken up
    31  	// and returned in parts by two different reads.
    32  	// We allocate a tiny buffer (at least len(id)) and a big buffer (bufSize bytes)
    33  	// next to each other in memory and then copy the tail of
    34  	// one read into the tiny buffer before reading new data into the big buffer.
    35  	// The search for id is over the entire tiny+big buffer.
    36  	tiny := (len(id) + 127) &^ 127 // round up to 128-aligned
    37  	buf := make([]byte, tiny+bufSize)
    38  	h := sha256.New()
    39  	start := tiny
    40  	for offset := int64(0); ; {
    41  		// The file offset maintained by the loop corresponds to &buf[tiny].
    42  		// buf[start:tiny] is left over from previous iteration.
    43  		// After reading n bytes into buf[tiny:], we process buf[start:tiny+n].
    44  		n, err := io.ReadFull(r, buf[tiny:])
    45  		if err != io.ErrUnexpectedEOF && err != io.EOF && err != nil {
    46  			return nil, [32]byte{}, err
    47  		}
    48  
    49  		// Process any matches.
    50  		for {
    51  			i := bytes.Index(buf[start:tiny+n], idBytes)
    52  			if i < 0 {
    53  				break
    54  			}
    55  			matches = append(matches, offset+int64(start+i-tiny))
    56  			h.Write(buf[start : start+i])
    57  			h.Write(zeros)
    58  			start += i + len(id)
    59  		}
    60  		if n < bufSize {
    61  			// Did not fill buffer, must be at end of file.
    62  			h.Write(buf[start : tiny+n])
    63  			break
    64  		}
    65  
    66  		// Process all but final tiny bytes of buf (bufSize = len(buf)-tiny).
    67  		// Note that start > len(buf)-tiny is possible, if the search above
    68  		// found an id ending in the final tiny fringe. That's OK.
    69  		if start < len(buf)-tiny {
    70  			h.Write(buf[start : len(buf)-tiny])
    71  			start = len(buf) - tiny
    72  		}
    73  
    74  		// Slide ending tiny-sized fringe to beginning of buffer.
    75  		copy(buf[0:], buf[bufSize:])
    76  		start -= bufSize
    77  		offset += int64(bufSize)
    78  	}
    79  	h.Sum(hash[:0])
    80  	return matches, hash, nil
    81  }
    82  
    83  func Rewrite(w io.WriterAt, pos []int64, id string) error {
    84  	b := []byte(id)
    85  	for _, p := range pos {
    86  		if _, err := w.WriteAt(b, p); err != nil {
    87  			return err
    88  		}
    89  	}
    90  	return nil
    91  }