slicewriter.go

     1  // Copyright 2022 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 slicewriter
     6  
     7  import (
     8  	"fmt"
     9  	"io"
    10  )
    11  
    12  // WriteSeeker is a helper object that implements the io.WriteSeeker
    13  // interface. Clients can create a WriteSeeker, make a series of Write
    14  // calls to add data to it (and possibly Seek calls to update
    15  // previously written portions), then finally invoke BytesWritten() to
    16  // get a pointer to the constructed byte slice.
    17  type WriteSeeker struct {
    18  	payload []byte
    19  	off     int64
    20  }
    21  
    22  func (sws *WriteSeeker) Write(p []byte) (n int, err error) {
    23  	amt := len(p)
    24  	towrite := sws.payload[sws.off:]
    25  	if len(towrite) < amt {
    26  		sws.payload = append(sws.payload, make([]byte, amt-len(towrite))...)
    27  		towrite = sws.payload[sws.off:]
    28  	}
    29  	copy(towrite, p)
    30  	sws.off += int64(amt)
    31  	return amt, nil
    32  }
    33  
    34  // Seek repositions the read/write position of the WriteSeeker within
    35  // its internally maintained slice. Note that it is not possible to
    36  // expand the size of the slice using SEEK_SET; trying to seek outside
    37  // the slice will result in an error.
    38  func (sws *WriteSeeker) Seek(offset int64, whence int) (int64, error) {
    39  	switch whence {
    40  	case io.SeekStart:
    41  		if sws.off != offset && (offset < 0 || offset > int64(len(sws.payload))) {
    42  			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(sws.payload))
    43  		}
    44  		sws.off = offset
    45  		return offset, nil
    46  	case io.SeekCurrent:
    47  		newoff := sws.off + offset
    48  		if newoff != sws.off && (newoff < 0 || newoff > int64(len(sws.payload))) {
    49  			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
    50  		}
    51  		sws.off += offset
    52  		return sws.off, nil
    53  	case io.SeekEnd:
    54  		newoff := int64(len(sws.payload)) + offset
    55  		if newoff != sws.off && (newoff < 0 || newoff > int64(len(sws.payload))) {
    56  			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
    57  		}
    58  		sws.off = newoff
    59  		return sws.off, nil
    60  	}
    61  	// other modes not supported
    62  	return 0, fmt.Errorf("unsupported seek mode %d", whence)
    63  }
    64  
    65  // BytesWritten returns the underlying byte slice for the WriteSeeker,
    66  // containing the data written to it via Write/Seek calls.
    67  func (sws *WriteSeeker) BytesWritten() []byte {
    68  	return sws.payload
    69  }
    70  
    71  func (sws *WriteSeeker) Read(p []byte) (n int, err error) {
    72  	amt := len(p)
    73  	toread := sws.payload[sws.off:]
    74  	if len(toread) < amt {
    75  		amt = len(toread)
    76  	}
    77  	copy(p, toread)
    78  	sws.off += int64(amt)
    79  	return amt, nil
    80  }
    81
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