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, []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