Or the caller can wrap the object with io.NewSectionReader
<https://pkg.go.dev/io#NewSectionReader>(f, 0, len)
On Saturday, 1 April 2023 at 10:51:11 UTC+1 Brian Candler wrote:
> Could you make your function accept some other interface? e.g.
>
> type SizeReaderAt interface {
> io.ReaderAt
> Size() int64
> }
>
> Values of this type will still be able to satisfy io.ReaderAt at point of
> use.
>
> You could also use the existing io.SectionReader
> <https://pkg.go.dev/io#SectionReader> which has has Read(), ReadAt(),
> Seek() and Size() methods.
>
> Another option is to define your own interface:
>
> type StatReaderAt interface {
> io.ReaderAt
> Stat() (os.FileInfo, error)
> }
>
> This interface also already satisfied if the caller is passing a concrete
> *os.File. However if they aren't, then the caller has a bit of work to wrap
> their value so that it carries a Stat() method which returns a value that
> satisfies FileInfo <https://pkg.go.dev/io/fs#FileInfo> (with 6 methods,
> although 5 of those could be dummies).
>
> It's unfortunate that *os.File doesn't automatically satisfy
> io.SectionReader (since File doesn't have a Size() method). But I guess you
> could make a type which embeds *os.File and adds a Size() method which
> calls Stat().Size().
>
> On Saturday, 1 April 2023 at 09:58:28 UTC+1 Jochen Voss wrote:
>
>> Dear Bruno,
>>
>> Thanks for your answer. Originally I had a separate size argument for
>> the function which opens a new reader, something like this:
>>
>> func NewReader(data io.ReaderAt, size int64, opt *ReaderOptions)
>> (*Reader, error)
>>
>> But this makes it a bit annoying to create a reader for a file, because
>> you always need the additional Stat() call before, and also it looks a bit
>> weird, I though. So my idea was to move the code for checking the size
>> into the function, and to simplify the API:
>>
>> func NewReader(data io.ReaderAt, opt *ReaderOptions) (*Reader, error)
>>
>> This way the library does the work once, rather than every caller having
>> to do this separately.
>>
>> All the best,
>> Jochen
>>
>> On Friday, 31 March 2023 at 21:38:04 UTC+1 Bruno Albuquerque wrote:
>>
>>> Not a direct answer to your question (your code looks like a reasonable
>>> implementation modulo any bugs I did not notice) but: Aren't you over
>>> engineering things? If the source is a PDF file, why not also pass the size
>>> to the function instead of doing all this work to get it? At some point you
>>> have to open the file, right? And checking the size of a file is trivial
>>> (it is just a stat call) and then you do not need to do that.
>>>
>>>
>>> On Fri, Mar 31, 2023 at 4:30 PM Jochen Voss <joche...@gmail.com> wrote:
>>>
>>>> Dear all,
>>>>
>>>> I am trying to get the size of an io.ReaderAt, i.e. the offset after
>>>> which no more data can be read. I have some working (?) code (
>>>> https://go.dev/play/p/wTouYbaJ7RG , also reproduced below), but I am
>>>> not sure whether what I do is correct and the best way to do this. Some
>>>> questions:
>>>>
>>>> - Does the standard library provide a way to get the size of an
>>>> io,ReaderAt?
>>>> - Is my code correct?
>>>> - Is there a better way to do this?
>>>> - If a type provides not only a ReadAt() method, but also a Size()
>>>> method, would it be save to assume that Size() returns how many bytes
>>>> are
>>>> accessible via ReadAt()? This seems to work for bytes.Reader and
>>>> strings.Reader, but there may be types out there where Size() does
>>>> something different?
>>>> - If ReadAt(p, x) returns io.EOF for an offset x, is it then
>>>> guaranteed that then ReadAt(p, y) also returns io.EOF for all y >
>>>> x? Or could there be different error messages, or "files with holes",
>>>> or
>>>> whatnot?
>>>> - Which types in the standard library provide ReadAt methods? I
>>>> know of os.File, strings.Reader, and bytes.Reader. Any others?
>>>>
>>>> For context: this is for reading the cross reference table of PDF
>>>> files, which have to be located by following some convoluted route
>>>> starting *from
>>>> the end* of the PDF file.
>>>>
>>>> Many thanks,
>>>> Jochen
>>>>
>>>> func getSize(r io.ReaderAt) (int64, error) {
>>>> if f, ok := r.(*os.File); ok {
>>>> fi, err := f.Stat()
>>>> if err != nil {
>>>> return 0, err
>>>> }
>>>> return fi.Size(), nil
>>>> }
>>>> if b, ok := r.(*bytes.Reader); ok {
>>>> return int64(b.Size()), nil
>>>> }
>>>> if s, ok := r.(*strings.Reader); ok {
>>>> return int64(s.Size()), nil
>>>> }
>>>>
>>>> buf := make([]byte, 1024)
>>>> n, err := r.ReadAt(buf, 0)
>>>> if err == io.EOF {
>>>> return int64(n), nil
>>>> } else if err != nil {
>>>> return 0, err
>>>> }
>>>>
>>>> lowerBound := int64(n) // all bytes before lowerBound are known to be
>>>> present
>>>> var upperBound int64 // at least one byte before upperBound is known
>>>> to be missing
>>>> for {
>>>> test := 2 * lowerBound
>>>> _, err := r.ReadAt(buf[:1], test-1)
>>>> if err == io.EOF {
>>>> upperBound = test
>>>> break
>>>> } else if err != nil {
>>>> return 0, err
>>>> }
>>>> lowerBound = test
>>>> }
>>>>
>>>> for lowerBound+1 < upperBound {
>>>> test := (lowerBound + upperBound + 1) / 2
>>>> _, err := r.ReadAt(buf[:1], test-1)
>>>> if err == io.EOF {
>>>> upperBound = test
>>>> } else if err != nil {
>>>> return 0, err
>>>> } else {
>>>> lowerBound = test
>>>> }
>>>> }
>>>> return lowerBound, nil
>>>> }
>>>>
>>>> --
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>>>>
>>>> <https://groups.google.com/d/msgid/golang-nuts/aa604ac0-3542-4a9e-adef-a9eaecfd8170n%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>> .
>>>>
>>>
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