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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>>
>
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