On 2014-01-03 23:56, Justin Whear wrote:
I've run into a design issue surrounding ranges and am looking for advice on the best way to proceed. To illustrate the issue, consider the Shapefile format: a 100 byte header followed by variable-length records. The tricky bit is that the header includes a field which contains the total length of the file (as measured in 16-bit words, curiously). The header must be written first, but the total length of the file isn't known until all the records have been encoded. When writing to a File this isn't a problem: write 100 bytes of padding, write the records, use rewind(), and write the proper header. It's in the context of an OutputRange that I don't know how to proceed. Consider the most flexible range type: the array. An array is not an OutputRange, so it needs to be wrapped in something like std.array.Appender. Ideally I could save off the initial state of the range, write a bogus header, write the records, then jump back and write the proper header. Unfortunately, Appender is not a ForwardRange, nor does it appear that the field of OutputRanges which are also ForwardRanges has been explored. I'm using the excellent read, write, and append functions from std.bitmanip, so write() would fit the bill if only Appender supported slicing.
If you're satisfied with just using arrays and not a general output range you can do something similar to what you did with a file. Add the 100 extra bytes for the header to Appender, then add the contents. Extract the array used for backing in the Appender and fill the proper header.
-- /Jacob Carlborg
