tracks (line, column) positions in streams, suitable for error messages
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streampos: a writer that tracks lines and columns

Or "How I learned to love composition even more!"

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Package streampos exports an io.Writer that tracks (line, column) positions in streams, suitable for error messages. It looks for newline characters and builds a mapping from byte offset ranges (which start at 0) to line numbers (which start at 1). The Position method returns the line and column number that corresponds to a given offset in the stream. The offset has to be between 0 and whatever the Length method returns.

If all of this strikes you as rather strange, you may want to read my blog post on the genesis of this package. (Also if I "shot myself in the foot" and there's a much better way of doing this, please comment there!)


You should be able to just say this:

go get -u

Then you can import and use the package as follows:

package main

import (

func main() {
	b := []byte("Write\nmore\nGo!\n")

	w := &streampos.Writer{}
	w.Write(b) // never fails

	for i := int64(0); i < int64(len(b)); i++ {
		l, c, err := w.Position(i)
		fmt.Printf("offset %v maps to line %v, column %v (error: %v)\n", i, l, c, err)



Column numbers are currently computed in terms of bytes just like in the Go compiler. The advantage is a clear definition of what "column" actually means. The drawback is that users may not agree when it comes to TAB characters or multi-byte runes. However, given that GCC's bug-tracker is full of column-related issues, maybe it's better to keep it simple?

Maybe we could at least make column numbers somewhat configurable. The text/scanner package for example uses character counts instead of byte counts, at least that's how I understand the documentation. Adding a small constructor function with an (optional) parameter to specify options could achieve that in a moderately straightforward way. I don't care very much one way or the other, I am happy with columns in terms of bytes. At least for now.

Currently I use a slice of byte offset ranges and a linear search. For large streams, that's obviously slow. Binary search would help, as would some tree-based data-structure that supports range queries. So far I don't care enough for either, but I'd be happy to accept a pull-request.

Concurrency safety, anyone? :-)


The MIT License.