blog.kempkens.io/content/posts/2015-03-12-encoding-and-decoding-polylines-with-erlang.md

date	description	tags	slug	title
2015-03-12T22:03:00Z	Description and implementation of functions that encode and decode polylines.	erlang programming english	encoding-and-decoding-polylines-with-erlang	Encoding and Decoding Polylines with Erlang

If you have ever worked with the Google Directions API you probably came across encoded polylines. I wanted to decode and encode these using Erlang but was unable to find an existing implementation. So I decided to write my own.

• A single point (latitude/longitude pair) is represented using a tuple
• A path, i.e. a decoded polyline, is represented as a list of point tuples
• An encoded polyline is represented as a simple binary string

{{< highlight erlang >}} Point = {Lng, Lat}, Path = [{Lng, Lat}, {Lng2, Lat2}]. {{< / highlight >}}

The encode/1 function takes a path and returns an encoded polyline.

{{< highlight erlang "linenos=table" >}} encode(Path) -> encode_acc(Path, 0, 0, <<>>).

% Private

encode_acc([], _PLat, _PLng, Acc) -> Acc; encode_acc([{Lng, Lat}|Rest], PLat, PLng, Acc) -> LatE5 = round(Lat * 1.0e5), LngE5 = round(Lng * 1.0e5), EncodedLat = encode_part(encode_sign(LatE5 - PLat), <<>>), EncodedLng = encode_part(encode_sign(LngE5 - PLng), <<>>), encode_acc(Rest, LatE5, LngE5, <<Acc/binary, EncodedLat/binary, EncodedLng/binary>>).

encode_sign(Num) when Num < 0 -> bnot (Num bsl 1); encode_sign(Num) -> Num bsl 1.

encode_part(Num, Result) when Num < 32 -> <<Result/binary, (Num + 63)>>; encode_part(Num, Result) -> Value = (32 bor (Num band 31)) + 63, encode_part(Num bsr 5, <<Result/binary, Value>>). {{< / highlight >}}

The decode/1 function takes an encoded polyline and returns a path.

{{< highlight erlang "linenos=table" >}} decode(Line) -> decode_acc(Line, 0, 0, []).

% Private

decode_acc(<<>>, _Lat, _Lng, Acc) -> lists:reverse(Acc); decode_acc(Line, Lat, Lng, Acc) -> {DLat, Rest} = decode_part(Line, 32, 0, 0), Lat2 = Lat + DLat, {DLng, Rest2} = decode_part(Rest, 32, 0, 0), Lng2 = Lng + DLng, decode_acc(Rest2, Lat2, Lng2, [{Lng2 / 1.0e5, Lat2 / 1.0e5} | Acc]).

decode_part(Line, B, _Shift, Result) when B < 32 -> Result2 = if Result band 1 == 0 -> Result bsr 1; true -> bnot (Result bsr 1) end, {Result2, Line}; decode_part(<<C:8, Rest/binary>>, _OldB, Shift, Result) -> B = C - 63, Result2 = Result bor ((B band 31) bsl Shift), decode_part(Rest, B, Shift + 5, Result2). {{< / highlight >}}

I have written these functions for noesis, which is a library that contains useful utility functions. Right now the implementation is only available in the development branch. It is tested using EUnit and QuickCheck.

If you're reading this a couple of months from now, you might find an updated implementation on GitHub.