75 lines
3.2 KiB
Markdown
75 lines
3.2 KiB
Markdown
---
|
|
layout: post
|
|
title: Encoding and Decoding Polylines with Erlang
|
|
description: "Description and implementation of functions that encode and decode polylines."
|
|
date: 2015-03-12 22:03:00 CET
|
|
category: posts
|
|
tags: [erlang, programming, english]
|
|
comments: true
|
|
---
|
|
|
|
If you have ever worked with the [Google Directions API](https://developers.google.com/maps/documentation/directions/?csw=1) you probably came across [encoded polylines](https://developers.google.com/maps/documentation/utilities/polylinealgorithm). 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}].
|
|
{% endhighlight %}
|
|
|
|
The `encode/1` function takes a path and returns an encoded polyline.
|
|
|
|
{% highlight erlang linenos %}
|
|
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>>).
|
|
{% endhighlight %}
|
|
|
|
The `decode/1` function takes an encoded polyline and returns a path.
|
|
|
|
{% highlight erlang linenos %}
|
|
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).
|
|
{% endhighlight %}
|
|
|
|
I have written these functions for [noesis](https://github.com/nifoc/noesis), which is a library that contains useful utility functions. Right now the implementation is only available in the [development branch](https://github.com/nifoc/noesis/blob/development/src/noesis_polyline.erl). It is tested using [EUnit](https://github.com/nifoc/noesis/blob/f3e9ae21d53e09bea9ca48fe4a56ddd006952e0a/test/noesis_polyline_test.erl) and [QuickCheck](https://github.com/nifoc/noesis/blob/f3e9ae21d53e09bea9ca48fe4a56ddd006952e0a/test/noesis_polyline_triq.erl).
|
|
|
|
If you're reading this a couple of months from now, you might find an updated implementation on GitHub.
|