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Hexagons! #31

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Commits on Mar 28, 2019

  1. Add HexJSON representation of constituencies 

    This file represents every UK parliamentary constituency as a hexagon
arranged on a grid that maintains a semblance of geography.  This will let
us show a different view of the UK that won't be overwhelmed by all the
sparsely populated rural constituencies and hide all the densely populated
urban constituencies.  The data was created by the [ODI Leeds][odi-leeds]
as part of their [Hexmaps project][odi-hexmaps], thanks to them for making
the data open; it makes things better!

[odi-leeds]: https://odileeds.org
[odi-leeds-hexmaps]: https://odileeds.org/projects/hexmaps/constituencies/
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  2. Add D3 plugin for rendering HexJSON files 

    [This plugin][d3-hexjson], by [Oli Hawkins][oli-hawkins], lets us render
the hexjson file via D3.

[d3-hexjson]: https://github.com/olihawkins/d3-hexjson
[oli-hawkins]: https://olihawkins.com/
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  3. Pass constituency id, not constituency object in event 

    The object we send to the events is the constituency representation from
the topo json file.  The structure of this object will be different when
we introduce the hex json version.  As it happens we only use the id of
the constituency in any of our handlers, and this will be present on the
hex json objects, so we can just pass that instead.
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  4. Introduce PetitionMap.mapMode and draw some hexagons 

    If the mapMode is set to `topo` (the default) we draw the geographic maps
we always used to.  If we set it to `hex` though, we'll draw the hexagon
map based on the hexJSON file from the ODI.  We don't need the projection
and path stuff from the topo json maps because those are to do with
translating spherical maps onto a flat plane.  Other than that though, it
is pretty straightforward.  Our hex rendering is based pretty strongly on
the examples from the [d3-hexjson project][d3-hexjson].

[d3-hexjson]: https://github.com/olihawkins/d3-hexjson/
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  5. Hook up UI for PetitionMap.map_mode

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  6. Remove arbitrary zoom limit 

    I'm not sure why we have it and I can't see any downsides to letting
people zoom in or out as much as they want.
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  7. Give credit for hex maps in about page 

    Also for the d3-hexjson library.
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  8. Add hexjson files for map subdivisions 

    Originally I hoped to just filter the uk map to only include the relevant
constituencies, but for some reason the d3-hexjson library would render
some of the files incorrectly.  The problem seems to be that the reduced
number of hexagons in each file means we have to translate the `q` and `r`
attributes of each hexagon into an absolute grid reference, and sometimes
this means we move a hexagon from an odd or even `q` or `r` onto an even
or odd one.  As [rendering hexagons by grid reference][hex-coords] is
dependent on the odd and even state of the `q` and `r` attributes, this
meant that for some of the smaller maps some rows were drawn incorrectly,
making the whole map wrong.

I tried detecting when the absolute `q` and `r` had a different odd or
even state compared to the original, but this meant that the rendering of
the map was offset by a row or column, and so the arrangement was good,
but it wasn't centered on the screen.

The solution I ended up with was to manually translate the `q` and `r`
attributes of each map to an absolute space that preserved the correct
odd and even states and didn't cause a problem.

This was painful, but perhaps could be automated later if we need to
regenerate the subdivisions for any future constituency changes.

[hex-coords]: https://www.redblobgames.com/grids/hexagons/#coordinates
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