From 59086cc728cc2e705a7d2ac8169afff9195f7c24 Mon Sep 17 00:00:00 2001
From: Paul Chavard <github@paul.chavard.net>
Date: Thu, 1 Oct 2020 11:33:15 +0200
Subject: [PATCH] Fix geo length computations

---
 app/models/geo_area.rb          |  4 +--
 app/services/geojson_service.rb | 43 +++++++++++++++++++++++++++++++++
 spec/models/geo_area_spec.rb    |  2 +-
 3 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/app/models/geo_area.rb b/app/models/geo_area.rb
index bb535d04b..19b16ce21 100644
--- a/app/models/geo_area.rb
+++ b/app/models/geo_area.rb
@@ -84,8 +84,8 @@ class GeoArea < ApplicationRecord
   end
 
   def length
-    if line? && RGeo::Geos.supported?
-      rgeo_geometry.length.round(1)
+    if line?
+      GeojsonService.length(geometry.deep_symbolize_keys).round(1)
     end
   end
 
diff --git a/app/services/geojson_service.rb b/app/services/geojson_service.rb
index 6c2041224..d2a78d884 100644
--- a/app/services/geojson_service.rb
+++ b/app/services/geojson_service.rb
@@ -24,6 +24,27 @@ class GeojsonService
     calculate_area(geojson)
   end
 
+  def self.length(geojson)
+    segment_reduce(geojson, 0) do |previous_value, segment|
+      coordinates = segment[:geometry][:coordinates]
+      previous_value + distance(coordinates[0], coordinates[1])
+    end
+  end
+
+  def self.distance(from, to)
+    coordinates1 = from
+    coordinates2 = to
+    d_lat = degrees_to_radians(coordinates2[1] - coordinates1[1])
+    d_lon = degrees_to_radians(coordinates2[0] - coordinates1[0])
+    lat1 = degrees_to_radians(coordinates1[1])
+    lat2 = degrees_to_radians(coordinates2[1])
+
+    a = (Math.sin(d_lat / 2)**2) + (Math.sin(d_lon / 2)**2) * Math.cos(lat1) * Math.cos(lat2)
+
+    radians = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))
+    radians * EQUATORIAL_RADIUS
+  end
+
   def self.calculate_area(geom)
     total = 0
     case geom[:type]
@@ -81,7 +102,29 @@ class GeojsonService
     total
   end
 
+  def self.segment_reduce(geojson, initial_value)
+    previous_value = initial_value
+    started = false
+    coordinates = geojson[:coordinates].dup
+    from = coordinates.shift
+    coordinates.each do |to|
+      current_segment = { type: 'Feature', geometry: { type: 'LineString', coordinates: [from, to] } }
+      from = to
+      if started == false && initial_value.blank?
+        previous_value = current_segment
+      else
+        previous_value = yield previous_value, current_segment
+      end
+      started = true
+    end
+    previous_value
+  end
+
   def self.rad(num)
     num * Math::PI / 180
   end
+
+  def self.degrees_to_radians(degrees)
+    rad(degrees % 360)
+  end
 end
diff --git a/spec/models/geo_area_spec.rb b/spec/models/geo_area_spec.rb
index aef3a13de..377b2bf00 100644
--- a/spec/models/geo_area_spec.rb
+++ b/spec/models/geo_area_spec.rb
@@ -14,7 +14,7 @@ RSpec.describe GeoArea, type: :model do
   describe '#length' do
     let(:geo_area) { build(:geo_area, :line_string) }
 
-    it { expect(geo_area.length).to eq(30.8) }
+    it { expect(geo_area.length).to eq(21.2) }
   end
 
   describe '#location' do