From 4dfc1c719f71d2990d6702836a1433ec97a598ec Mon Sep 17 00:00:00 2001
From: Paul Chavard <github@paul.chavard.net>
Date: Thu, 1 Oct 2020 11:32:56 +0200
Subject: [PATCH] Fix geo areas computations

---
 app/models/geo_area.rb          |  4 +-
 app/services/geojson_service.rb | 70 +++++++++++++++++++++++++++++++++
 spec/models/dossier_spec.rb     |  2 +-
 spec/models/geo_area_spec.rb    |  6 +--
 4 files changed, 75 insertions(+), 7 deletions(-)

diff --git a/app/models/geo_area.rb b/app/models/geo_area.rb
index c2693e80a..bb535d04b 100644
--- a/app/models/geo_area.rb
+++ b/app/models/geo_area.rb
@@ -78,8 +78,8 @@ class GeoArea < ApplicationRecord
   end
 
   def area
-    if polygon? && RGeo::Geos.supported?
-      rgeo_geometry&.area&.round(1)
+    if polygon?
+      GeojsonService.area(geometry.deep_symbolize_keys).round(1)
     end
   end
 
diff --git a/app/services/geojson_service.rb b/app/services/geojson_service.rb
index 0e4c04914..6c2041224 100644
--- a/app/services/geojson_service.rb
+++ b/app/services/geojson_service.rb
@@ -14,4 +14,74 @@ class GeojsonService
 
     polygon.to_json
   end
+
+  # The following code is ported from turfjs
+  # https://github.com/Turfjs/turf/blob/master/packages/turf-area/index.ts
+
+  EQUATORIAL_RADIUS = 6378137
+
+  def self.area(geojson)
+    calculate_area(geojson)
+  end
+
+  def self.calculate_area(geom)
+    total = 0
+    case geom[:type]
+    when 'Polygon'
+      polygon_area(geom[:coordinates]);
+    when 'MultiPolygon'
+      geom[:coordinates].each do |coordinates|
+        total += polygon_area(coordinates)
+      end
+      total
+    else
+      total
+    end
+  end
+
+  def self.polygon_area(coordinates)
+    total = 0
+    if coordinates.present?
+      coordinates = coordinates.dup
+      total += ring_area(coordinates.shift).abs
+      coordinates.each do |coordinates|
+        total -= ring_area(coordinates).abs
+      end
+    end
+    total
+  end
+
+  def self.ring_area(coordinates)
+    total = 0
+    coords_length = coordinates.size
+
+    if coords_length > 2
+      coords_length.times do |i|
+        if i == coords_length - 2 # i = N-2
+          lower_index = coords_length - 2
+          middle_index = coords_length - 1
+          upper_index = 0
+        elsif i == coords_length - 1 # i = N-1
+          lower_index = coords_length - 1
+          middle_index = 0
+          upper_index = 1
+        else # i = 0 to N-3
+          lower_index = i
+          middle_index = i + 1
+          upper_index = i + 2
+        end
+        p1 = coordinates[lower_index]
+        p2 = coordinates[middle_index]
+        p3 = coordinates[upper_index]
+        total += (rad(p3[0]) - rad(p1[0])) * Math.sin(rad(p2[1]))
+      end
+      total = total * EQUATORIAL_RADIUS * EQUATORIAL_RADIUS / 2
+    end
+
+    total
+  end
+
+  def self.rad(num)
+    num * Math::PI / 180
+  end
 end
diff --git a/spec/models/dossier_spec.rb b/spec/models/dossier_spec.rb
index b3f8966a8..0ab322673 100644
--- a/spec/models/dossier_spec.rb
+++ b/spec/models/dossier_spec.rb
@@ -1305,7 +1305,7 @@ describe Dossier do
               'type' => 'Polygon'
             },
             properties: {
-              area: 219.0,
+              area: 103.6,
               champ_id: champ_carte.stable_id,
               dossier_id: dossier.id,
               id: geo_area.id,
diff --git a/spec/models/geo_area_spec.rb b/spec/models/geo_area_spec.rb
index 4a42e972b..aef3a13de 100644
--- a/spec/models/geo_area_spec.rb
+++ b/spec/models/geo_area_spec.rb
@@ -2,15 +2,13 @@ RSpec.describe GeoArea, type: :model do
   describe '#area' do
     let(:geo_area) { build(:geo_area, :polygon) }
 
-    it { expect(geo_area.area).to eq(219.0) }
+    it { expect(geo_area.area).to eq(103.6) }
   end
 
   describe '#area (hourglass polygon)' do
     let(:geo_area) { build(:geo_area, :hourglass_polygon) }
 
-    # This test fails in my local environement end the problem exists in production.
-    # Must be some mismatch between CI/production. I still want this fix in production.
-    it.pending { expect(geo_area.area).to be_nil }
+    it { expect(geo_area.area).to eq(32.4) }
   end
 
   describe '#length' do