diff --git a/areas.cpp b/areas.cpp
index 5c47ac5..ea8c8f0 100644
--- a/areas.cpp
+++ b/areas.cpp
@@ -7,6 +7,7 @@
 
 #include "geometry.h"
 #include "areas.h"
+#include "segment_tree.h"
 
 
 Unit get_area_union(std::vector<Rectangle> rectangles) {
@@ -15,9 +16,6 @@ Unit get_area_union(std::vector<Rectangle> rectangles) {
         assert(rect.bottom_left.x <= rect.top_right.x);
         assert(rect.bottom_left.y <= rect.top_right.y);
     }
-    // entry i will describe coverage of the interval (x_points[i], x_points[i+1])
-    std::vector<unsigned> coverage_numbers;
-    coverage_numbers.reserve(2*rectangles.size() -1);
 
     //sorted vector of all x coordinates of all rectangles
     std::vector<RectCoord> x_points;
@@ -48,39 +46,33 @@ Unit get_area_union(std::vector<Rectangle> rectangles) {
     }
     std::sort(y_points.begin(), y_points.end());
 
-    // run sweepline
-    Unit current_cross_section_length = 0;
+    // init data and run sweepline
     Unit total_area = 0;
+    SegmentTree segmentTree(x_points);
+
     for(Index y_index = 0 ; y_index < y_points.size() ; ++y_index) {
         // first, update cross section
         if (y_points[y_index].rect->bottom_left.y == y_points[y_index].coord) {
             // rectangle is starting now, add its cross section
-            for (Index index = y_points[y_index].rect->i_left; index < y_points[y_index].rect->i_right; ++index) {
-                ++coverage_numbers[index];
-                if (coverage_numbers[index] == 1) {
-                    current_cross_section_length += (x_points[index + 1].coord - x_points[index].coord);
-                }
-            }
+            segmentTree.add_interval(
+                    {x_points[y_points[y_index].rect->i_left].coord,
+                     x_points[y_points[y_index].rect->i_right].coord}
+                    );
         } else {
             assert(y_points[y_index].rect->top_right.y == y_points[y_index].coord);
             // rectangle stopping, remove its cross section
-            for (Index index = y_points[y_index].rect->i_left; index < y_points[y_index].rect->i_right; ++index) {
-                --coverage_numbers[index];
-                if (coverage_numbers[index] == 0) {
-                    current_cross_section_length -= (x_points[index + 1].coord - x_points[index].coord);
-                }
-            }
+            segmentTree.remove_interval(
+                    {x_points[y_points[y_index].rect->i_left].coord,
+                     x_points[y_points[y_index].rect->i_right].coord}
+            );
         }
         //cross section is now up to date
         if(y_index + 1 < y_points.size()) {
             // add area up to next y point
-            total_area += current_cross_section_length * (y_points[y_index+1].coord - y_points[y_index].coord);
+            total_area += segmentTree.length_covered_intervals();
         }
     }
-    // sanity check that nothing is covered when arriving at the end
-    for(auto coverage : coverage_numbers) {
-        assert(coverage == 0);
-    }
+    // TODO: sanity check that nothing is covered when arriving at the end
     return total_area;
 }
 
diff --git a/segment_tree.cpp b/segment_tree.cpp
index 6d89a67..c0811d8 100644
--- a/segment_tree.cpp
+++ b/segment_tree.cpp
@@ -15,7 +15,7 @@ SegmentTree::SegmentTree(const std::vector<RectCoord> &coords):
     size_t num_meta_nodes = std::bit_ceil(num_leaf_nodes) - 1;
 
     // Initialize all nodes with zero coverage and dummy interval
-    _nodes.resize(2 * num_meta_nodes - 1, {0, coords[num_leaf_nodes].coord, coords[num_leaf_nodes].coord});
+    _nodes.resize(2 * num_meta_nodes + 1, {0, coords[num_leaf_nodes].coord, coords[num_leaf_nodes].coord});
 
     // We initialize the tree from bottom up, keeping track of the index ranges
     // and the length of the corresponding segment
@@ -25,7 +25,7 @@ SegmentTree::SegmentTree(const std::vector<RectCoord> &coords):
         _nodes[leaf_node_idx + num_meta_nodes].right_coord = coords[leaf_node_idx +1].coord;
     }
     // Note that there are remaining leafs that represent dummy ranges
-    for(Index node_idx = num_meta_nodes - 1; node_idx >= 0; ++node_idx) {
+    for(Index node_idx = num_meta_nodes - 1; node_idx != -1 ; --node_idx) {
         _nodes[node_idx].left_coord = left_child(node_idx).left_coord;
         _nodes[node_idx].right_coord = right_child(node_idx).right_coord;
     }