Improve country indexing (fix #13)

Author: tananaev

builder/src/build_index.cpp

@@ -189,19 +189,39 @@ static std::vector<std::pair<S2CellId, bool>> cover_polygon(const std::vector<st
         }
     }
 
+    // Only mark a cell as interior if all its neighbors are also interior.
+    // This erodes the interior by one cell from the border, avoiding
+    // false positives from spherical/flat geometry mismatch near edges.
+    std::unordered_set<uint64_t> safe_interior;
+    for (uint64_t id : interior_set) {
+        S2CellId cell(id);
+        S2CellId neighbors[4];
+        cell.GetEdgeNeighbors(neighbors);
+        bool all_interior = true;
+        for (const auto& n : neighbors) {
+            if (interior_set.count(n.id()) == 0) {
+                all_interior = false;
+                break;
+            }
+        }
+        if (all_interior) {
+            safe_interior.insert(id);
+        }
+    }
+
     // Normalize all covering cells to kAdminCellLevel
     std::vector<std::pair<S2CellId, bool>> result;
     for (const auto& cell : covering.cell_ids()) {
         if (cell.level() <= kAdminCellLevel) {
             auto begin = cell.range_min().parent(kAdminCellLevel);
             auto end = cell.range_max().parent(kAdminCellLevel);
             for (auto c = begin; c != end; c = c.next()) {
-                result.emplace_back(c, interior_set.count(c.id()) > 0);
+                result.emplace_back(c, safe_interior.count(c.id()) > 0);
             }
-            result.emplace_back(end, interior_set.count(end.id()) > 0);
+            result.emplace_back(end, safe_interior.count(end.id()) > 0);
         } else {
             auto parent = cell.parent(kAdminCellLevel);
-            result.emplace_back(parent, interior_set.count(parent.id()) > 0);
+            result.emplace_back(parent, safe_interior.count(parent.id()) > 0);
         }
     }
 
@@ -364,8 +384,9 @@ static void add_addr_point(double lat, double lng, const char* housenumber, cons
 static void add_admin_polygon(const std::vector<std::pair<double,double>>& vertices,
                                const char* name, uint8_t admin_level,
                                const char* country_code) {
-    // Simplify large polygons
-    auto simplified = simplify_polygon(vertices, 500);
+    // Simplify large polygons (proportional to preserve border accuracy)
+    size_t max_vertices = std::clamp(vertices.size() / 5, size_t(500), size_t(65000));
+    auto simplified = simplify_polygon(vertices, max_vertices);
     if (simplified.size() < 3) return;
 
     uint32_t poly_id = static_cast<uint32_t>(admin_polygons.size());
@@ -471,6 +492,8 @@ class BuildHandler : public osmium::handler::Handler {
         const char* country_code = (admin_level == 2)
             ? area.tags()["ISO3166-1:alpha2"]
             : nullptr;
+        if (admin_level == 2 && !country_code) return;
+
         // Extract outer ring vertices
         for (const auto& outer_ring : area.outer_rings()) {
             std::vector<std::pair<double,double>> vertices;