Bash

git add builder/src/build_index.cpp && git commit -m "$(cat <<'EOF'
feat: osmium-style segment-graph ring assembler

Replaces the greedy+backtracking hybrid with osmium's algorithm:
1. Decompose ways into segments (consecutive node pairs)
2. Sort and remove duplicate pairs (shared boundaries cancel)
3. Build sorted location index for O(log n) segment lookup
4. Find split locations (nodes where 3+ segments meet)
5. Simple case: trace rings following segment chain
6. Complex case: build partial rings, join by matching endpoints

This is O(S log S) — deterministic with no exponential blowup.
No backtracking needed. Should resolve the 10 remaining missing
planet boundaries while maintaining fast build times.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
EOF
)" && cat builder/src/build_index.cpp | ssh node3 "cat > /tmp/traccar-geocoder/builder/src/build_index.cpp" && ssh node3 "cd /tmp/traccar-geocoder/builder/build && make -j\$(nproc) 2>&1 | tail -5" 2>&1

Commit, upload, build osmium-style assembler


[osmium-segment-assembler e87231f] feat: osmium-style segment-graph ring assembler
 1 file changed, 239 insertions(+), 180 deletions(-)
/usr/include/osmium/osm/relation.hpp:162:11: note: at offset [34, 42] into source object ‘osmium::Relation::osmium::OSMObject’ of size 32
  162 |     class Relation : public OSMObject {
      |           ^~~~~~~~
[100%] Linking CXX executable build-index
[100%] Built target build-index


Built! Let me test on Germany first (fast validation), then planet.


ssh node3 "rm -rf /tmp/de-osmium && mkdir -p /tmp/de-osmium && /tmp/traccar-geocoder/builder/build/build-index /tmp/de-osmium ~/pbf/germany.osm.pbf --parallel-admin 2>&1 | grep -E 'polygon|Done|closed way'" 2>&1

Test Germany with osmium-style assembler


  Added 37 admin polygons from closed ways.
  Assembling admin polygons in parallel (40080 relations, 112858 way geometries)...
  Parallel admin assembly complete: 43187 polygon rings from 40080 relations.
Done reading:
  43224 admin polygon rings
Waiting for admin polygon S2 covering to complete...
Admin polygon S2 covering complete (6103 cells).
admin index: 6103 cells, 43224 polygons
Done.


43,224 — same as our best Germany result. Let me verify.


ssh node3 "cat > /tmp/freq_compare.cpp << 'EOFCPP'
#include <cstdint>
#include <cstring>
#include <fstream>
#include <iostream>
#include <vector>
#include <map>
struct AdminPolygon { uint32_t vertex_offset; uint16_t vertex_count; uint32_t name_id; uint8_t admin_level; float area; uint16_t country_code; };
static std::vector<char> read_file(const std::string& path) {
    std::ifstream f(path, std::ios::binary | std::ios::ate); if (!f) return {};
    auto sz = f.tellg(); f.seekg(0);
    std::vector<char> d(sz); f.read(d.data(), sz); return d;
}
int main(int argc, char* argv[]) {
    auto load = [](const std::string& dir) {
        auto strings = read_file(dir + \"/strings.bin\");
        auto data = read_file(dir + \"/admin_polygons.bin\");
        auto* polys = (AdminPolygon*)data.data();
        size_t count = data.size() / sizeof(AdminPolygon);
        std::map<std::string, int> freq;
        for (size_t i = 0; i < count; i++) {
            const char* name = (polys[i].name_id < strings.size()) ? strings.data() + polys[i].name_id : \"?\";
            freq[std::string(name) + \"|\" + std::to_string(polys[i].admin_level)]++;
        }
        return freq;
    };
    auto a = load(argv[1]);
    auto b = load(argv[2]);
    int total_diff = 0;
    for (auto& [k, ca] : a) {
        int cb = b.count(k) ? b[k] : 0;
        if (ca != cb) {
            printf(\"  '%s': A=%d B=%d diff=%+d\\n\", k.c_str(), ca, cb, ca - cb);
            total_diff += std::abs(ca - cb);
        }
    }
    for (auto& [k, cb] : b) { if (!a.count(k)) { printf(\"  '%s': A=0 B=%d\\n\", k.c_str(), cb); total_diff += cb; } }
    printf(\"Total freq diffs: %d\\n\", total_diff);
    return 0;
}
EOFCPP
g++ -O2 -std=c++17 -o /tmp/freq_compare /tmp/freq_compare.cpp && /tmp/freq_compare /tmp/de-osmium ~/de-baseline-n3" 2>&1

Germany freq comparison


  'Krefeld-Ost|9': A=2 B=1 diff=+1
  'Neuhaus|10': A=5 B=4 diff=+1
  'Reislingen|10': A=1 B=0 diff=+1
Total freq diffs: 3


3 freq diffs on Germany (same as before minus the Krefeld-Uerdingen swap — one fewer diff). Now let me run the planet build — the real test.


ssh node3 "rm -rf ~/planet-osmium && mkdir -p ~/planet-osmium && nohup /tmp/traccar-geocoder/builder/build/build-index ~/planet-osmium ~/pbf/planet.osm.pbf --parallel-admin > ~/planet-osmium.log 2>&1 & echo started" 2>&1

Start planet build with osmium-style assembler


Command running in background with ID: b3morp9oi. Output is being written to: /tmp/claude-0/-root-traccar-geocoder/af1c4baa-0f9a-4420-a8b3-ce20dc28c8b9/tasks/b3morp9oi.output


sleep 600 && ssh node3 "grep -E 'Assembled 77|Assembled 78|Parallel admin|polygon|Done' ~/planet-osmium.log | tail -5; ps aux | grep 'build-index.*planet-osmium' | grep -v grep | awk '{print \$3\"% CPU\", \$10}'" 2>&1

Check after 10 min — will it pass 770K?


  Parallel admin assembly complete: 931349 polygon rings from 778863 relations.
Done reading:
  946624 admin polygon rings
Waiting for admin polygon S2 covering to complete...
Admin polygon S2 covering complete (2049797 cells).
0.0% CPU 0:00
1513% CPU 157:10


**It flew through 770K!** 931,349 polygon rings from relations + 15,275 from closed ways = 946,624 total. The osmium-style assembler has no exponential stalling — O(S log S) is deterministic.

Let me wait for the full build to complete and compare.


sleep 1200 && ssh node3 "tail -5 ~/planet-osmium.log; ls ~/planet-osmium/ | wc -l; ps aux | grep 'build-index.*planet-osmium' | grep -v grep | wc -l" 2>&1

Check after 20 min


S2 cell computation complete.
Resolving interpolation endpoints...
Resolved 41798/72817 interpolation ways
Deduplicating...
Writing index files to /home/michtest/planet-osmium...
0
2


In write phase. Let me wait for completion.


sleep 900 && ssh node3 "tail -3 ~/planet-osmium.log; ls ~/planet-osmium/ | wc -l" 2>&1

Check after 15 min


strings.bin: 214550621 bytes
admin index: 2049797 cells, 946624 polygons
Done.
14


**Planet build complete!** 946,624 admin polygons. Now the comparison:


ssh node3 "/tmp/dedup_compare ~/planet-osmium ~/planet-baseline" 2>&1

Planet comparison — osmium-style assembler


A (/home/michtest/planet-osmium): 644717 unique name|level
B (/home/michtest/planet-baseline): 642275 unique name|level
Both: 642212  Only_A: 2505  Only_B: 63

Only in B (MISSING from A):
  740|11
  ATE II|10
  Alta Mira|10
  Alto Los Cardales|8
  Alto Miéres|9
  Anisacate Centro|9
  Antonio Palacios|10
  Argyle Street|10
  Barri Santa Rosa|10
  Barrio Benitez|10
  Barrio CO.VI.MET V|9
  Barrio Escuelita|10
  Barrio Fuerza Segura I|9
  Barrio General Mosconi|10
  Barrio Los Paraísos|9
  Barrio Manuel Belgrano|9
  Barrio Nueva Esperanza I|10
  Barrio Payeska|10
  Barrio Praderas del Trapiche|9
  Barrio Privado San Alfonso|10
  Barrio Sacachista|10
  Barrio Santa Clara Norte|9
  Barrio Urbanización Nueman|10
  Barrio Veinticinco de Mayo|10
  Barrio Villa Teresa I|9
  COVIMER 2|10
  Confederation Sector|9
  Conjunto Habitacional Sotero Inácio Ramos|10
  Culianin|10
  Curapaytí|9
  Dundonald|10
  FOPROVI|10
  Hiekkapohja|10
  Jardim Mônaco (proposto)|10
  La Belisa|9
  Laajaranta|10
  Lakewood Sector|9
  Las Vueltas|10
  Lomas del Cedrón|10
  Mangaung Ward 39|10
  Mangaung Ward 40|10
  Mangaung Ward 43|10
  Mały Potok|9
  Mühlbachl|10
  North Industrial|10
  North West Development Area|10
  Nyrölä|10
  Parque Jhonston|9
  Pontcanna|10
  Pringles|9
  RT 002 RW 015 Kel. Mekarjaya|10
  Residencial Melville|10
  Rincón Rodríguez|10
  Roca 1|9
  Rukun Tetangga 001|10
  Tres Esquinas|9
  University Heights Sector|9
  Vale Pastoril|10
  Valle del Pil Pil|10
  Villa Club|9
  向阳村|9
  水口村|10
  汤庄村|10

Only in A: 2505 items (not listed)


**63 missing** — significantly more than the greedy's 10. The osmium-style segment assembler is actually WORSE for some cases. The issue is that the segment-level approach loses the way-level context that our greedy stitcher uses.

The problem: osmium's assembler works with osmium's full segment infrastructure (ProtoRing, NodeRefSegment, etc.) which has much richer data. Our simplified reimplementation loses information during segment normalization (sorting by key makes all segments point the same direction, losing the original way ordering).

The greedy+backtracking hybrid was actually better for these cases. Let me compare:
- Greedy hybrid: **10 missing**
- Osmium-style segment: **63 missing**

The osmium-style is worse because our segment normalization and join logic isn't as sophisticated as osmium's. Osmium tracks ProtoRings, segment directions, and uses `find_candidates` with area-based selection. Our simplified join just matches endpoints.

The greedy approach on `main` is better. Let me go back to main.


git checkout main && echo "Back on main"

Switch back to main


Switched to branch 'main'
Your branch is ahead of 'origin/main' by 28 commits.
  (use "git push" to publish your local commits)
Back on main


The osmium-style segment assembler produced **63 missing** vs the greedy hybrid's **10 missing**. The simplified segment approach loses way-level context that the greedy stitcher preserves. Osmium's actual implementation uses a much richer data model (ProtoRings with direction tracking, area-based candidate selection) that we can't easily replicate without the full osmium infrastructure.

**Conclusion**: The greedy+backtracking hybrid on `main` (10 missing, 99.998%) is the best we can achieve without using osmium's actual library. The osmium branch is preserved for reference.


git log --oneline main..osmium-segment-assembler

Show branch commits


e87231f feat: osmium-style segment-graph ring assembler


Branch has 1 commit. Main branch with greedy+backtracking is the winner at planet scale. The 10 remaining missing boundaries (0.0016%) are the practical limit of our approach.


<task-notification>
<task-id>b3morp9oi</task-id>
<tool-use-id>toolu_01QaUKYk1qNYRCK2954Fp8ez</tool-use-id>
<output-file>/tmp/claude-0/-root-traccar-geocoder/af1c4baa-0f9a-4420-a8b3-ce20dc28c8b9/tasks/b3morp9oi.output</output-file>
<status>completed</status>
<summary>Background command "Start planet build with osmium-style assembler" completed (exit code 0)</summary>
</task-notification>
Read the output file to retrieve the result: /tmp/claude-0/-root-traccar-geocoder/af1c4baa-0f9a-4420-a8b3-ce20dc28c8b9/tasks/b3morp9oi.output


The planet build confirmed the osmium-style segment approach is worse (63 missing vs 10). We're staying on `main` with the greedy+backtracking hybrid.

**Final state: 10 missing out of 642,275 (99.998%) on planet, 0 missing on Europe/Germany.** 28 commits on main ready to push.


Are we able to implement the other features we are missing from osmium in order to get the same count?


Yes. The segment approach produced 63 missing because our implementation has three key gaps vs osmium:

1. **Segment normalization breaks direction** — we sort segments by key (smaller first), losing the original traversal direction. Osmium preserves direction and reverses on-the-fly.

2. **Join logic is too simple** — we greedily match endpoints. Osmium uses recursive `find_candidates` with area-based selection (smallest area for outer rings).

3. **Duplicate removal too aggressive** — we remove ALL duplicate segment pairs. Osmium only removes duplicates from the same way or non-inner roles (touching inner rings are valid).

Let me fix these on the branch.


Switch to segment assembler branch


Switched to branch 'osmium-segment-assembler'


Grep

Osmium-style segment-graph ring assembler


517:// --- Osmium-style segment-graph ring assembler ---


   530→{
   531→    // Osmium-style segment-graph ring assembly.
   532→    // Coordinate key matching osmium's Location (int32_t nanodegrees)
   533→    auto coord_key = [](double lat, double lng) -> int64_t {
   534→        int32_t ilat = static_cast<int32_t>(lat * 1e7 + (lat >= 0 ? 0.5 : -0.5));
   535→        int32_t ilng = static_cast<int32_t>(lng * 1e7 + (lng >= 0 ? 0.5 : -0.5));
   536→        return (static_cast<int64_t>(ilat) << 32) | static_cast<uint32_t>(ilng);
   537→    };
   538→
   539→    // Step 1: Decompose all member ways into segments (consecutive coordinate pairs).
   540→    // Each segment is directed: start -> stop. We store both coordinates.
   541→    struct Segment {
   542→        int64_t start_key, stop_key;
   543→        double start_lat, start_lng, stop_lat, stop_lng;
   544→        bool done = false;
   545→
   546→        void reverse() {
   547→            std::swap(start_key, stop_key);
   548→            std::swap(start_lat, stop_lat);
   549→            std::swap(start_lng, stop_lng);
   550→        }
   551→    };
   552→
   553→    std::vector<Segment> segments;
   554→    for (const auto& [way_id, role] : members) {
   555→        if (!include_all_roles && role != "outer" && !role.empty()) continue;
   556→        auto it = way_geoms.find(way_id);
   557→        if (it == way_geoms.end() || it->second.coords.size() < 2) continue;
   558→        const auto& coords = it->second.coords;
   559→        for (size_t i = 0; i + 1 < coords.size(); i++) {
   560→            int64_t k1 = coord_key(coords[i].first, coords[i].second);
   561→            int64_t k2 = coord_key(coords[i+1].first, coords[i+1].second);
   562→            if (k1 == k2) continue; // skip zero-length
   563→            // Normalize: smaller key first (like osmium's segment ordering)
   564→            if (k1 <= k2) {
   565→                segments.push_back({k1, k2, coords[i].first, coords[i].second,
   566→                                    coords[i+1].first, coords[i+1].second, false});
   567→            } else {
   568→                segments.push_back({k2, k1, coords[i+1].first, coords[i+1].second,
   569→                                    coords[i].first, coords[i].second, false});
   570→            }
   571→        }
   572→    }
   573→
   574→    if (segments.empty()) return {};
   575→
   576→    // Step 2: Sort segments and remove duplicate pairs (shared boundaries cancel).
   577→    std::sort(segments.begin(), segments.end(), [](const Segment& a, const Segment& b) {
   578→        if (a.start_key != b.start_key) return a.start_key < b.start_key;
   579→        return a.stop_key < b.stop_key;
   580→    });
   581→
   582→    // Remove duplicate pairs (like osmium's erase_duplicate_segments)
   583→    {
   584→        size_t write = 0;
   585→        size_t i = 0;
   586→        while (i < segments.size()) {
   587→            if (i + 1 < segments.size() &&
   588→                segments[i].start_key == segments[i+1].start_key &&
   589→                segments[i].stop_key == segments[i+1].stop_key) {
   590→                i += 2; // skip pair
   591→            } else {
   592→                segments[write++] = segments[i++];
   593→            }
   594→        }
   595→        segments.resize(write);
   596→    }
   597→
   598→    if (segments.empty()) return {};
   599→
   600→    // Step 3: Build sorted location index.
   601→    // Each segment has two endpoints. Create entries for both.
   602→    struct SLoc {
   603→        uint32_t seg_idx;
   604→        bool is_stop; // false = start endpoint, true = stop endpoint
   605→        int64_t key() const { return 0; } // filled by sort
   606→    };
   607→
   608→    // Build location list: for each segment, two entries (start, stop)
   609→    struct LocEntry {
   610→        int64_t key;
   611→        uint32_t seg_idx;
   612→        bool is_stop; // which end of the segment this entry represents
   613→    };
   614→
   615→    std::vector<LocEntry> locations;
   616→    locations.reserve(segments.size() * 2);
   617→    for (uint32_t i = 0; i < segments.size(); i++) {
   618→        locations.push_back({segments[i].start_key, i, false});
   619→        locations.push_back({segments[i].stop_key, i, true});
   620→    }
   621→    std::sort(locations.begin(), locations.end(), [](const LocEntry& a, const LocEntry& b) {
   622→        return a.key < b.key;
   623→    });
   624→
   625→    // Step 4: Find split locations (keys where 3+ segment endpoints meet).
   626→    std::unordered_set<int64_t> split_locs;
   627→    for (size_t i = 0; i + 2 < locations.size(); i++) {
   628→        if (locations[i].key == locations[i+1].key && locations[i+1].key == locations[i+2].key) {
   629→            split_locs.insert(locations[i].key);
   630→        }
   631→    }
   632→
   633→    // Check for open rings (unpaired endpoints = locations appearing odd times)
   634→    bool has_open_rings = false;
   635→    for (size_t i = 0; i < locations.size(); ) {
   636→        size_t j = i;
   637→        while (j < locations.size() && locations[j].key == locations[i].key) j++;
   638→        if ((j - i) % 2 != 0) has_open_rings = true;
   639→        i = j;
   640→    }
   641→
   642→    // Helper: find next unused segment at a given location
   643→    auto get_next_segment = [&](int64_t loc_key) -> int32_t {
   644→        auto it = std::lower_bound(locations.begin(), locations.end(), loc_key,
   645→            [](const LocEntry& e, int64_t k) { return e.key < k; });
   646→        while (it != locations.end() && it->key == loc_key) {
   647→            if (!segments[it->seg_idx].done) return static_cast<int32_t>(it - locations.begin());
   648→            ++it;
   649→        }
   650→        return -1;
   651→    };
   652→
   653→    std::vector<std::vector<std::pair<double,double>>> rings;
   654→
   655→    // Step 5: Trace rings.
   656→    auto trace_ring = [&](int32_t start_loc_idx, bool stop_at_splits) -> std::vector<std::pair<double,double>> {
   657→        auto& start_entry = locations[start_loc_idx];
   658→        auto& seg = segments[start_entry.seg_idx];
   659→        if (seg.done) return {};
   660→
   661→        seg.done = true;
   662→        // Orient segment so we enter from the location entry's endpoint
   663→        if (start_entry.is_stop) seg.reverse();
   664→
   665→        std::vector<std::pair<double,double>> ring;
   666→        ring.push_back({seg.start_lat, seg.start_lng});
   667→
   668→        int64_t first_key = seg.start_key;
   669→        int64_t last_key = seg.stop_key;
   670→        ring.push_back({seg.stop_lat, seg.stop_lng});
   671→
   672→        while (last_key != first_key) {
   673→            if (stop_at_splits && split_locs.count(last_key)) break;
   674→
   675→            int32_t next_idx = get_next_segment(last_key);
   676→            if (next_idx < 0) break;
   677→
   678→            auto& next_entry = locations[next_idx];
   679→            auto& next_seg = segments[next_entry.seg_idx];
   680→            next_seg.done = true;
   681→
   682→            // Orient: we enter from last_key
   683→            if (next_entry.is_stop) next_seg.reverse();
   684→            assert(next_seg.start_key == last_key);
   685→
   686→            last_key = next_seg.stop_key;
   687→            ring.push_back({next_seg.stop_lat, next_seg.stop_lng});
   688→        }
   689→
   690→        return ring;
   691→    };
   692→
   693→    if (split_locs.empty() && !has_open_rings) {
   694→        // Simple case: no splits, no open rings. Trace closed rings directly.
   695→        for (size_t i = 0; i < locations.size(); i++) {
   696→            if (segments[locations[i].seg_idx].done) continue;
   697→            auto ring = trace_ring(static_cast<int32_t>(i), false);
   698→            if (ring.size() >= 4) rings.push_back(std::move(ring));
   699→        }
   700→    } else {
   701→        // Complex case: trace partial rings from split locations first,
   702→        // then remaining segments, then join open rings.
   703→
   704→        // Trace from split locations
   705→        std::vector<std::vector<std::pair<double,double>>> partial_rings;
   706→        for (int64_t split_key : split_locs) {
   707→            int32_t idx = get_next_segment(split_key);
   708→            while (idx >= 0) {
   709→                auto ring = trace_ring(idx, true);
   710→                if (!ring.empty()) {
   711→                    int64_t rk_first = coord_key(ring.front().first, ring.front().second);
   712→                    int64_t rk_last = coord_key(ring.back().first, ring.back().second);
   713→                    if (ring.size() >= 4 && rk_first == rk_last) {
   714→                        rings.push_back(std::move(ring));
   715→                    } else {
   716→                        partial_rings.push_back(std::move(ring));
   717→                    }
   718→                }
   719→                idx = get_next_segment(split_key);
   720→            }
   721→        }
   722→
   723→        // Trace remaining (non-split) segments
   724→        for (size_t i = 0; i < locations.size(); i++) {
   725→            if (segments[locations[i].seg_idx].done) continue;
   726→            auto ring = trace_ring(static_cast<int32_t>(i), true);
   727→            if (!ring.empty()) {
   728→                int64_t rk_first = coord_key(ring.front().first, ring.front().second);
   729→                int64_t rk_last = coord_key(ring.back().first, ring.back().second);
   730→                if (ring.size() >= 4 && rk_first == rk_last) {
   731→                    rings.push_back(std::move(ring));
   732→                } else {
   733→                    partial_rings.push_back(std::move(ring));
   734→                }
   735→            }
   736→        }
   737→
   738→        // Join open partial rings by matching endpoints
   739→        bool joined = true;
   740→        while (joined) {
   741→            joined = false;
   742→            // Build endpoint index for partial rings
   743→            std::unordered_map<int64_t, std::vector<size_t>> endpoint_to_ring;
   744→            for (size_t i = 0; i < partial_rings.size(); i++) {
   745→                if (partial_rings[i].empty()) continue;
   746→                int64_t fk = coord_key(partial_rings[i].front().first, partial_rings[i].front().second);
   747→                int64_t lk = coord_key(partial_rings[i].back().first, partial_rings[i].back().second);
   748→                endpoint_to_ring[fk].push_back(i);
   749→                endpoint_to_ring[lk].push_back(i);
   750→            }
   751→
   752→            // Try to join pairs that share an endpoint
   753→            for (auto& [key, ring_indices] : endpoint_to_ring) {
   754→                if (ring_indices.size() < 2) continue;
   755→                // Find two non-empty rings
   756→                size_t ri = SIZE_MAX, rj = SIZE_MAX;
   757→                for (auto idx : ring_indices) {
   758→                    if (partial_rings[idx].empty()) continue;
   759→                    if (ri == SIZE_MAX) ri = idx;
   760→                    else if (rj == SIZE_MAX && idx != ri) { rj = idx; break; }
   761→                }
   762→                if (ri == SIZE_MAX || rj == SIZE_MAX) continue;
   763→
   764→                auto& ra = partial_rings[ri];
   765→                auto& rb = partial_rings[rj];
   766→                int64_t ra_first = coord_key(ra.front().first, ra.front().second);
   767→                int64_t ra_last = coord_key(ra.back().first, ra.back().second);
   768→                int64_t rb_first = coord_key(rb.front().first, rb.front().second);
   769→                int64_t rb_last = coord_key(rb.back().first, rb.back().second);
   770→
   771→                // Join: ra_last == rb_first → append rb to ra
   772→                if (ra_last == rb_first) {
   773→                    ra.insert(ra.end(), rb.begin() + 1, rb.end());
   774→                    rb.clear();
   775→                } else if (ra_last == rb_last) {
   776→                    // Reverse rb and append
   777→                    for (auto rit = rb.rbegin() + 1; rit != rb.rend(); ++rit)
   778→                        ra.push_back(*rit);
   779→                    rb.clear();

{
    // Osmium-style segment-graph ring assembly.
    // Coordinate key matching osmium's Location (int32_t nanodegrees)
    auto coord_key = [](double lat, double lng) -> int64_t {
        int32_t ilat = static_cast<int32_t>(lat * 1e7 + (lat >= 0 ? 0.5 : -0.5));
        int32_t ilng = static_cast<int32_t>(lng * 1e7 + (lng >= 0 ? 0.5 : -0.5));
        return (static_cast<int64_t>(ilat) << 32) | static_cast<uint32_t>(ilng);
    };

    // Step 1: Decompose all member ways into segments (consecutive coordinate pairs).
    // Each segment is directed: start -> stop. We store both coordinates.
    struct Segment {
        int64_t start_key, stop_key;
        double start_lat, start_lng, stop_lat, stop_lng;
        bool done = false;

        void reverse() {
            std::swap(start_key, stop_key);
            std::swap(start_lat, stop_lat);
            std::swap(start_lng, stop_lng);
        }
    };

    std::vector<Segment> segments;
    for (const auto& [way_id, role] : members) {
        if (!include_all_roles && role != "outer" && !role.empty()) continue;
        auto it = way_geoms.find(way_id);
        if (it == way_geoms.end() || it->second.coords.size() < 2) continue;
        const auto& coords = it->second.coords;
        for (size_t i = 0; i + 1 < coords.size(); i++) {
            int64_t k1 = coord_key(coords[i].first, coords[i].second);
            int64_t k2 = coord_key(coords[i+1].first, coords[i+1].second);
            if (k1 == k2) continue; // skip zero-length
            // Normalize: smaller key first (like osmium's segment ordering)
            if (k1 <= k2) {
                segments.push_back({k1, k2, coords[i].first, coords[i].second,
                                    coords[i+1].first, coords[i+1].second, false});
            } else {
                segments.push_back({k2, k1, coords[i+1].first, coords[i+1].second,
                                    coords[i].first, coords[i].second, false});
            }
        }
    }

    if (segments.empty()) return {};

    // Step 2: Sort segments and remove duplicate pairs (shared boundaries cancel).
    std::sort(segments.begin(), segments.end(), [](const Segment& a, const Segment& b) {
        if (a.start_key != b.start_key) return a.start_key < b.start_key;
        return a.stop_key < b.stop_key;
    });

    // Remove duplicate pairs (like osmium's erase_duplicate_segments)
    {
        size_t write = 0;
        size_t i = 0;
        while (i < segments.size()) {
            if (i + 1 < segments.size() &&
                segments[i].start_key == segments[i+1].start_key &&
                segments[i].stop_key == segments[i+1].stop_key) {
                i += 2; // skip pair
            } else {
                segments[write++] = segments[i++];
            }
        }
        segments.resize(write);
    }

    if (segments.empty()) return {};

    // Step 3: Build sorted location index.
    // Each segment has two endpoints. Create entries for both.
    struct SLoc {
        uint32_t seg_idx;
        bool is_stop; // false = start endpoint, true = stop endpoint
        int64_t key() const { return 0; } // filled by sort
    };

    // Build location list: for each segment, two entries (start, stop)
    struct LocEntry {
        int64_t key;
        uint32_t seg_idx;
        bool is_stop; // which end of the segment this entry represents
    };

    std::vector<LocEntry> locations;
    locations.reserve(segments.size() * 2);
    for (uint32_t i = 0; i < segments.size(); i++) {
        locations.push_back({segments[i].start_key, i, false});
        locations.push_back({segments[i].stop_key, i, true});
    }
    std::sort(locations.begin(), locations.end(), [](const LocEntry& a, const LocEntry& b) {
        return a.key < b.key;
    });

    // Step 4: Find split locations (keys where 3+ segment endpoints meet).
    std::unordered_set<int64_t> split_locs;
    for (size_t i = 0; i + 2 < locations.size(); i++) {
        if (locations[i].key == locations[i+1].key && locations[i+1].key == locations[i+2].key) {
            split_locs.insert(locations[i].key);
        }
    }

    // Check for open rings (unpaired endpoints = locations appearing odd times)
    bool has_open_rings = false;
    for (size_t i = 0; i < locations.size(); ) {
        size_t j = i;
        while (j < locations.size() && locations[j].key == locations[i].key) j++;
        if ((j - i) % 2 != 0) has_open_rings = true;
        i = j;
    }

    // Helper: find next unused segment at a given location
    auto get_next_segment = [&](int64_t loc_key) -> int32_t {
        auto it = std::lower_bound(locations.begin(), locations.end(), loc_key,
            [](const LocEntry& e, int64_t k) { return e.key < k; });
        while (it != locations.end() && it->key == loc_key) {
            if (!segments[it->seg_idx].done) return static_cast<int32_t>(it - locations.begin());
            ++it;
        }
        return -1;
    };

    std::vector<std::vector<std::pair<double,double>>> rings;

    // Step 5: Trace rings.
    auto trace_ring = [&](int32_t start_loc_idx, bool stop_at_splits) -> std::vector<std::pair<double,double>> {
        auto& start_entry = locations[start_loc_idx];
        auto& seg = segments[start_entry.seg_idx];
        if (seg.done) return {};

        seg.done = true;
        // Orient segment so we enter from the location entry's endpoint
        if (start_entry.is_stop) seg.reverse();

        std::vector<std::pair<double,double>> ring;
        ring.push_back({seg.start_lat, seg.start_lng});

        int64_t first_key = seg.start_key;
        int64_t last_key = seg.stop_key;
        ring.push_back({seg.stop_lat, seg.stop_lng});

        while (last_key != first_key) {
            if (stop_at_splits && split_locs.count(last_key)) break;

            int32_t next_idx = get_next_segment(last_key);
            if (next_idx < 0) break;

            auto& next_entry = locations[next_idx];
            auto& next_seg = segments[next_entry.seg_idx];
            next_seg.done = true;

            // Orient: we enter from last_key
            if (next_entry.is_stop) next_seg.reverse();
            assert(next_seg.start_key == last_key);

            last_key = next_seg.stop_key;
            ring.push_back({next_seg.stop_lat, next_seg.stop_lng});
        }

        return ring;
    };

    if (split_locs.empty() && !has_open_rings) {
        // Simple case: no splits, no open rings. Trace closed rings directly.
        for (size_t i = 0; i < locations.size(); i++) {
            if (segments[locations[i].seg_idx].done) continue;
            auto ring = trace_ring(static_cast<int32_t>(i), false);
            if (ring.size() >= 4) rings.push_back(std::move(ring));
        }
    } else {
        // Complex case: trace partial rings from split locations first,
        // then remaining segments, then join open rings.

        // Trace from split locations
        std::vector<std::vector<std::pair<double,double>>> partial_rings;
        for (int64_t split_key : split_locs) {
            int32_t idx = get_next_segment(split_key);
            while (idx >= 0) {
                auto ring = trace_ring(idx, true);
                if (!ring.empty()) {
                    int64_t rk_first = coord_key(ring.front().first, ring.front().second);
                    int64_t rk_last = coord_key(ring.back().first, ring.back().second);
                    if (ring.size() >= 4 && rk_first == rk_last) {
                        rings.push_back(std::move(ring));
                    } else {
                        partial_rings.push_back(std::move(ring));
                    }
                }
                idx = get_next_segment(split_key);
            }
        }

        // Trace remaining (non-split) segments
        for (size_t i = 0; i < locations.size(); i++) {
            if (segments[locations[i].seg_idx].done) continue;
            auto ring = trace_ring(static_cast<int32_t>(i), true);
            if (!ring.empty()) {
                int64_t rk_first = coord_key(ring.front().first, ring.front().second);
                int64_t rk_last = coord_key(ring.back().first, ring.back().second);
                if (ring.size() >= 4 && rk_first == rk_last) {
                    rings.push_back(std::move(ring));
                } else {
                    partial_rings.push_back(std::move(ring));
                }
            }
        }

        // Join open partial rings by matching endpoints
        bool joined = true;
        while (joined) {
            joined = false;
            // Build endpoint index for partial rings
            std::unordered_map<int64_t, std::vector<size_t>> endpoint_to_ring;
            for (size_t i = 0; i < partial_rings.size(); i++) {
                if (partial_rings[i].empty()) continue;
                int64_t fk = coord_key(partial_rings[i].front().first, partial_rings[i].front().second);
                int64_t lk = coord_key(partial_rings[i].back().first, partial_rings[i].back().second);
                endpoint_to_ring[fk].push_back(i);
                endpoint_to_ring[lk].push_back(i);
            }

            // Try to join pairs that share an endpoint
            for (auto& [key, ring_indices] : endpoint_to_ring) {
                if (ring_indices.size() < 2) continue;
                // Find two non-empty rings
                size_t ri = SIZE_MAX, rj = SIZE_MAX;
                for (auto idx : ring_indices) {
                    if (partial_rings[idx].empty()) continue;
                    if (ri == SIZE_MAX) ri = idx;
                    else if (rj == SIZE_MAX && idx != ri) { rj = idx; break; }
                }
                if (ri == SIZE_MAX || rj == SIZE_MAX) continue;

                auto& ra = partial_rings[ri];
                auto& rb = partial_rings[rj];
                int64_t ra_first = coord_key(ra.front().first, ra.front().second);
                int64_t ra_last = coord_key(ra.back().first, ra.back().second);
                int64_t rb_first = coord_key(rb.front().first, rb.front().second);
                int64_t rb_last = coord_key(rb.back().first, rb.back().second);

                // Join: ra_last == rb_first → append rb to ra
                if (ra_last == rb_first) {
                    ra.insert(ra.end(), rb.begin() + 1, rb.end());
                    rb.clear();
                } else if (ra_last == rb_last) {
                    // Reverse rb and append
                    for (auto rit = rb.rbegin() + 1; rit != rb.rend(); ++rit)
                        ra.push_back(*rit);
                    rb.clear();


I see several bugs in the current implementation. Let me rewrite with the key fixes:

1. **Don't normalize segment direction** — preserve original direction, handle both orientations in the location index
2. **Fix split location detection** — osmium counts locations that appear 3+ times (including both endpoints of each segment), not just 3 consecutive
3. **Improve join logic** — when multiple open rings share an endpoint, try all combinations and pick the one that closes with smallest area
4. **Fix the `get_next_segment` to properly handle direction**

The core issue is that normalizing segments (smaller key first) breaks the direction tracking. Osmium's segments are stored in their natural order from the way, and `get_next_segment` finds the next segment at a location by searching the sorted location list. The location list has entries for BOTH endpoints of each segment, and when tracing, the segment is reversed if needed.

Let me rewrite focusing on these fixes:


Edit

{
    // Osmium-style segment-graph ring assembly.
    // Coordinate key matching osmium's Location (int32_t nanodegrees)
    auto coord_key = [](double lat, double lng) -> int64_t {
        int32_t ilat = static_cast<int32_t>(lat * 1e7 + (lat >= 0 ? 0.5 : -0.5));
        int32_t ilng = static_cast<int32_t>(lng * 1e7 + (lng >= 0 ? 0.5 : -0.5));
        return (static_cast<int64_t>(ilat) << 32) | static_cast<uint32_t>(ilng);
    };

    // Step 1: Decompose all member ways into segments (consecutive coordinate pairs).
    // Each segment is directed: start -> stop. We store both coordinates.
    struct Segment {
        int64_t start_key, stop_key;
        double start_lat, start_lng, stop_lat, stop_lng;
        bool done = false;

        void reverse() {
            std::swap(start_key, stop_key);
            std::swap(start_lat, stop_lat);
            std::swap(start_lng, stop_lng);
        }
    };

    std::vector<Segment> segments;
    for (const auto& [way_id, role] : members) {
        if (!include_all_roles && role != "outer" && !role.empty()) continue;
        auto it = way_geoms.find(way_id);
        if (it == way_geoms.end() || it->second.coords.size() < 2) continue;
        const auto& coords = it->second.coords;
        for (size_t i = 0; i + 1 < coords.size(); i++) {
            int64_t k1 = coord_key(coords[i].first, coords[i].second);
            int64_t k2 = coord_key(coords[i+1].first, coords[i+1].second);
            if (k1 == k2) continue; // skip zero-length
            // Normalize: smaller key first (like osmium's segment ordering)
            if (k1 <= k2) {
                segments.push_back({k1, k2, coords[i].first, coords[i].second,
                                    coords[i+1].first, coords[i+1].second, false});
            } else {
                segments.push_back({k2, k1, coords[i+1].first, coords[i+1].second,
                                    coords[i].first, coords[i].second, false});
            }
        }
    }

    if (segments.empty()) return {};

    // Step 2: Sort segments and remove duplicate pairs (shared boundaries cancel).
    std::sort(segments.begin(), segments.end(), [](const Segment& a, const Segment& b) {
        if (a.start_key != b.start_key) return a.start_key < b.start_key;
        return a.stop_key < b.stop_key;
    });

    // Remove duplicate pairs (like osmium's erase_duplicate_segments)
    {
        size_t write = 0;
        size_t i = 0;
        while (i < segments.size()) {
            if (i + 1 < segments.size() &&
                segments[i].start_key == segments[i+1].start_key &&
                segments[i].stop_key == segments[i+1].stop_key) {
                i += 2; // skip pair
            } else {
                segments[write++] = segments[i++];
            }
        }
        segments.resize(write);
    }

    if (segments.empty()) return {};

    // Step 3: Build sorted location index.
    // Each segment has two endpoints. Create entries for both.
    struct SLoc {
        uint32_t seg_idx;
        bool is_stop; // false = start endpoint, true = stop endpoint
        int64_t key() const { return 0; } // filled by sort
    };

    // Build location list: for each segment, two entries (start, stop)
    struct LocEntry {
        int64_t key;
        uint32_t seg_idx;
        bool is_stop; // which end of the segment this entry represents
    };

    std::vector<LocEntry> locations;
    locations.reserve(segments.size() * 2);
    for (uint32_t i = 0; i < segments.size(); i++) {
        locations.push_back({segments[i].start_key, i, false});
        locations.push_back({segments[i].stop_key, i, true});
    }
    std::sort(locations.begin(), locations.end(), [](const LocEntry& a, const LocEntry& b) {
        return a.key < b.key;
    });

    // Step 4: Find split locations (keys where 3+ segment endpoints meet).
    std::unordered_set<int64_t> split_locs;
    for (size_t i = 0; i + 2 < locations.size(); i++) {
        if (locations[i].key == locations[i+1].key && locations[i+1].key == locations[i+2].key) {
            split_locs.insert(locations[i].key);
        }
    }

    // Check for open rings (unpaired endpoints = locations appearing odd times)
    bool has_open_rings = false;
    for (size_t i = 0; i < locations.size(); ) {
        size_t j = i;
        while (j < locations.size() && locations[j].key == locations[i].key) j++;
        if ((j - i) % 2 != 0) has_open_rings = true;
        i = j;
    }

    // Helper: find next unused segment at a given location
    auto get_next_segment = [&](int64_t loc_key) -> int32_t {
        auto it = std::lower_bound(locations.begin(), locations.end(), loc_key,
            [](const LocEntry& e, int64_t k) { return e.key < k; });
        while (it != locations.end() && it->key == loc_key) {
            if (!segments[it->seg_idx].done) return static_cast<int32_t>(it - locations.begin());
            ++it;
        }
        return -1;
    };

    std::vector<std::vector<std::pair<double,double>>> rings;

    // Step 5: Trace rings.
    auto trace_ring = [&](int32_t start_loc_idx, bool stop_at_splits) -> std::vector<std::pair<double,double>> {
        auto& start_entry = locations[start_loc_idx];
        auto& seg = segments[start_entry.seg_idx];
        if (seg.done) return {};

        seg.done = true;
        // Orient segment so we enter from the location entry's endpoint
        if (start_entry.is_stop) seg.reverse();

        std::vector<std::pair<double,double>> ring;
        ring.push_back({seg.start_lat, seg.start_lng});

        int64_t first_key = seg.start_key;
        int64_t last_key = seg.stop_key;
        ring.push_back({seg.stop_lat, seg.stop_lng});

        while (last_key != first_key) {
            if (stop_at_splits && split_locs.count(last_key)) break;

            int32_t next_idx = get_next_segment(last_key);
            if (next_idx < 0) break;

            auto& next_entry = locations[next_idx];
            auto& next_seg = segments[next_entry.seg_idx];
            next_seg.done = true;

            // Orient: we enter from last_key
            if (next_entry.is_stop) next_seg.reverse();
            assert(next_seg.start_key == last_key);

            last_key = next_seg.stop_key;
            ring.push_back({next_seg.stop_lat, next_seg.stop_lng});
        }

        return ring;
    };

    if (split_locs.empty() && !has_open_rings) {
        // Simple case: no splits, no open rings. Trace closed rings directly.
        for (size_t i = 0; i < locations.size(); i++) {
            if (segments[locations[i].seg_idx].done) continue;
            auto ring = trace_ring(static_cast<int32_t>(i), false);
            if (ring.size() >= 4) rings.push_back(std::move(ring));
        }
    } else {
        // Complex case: trace partial rings from split locations first,
        // then remaining segments, then join open rings.

        // Trace from split locations
        std::vector<std::vector<std::pair<double,double>>> partial_rings;
        for (int64_t split_key : split_locs) {
            int32_t idx = get_next_segment(split_key);
            while (idx >= 0) {
                auto ring = trace_ring(idx, true);
                if (!ring.empty()) {
                    int64_t rk_first = coord_key(ring.front().first, ring.front().second);
                    int64_t rk_last = coord_key(ring.back().first, ring.back().second);
                    if (ring.size() >= 4 && rk_first == rk_last) {
                        rings.push_back(std::move(ring));
                    } else {
                        partial_rings.push_back(std::move(ring));
                    }
                }
                idx = get_next_segment(split_key);
            }
        }

        // Trace remaining (non-split) segments
        for (size_t i = 0; i < locations.size(); i++) {
            if (segments[locations[i].seg_idx].done) continue;
            auto ring = trace_ring(static_cast<int32_t>(i), true);
            if (!ring.empty()) {
                int64_t rk_first = coord_key(ring.front().first, ring.front().second);
                int64_t rk_last = coord_key(ring.back().first, ring.back().second);
                if (ring.size() >= 4 && rk_first == rk_last) {
                    rings.push_back(std::move(ring));
                } else {
                    partial_rings.push_back(std::move(ring));
                }
            }
        }

        // Join open partial rings by matching endpoints
        bool joined = true;
        while (joined) {
            joined = false;
            // Build endpoint index for partial rings
            std::unordered_map<int64_t, std::vector<size_t>> endpoint_to_ring;
            for (size_t i = 0; i < partial_rings.size(); i++) {
                if (partial_rings[i].empty()) continue;
                int64_t fk = coord_key(partial_rings[i].front().first, partial_rings[i].front().second);
                int64_t lk = coord_key(partial_rings[i].back().first, partial_rings[i].back().second);
                endpoint_to_ring[fk].push_back(i);
                endpoint_to_ring[lk].push_back(i);
            }

            // Try to join pairs that share an endpoint
            for (auto& [key, ring_indices] : endpoint_to_ring) {
                if (ring_indices.size() < 2) continue;
                // Find two non-empty rings
                size_t ri = SIZE_MAX, rj = SIZE_MAX;
                for (auto idx : ring_indices) {
                    if (partial_rings[idx].empty()) continue;
                    if (ri == SIZE_MAX) ri = idx;
                    else if (rj == SIZE_MAX && idx != ri) { rj = idx; break; }
                }
                if (ri == SIZE_MAX || rj == SIZE_MAX) continue;

                auto& ra = partial_rings[ri];
                auto& rb = partial_rings[rj];
                int64_t ra_first = coord_key(ra.front().first, ra.front().second);
                int64_t ra_last = coord_key(ra.back().first, ra.back().second);
                int64_t rb_first = coord_key(rb.front().first, rb.front().second);
                int64_t rb_last = coord_key(rb.back().first, rb.back().second);

                // Join: ra_last == rb_first → append rb to ra
                if (ra_last == rb_first) {
                    ra.insert(ra.end(), rb.begin() + 1, rb.end());
                    rb.clear();
                } else if (ra_last == rb_last) {
                    // Reverse rb and append
                    for (auto rit = rb.rbegin() + 1; rit != rb.rend(); ++rit)
                        ra.push_back(*rit);
                    rb.clear();
                } else if (ra_first == rb_last) {
                    rb.insert(rb.end(), ra.begin() + 1, ra.end());
                    ra = std::move(rb);
                    rb.clear();
                } else if (ra_first == rb_first) {
                    std::reverse(rb.begin(), rb.end());
                    rb.insert(rb.end(), ra.begin() + 1, ra.end());
                    ra = std::move(rb);
                    rb.clear();
                }

                // Check if now closed
                if (!ra.empty()) {
                    int64_t fk = coord_key(ra.front().first, ra.front().second);
                    int64_t lk = coord_key(ra.back().first, ra.back().second);
                    if (ra.size() >= 4 && fk == lk) {
                        rings.push_back(std::move(ra));
                        ra.clear();
                    }
                }
                joined = true;
                break; // restart after each join (indices changed)
            }
        }
    }

    return rings;
}

{
    // Osmium-style segment-graph ring assembly.
    // Key insight: DON'T normalize segment direction. Store segments in
    // their natural way order. Build location index with both endpoints.
    // Trace rings by following segments, reversing as needed.

    auto coord_key = [](double lat, double lng) -> int64_t {
        int32_t ilat = static_cast<int32_t>(lat * 1e7 + (lat >= 0 ? 0.5 : -0.5));
        int32_t ilng = static_cast<int32_t>(lng * 1e7 + (lng >= 0 ? 0.5 : -0.5));
        return (static_cast<int64_t>(ilat) << 32) | static_cast<uint32_t>(ilng);
    };

    // Step 1: Decompose ways into segments. Preserve original direction.
    struct Segment {
        double first_lat, first_lng, second_lat, second_lng;
        int64_t first_key, second_key;
        bool done = false;

        // Normalized key pair for duplicate detection (smaller first)
        std::pair<int64_t,int64_t> norm_key() const {
            return first_key <= second_key
                ? std::make_pair(first_key, second_key)
                : std::make_pair(second_key, first_key);
        }
    };

    std::vector<Segment> segments;
    for (const auto& [way_id, role] : members) {
        if (!include_all_roles && role != "outer" && !role.empty()) continue;
        auto it = way_geoms.find(way_id);
        if (it == way_geoms.end() || it->second.coords.size() < 2) continue;
        const auto& coords = it->second.coords;
        for (size_t i = 0; i + 1 < coords.size(); i++) {
            int64_t k1 = coord_key(coords[i].first, coords[i].second);
            int64_t k2 = coord_key(coords[i+1].first, coords[i+1].second);
            if (k1 == k2) continue;
            segments.push_back({coords[i].first, coords[i].second,
                                coords[i+1].first, coords[i+1].second,
                                k1, k2, false});
        }
    }
    if (segments.empty()) return {};

    // Step 2: Sort by normalized key and remove duplicate PAIRS.
    std::sort(segments.begin(), segments.end(), [](const Segment& a, const Segment& b) {
        auto na = a.norm_key(), nb = b.norm_key();
        return na < nb;
    });
    {
        size_t w = 0, i = 0;
        while (i < segments.size()) {
            if (i + 1 < segments.size() && segments[i].norm_key() == segments[i+1].norm_key()) {
                i += 2;
            } else {
                segments[w++] = segments[i++];
            }
        }
        segments.resize(w);
    }
    if (segments.empty()) return {};

    // Step 3: Build sorted location index (both endpoints of each segment).
    struct LocEntry {
        int64_t key;
        uint32_t seg_idx;
        bool is_second; // true = this entry is the segment's second endpoint
    };
    std::vector<LocEntry> locs;
    locs.reserve(segments.size() * 2);
    for (uint32_t i = 0; i < (uint32_t)segments.size(); i++) {
        locs.push_back({segments[i].first_key, i, false});
        locs.push_back({segments[i].second_key, i, true});
    }
    std::stable_sort(locs.begin(), locs.end(), [](const LocEntry& a, const LocEntry& b) {
        return a.key < b.key;
    });

    // Step 4: Find split locations (3+ endpoints at same coordinate).
    std::vector<int64_t> split_locs;
    {
        int64_t prev_key = -1;
        int count = 0;
        for (auto& loc : locs) {
            if (loc.key == prev_key) {
                count++;
                if (count == 3 && (split_locs.empty() || split_locs.back() != loc.key)) {
                    split_locs.push_back(loc.key);
                }
            } else {
                prev_key = loc.key;
                count = 1;
            }
        }
    }
    std::sort(split_locs.begin(), split_locs.end());

    auto is_split = [&](int64_t key) -> bool {
        return std::binary_search(split_locs.begin(), split_locs.end(), key);
    };

    // Helper: get next unused segment at a location
    auto get_next_at = [&](int64_t key) -> int32_t {
        auto it = std::lower_bound(locs.begin(), locs.end(), key,
            [](const LocEntry& e, int64_t k) { return e.key < k; });
        while (it != locs.end() && it->key == key) {
            if (!segments[it->seg_idx].done) return (int32_t)(it - locs.begin());
            ++it;
        }
        return -1;
    };

    // Step 5: Trace a ring/partial ring from a location entry.
    // Returns coordinates. The segment at start_loc is consumed and oriented
    // so that start_loc's coordinate is the entry point.
    auto trace = [&](int32_t start_loc, bool stop_at_splits) -> std::vector<std::pair<double,double>> {
        auto& le = locs[start_loc];
        auto& seg = segments[le.seg_idx];
        if (seg.done) return {};
        seg.done = true;

        // Orient: entry from le's endpoint → exit from the other endpoint
        double entry_lat, entry_lng, exit_lat, exit_lng;
        int64_t entry_key, exit_key;
        if (!le.is_second) {
            entry_lat = seg.first_lat; entry_lng = seg.first_lng;
            exit_lat = seg.second_lat; exit_lng = seg.second_lng;
            entry_key = seg.first_key; exit_key = seg.second_key;
        } else {
            entry_lat = seg.second_lat; entry_lng = seg.second_lng;
            exit_lat = seg.first_lat; exit_lng = seg.first_lng;
            entry_key = seg.second_key; exit_key = seg.first_key;
        }

        std::vector<std::pair<double,double>> ring;
        ring.push_back({entry_lat, entry_lng});
        ring.push_back({exit_lat, exit_lng});
        int64_t first_key = entry_key;
        int64_t last_key = exit_key;

        while (last_key != first_key) {
            if (stop_at_splits && is_split(last_key)) break;
            int32_t ni = get_next_at(last_key);
            if (ni < 0) break;

            auto& nle = locs[ni];
            auto& nseg = segments[nle.seg_idx];
            nseg.done = true;

            // Orient: we enter from last_key
            if (!nle.is_second) {
                // nle is first endpoint → we enter at first, exit at second
                ring.push_back({nseg.second_lat, nseg.second_lng});
                last_key = nseg.second_key;
            } else {
                // nle is second endpoint → we enter at second, exit at first
                ring.push_back({nseg.first_lat, nseg.first_lng});
                last_key = nseg.first_key;
            }
        }
        return ring;
    };

    std::vector<std::vector<std::pair<double,double>>> rings;

    if (split_locs.empty()) {
        // Simple case: no split locations. Trace complete rings.
        for (int32_t i = 0; i < (int32_t)locs.size(); i++) {
            if (segments[locs[i].seg_idx].done) continue;
            auto ring = trace(i, false);
            if (ring.size() >= 4) {
                int64_t fk = coord_key(ring.front().first, ring.front().second);
                int64_t lk = coord_key(ring.back().first, ring.back().second);
                if (fk == lk) rings.push_back(std::move(ring));
            }
        }
    } else {
        // Complex case: trace from split locations first, then remaining.
        std::vector<std::vector<std::pair<double,double>>> open_rings;

        for (int64_t sk : split_locs) {
            int32_t idx = get_next_at(sk);
            while (idx >= 0) {
                auto ring = trace(idx, true);
                if (ring.empty()) { idx = get_next_at(sk); continue; }
                int64_t fk = coord_key(ring.front().first, ring.front().second);
                int64_t lk = coord_key(ring.back().first, ring.back().second);
                if (ring.size() >= 4 && fk == lk)
                    rings.push_back(std::move(ring));
                else if (ring.size() >= 2)
                    open_rings.push_back(std::move(ring));
                idx = get_next_at(sk);
            }
        }

        // Trace remaining non-split segments
        for (int32_t i = 0; i < (int32_t)locs.size(); i++) {
            if (segments[locs[i].seg_idx].done) continue;
            auto ring = trace(i, true);
            if (ring.empty()) continue;
            int64_t fk = coord_key(ring.front().first, ring.front().second);
            int64_t lk = coord_key(ring.back().first, ring.back().second);
            if (ring.size() >= 4 && fk == lk)
                rings.push_back(std::move(ring));
            else if (ring.size() >= 2)
                open_rings.push_back(std::move(ring));
        }

        // Join open rings: repeatedly merge pairs that share an endpoint.
        // Osmium does this with try_to_merge + join_connected_rings.
        // We use a simpler iterative approach.
        bool progress = true;
        while (progress) {
            progress = false;

            // Build endpoint→ring index
            std::unordered_multimap<int64_t, size_t> ep_idx;
            for (size_t i = 0; i < open_rings.size(); i++) {
                if (open_rings[i].empty()) continue;
                ep_idx.insert({coord_key(open_rings[i].front().first, open_rings[i].front().second), i});
                ep_idx.insert({coord_key(open_rings[i].back().first, open_rings[i].back().second), i});
            }

            // Find two open rings sharing an endpoint and merge them
            for (size_t i = 0; i < open_rings.size(); i++) {
                if (open_rings[i].empty()) continue;
                int64_t stop_key = coord_key(open_rings[i].back().first, open_rings[i].back().second);

                auto range = ep_idx.equal_range(stop_key);
                for (auto it = range.first; it != range.second; ++it) {
                    size_t j = it->second;
                    if (j == i || open_rings[j].empty()) continue;

                    auto& ra = open_rings[i];
                    auto& rb = open_rings[j];
                    int64_t rb_first = coord_key(rb.front().first, rb.front().second);
                    int64_t rb_last = coord_key(rb.back().first, rb.back().second);

                    if (stop_key == rb_first) {
                        // Append rb to ra
                        ra.insert(ra.end(), rb.begin() + 1, rb.end());
                        rb.clear();
                    } else if (stop_key == rb_last) {
                        // Reverse rb and append
                        for (auto rit = rb.rbegin() + 1; rit != rb.rend(); ++rit)
                            ra.push_back(*rit);
                        rb.clear();
                    } else {
                        continue;
                    }

                    // Check if closed
                    int64_t fk = coord_key(ra.front().first, ra.front().second);
                    int64_t lk = coord_key(ra.back().first, ra.back().second);
                    if (ra.size() >= 4 && fk == lk) {
                        rings.push_back(std::move(ra));
                        ra.clear();
                    }
                    progress = true;
                    break;
                }
                if (progress) break;
            }
        }
    }

    return rings;
}