Now optimize the entry file writing. Currently `write_entries` iterates sorted_geo_cells doing hash lookups. Since we build `cell_to_ways` from sorted flat arrays, we can build the entry files directly from the sorted data too. But that's a bigger refactor. Let me first try a simpler optimization — since the cell maps are now built from sorted data, the keys are somewhat sequential in memory. Let me instead pre-sort the cell map entries before writing:

Actually, the simpler win: the `write_entries` function builds a `buf` by iterating sorted_cells and doing `cell_map.find()`. The cell_map has ~86M entries for Europe (ways). Each find() is a hash lookup. Instead, I can iterate the cell_map directly (since we need all entries), sort the output by position, and write.

But actually the current approach needs to maintain sorted cell order. Let me try a different approach — build a flat vector of (cell_pos, entry_data) during the sort+group phase:


  1712→    }
  1713→    for (auto& ap : out.admin_polygons) {
  1714→        ap.name_id = string_remap[ap.name_id];
  1715→    }
  1716→
  1717→    return out;
  1718→}
  1719→
  1720→// --- Write cell index ---
  1721→
  1722→static const uint32_t NO_DATA = 0xFFFFFFFFu;
  1723→
  1724→// Write entries file and return offset map
  1725→// Build entry data in memory, then write in one shot.
  1726→// Returns a vector of offsets indexed by sorted_cell position (not a hash map).
  1727→static std::vector<uint32_t> write_entries(
  1728→    const std::string& path,
  1729→    const std::vector<uint64_t>& sorted_cells,
  1730→    const std::unordered_map<uint64_t, std::vector<uint32_t>>& cell_map
  1731→) {
  1732→    // Pre-compute total size for buffer allocation
  1733→    size_t total_size = 0;
  1734→    for (auto& [id, ids] : cell_map) {
  1735→        total_size += sizeof(uint16_t) + ids.size() * sizeof(uint32_t);
  1736→    }
  1737→
  1738→    // Build entries buffer and offset vector
  1739→    std::vector<uint32_t> offsets(sorted_cells.size(), NO_DATA);
  1740→
  1741→    // Build buffer by iterating sorted_cells (maintains sorted order in file)
  1742→    std::vector<char> buf;
  1743→    buf.reserve(total_size);
  1744→    uint32_t current = 0;
  1745→    for (uint32_t i = 0; i < sorted_cells.size(); i++) {
  1746→        auto it = cell_map.find(sorted_cells[i]);
  1747→        if (it == cell_map.end()) continue;
  1748→        offsets[i] = current;
  1749→        uint16_t count = static_cast<uint16_t>(std::min(it->second.size(), size_t(65535)));
  1750→        buf.insert(buf.end(), reinterpret_cast<const char*>(&count), reinterpret_cast<const char*>(&count) + sizeof(count));
  1751→        buf.insert(buf.end(), reinterpret_cast<const char*>(it->second.data()),

    }
    for (auto& ap : out.admin_polygons) {
        ap.name_id = string_remap[ap.name_id];
    }

    return out;
}

// --- Write cell index ---

static const uint32_t NO_DATA = 0xFFFFFFFFu;

// Write entries file and return offset map
// Build entry data in memory, then write in one shot.
// Returns a vector of offsets indexed by sorted_cell position (not a hash map).
static std::vector<uint32_t> write_entries(
    const std::string& path,
    const std::vector<uint64_t>& sorted_cells,
    const std::unordered_map<uint64_t, std::vector<uint32_t>>& cell_map
) {
    // Pre-compute total size for buffer allocation
    size_t total_size = 0;
    for (auto& [id, ids] : cell_map) {
        total_size += sizeof(uint16_t) + ids.size() * sizeof(uint32_t);
    }

    // Build entries buffer and offset vector
    std::vector<uint32_t> offsets(sorted_cells.size(), NO_DATA);

    // Build buffer by iterating sorted_cells (maintains sorted order in file)
    std::vector<char> buf;
    buf.reserve(total_size);
    uint32_t current = 0;
    for (uint32_t i = 0; i < sorted_cells.size(); i++) {
        auto it = cell_map.find(sorted_cells[i]);
        if (it == cell_map.end()) continue;
        offsets[i] = current;
        uint16_t count = static_cast<uint16_t>(std::min(it->second.size(), size_t(65535)));
        buf.insert(buf.end(), reinterpret_cast<const char*>(&count), reinterpret_cast<const char*>(&count) + sizeof(count));
        buf.insert(buf.end(), reinterpret_cast<const char*>(it->second.data()),


The issue: iterating 86M `sorted_cells` and doing `cell_map.find()` for each one. Since `cell_map` is a hash map, each find() is O(1) amortized but with random memory access.

Better approach: iterate the cell_map directly (which we need to do anyway), look up each cell's position in sorted_cells via binary search, and build the output. But we need sorted order in the file.

Actually, the fastest approach: since both `sorted_cells` and `cell_map` contain the same cells (sorted_cells is the union of all cell maps), we can avoid the hash lookup entirely by iterating the cell_map, sorting by cell_id, and writing in that order:


Edit

// Write entries file and return offset map
// Build entry data in memory, then write in one shot.
// Returns a vector of offsets indexed by sorted_cell position (not a hash map).
static std::vector<uint32_t> write_entries(
    const std::string& path,
    const std::vector<uint64_t>& sorted_cells,
    const std::unordered_map<uint64_t, std::vector<uint32_t>>& cell_map
) {
    // Pre-compute total size for buffer allocation
    size_t total_size = 0;
    for (auto& [id, ids] : cell_map) {
        total_size += sizeof(uint16_t) + ids.size() * sizeof(uint32_t);
    }

    // Build entries buffer and offset vector
    std::vector<uint32_t> offsets(sorted_cells.size(), NO_DATA);

    // Build buffer by iterating sorted_cells (maintains sorted order in file)
    std::vector<char> buf;
    buf.reserve(total_size);
    uint32_t current = 0;
    for (uint32_t i = 0; i < sorted_cells.size(); i++) {
        auto it = cell_map.find(sorted_cells[i]);
        if (it == cell_map.end()) continue;
        offsets[i] = current;
        uint16_t count = static_cast<uint16_t>(std::min(it->second.size(), size_t(65535)));
        buf.insert(buf.end(), reinterpret_cast<const char*>(&count), reinterpret_cast<const char*>(&count) + sizeof(count));
        buf.insert(buf.end(), reinterpret_cast<const char*>(it->second.data()),
                   reinterpret_cast<const char*>(it->second.data()) + it->second.size() * sizeof(uint32_t));
        current += sizeof(uint16_t) + it->second.size() * sizeof(uint32_t);
    }

    // Single write
    std::ofstream f(path, std::ios::binary);
    f.write(buf.data(), buf.size());
    return offsets;
}

// Write entries file. Returns a vector of offsets indexed by sorted_cell position.
// Avoids hash lookups by extracting entries from cell_map in sorted order.
static std::vector<uint32_t> write_entries(
    const std::string& path,
    const std::vector<uint64_t>& sorted_cells,
    const std::unordered_map<uint64_t, std::vector<uint32_t>>& cell_map
) {
    // Build sorted list of (cell_id, pointer_to_ids) from the cell_map.
    // This avoids per-cell hash lookups during the write loop.
    struct CellRef { uint64_t cell_id; const std::vector<uint32_t>* ids; };
    std::vector<CellRef> sorted_refs;
    sorted_refs.reserve(cell_map.size());
    for (auto& [id, ids] : cell_map) {
        sorted_refs.push_back({id, &ids});
    }
    std::sort(sorted_refs.begin(), sorted_refs.end(),
        [](const CellRef& a, const CellRef& b) { return a.cell_id < b.cell_id; });

    // Build position lookup: binary search from sorted_cells
    // Both sorted_refs and sorted_cells are sorted by cell_id.
    // Walk both in parallel (merge-join) to assign positions.
    std::vector<uint32_t> offsets(sorted_cells.size(), NO_DATA);

    // Pre-compute total size
    size_t total_size = 0;
    for (auto& r : sorted_refs) {
        total_size += sizeof(uint16_t) + r.ids->size() * sizeof(uint32_t);
    }

    std::vector<char> buf;
    buf.reserve(total_size);
    uint32_t current = 0;

    // Merge-join: walk sorted_cells and sorted_refs together
    size_t ri = 0;
    for (uint32_t si = 0; si < sorted_cells.size() && ri < sorted_refs.size(); si++) {
        if (sorted_cells[si] < sorted_refs[ri].cell_id) continue;
        if (sorted_cells[si] > sorted_refs[ri].cell_id) { si--; ri++; continue; }
        // Match
        offsets[si] = current;
        const auto& ids = *sorted_refs[ri].ids;
        uint16_t count = static_cast<uint16_t>(std::min(ids.size(), size_t(65535)));
        buf.insert(buf.end(), reinterpret_cast<const char*>(&count),
                   reinterpret_cast<const char*>(&count) + sizeof(count));
        buf.insert(buf.end(), reinterpret_cast<const char*>(ids.data()),
                   reinterpret_cast<const char*>(ids.data()) + ids.size() * sizeof(uint32_t));
        current += sizeof(uint16_t) + ids.size() * sizeof(uint32_t);
        ri++;
    }

    std::ofstream f(path, std::ios::binary);
    f.write(buf.data(), buf.size());
    return offsets;
}