"Bit trie" a trie-like structure

bit_trie.cpp

#include <algorithm>
#include <climits>
#include <cstdlib>
#include <iostream>
#include <stdexcept>
#include <string>
#include <vector>

/*
bit_trie
  bit_tries are associative containers that store elements formed
  by the combination of a key value and a mapped value, and which
  allows for fast retrieval of individual elements based on their keys.
 Key
  Type of the key values. Each element in an bit_tire is uniquely
  identified by its key value.
 Value
  Type of the mapped values.
 BitTester
  A binary function object type that takes an object of type Key
  and a size_t integer (bit address) as arguments and returns a bool.
  The bit address is an integer less than bit length. The function
  should return a consistent result for the same key and the same
  bit address. The results for two differents key should be different
  for at least one bit address.
*/
template <
    typename Key,
    typename Value,
    typename BitTester
>
class bit_trie {
    struct Branch{
        int offset;
        bool end;
    };
    struct Node {
        std::size_t bit_address;
        Branch left;
        Branch right;
        Key key; // this can be removed if use only at_no_verify to retrieve elements
        Value value;
    };
    std::vector<Node> nodes;
    BitTester tester;
    Node& at_node(const Key& key) {
        size_t cur_pos = 0;
        Branch next = nodes[0].left;
        while (true) {
            cur_pos += next.offset;
            if (next.end)
                return nodes[cur_pos];
            if (tester(key, nodes[cur_pos].bit_address)) {
                next = nodes[cur_pos].right;
            } else {
                next = nodes[cur_pos].left;
            }
        }
    }
    void build(typename std::vector<Node>::iterator begin, typename std::vector<Node>::iterator end, std::size_t bit_length) {
        // counting number of elements and numbers passing each bit test
        std::vector<std::size_t> pass_count(bit_length);
        std::size_t count = 0;
        std::for_each(begin, end, [&](const Node& node){
            for (std::size_t bit = 0; bit < bit_length; ++bit) {
                if (tester(node.key, bit))
                    ++pass_count[bit];
            }
            ++count;
        });
        if (count == 1)
            return;

        // find the best address that partition the elements evenly
        long badness = LONG_MAX;
        std::size_t best_address;
        for (std::size_t address = 0; address < bit_length; ++ address) {
            long current = std::abs((long)(pass_count[address] - count / 2));
            if (current < badness) {
                badness = current;
                best_address = address;
            }
        }

        // partition
        auto partition_pos = std::partition(begin, end, [&](const Node& node){
            return !tester(node.key, best_address);
        });
        auto partition_distance = std::distance(begin, partition_pos);
        if (partition_distance == count || partition_distance == 0)
            throw std::invalid_argument("Duplicated keys.");

        // build trie for each partition
        build(begin, partition_pos, bit_length);
        build(partition_pos, end, bit_length);

        // let right guiding node be the first level node
        partition_pos->right = partition_pos->left;
        partition_pos->left = begin->left;
        partition_pos->left.offset -= partition_distance;
        partition_pos->bit_address = best_address;
        // and left guiding node be the main guiding node
        begin->left.offset = partition_distance;
        begin->left.end = false;
    }
public:
    template <typename InputInterator>
    bit_trie(
        InputInterator begin,
        InputInterator end,
        std::size_t bit_length,
        const BitTester& tester_ = BitTester()
    ) : nodes(std::distance(begin, end)), tester(tester_) {
        constexpr std::size_t invalid_bit_address = static_cast<std::size_t>(-1);
        std::transform(begin, end, nodes.begin(), [](std::pair<Key, Value> pair){
            return Node {
                invalid_bit_address,
                {0, true},
                {0, true},
                pair.first,
                pair.second
            };
        });
        if (nodes.empty())
            throw std::length_error("Empty range.");

        build(nodes.begin(), nodes.end(), bit_length);
    }

    Value& at(const Key& key) {
        Node& node = at_node(key);
        if (node.key == key)
            return node.value;
        throw std::out_of_range("Element not found.");
    }

    Value& at_no_verify(const Key& key) {
        return at_node(key).value;
    }

};

///////////////////////////////////////////////////////////////////////////////
/// TEST

const std::pair<std::string, std::string> example_map[] = {
    {"Hydrogen", "氢"},
    {"Helium", "氦"},
    {"Lithium", "锂"},
    {"Beryllium", "铍"},
    {"Boron", "硼"},
    {"Carbon", "碳"},
    {"Nitrogen", "氮"},
    {"Oxygen", "氧"},
    {"Fluorine", "氟"},
    {"Neon", "氖"},
    {"Sodium", "钠"},
    {"Magnesium", "镁"},
    {"Aluminium", "铝"},
    {"Silicon", "硅"},
    {"Phosphorus", "磷"},
    {"Sulfur", "硫"},
    {"Chlorine", "氯"},
    {"Argon", "氩"},
    {"Potassium", "钾"},
    {"Calcium", "钙"},
    {"Scandium", "钪"},
    {"Titanium", "钛"},
    {"Vanadium", "钒"},
    {"Chromium", "铬"},
    {"Manganese", "锰"},
    {"Iron", "铁"},
    {"Cobalt", "钴"},
    {"Nickel", "镍"},
    {"Copper", "铜"},
    {"Zinc", "锌"},
    {"Gallium", "镓"},
    {"Germanium", "锗"},
    {"Arsenic", "砷"},
    {"Selenium", "硒"},
    {"Bromine", "溴"},
    {"Krypton", "氪"},
    {"Rubidium", "铷"},
    {"Strontium", "锶"},
    {"Yttrium", "钇"},
    {"Zirconium", "锆"},
    {"Niobium", "铌"},
    {"Molybdenum", "钼"},
    {"Technetium", "锝"},
    {"Ruthenium", "钌"},
    {"Rhodium", "铑"},
    {"Palladium", "钯"},
    {"Silver", "银"},
    {"Cadmium", "镉"},
    {"Indium", "铟"},
    {"Tin", "锡"},
    {"Antimony", "锑"},
    {"Tellurium", "碲"},
    {"Iodine", "碘"},
    {"Xenon", "氙"},
    {"Caesium", "铯"},
    {"Barium", "钡"},
    {"Hafnium", "铪"},
    {"Tantalum", "钽"},
    {"Tungsten", "钨"},
    {"Rhenium", "铼"},
    {"Osmium", "锇"},
    {"Iridium", "铱"},
    {"Platinum", "铂"},
    {"Gold", "金"},
    {"Mercury", "汞"},
    {"Thallium", "铊"},
    {"Lead", "铅"},
    {"Bismuth", "铋"},
    {"Polonium", "钋"},
    {"Astatine", "砹"},
    {"Radon", "氡"},
    {"Francium", "钫"},
    {"Radium", "镭"},
    {"Rutherfordium", "鑪"},
    {"Meitnerium", "䥑"},
    {"Darmstadtium", "鐽"},
    {"Roentgenium", "錀"},
    {"Copernicium", "鎶"},
    {"Flerovium", "鈇"},
    {"Livermorium", "鉝"},
    {"Lanthanum", "镧"},
    {"Cerium", "铈"},
    {"Praseodymium", "镨"},
    {"Neodymium", "钕"},
    {"Promethium", "钷"},
    {"Samarium", "钐"},
    {"Europium", "铕"},
    {"Gadolinium", "钆"},
    {"Terbium", "铽"},
    {"Dysprosium", "镝"},
    {"Holmium", "钬"},
    {"Erbium", "铒"},
    {"Thulium", "铥"},
    {"Ytterbium", "镱"},
    {"Lutetium", "镏"},
    {"Actinium", "锕"},
    {"Thorium", "钍"},
    {"Protactinium", "镤"},
    {"Uranium", "铀"},
    {"Neptunium", "镎"},
    {"Plutonium", "钚"},
    {"Americium", "銤"},
    {"Curium", "锔"},
    {"Berkelium", "锫"},
    {"Californium", "锎"},
    {"Einsteinium", "锿"},
    {"Fermium", "镄"},
    {"Mendelevium", "钔"},
    {"Nobelium", "锘"},
    {"Lawrencium", "铹"}
};

bool string_tester(const std::string& key, std::size_t position) {
    std::size_t byte = position >> 3;
    if (byte >= key.size())
        return false;
    return (key[byte] >> (position & 7)) & 1;
}

int main() {
    std::size_t max_bit_length = 0;
    for (const auto& pair : example_map) {
        if (pair.first.size() > max_bit_length)
            max_bit_length = pair.first.size();
    }
    max_bit_length *= 8;

    bit_trie<std::string, std::string, decltype(&string_tester)>
        example(std::begin(example_map), std::end(example_map), max_bit_length, &string_tester);

    for (const auto& pair : example_map) {
        auto value = example.at_no_verify(pair.first);
        std::cout << "(" << (value == pair.second ? "✓" : "✗") << ")"
            << pair.first << " : " << value << std::endl;
    }

    std::string bad_key = "abcdefg";
    try {
        auto value = example.at(bad_key);
        std::cout << bad_key << " : " << value << std::endl;
    } catch (std::out_of_range e) {
        std::cout << bad_key << " : " << e.what() << std::endl;
    }
    auto value = example.at_no_verify(bad_key);
    std::cout << bad_key << " : " << value << std::endl;

    return 0;
}

@yuriks this trie doesn't match prefix of the keys. Each level choose a bit in the middle of the key to match. The bit choosing method is to find the bit that balance the trie most. So the it doesn't matter if I reverse the key.