Author: Oliver Keyes
Date: 2016-08-03

# Radix trees in Rcpp

#### Oliver Keyes

#### 2016-08-03

A radix tree is a data structure optimised for storing key-value pairs in a way optimised for searching. This makes them very, very good for efficiently matching data against keys, and retrieving the values associated with those keys.

`triebeard` provides an implementation of radix trees for Rcpp (and also for use directly in R). To start using radix trees in your Rcpp development, simply modify your C++ file to include at the top:

    //[[Rcpp::depends(triebeard)]]
    #include <radix.h>

## Constructing trees

Trees are constructed using the syntax:

    radix_tree<type1, type2> radix;

Where `type` represents the type of the keys (for example, `std::string`) and `type2` the type of the values.

Radix trees can have any scalar type as keys, although strings are most typical; they can also have any scalar type for values. Once you’ve constructed a tree, new entries can be added in a very R-like way: `radix[new_key] = new_value;`. Entries can also be removed, with `radix.erase(key)`.

## Matching against trees

We then move on to the fun bit: matching! As mentioned, radix trees are really good for matching arbitrary values against keys (well, keys of the same type) and retrieving the associated values.

There are three types of supported matching; longest, prefix, and greedy. Longest does exactly what it says on the tin: it finds the key-value pair where the longest initial part of the key matches the arbitrary value:

    radix_tree<std::string, std::string> radix;
    radix["turnin"] = "entry the first";
    radix["turin"] = "entry the second";

    radix_tree<std::string, std::string>::iterator it;

    it = radix.longest_match("turing");

    if(it = radix.end()){
      printf("No match was found :(");
    } else {
      std::string result = "Key of longest match: " + it->first + " , value of longest match: " + it->second;
    }

Prefix matching provides all trie entries where the value-to-match is a prefix of the key:

    radix_tree<std::string, std::string> radix;
    radix["turnin"] = "entry the first";
    radix["turin"] = "entry the second";

    std::vector<radix_tree<std::string, std::string>::iterator> vec;
    std::vector<radix_tree<std::string, std::string>::iterator>::iterator it;

    it = radix.prefix_match("tur");

    if(it == vec.end()){
      printf("No match was found :(");
    } else {
      for (it = vec.begin(); it != vec.end(); ++it) {
        std::string result = "Key of a prefix match: " + it->first + " , value of a prefix match: " + it->second;
      }
    }

Greedy matching matches very, very fuzzily (a value of ‘bring’, for example, will match ‘blind’, ‘bind’ and ‘binary’) and, syntactically, looks exactly the same as prefix-matching, albeit with `radix.greedy_match()` instead of `radix.prefix_match()`.

### Other trie things

If you have ideas for other trie-like structures, or functions that would be useful with these tries, the best approach is to either request it or add it!