github.com/alexclewontin/mph

Minimal Perfect Hashing for Go

This library provides Minimal Perfect Hashing (MPH) using the Compress, Hash and Displace (CHD) algorithm.

What is this useful for?

Primarily, extremely efficient access to potentially very large static datasets, such as geographical data, NLP data sets, etc.

On my 2012 vintage MacBook Air, a benchmark against a wikipedia index with 300K keys against a 2GB TSV dump takes about ~200ns per lookup.

How would it be used?

Typically, the table would be used as a fast index into a (much) larger data set, with values in the table being file offsets or similar.

The tables can be serialized. Numeric values are written in little endian form.

Example code

Building and serializing an MPH hash table (error checking omitted for clarity):

b := mph.Builder()
for k, v := range data {
    b.Add(k, v)
}
h, _ := b.Build()
w, _ := os.Create("data.idx")
_ := h.Write(w)

Deserializing the hash table and performing lookups:

r, _ := os.Open("data.idx")
h, _ := mph.Read(r)

v := h.Get([]byte("some key"))
if v == nil {
    // Key not found
}

MMAP is also indirectly supported, by deserializing from a byte slice and slicing the keys and values.

The API documentation has more details.