go-ncrlite

ncrlite is a simple and fast compression format specifically designed to compress an unordered set of positive integers (below 2⁶⁴). This repository contains a Go package that implements it and a commandline tool.

Warning. The file format is not yet final.

Performance

ncrlite achieves smaller compressed sizes than general-purpose compressors.

Compared to more specialized compressors, ncrlite outperforms Elias–Fano. nrclite performs slightly worse than Rice coding on random sets, but is still close to the theoretical limit of lg N choose k. ncrlite does perform better than Rice coding on skewed sets like {9900, ..., 10000}.

Theoretical limit for random sets

There are N choose k subsets of k positive integers below N. Thus there is a hard limit: no compression method can encode every such set in less than lg N choose k bits.

Of course a compression method can beat the limit for specific sets, but it will have to compensate by using more bits for others.

Commandline tool

Installation

Install Go and run

$ go install github.com/bwesterb/go-ncrlite/cmd/ncrlite@latest

Now you can use ncrlite.

Basic operation

ncrlite takes as input a textfile with a positive number on each line.

$ cat dunbar
5
15
35
150
500
1500

To compress simply run:

$ ncrlite dunbar

This will create dunbar.ncrlite and remove dunbar.

The input file does not have to be sorted (numerically). If it is not, ncrlite will sort the input first, which is slower.

To decompress, run:

$ ncrlite -d dunbar.ncrlite

This will create dunbar and remove dunbar.ncrlite. The output file is always sorted.

Other formats

At the moment, the ncrlite commandline tool only supports the simple text format. Reach out if another is useful.

Other flags

ncrlite supports several familiar flags.

  -f, --force
        overwrite output
  -k, --keep
        keep (don't delete) input file
  -c, --stdout
        write to stdout; implies -k

Without specifying a filename (or using -), ncrlite will read from stdin and write to stdout.

Inspect compressed file

With -i we can inspect a compressed file:

$ ncrlite -i le.csv.ncrlite 
max bitlength        14
codelength h[0]      9
dictionary size      56b

Codebook bitlengths:
 0 111111110
 1 11111110
 2 1111100
 3 1111101
 4 11110
 5 1100
 6 1101
 7 100
 8 00
 9 01
10 101
11 1110
12 1111110
13 1111111110
14 1111111111

Maximum value    (N)  382584265
Number of values (k)  512652
Theoretical best avg  703953.8B
Overhead              0.4%

Format

In short: we store the deltas (differences) which are each prefixed by a Huffman code for their bitlength. The Huffman code is stored using bzip2's method.

Now, in detail. The file starts with the size of the set as an unsigned varint.

There are two special cases.

1. If the size of the set is zero, the file ends immediately after the size (without endmarker.)

2. If the size of the set is one, then the value of that element is encoded as an unsigned varint after it and the file ends (without endmarker.)

The values of the set are not encoded directly, but instead their deltas are encoded. The nth delta is the difference between the nth and the n-1th value, considering the set as a sorted list.

The first delta is special: it's the minimum value of the set plus one so that a delta is never zero.

For each delta d, we consider its bitlength. That is the least l such that 2^(l+1) > d. Note that this is different from the typical definition of bitlength being one smaller: the length of 1 is 0 and of 4 is 2.

The six least significant bits of the next byte encode the largest bitlength of any delta that occurs. We assign to that bitlength, and each smaller bitlength, a canonical Huffman code by encoding the length of each of their codewords.

The next six bits (that is: the two most significant bits of the byte used for the largest bitlength, and the four least significant bits of the byte afterward) encode the length of the codeword for the zero bitlength.

We continue with the remaining bitlengths in order. If the next bitlength has a codeword of the same length as the previous codeword, we encode this with a single bit 1.

Instead, if the codeword is one larger we encode this as first a single bit 0 to say we're not done; then a single bit 1 to say the next is larger; and finally a 1 to say we're done. Together: 0b101. If it's two larger we repeat twice: 0b10101. And so on. If the codeword is one smaller we use 0b001, and repeat 00 if the difference is larger.

After having encoded the Huffman code for the bitlengths, we encode the deltas themselves. First we write the Huffman code for the bitlength. Then we write the delta with that many bits, without its most significant bit as it's implied.

Finally, we write the endmarker 0xaa = 0b10101010. This allows for simpler decompression using prefix tables. The remaining high bits in the final byte are set to zero.