dsrs 0.3.0

Rusty wrapper for Apache DataSketches
Documentation 
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# DataSketches in Rust

A Rust binding for the [Apache DataSketches](https://datasketches.apache.org/) library.

At this point, this package only wraps the count-distinct CPC sketch and provides a command-line tool, `dsrs`, for approximate distinct line-counting functionality (I personally find this useful when collecting statistics from logs). Examples require [GNU Parallel](https://www.gnu.org/software/parallel). The command-line part probably doesn't work on Windows.

```bash
(seq $((100 * 1000 * 1000)) && seq $((100 * 1000 * 1000))) | \
  /usr/bin/time -f "%e sec %M KB" dsrs
102055590
5.22 sec 4288 KB

(seq $((100 * 1000 * 1000)) && seq $((100 * 1000 * 1000))) | \
  /usr/bin/time -f "%e sec %M KB" sort -u | wc -l
438.66 sec 12880 KB
100000000

(seq $((100 * 1000 * 1000)) && seq $((100 * 1000 * 1000))) | \
  /usr/bin/time -f "%e sec %M KB" awk '{a[$0]=1}END{print length(a)}'
100000000
39.28 sec 898240 KB
```

```bash
TODO: some really fast keyed approx count in parallel, compare memory and time
```

## Installation

Assumes a modern Rust `cargo` is installed. The command line tool `dsrs` can be installed with:

```
cargo install dsrs
```

The library may be used as a regular Rust dependency by adding it to your `Cargo.toml` file.

## Embedded C++ Library

This Rust library contains manually-copied header files from the header-only `datasketches-cpp` library at commit [043b947f](https://github.com/apache/datasketches-cpp/tree/043b947fe5b1f9b82527deb0eea4da32f5764f6c).

This was done by extracting all headers. Assuming you're in the `datasketches-rs` directory, which has a sibling `datasketches-cpp`:

```
# make all required directories
find ../datasketches-cpp/ -name "*.h" -or -name "*.hpp" | \
  xargs dirname | \
  sort -u |
  cut -d/ -f2- | \
  xargs mkdir -p
# copy over the actual headers
find ../datasketches-cpp/ -name "*.h" -or -name "*.hpp" | \
  cut -d/ -f2- | \
  xargs -I {} cp ../{} {}
# and the license info too
cp ../datasketches-cpp/{NOTICE,LICENSE} datasketches-cpp/
```

This is all only possible thanks to the excellent [dtolnay/cxx](https://github.com/dtolnay/cxx) library!

## Why DataSketches in Rust?

There are quite a few crates containing HyperLogLog sketches. However, when I attempted to use them (as of 2021-06-20), I found that their APIs panicked on certain inputs (e.g., try `amadeus_streaming::HyperLogLog::<u64>::new(0.0001);`), or did not have merge operations. A very rudimentary `cargo criterion` on 1M unique keys finds that CPC has better accuracy anyway (for all of the below, the same nominal accuracy configuration was set, so these should be using roughly the same amount of memory):

```
repeat-ten/dsrs::CpcSketch/1000000
                        time:   [144.95 ms 149.27 ms 155.42 ms]
repeat-ten/amadeus_streaming::HyperLogLog/1000000
                        time:   [132.89 ms 134.01 ms 135.49 ms]
repeat-ten/probabilistic_collections::HyperLogLog/1000000
                        time:   [159.99 ms 165.94 ms 172.32 ms]
repeat-ten/probably::HyperLogLog/1000000
                        time:   [119.47 ms 123.95 ms 127.84 ms]
repeat-ten/hyperloglogplus::HyperLogLogPlus/1000000
                        time:   [120.74 ms 121.32 ms 122.10 ms]

relative errors
size: 1000000
  relerr:   1.1% name: dsrs::CpcSketch
  relerr:   3.3% name: amadeus_streaming::HyperLogLog
  relerr:   4.3% name: hyperloglogplus::HyperLogLogPlus
  relerr:  50.7% name: probably::HyperLogLog
  relerr:   inf% name: probabilistic_collections::HyperLogLog
```

while overall update speed doesn't change too much between implementations.