Cargo.toml

[package]
name = "fst-int"
version = "0.1.0"
authors = ["Andrew Gallant <jamslam@gmail.com>"]

[dependencies]
byteorder = "0.4"
fst = "0.1"

main.rs

extern crate byteorder;
extern crate fst;

use byteorder::{ByteOrder, BigEndian};
use fst::{IntoStreamer, Streamer};

fn main() {
    let primes = sieve(100_000_000);
    println!("# primes: {}, size: {} bytes", primes.len(), primes.len() * 4);

    let fst = FstIntSet::from_vec(primes);
    println!("FST size: {} bytes", fst.size());

    // Very quickly query for whether an integer is prime.
    println!("prime? {} => {}", 1, fst.contains(1));
    println!("prime? {} => {}", 2, fst.contains(2));
    println!("prime? {} => {}", 999613, fst.contains(999613));
    println!("prime? {} => {}", 999614, fst.contains(999614));
    println!("prime? {} => {}", 9999973, fst.contains(9999973));

    // Range queries are totally cool because big-endian preserves
    // integer ordering. If we encoded integers as little-endian, this would
    // not work!
    let (start, end) = (999_800, 1_000_000);
    println!("primes in range {}-{}", start, end);
    let mut stream = fst.range(start, end);
    while let Some(key) = stream.next() {
        let n = BigEndian::read_u32(&key);
        println!("{}", n);
    }
}

struct FstIntSet {
    set: fst::Set,
}

impl FstIntSet {
    fn from_vec(mut nums: Vec<u32>) -> FstIntSet {
        nums.sort();
        nums.dedup();

        let mut build = fst::SetBuilder::memory();
        for n in nums {
            let mut buf = [0; 4];
            BigEndian::write_u32(&mut buf, n);
            build.insert(buf).unwrap();
        }
        let bytes = build.into_inner().unwrap();
        FstIntSet {
            set: fst::Set::from_bytes(bytes).unwrap(),
        }
    }

    fn contains(&self, n: u32) -> bool {
        let mut buf = [0; 4];
        BigEndian::write_u32(&mut buf, n);
        self.set.contains(buf)
    }

    fn range(&self, start: u32, end: u32) -> fst::set::Stream {
        let (mut bstart, mut bend) = ([0; 4], [0; 4]);
        BigEndian::write_u32(&mut bstart, start);
        BigEndian::write_u32(&mut bend, end);
        self.set.range().ge(bstart).lt(bend).into_stream()
    }

    fn size(&self) -> usize {
        self.set.as_fst().size()
    }
}

fn sieve(n: usize) -> Vec<u32> {
    if n <= 1 {
        return vec![];
    }

    // Taken from:
    // https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_and_variants
    let mut marked = vec![true; n+1];
    marked[0] = false;
    marked[1] = false;
    for i in 2..((n as f64).sqrt().ceil() as usize) {
        if marked[i] {
            let mut j = i * i;
            let mut k = 0;
            while j <= n {
                marked[j] = false;
                k += 1;
                j = (i * i) + k * i;
            }
        }
    }
    marked.iter()
          .enumerate()
          .filter_map(|(i, &m)| if !m { None } else { Some(i as u32) })
          .collect()
}