rust-ngram.rs

use std::error;
use std::fs::File;
use std::io::{self, BufRead};
use std::result;
use std::process;

type Result<T> = result::Result<T, Box<error::Error>>;

fn process_line(bytes: &[u8]) -> (usize, usize) {
    let (mut i, mut key, mut val) = (0, 0, 0);
    while bytes[i] != b'\t' {
        i += 1;
    }
    i += 1;
    while bytes[i] != b'\t' {
        key = key * 10 + (bytes[i] as usize) - (b'0' as usize);
        i += 1;
    }
    i += 1;
    while bytes[i] != b'\t' {
        val = val * 10 + (bytes[i] as usize) - (b'0' as usize);
        i += 1;
    }
    (key, val)
}

fn process_file(file: &File) -> Result<(usize, usize)> {
    let mut sum_by_key = [0; 2009];
    let mut buf = io::BufReader::new(file);
    let mut line = vec![];
    while buf.read_until(b'\n', &mut line)? > 0 {
        let (k, v) = process_line(&line);
        sum_by_key[k] += v;
        line.clear();
    }
    let (mut k, mut v) = (0, 0);
    for (i, &val) in sum_by_key.iter().enumerate() {
        if val > v {
            k = i;
            v = val;
        }
    }
    Ok((k, v))
}

fn try_main() -> Result<()> {
    let file = File::open("/tmp/ngrams.tsv")?;
    let (k, v) = process_file(&file)?;
    if k != 2006 || v != 22569013 {
        Err(format!("bad result {:?} | {:?}", k, v).into())
    } else {
        Ok(())
    }
}

fn main() {
    if let Err(err) = try_main() {
        println!("{}", err);
        process::exit(1);
    }
}