build.rs

use std::env;
use std::fs::File;
use std::io::Write;
use std::mem;
use std::path::Path;

fn log2(x: usize) -> usize {
    mem::size_of::<usize>() * 8 - (x.leading_zeros() as usize) - 1
}

const SBLOCK_WIDTH: usize = 64;
const MAX_CODE_SIZE: usize = 48;

fn main() {
    let mut combination = vec![vec![0u64; SBLOCK_WIDTH + 1]; SBLOCK_WIDTH + 1];
    for n in 0..=SBLOCK_WIDTH {
        combination[n][0] = 1;
        for r in 1..=n {
            combination[n][r] = combination[n - 1][r - 1] + combination[n - 1][r];
        }
    }

    let mut code_size = vec![0; SBLOCK_WIDTH + 1];
    code_size[0] = 0;
    code_size[SBLOCK_WIDTH] = 0;
    for n in 1..SBLOCK_WIDTH {
        let size = log2((combination[SBLOCK_WIDTH][n] - 1) as usize) + 1;
        code_size[n] = if size <= MAX_CODE_SIZE {
            size
        } else {
            SBLOCK_WIDTH
        };
    }

    let out_dir = env::var("OUT_DIR").unwrap();
    let out_path = Path::new(&out_dir).join("const.rs");
    let mut f = File::create(&out_path).unwrap();

    f.write_all(
        format!(
            "const COMBINATION: [[u64; {size}]; {size}] = {:?};
            const CODE_SIZE: [u64; {size}] = {:?};",
            combination,
            code_size,
            size = SBLOCK_WIDTH + 1
        ).as_bytes(),
    ).unwrap();
}