num-bigint 0.2.3

// The Computer Language Benchmarks Game
// http://benchmarksgame.alioth.debian.org/
//
// contributed by the Rust Project Developers

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extern crate num_bigint;
extern crate num_integer;
extern crate num_traits;

use std::io;
use std::str::FromStr;

use num_bigint::BigInt;
use num_integer::Integer;
use num_traits::{FromPrimitive, One, ToPrimitive, Zero};

struct Context {
    numer: BigInt,
    accum: BigInt,
    denom: BigInt,
}

impl Context {
    fn new() -> Context {
        Context {
            numer: One::one(),
            accum: Zero::zero(),
            denom: One::one(),
        }
    }

    fn from_i32(i: i32) -> BigInt {
        FromPrimitive::from_i32(i).unwrap()
    }

    fn extract_digit(&self) -> i32 {
        if self.numer > self.accum {
            return -1;
        }
        let (q, r) = (&self.numer * Context::from_i32(3) + &self.accum).div_rem(&self.denom);
        if r + &self.numer >= self.denom {
            return -1;
        }
        q.to_i32().unwrap()
    }

    fn next_term(&mut self, k: i32) {
        let y2 = Context::from_i32(k * 2 + 1);
        self.accum = (&self.accum + (&self.numer << 1)) * &y2;
        self.numer = &self.numer * Context::from_i32(k);
        self.denom = &self.denom * y2;
    }

    fn eliminate_digit(&mut self, d: i32) {
        let d = Context::from_i32(d);
        let ten = Context::from_i32(10);
        self.accum = (&self.accum - &self.denom * d) * &ten;
        self.numer = &self.numer * ten;
    }
}

fn pidigits(n: isize, out: &mut dyn io::Write) -> io::Result<()> {
    let mut k = 0;
    let mut context = Context::new();

    for i in 1..(n + 1) {
        let mut d;
        loop {
            k += 1;
            context.next_term(k);
            d = context.extract_digit();
            if d != -1 {
                break;
            }
        }

        write!(out, "{}", d)?;
        if i % 10 == 0 {
            write!(out, "\t:{}\n", i)?;
        }

        context.eliminate_digit(d);
    }

    let m = n % 10;
    if m != 0 {
        for _ in m..10 {
            write!(out, " ")?;
        }
        write!(out, "\t:{}\n", n)?;
    }
    Ok(())
}

const DEFAULT_DIGITS: isize = 512;

fn main() {
    let args = std::env::args().collect::<Vec<_>>();
    let n = if args.len() < 2 {
        DEFAULT_DIGITS
    } else if args[1] == "--bench" {
        return pidigits(DEFAULT_DIGITS, &mut std::io::sink()).unwrap();
    } else {
        FromStr::from_str(&args[1]).unwrap()
    };
    pidigits(n, &mut std::io::stdout()).unwrap();
}