You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: precompiled.rs
    path of file: ./repos/akula/src/execution
    
    the code of the file until where you have to start completion: use crate::{chain::protocol_param::param, crypto::*, models::*, util::*};
use arrayref::array_ref;
use bytes::{Buf, Bytes};
use num_bigint::BigUint;
use num_traits::Zero;
use ripemd::*;
use secp256k1::{
    ecdsa::{RecoverableSignature, RecoveryId},
    Message, SECP256K1,
};
use sha2::*;
use sha3::*;
use std::{
    cmp::min,
    convert::TryFrom,
    io::{repeat, Read},
};
use substrate_bn::*;

pub type GasFunction = fn(Bytes, Revision) -> Option<u64>;
pub type RunFunction = fn(Bytes) -> Option<Bytes>;

pub struct Contract {
    pub gas
    
fn expmod_run(input: Bytes) -> Option<Bytes> {
    let mut input = right_pad(input, 3 * 32);

    let base_len = usize::try_from(u64::from_be_bytes(*array_ref!(input, 24, 8))).unwrap();
    let exponent_len = usize::try_from(u64::from_be_bytes(*array_ref!(input, 56, 8))).unwrap();
    let modulus_len = usize::try_from(u64::from_be_bytes(*array_ref!(input, 88, 8))).unwrap();

    if modulus_len == 0 {
        return Some(Bytes::new());
    }

    input.advance(96);
    let input = right_pad(input, base_len + exponent_len + modulus_len);

    let base = BigUint::from_bytes_be(&input[..base_len]);
    let exponent = BigUint::from_bytes_be(&input[base_len..base_len + exponent_len]);
    let modulus = BigUint::from_bytes_be(
        &input[base_len + exponent_len..base_len + exponent_len + modulus_len],
    );

    let mut out = vec![0; modulus_len];
    if modulus.is_zero() {
        return Some(out.into());
    }

    let b = base.modpow(&exponent, &modulus).to_bytes_be();

    out[modulus_len - b.len()..].copy_from_slice(&b);

    Some(out.into())
}