A translation from Ruby to Rust in the book :pp.013-017[「もっとプログラマ脳を鍛える数学パズル」](https://www.shoeisha.co.jp/book/detail/9784798153612)

main.rs

use std::collections::HashMap;

struct Checker {
    memo: HashMap<(i64, i64), i64>,
    max_seat_at_one_table: i64,
}

impl Checker {
    pub fn check(&mut self, remain: i64, pre: i64) -> i64 {
        match &self.memo.get(&(remain, pre)) {
            Some(cnt) => return **cnt,
            _ => {
                if remain < 0 {
                    return 0;
                } else if remain == 0 {
                    return 1;
                } else {
                    let mut count = 0;
                    for i in pre..=self.max_seat_at_one_table {
                        count += self.check(remain - i, i);
                    }
                    &self.memo.insert((remain, pre), count);
                    return count;
                }
            }
        }
    }
}

fn main() {
    let mut chk = Checker {
        memo: HashMap::new(),
        max_seat_at_one_table: 10,
    };
    println!("{}", chk.check(100, 2));
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_check() {
        let number_of_people = 100;
        let mut chk = Checker {
            memo: HashMap::new(),
            max_seat_at_one_table: 10,
        };
        assert_eq!(chk.check(number_of_people, 2), 437_420);
    }
}