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# o0Ignition0o/bastion_floating_on_tide1.rs

Created Feb 23, 2020
Bastion floating on tide example!
 use std::iter; // in order to determine if n is prime // we will use a primality test. // https://en.wikipedia.org/wiki/Primality_test#Pseudocode fn is_prime(n: u128) -> bool { if n <= 3 { n > 1 } else if n % 2 == 0 || n % 3 == 0 { false } else { for i in (5..=(n as f64).sqrt() as u128).step_by(6) { if n % i == 0 || n % (i + 2) == 0 { return false; } } true } } // given a sequence of digits, return the corresponding number // eg: assert_eq!(1234, digits_to_number(vec![1,2,3,4])) fn digits_to_number(iter: impl Iterator) -> u128 { iter.fold(0, |acc, b| acc * 10 + b as u128) } fn get_min_bound(num_digits: usize) -> u128 { let lower_bound_iter = iter::once(1usize).chain(iter::repeat(0usize).take(num_digits - 1 as usize)); digits_to_number(lower_bound_iter) } fn get_max_bound(num_digits: usize) -> u128 { let lower_bound_iter = iter::once(1usize).chain(iter::repeat(0usize).take(num_digits)); digits_to_number(lower_bound_iter) } fn number_or_panic(number_to_return: u128) -> u128 { // Let's roll a dice if rand::random::() % 6 == 0 { panic!(format!( "I was about to return {} but I chose to panic instead!", number_to_return )) } number_to_return } fn get_prime(num_digits: usize) -> u128 { let min_bound = get_min_bound(num_digits); // with num_digits = 4, max_bound == 10000 let max_bound = get_max_bound(num_digits); // maybe_prime is a number in range [1000, 10000) // the closing parenthesiss means it won't reach the number. // the maximum allowed value for maybe_prime is 9999. use rand::Rng; let mut maybe_prime = rand::thread_rng().gen_range(min_bound, max_bound); loop { if is_prime(maybe_prime) { return number_or_panic(maybe_prime); } // for any integer n > 3, // there always exists at least one prime number p // with n < p < 2n - 2 maybe_prime += 1; // We don't want to return a number // that doesn't have the right number of digits if maybe_prime == max_bound { maybe_prime = min_bound; } } } async fn prime_number(req: tide::Request<()>) -> String { use std::time::Instant; let d: usize = req.param("digits").unwrap_or(1); // Start a stopwatch let start = Instant::now(); // Get a prime number let prime_number = get_prime(d); // Stop the stopwatch let elapsed = Instant::now().duration_since(start).as_secs(); format!( "{} is a prime number with {} digits.\nIt was computed in {} seconds.\n", prime_number, d, elapsed ) } fn main() -> Result<(), std::io::Error> { use async_std::task; task::block_on(async { let mut app = tide::new(); app.at("/prime/:digits").get(prime_number); app.listen("127.0.0.1:8080").await?; Ok(()) }) }

### o0Ignition0o commented Feb 23, 2020

 It order to run this example, several crates are required: ```async-std = "1.5.0" tide = "0.6.0" rand = "0.7.3"```

### cedric-h commented Sep 12, 2020 • edited

 why: ``` panic!(format!( "I was about to return {} but I chose to panic instead!", number_to_return ))``` instead of: `panic!("I was about to return {} but I chose to panic instead!", number_to_return);`

### o0Ignition0o commented Sep 12, 2020

 Haha indeed! That would work and look better! Maybe we should update the blog post as well ? Nice catch!
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