looking for long sequence

rust/insert_digit_anywhere_primes_ev/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "insert_digit_anywhere_primes_constant"
+version = "0.1.0"
+authors = ["William Ball <wball1@swarthmore.edu>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+uint = "0.8.3"
+rand = "0.7.3"

rust/insert_digit_anywhere_primes_ev/log

@@ -0,0 +1 @@
+0, 7, 17, 137, 1637, 18637, 198637, 1986037, 19986037, 199860337, 1998660337, 19998660337, 199098660337, 1949098660337, 19490986560337, 194909865603317, 1949098656033817, 19490983656033817, 

rust/insert_digit_anywhere_primes_ev/src/main.rs

@@ -0,0 +1,63 @@
+use std::io::prelude::*;
+use uint::construct_uint;
+use rand::prelude::*;
+
+construct_uint! {
+    pub struct U256(4);
+}
+
+fn is_prime(x: &U256) -> bool {
+    let zero = U256::from_dec_str("0").unwrap();
+    let six = U256::from_dec_str("6").unwrap();
+    if x % 2 == zero || x % 3 == zero {
+        return false;
+    }
+    let mut i = U256::from_dec_str("5").unwrap();
+    while i * i < *x {
+        if x % i == zero || x % (i + 2) == zero {
+            return false;
+        }
+        i += six;
+    }
+    true
+}
+
+fn step(x: &U256, rng: &mut ThreadRng) -> Option<U256> {
+    let mut count = 0;
+    loop {
+        let mut str_x = format!("{}", x);
+        count += 1;
+        if count > 10 * str_x.len() {
+            return None;
+        }
+        let i = rng.gen_range(0, str_x.len() + 1);
+        let d = rng.gen_range(0, 10) as u8;
+        str_x.insert(i, (d + '0' as u8) as char);
+        let temp = U256::from_dec_str(&str_x).unwrap();
+        if !(i == 0 && d == 0) {
+            if is_prime(&temp) {
+                return Some(temp);
+            }
+        }
+    }
+}
+
+fn main() {
+    let mut x = U256::from_dec_str("0").unwrap();
+    let mut rng = thread_rng();
+    let mut length = 0;
+    loop {
+        match step(&x, &mut rng) {
+            Some(val) => {
+                length += 1;
+                print!("{}, ", x);
+                std::io::stdout().flush().ok().expect("Could not flush stdout");
+                x = val;
+            },
+            None => {
+                x = U256::from_dec_str("0").unwrap();
+                println!(" {}\n\n", length);
+            }
+        }
+    }
+}