Finish challenge 22.

src/main.rs

@@ -34,7 +34,8 @@ fn main() {
         set3::challenge20();
         set3::challenge21();
         set3::challenge22();
+        set3::challenge23();
     } else {
-        set3::challenge22();
+        set3::challenge23();
     }
 }

src/set3.rs

@@ -8,6 +8,7 @@ use std::cell::RefCell;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::io::{BufRead, BufReader};
+use std::time::{SystemTime, UNIX_EPOCH};
 
 pub fn challenge17() {
     fn read(path: &str) -> Vec<Bytes> {
@@ -317,5 +318,46 @@ pub fn challenge21() {
 }
 
 pub fn challenge22() {
-    println!("[xxxx] Challenge 22: TBD!");
+    // let mut mt = mt19937::MT19937::new();
+    fn unix_timestamp() -> u32 {
+        let start = SystemTime::now();
+        let since_the_epoch = start
+            .duration_since(UNIX_EPOCH)
+            .expect("Time went backwards");
+        since_the_epoch.as_secs() as u32
+    }
+
+    // Wait a random number of seconds between, I don't know, 40 and 1000.
+    let now = unix_timestamp();
+    let wait_time: u32 = rand::thread_rng().gen_range(40..1000);
+    let seed = now + wait_time;
+
+    // Seeds the RNG with the current Unix timestamp.
+    let mut mt = mt19937::MT19937::new();
+    mt.seed(seed);
+
+    // Returns the first 32 bit output of the RNG.
+    let rngout = mt.extract_number();
+
+    // From the 32 bit RNG output, discover the seed.
+    fn find_seed(rngout: u32) -> Option<u32> {
+        let mut mt = mt19937::MT19937::new();
+        let start = unix_timestamp() - 2000;
+        for seed in start..(start + 4000) {
+            mt.seed(seed);
+            if rngout == mt.extract_number() {
+                return Some(seed);
+            }
+        }
+        None
+    }
+
+    let found_seed = find_seed(rngout);
+    assert_eq!(seed, found_seed.unwrap());
+
+    println!("[done] Challenge 22: cracked MT19937 seed.");
+}
+
+pub fn challenge23() {
+    println!("[xxxx] Challenge 23: tbd");
 }