RNG (#77)

* quick first draft for random

* update init method

* nits

* fix

* rename

* add source

* fix

---------

Co-authored-by: Luke Videckis <lukevideckis@gmail.com>

Author: lrvideckis

src/helpers/untested_rng.rs

@@ -0,0 +1,60 @@
+//! # Random Number Generator
+
+/// see <https://en.wikipedia.org/wiki/Xorshift>
+///
+/// # Example
+/// ```
+/// use std::time::{SystemTime, UNIX_EPOCH};
+/// use programming_team_code_rust::helpers::untested_rng::UntestedRNG;
+///
+/// // fixed seed for debugging
+/// let mut rng1 = UntestedRNG::new(12345);
+/// // for submitting
+/// let mut rng2 = UntestedRNG::new(
+///     SystemTime::now()
+///         .duration_since(UNIX_EPOCH)
+///         .unwrap()
+///         .subsec_nanos() as u64,
+/// );
+///
+/// assert_eq!(rng1.get(), 13289605635609);
+/// assert_eq!(rng1.get_in_range(-3..4), 0);
+/// ```
+struct UntestedRNG {
+    state: u64,
+}
+
+impl UntestedRNG {
+    /// Creates a new RNG
+    ///
+    /// # Complexity
+    /// - Time: O(1)
+    /// - Space: O(1)
+    fn new(seed: u64) -> Self {
+        assert_ne!(seed, 0);
+        Self { state: seed }
+    }
+
+    /// Gets a new random number
+    ///
+    /// # Complexity
+    /// - Time: O(1)
+    /// - Space: O(1)
+    fn get(&mut self) -> u64 {
+        let mut x = self.state;
+        x ^= x << 13;
+        x ^= x >> 7;
+        x ^= x << 17;
+        self.state = x;
+        x
+    }
+
+    /// Gets a new random number in range
+    ///
+    /// # Complexity
+    /// - Time: O(1)
+    /// - Space: O(1)
+    fn get_in_range(&mut self, range: std::ops::Range<i64>) -> i64 {
+        range.start + (self.get() % (range.end - range.start) as u64) as i64
+    }
+}