Prüfsumme und wiederholbarer Zufall

FNV-1a für Zustandsprüfsummen und SplitMix64 als einzige Zufallsquelle
des Kerns. Beide sind gegen veröffentlichte Referenzwerte geprüft.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

core/src/rng.rs

@@ -0,0 +1,34 @@
+//! Repeatable pseudo random numbers (SplitMix64).
+//!
+//! The core never uses any other source of randomness. The same seed always
+//! gives the same sequence, on every machine.
+
+/// Pseudo random number generator with 64 bits of state.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Rng {
+    state: u64,
+}
+
+impl Rng {
+    pub fn new(seed: u64) -> Self {
+        Self { state: seed }
+    }
+
+    /// Returns the next value of the sequence.
+    pub fn next_u64(&mut self) -> u64 {
+        self.state = self.state.wrapping_add(0x9E37_79B9_7F4A_7C15);
+        let mut z = self.state;
+        z = (z ^ (z >> 30)).wrapping_mul(0xBF58_476D_1CE4_E5B9);
+        z = (z ^ (z >> 27)).wrapping_mul(0x94D0_49BB_1331_11EB);
+        z ^ (z >> 31)
+    }
+
+    /// Returns a value in `0..bound`.
+    ///
+    /// # Panics
+    /// Panics if `bound` is zero.
+    pub fn below(&mut self, bound: u32) -> u32 {
+        assert!(bound > 0, "bound must be greater than zero");
+        ((u128::from(self.next_u64()) * u128::from(bound)) >> 64) as u32
+    }
+}