Implement RngCore for RingOscillator (#135)

Implement rand_core::RngCore for RingOscillator from the get_random_bit
function. This is not suitable for security purposes so
rand_core::CryptoRng has not been implemented.

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- implement `rng_core::RngCore` for `RingOscillator`
+
 ## [0.3.0] - 2021-09-20
 
 ### Added

rp2040-hal/Cargo.toml

@@ -21,6 +21,7 @@ pio = { git = "https://github.com/rp-rs/pio-rs.git", branch = "main" }
 usb-device = "0.2.8"
 vcell = "0.1"
 void = { version = "1.0.2", default-features = false }
+rand_core = "0.6.3"
 
 [dev-dependencies]
 cortex-m-rt = "0.7"

rp2040-hal/src/rosc.rs

@@ -97,3 +97,28 @@ impl RingOscillator<Enabled> {
         self.transition(Dormant)
     }
 }
+
+impl rand_core::RngCore for RingOscillator<Enabled> {
+    fn next_u32(&mut self) -> u32 {
+        rand_core::impls::next_u32_via_fill(self)
+    }
+
+    fn next_u64(&mut self) -> u64 {
+        rand_core::impls::next_u64_via_fill(self)
+    }
+
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        for chunk in dest.iter_mut() {
+            *chunk = 0_u8;
+            for _ in 0..8 {
+                *chunk <<= 1;
+                *chunk ^= self.get_random_bit() as u8;
+            }
+        }
+    }
+
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand_core::Error> {
+        self.fill_bytes(dest);
+        Ok(())
+    }
+}