Implement BufOutputReader; add rand_core impls for BufOutputReader

Cargo.toml

@@ -39,6 +39,10 @@ mmap = ["std", "dep:memmap2"]
 # Implement the zeroize::Zeroize trait for types in this crate.
 zeroize = ["dep:zeroize", "arrayvec/zeroize"]
 
+# Implement the rand_core Rng traits for OutputReader, allowing using it as
+# a rand::Rng.
+rand = ["dep:rand_core"]
+
 # This crate implements traits from the RustCrypto project, exposed here as the
 # "traits-preview" feature. However, these traits aren't stable, and they're
 # expected to change in incompatible ways before they reach 1.0. For that
@@ -101,6 +105,7 @@ memmap2 = { version = "0.7.1", optional = true }
 rayon = { version = "1.2.1", optional = true }
 serde = { version = "1.0", default-features = false, features = ["derive"], optional = true }
 zeroize = { version = "1", default-features = false, features = ["zeroize_derive"], optional = true }
+rand_core = { version = "0.6", default-features = false, optional = true }
 
 [dev-dependencies]
 hmac = "0.12.0"

src/lib.rs

@@ -1643,3 +1643,42 @@ impl std::io::Seek for OutputReader {
         Ok(self.position())
     }
 }
+
+#[cfg(feature = "rand")]
+impl rand_core::SeedableRng for OutputReader {
+    type Seed = [u8; 32];
+
+    #[inline]
+    fn from_seed(seed: Self::Seed) -> Self {
+        let mut hasher = Hasher::new();
+        hasher.update(&seed);
+        hasher.finalize_xof()
+    }
+}
+
+#[cfg(feature = "rand")]
+impl rand_core::RngCore for OutputReader {
+    #[inline]
+    fn next_u32(&mut self) -> u32 {
+        rand_core::impls::next_u32_via_fill(self)
+    }
+
+    #[inline]
+    fn next_u64(&mut self) -> u64 {
+        rand_core::impls::next_u64_via_fill(self)
+    }
+
+    #[inline]
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        self.fill(dest);
+    }
+
+    #[inline]
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand_core::Error> {
+        self.fill(dest);
+        Ok(())
+    }
+}
+
+#[cfg(feature = "rand")]
+impl rand_core::CryptoRng for OutputReader {}

src/test.rs

@@ -820,3 +820,25 @@ fn test_serde() {
     let hash2: crate::Hash = serde_json::from_str(&json).unwrap();
     assert_eq!(hash, hash2);
 }
+
+#[test]
+#[cfg(feature = "rand")]
+fn test_rand_core() {
+    let mut seeded = crate::OutputReader::from_seed([b'0'; 32]);
+    let mut buf = [0u8; 64];
+    seeded.fill_bytes(&mut buf);
+    // Verified using: printf 00000000000000000000000000000000 | b3sum -l 76
+    assert_eq!(
+        &buf,
+        b"\
+        \x9a\x91\x3b\xc3\x24\xb1\x7e\x97\x31\x3a\x3e\x6b\x1d\x24\x05\x44\
+        \xbd\xab\xb7\x0e\xe2\xd0\xdd\x0f\x80\x25\x8c\x95\x70\x43\x1e\xb1\
+        \x43\x9a\x91\x99\xca\x39\xbe\xae\x7f\x16\xe7\x0a\x96\xc4\x60\xba\
+        \x11\x57\xb6\xc9\xd7\x85\x07\xd7\x37\xef\xae\x55\x23\x1f\x08\x6f\
+        ",
+    );
+
+    // defers to rand_core::impls, which interpret bytes little-endian.
+    assert_eq!(seeded.gen::<u32>(), 0x91bd7fa7u32);
+    assert_eq!(seeded.gen::<u64>(), 0x81f88d825bee930fu64);
+}