digest: replace Reset trait with methods; add *into* methods

Closes RustCrypto/hashes#86

Replaces the `Reset` trait with a set of `_reset` methods on all of the
various traits, and uses these as the default impl for the methods which
consume hashers.

Also adds a set of methods to `FixedOutput` (`finalize_fixed_into*`)
which eliminate copies by accepting an existing buffer as an argument.

Author: tarcieri

digest/src/dev.rs

@@ -2,7 +2,7 @@
 
 pub use blobby;
 
-use super::{ExtendableOutput, Reset, Update, VariableOutput, XofReader};
+use super::{ExtendableOutput, Update, VariableOutput, XofReader};
 use core::fmt::Debug;
 
 /// Define test
@@ -51,7 +51,7 @@ mod foo {
         }
 
         // Test if reset works correctly
-        hasher2.reset();
+        hasher2.finalize_reset();
         hasher2.update(input);
         if hasher2.finalize().as_slice() != output {
             return Some("whole message after reset");
@@ -101,7 +101,7 @@ pub use self::foo::{digest_test, one_million_a};
 /// XOF test
 pub fn xof_test<D>(input: &[u8], output: &[u8]) -> Option<&'static str>
 where
-    D: Update + ExtendableOutput + Default + Debug + Reset + Clone,
+    D: Update + ExtendableOutput + Default + Debug + Clone,
 {
     let mut hasher = D::default();
     let mut buf = [0u8; 1024];
@@ -119,7 +119,7 @@ where
     }
 
     // Test if hasher resets correctly
-    hasher2.reset();
+    hasher2.finalize_xof_reset();
     hasher2.update(input);
 
     {
@@ -168,7 +168,7 @@ where
 /// Variable-output digest test
 pub fn variable_test<D>(input: &[u8], output: &[u8]) -> Option<&'static str>
 where
-    D: Update + VariableOutput + Reset + Debug + Clone,
+    D: Update + VariableOutput + Debug + Clone,
 {
     let mut hasher = D::new(output.len()).unwrap();
     let mut buf = [0u8; 128];
@@ -182,7 +182,7 @@ where
     }
 
     // Test if reset works correctly
-    hasher2.reset();
+    hasher2.finalize_variable_reset(|_| ());
     hasher2.update(input);
     hasher2.finalize_variable(|res| buf.copy_from_slice(res));
     if buf != output {

digest/src/digest.rs

@@ -1,4 +1,4 @@
-use super::{FixedOutput, Reset, Update};
+use super::{FixedOutput, Update};
 use generic_array::typenum::Unsigned;
 use generic_array::{ArrayLength, GenericArray};
 
@@ -32,9 +32,6 @@ pub trait Digest {
     /// re-creation.
     fn finalize_reset(&mut self) -> Output<Self>;
 
-    /// Reset hasher instance to its initial state.
-    fn reset(&mut self);
-
     /// Get output size of the hasher
     fn output_size() -> usize;
 
@@ -49,7 +46,7 @@ pub trait Digest {
     fn digest(data: &[u8]) -> Output<Self>;
 }
 
-impl<D: Update + FixedOutput + Reset + Clone + Default> Digest for D {
+impl<D: Update + FixedOutput + Clone + Default> Digest for D {
     type OutputSize = <Self as FixedOutput>::OutputSize;
 
     fn new() -> Self {
@@ -72,13 +69,7 @@ impl<D: Update + FixedOutput + Reset + Clone + Default> Digest for D {
     }
 
     fn finalize_reset(&mut self) -> Output<Self> {
-        let res = self.clone().finalize_fixed();
-        self.reset();
-        res
-    }
-
-    fn reset(&mut self) {
-        <Self as Reset>::reset(self)
+        self.finalize_fixed_reset()
     }
 
     fn output_size() -> usize {

digest/src/dyn_digest.rs

@@ -1,7 +1,7 @@
 #![cfg(feature = "alloc")]
 use alloc::boxed::Box;
 
-use super::{FixedOutput, Reset, Update};
+use super::{FixedOutput, Update};
 use generic_array::typenum::Unsigned;
 
 /// The `DynDigest` trait is a modification of `Digest` trait suitable
@@ -18,33 +18,25 @@ pub trait DynDigest {
     /// Retrieve result and consume boxed hasher instance
     fn finalize(self: Box<Self>) -> Box<[u8]>;
 
-    /// Reset hasher instance to its initial state.
-    fn reset(&mut self);
-
     /// Get output size of the hasher
     fn output_size(&self) -> usize;
 
     /// Clone hasher state into a boxed trait object
     fn box_clone(&self) -> Box<dyn DynDigest>;
 }
 
-impl<D: Update + FixedOutput + Reset + Clone + 'static> DynDigest for D {
+impl<D: Update + FixedOutput + Clone + 'static> DynDigest for D {
     fn update(&mut self, data: &[u8]) {
         Update::update(self, data);
     }
 
+    #[inline]
     fn finalize_reset(&mut self) -> Box<[u8]> {
-        let res = self.clone().finalize_fixed().to_vec().into_boxed_slice();
-        Reset::reset(self);
-        res
-    }
-
-    fn finalize(self: Box<Self>) -> Box<[u8]> {
-        self.finalize_fixed().to_vec().into_boxed_slice()
+        self.finalize_fixed_reset().to_vec().into_boxed_slice()
     }
 
-    fn reset(&mut self) {
-        Reset::reset(self);
+    fn finalize(mut self: Box<Self>) -> Box<[u8]> {
+        self.finalize_reset()
     }
 
     fn output_size(&self) -> usize {

digest/src/lib.rs

@@ -80,7 +80,33 @@ pub trait FixedOutput {
     type OutputSize: ArrayLength<u8>;
 
     /// Retrieve result and consume hasher instance.
-    fn finalize_fixed(self) -> GenericArray<u8, Self::OutputSize>;
+    #[inline]
+    fn finalize_fixed(mut self) -> GenericArray<u8, Self::OutputSize>
+    where
+        Self: Sized,
+    {
+        self.finalize_fixed_reset()
+    }
+
+    /// Retrieve result and reset hasher instance.
+    #[inline]
+    fn finalize_fixed_reset(&mut self) -> GenericArray<u8, Self::OutputSize> {
+        let mut buf = Default::default();
+        self.finalize_fixed_into_reset(&mut buf);
+        buf
+    }
+
+    /// Retrieve result into provided buffer and consume hasher instance.
+    #[inline]
+    fn finalize_fixed_into(mut self, out: &mut GenericArray<u8, Self::OutputSize>)
+    where
+        Self: Sized,
+    {
+        self.finalize_fixed_into_reset(out);
+    }
+
+    /// Retrieve result and provided buffer and consume hasher instance.
+    fn finalize_fixed_into_reset(&mut self, out: &mut GenericArray<u8, Self::OutputSize>);
 }
 
 /// Trait for returning digest result with the variable size
@@ -99,7 +125,16 @@ pub trait VariableOutput: core::marker::Sized {
     ///
     /// Closure is guaranteed to be called, length of the buffer passed to it
     /// will be equal to `output_size`.
-    fn finalize_variable<F: FnOnce(&[u8])>(self, f: F);
+    #[inline]
+    fn finalize_variable(mut self, f: impl FnOnce(&[u8])) {
+        self.finalize_variable_reset(f);
+    }
+
+    /// Retrieve result via closure and reset hasher.
+    ///
+    /// Closure is guaranteed to be called, length of the buffer passed to it
+    /// will be equal to `output_size`.
+    fn finalize_variable_reset(&mut self, f: impl FnOnce(&[u8]));
 
     /// Retrieve result into vector and consume hasher.
     #[cfg(feature = "alloc")]
@@ -135,7 +170,13 @@ pub trait ExtendableOutput: core::marker::Sized {
     type Reader: XofReader;
 
     /// Retrieve XOF reader and consume hasher instance.
-    fn finalize_xof(self) -> Self::Reader;
+    #[inline]
+    fn finalize_xof(mut self) -> Self::Reader {
+        self.finalize_xof_reset()
+    }
+
+    /// Retrieve XOF reader and reset hasher instance.
+    fn finalize_xof_reset(&mut self) -> Self::Reader;
 
     /// Retrieve result into vector of specified length.
     #[cfg(feature = "alloc")]
@@ -147,12 +188,6 @@ pub trait ExtendableOutput: core::marker::Sized {
     }
 }
 
-/// Trait for resetting hash instances
-pub trait Reset {
-    /// Reset hasher instance to its initial state and return current state.
-    fn reset(&mut self);
-}
-
 #[macro_export]
 /// Implements `std::io::Write` trait for implementer of [`Update`]
 macro_rules! impl_write {