Merge rust-bitcoin/rust-bitcoin#1910: Make sha512::HashEngine fields private

96784b9cfa Make sha512::HashEngine fields private (Tobin C. Harding)

Pull request description:

  Recently we made the hash engine fields pub crate so that `sha512_256` could construct a hash engine with different constants. We can make the code slightly cleaner by adding a pub crate constructor and making the fields private again.

  Idea from Kixunil:

    https://github.com/rust-bitcoin/rust-bitcoin/pull/1413#pullrequestreview-1197207593

  This is a follow up to #1413, cc kcalvinalvin as the author of that one.

ACKs for top commit:
  apoelstra:
    ACK 96784b9cfa
  Kixunil:
    ACK 96784b9cfa

Tree-SHA512: 40faa969b2227e46ea66a4ce887f17d9a48a0bc18846fb6bb1a51dd117a4e49cc031196846e913e03c39597cacd30a61a55bd1ccde600be72583965d2faf090a

hashes/src/sha512.rs

@@ -17,9 +17,9 @@ pub(crate) const BLOCK_SIZE: usize = 128;
 /// Engine to compute SHA512 hash function.
 #[derive(Clone)]
 pub struct HashEngine {
-    pub(crate) h: [u64; 8],
-    pub(crate) length: usize,
-    pub(crate) buffer: [u8; BLOCK_SIZE],
+    h: [u64; 8],
+    length: usize,
+    buffer: [u8; BLOCK_SIZE],
 }
 
 impl Default for HashEngine {
@@ -36,6 +36,21 @@ impl Default for HashEngine {
     }
 }
 
+impl HashEngine {
+    /// Constructs a hash engine suitable for use inside the default `sha512_256::HashEngine`.
+    #[rustfmt::skip]
+    pub(crate) fn sha512_256() -> Self {
+        HashEngine {
+            h: [
+                0x22312194fc2bf72c, 0x9f555fa3c84c64c2, 0x2393b86b6f53b151, 0x963877195940eabd,
+                0x96283ee2a88effe3, 0xbe5e1e2553863992, 0x2b0199fc2c85b8aa, 0x0eb72ddc81c52ca2,
+            ],
+            length: 0,
+            buffer: [0; BLOCK_SIZE],
+        }
+    }
+}
+
 impl crate::HashEngine for HashEngine {
     type MidState = [u8; 64];
 

hashes/src/sha512_256.rs

@@ -11,7 +11,6 @@ use core::ops::Index;
 use core::slice::SliceIndex;
 use core::str;
 
-use crate::sha512::BLOCK_SIZE;
 use crate::{sha512, FromSliceError};
 
 /// Engine to compute SHA512/256 hash function.
@@ -26,14 +25,7 @@ pub struct HashEngine(sha512::HashEngine);
 impl Default for HashEngine {
     #[rustfmt::skip]
     fn default() -> Self {
-        HashEngine(sha512::HashEngine {
-            h: [
-                0x22312194fc2bf72c, 0x9f555fa3c84c64c2, 0x2393b86b6f53b151, 0x963877195940eabd,
-                0x96283ee2a88effe3, 0xbe5e1e2553863992, 0x2b0199fc2c85b8aa, 0x0eb72ddc81c52ca2,
-            ],
-            length: 0,
-            buffer: [0; BLOCK_SIZE],
-        })
+        HashEngine(sha512::HashEngine::sha512_256())
     }
 }
 
@@ -44,7 +36,7 @@ impl crate::HashEngine for HashEngine {
 
     const BLOCK_SIZE: usize = sha512::BLOCK_SIZE;
 
-    fn n_bytes_hashed(&self) -> usize { self.0.length }
+    fn n_bytes_hashed(&self) -> usize { self.0.n_bytes_hashed() }
 
     fn input(&mut self, inp: &[u8]) { self.0.input(inp); }
 }