From ba6d24d0ff24d5328b67e76f7d6065835be5ad59 Mon Sep 17 00:00:00 2001
From: Tony Arcieri <bascule@gmail.com>
Date: Sun, 24 Jul 2022 11:29:38 -0600
Subject: [PATCH] universal-hash: add `UhfBackend::blocks_needed_to_align`

Originally suggested by @str4d here:

https://github.com/RustCrypto/traits/pull/965#discussion_r857126434

The use case is ensuring unbuffered processing of the input AEAD message
body when the AAD does not line up with the number of blocks it's
processing in parallel.

This number of blocks can be input from the start of the message body to
finish completely filling the buffer, ensuring all subsequent processing
occurs at the start of the buffer.
---
 universal-hash/src/lib.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/universal-hash/src/lib.rs b/universal-hash/src/lib.rs
index a8b6b79e9..c64589977 100644
--- a/universal-hash/src/lib.rs
+++ b/universal-hash/src/lib.rs
@@ -53,6 +53,13 @@ pub trait UhfBackend: ParBlocksSizeUser {
             self.proc_block(block);
         }
     }
+
+    /// Returns the number of blocks that should be passed to `Self::proc_block` before
+    /// `Self::proc_par_blocks` can be used efficiently. This is always less than
+    /// `Self::ParBlocksSize`.
+    fn blocks_needed_to_align(&self) -> usize {
+        0
+    }
 }
 
 /// Trait for [`UhfBackend`] users.