From 564df5005fecc826c32adadaa77c38a5a7977511 Mon Sep 17 00:00:00 2001
From: Tom <tomrgowan@gmail.com>
Date: Mon, 20 Oct 2025 11:03:38 +1100
Subject: [PATCH] apply_transition helper Adds helper function and trait for
 updating the latest block

<!-- ps-id: a5413e9f-681f-4c8a-a790-3c78093f3b03 -->
---
 crates/app/src/lib.rs | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/crates/app/src/lib.rs b/crates/app/src/lib.rs
index 427be63..e73003b 100644
--- a/crates/app/src/lib.rs
+++ b/crates/app/src/lib.rs
@@ -292,6 +292,14 @@ pub trait VoidStream: Stream {
     }
 }
 
+/// Trait for updating the latest block height and hash in state.
+pub trait UpdateLatestBlock {
+    /// Error type for update_latest_block.
+    type Error;
+    /// Update the latest block height and hash in state.
+    fn update_latest_block(&mut self, height: u64, hash: [u8; 32]) -> Result<(), Self::Error>;
+}
+
 impl<T: ?Sized> VoidStream for T where T: Stream {}
 
 pin_project! {
@@ -728,3 +736,32 @@ impl Default for Notification {
         Self::new()
     }
 }
+
+/// Apply a state transition function with a block and update state.
+/// Updates the latest block height and hash in the state.
+/// Derives T from the updated state.
+/// Returns the height.
+pub fn apply_transition<S, T, F, Fe, E>(
+    block: Block,
+    state: &mut S,
+    state_transition_function: F,
+) -> Result<(Block, Height<T>), E>
+where
+    F: FnOnce(&Block, &mut S) -> Result<(), Fe>,
+    T: for<'a> TryFrom<&'a mut S>,
+    E: From<Fe>
+        + From<<S as UpdateLatestBlock>::Error>
+        + for<'a> From<<T as TryFrom<&'a mut S>>::Error>,
+    S: UpdateLatestBlock,
+{
+    state_transition_function(&block, state)?;
+    state.update_latest_block(block.height, void_hash::Hash::hash(&block))?;
+    let height = block.height;
+    Ok((
+        block,
+        Height {
+            data: T::try_from(state)?,
+            block_height: height,
+        },
+    ))
+}