feat(engine): FCU Version (#321)

### Description

Introduces an FCU version for downstream engine api use.

crates/rpc-types-engine/Cargo.toml

@@ -18,6 +18,7 @@ workspace = true
 # Workspace
 op-alloy-consensus.workspace = true
 op-alloy-protocol.workspace = true
+op-alloy-genesis.workspace = true
 
 # Alloy
 alloy-primitives.workspace = true

crates/rpc-types-engine/src/lib.rs

@@ -9,6 +9,9 @@
 
 extern crate alloc;
 
+mod versions;
+pub use versions::ForkchoiceUpdateVersion;
+
 mod attributes;
 pub use attributes::{OpAttributesWithParent, OpPayloadAttributes};
 

crates/rpc-types-engine/src/versions.rs

@@ -0,0 +1,69 @@
+//! Versions for the engine api.
+
+use op_alloy_genesis::RollupConfig;
+
+/// The version of the engine api.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(u32)]
+pub enum ForkchoiceUpdateVersion {
+    /// Version 1 of the engine api.
+    V1 = 1,
+    /// Version 2 of the engine api.
+    V2 = 2,
+    /// Version 3 of the engine api.
+    V3 = 3,
+    /// Version 4 of the engine api.
+    V4 = 4,
+}
+
+impl ForkchoiceUpdateVersion {
+    /// Constructs a `ForkchoiceUpdateVersion` from a [RollupConfig] and timestamp.
+    ///
+    /// See: <https://github.com/ethereum-optimism/optimism/blob/4ee839ae8996c2d421a2d85fd5471897840014fa/op-node/rollup/types.go#L514>
+    pub fn from_config(config: &RollupConfig, timestamp: u64) -> Self {
+        if config.is_ecotone_active(timestamp) {
+            // Cancun
+            Self::V3
+        } else if config.is_canyon_active(timestamp) {
+            // Shanghai
+            Self::V2
+        } else {
+            // According to Ethereum engine API spec, we can use fcuV2 here,
+            // but upstream Geth v1.13.11 does not accept V2 before Shanghai.
+            Self::V1
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_fcu_from_config_ecotone() {
+        let config =
+            RollupConfig { ecotone_time: Some(10), canyon_time: Some(5), ..Default::default() };
+        let expected = ForkchoiceUpdateVersion::V3;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 10), expected);
+        let expected = ForkchoiceUpdateVersion::V2;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 5), expected);
+        let expected = ForkchoiceUpdateVersion::V1;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 0), expected);
+    }
+
+    #[test]
+    fn test_fcu_from_config_canyon() {
+        let config = RollupConfig { canyon_time: Some(1), ..Default::default() };
+        let expected = ForkchoiceUpdateVersion::V2;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 1), expected);
+        let expected = ForkchoiceUpdateVersion::V1;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 0), expected);
+    }
+
+    #[test]
+    fn test_fcu_from_config() {
+        let config = RollupConfig::default();
+        let expected = ForkchoiceUpdateVersion::V1;
+        assert_eq!(ForkchoiceUpdateVersion::from_config(&config, 0), expected);
+    }
+}