From d4913fa5d880bab096c445f256de1af686cc344d Mon Sep 17 00:00:00 2001
From: Lucas Meier <lucas@cronokirby.com>
Date: Fri, 29 Mar 2024 13:49:58 -0700
Subject: [PATCH] Add mechanism for using meta params to update fmd params
 periodically

---
 .../src/component/shielded_pool.rs            | 26 +++++++++++++++---
 .../core/component/shielded-pool/src/fmd.rs   | 27 ++++++++++++++++++-
 2 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/crates/core/component/shielded-pool/src/component/shielded_pool.rs b/crates/core/component/shielded-pool/src/component/shielded_pool.rs
index a565a3c253..4c0c809fac 100644
--- a/crates/core/component/shielded-pool/src/component/shielded_pool.rs
+++ b/crates/core/component/shielded-pool/src/component/shielded_pool.rs
@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use crate::fmd::should_update_fmd_params;
 use crate::params::ShieldedPoolParameters;
 use crate::{fmd, genesis, state_key};
 use anyhow::anyhow;
@@ -62,11 +63,30 @@ impl Component for ShieldedPool {
     ) {
     }
 
-    #[instrument(name = "shielded_pool", skip(_state, _end_block))]
+    #[instrument(name = "shielded_pool", skip(state, end_block))]
     async fn end_block<S: StateWrite + 'static>(
-        _state: &mut Arc<S>,
-        _end_block: &abci::request::EndBlock,
+        state: &mut Arc<S>,
+        end_block: &abci::request::EndBlock,
     ) {
+        let height: u64 = end_block
+            .height
+            .try_into()
+            .expect("height should not be negative");
+        if should_update_fmd_params(height) {
+            let state = Arc::get_mut(state).expect("the state should not be shared");
+            let meta_params = state
+                .get_shielded_pool_params()
+                .await
+                .expect("should be able to read state")
+                .fmd_meta_params;
+            let old = state
+                .get_current_fmd_parameters()
+                .await
+                .expect("should be able to read state");
+            let new = meta_params.updated_fmd_params(&old, height);
+            state.put_previous_fmd_parameters(old);
+            state.put_current_fmd_parameters(new);
+        }
     }
 
     async fn end_epoch<S: StateWrite + 'static>(mut _state: &mut Arc<S>) -> Result<()> {
diff --git a/crates/core/component/shielded-pool/src/fmd.rs b/crates/core/component/shielded-pool/src/fmd.rs
index 0d0ac6671f..e8e1541949 100644
--- a/crates/core/component/shielded-pool/src/fmd.rs
+++ b/crates/core/component/shielded-pool/src/fmd.rs
@@ -1,10 +1,24 @@
 use anyhow::anyhow;
 use decaf377_fmd::Precision;
-use penumbra_proto::{core::component::shielded_pool::v1 as pb, DomainType};
+use penumbra_proto::{
+    core::component::shielded_pool::v1::{self as pb},
+    DomainType,
+};
 use serde::{Deserialize, Serialize};
 
 pub mod state_key;
 
+/// How long users have to switch to updated parameters.
+pub const FMD_GRACE_PERIOD_BLOCKS: u64 = 1 << 4;
+/// How often we update the params.
+pub const FMD_UPDATE_FREQUENCY_BLOCKS: u64 = 1 << 6;
+/// How many blocks we expect per day, approximately.
+const _BLOCKS_PER_DAY: u64 = 1 << 13;
+
+pub fn should_update_fmd_params(height: u64) -> bool {
+    height % FMD_UPDATE_FREQUENCY_BLOCKS == 0
+}
+
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(try_from = "pb::FmdParameters", into = "pb::FmdParameters")]
 pub struct Parameters {
@@ -88,3 +102,14 @@ impl Default for MetaParameters {
         Self::Fixed(Precision::default())
     }
 }
+
+impl MetaParameters {
+    pub fn updated_fmd_params(&self, _old: &Parameters, height: u64) -> Parameters {
+        match *self {
+            MetaParameters::Fixed(precision) => Parameters {
+                precision,
+                as_of_block_height: height,
+            },
+        }
+    }
+}