Add state key for LQT tallying

Author: cronokirby

crates/core/component/funding/src/component/state_key.rs

@@ -12,5 +12,60 @@ pub mod lqt {
                 format!("funding/lqt/v1/nullifier/{epoch_index:020}/lookup/{nullifier}")
             }
         }
+
+        pub mod votes {
+            use penumbra_sdk_asset::asset;
+            use penumbra_sdk_keys::{address::ADDRESS_LEN_BYTES, Address};
+
+            const PART0: &'static str = "funding/lqt/v1/votes/";
+            const EPOCH_LEN: usize = 20;
+            const PART1: &'static str = "/by_asset/";
+            const PREFIX_LEN: usize = PART0.len() + EPOCH_LEN + PART1.len();
+
+            /// A prefix for accessing the votes in a given epoch, c.f. [`power_asset_address`];
+            pub(crate) fn prefix(epoch_index: u64) -> [u8; PREFIX_LEN] {
+                let mut bytes = [0u8; PREFIX_LEN];
+
+                let rest = &mut bytes;
+                let (bytes_part0, rest) = rest.split_at_mut(PART0.len());
+                let (bytes_epoch_index, bytes_part1) = rest.split_at_mut(EPOCH_LEN);
+
+                bytes_part0.copy_from_slice(PART0.as_bytes());
+                bytes_epoch_index
+                    .copy_from_slice(format!("{epoch_index:0w$}", w = EPOCH_LEN).as_bytes());
+                bytes_part1.copy_from_slice(PART1.as_bytes());
+
+                bytes
+            }
+
+            const ASSET_LEN: usize = 32;
+            const POWER_LEN: usize = 8;
+            const RECEIPT_LEN: usize = PREFIX_LEN + ASSET_LEN + POWER_LEN + ADDRESS_LEN_BYTES;
+
+            #[allow(dead_code)] // TODO: remove once used
+            /// When present, indicates that an address voted for a particular asset, with a given power.
+            ///
+            /// To get the values ordered by descending voting power, use [`prefix`];
+            pub(crate) fn receipt(
+                epoch_index: u64,
+                asset: asset::Id,
+                power: u64,
+                voter: &Address,
+            ) -> [u8; RECEIPT_LEN] {
+                let mut bytes = [0u8; RECEIPT_LEN];
+
+                let rest = &mut bytes;
+                let (bytes_prefix, rest) = rest.split_at_mut(PREFIX_LEN);
+                let (bytes_asset, rest) = rest.split_at_mut(ASSET_LEN);
+                let (bytes_power, bytes_voter) = rest.split_at_mut(POWER_LEN);
+
+                bytes_prefix.copy_from_slice(&prefix(epoch_index));
+                bytes_asset.copy_from_slice(&asset.to_bytes());
+                bytes_power.copy_from_slice(&((!power).to_be_bytes()));
+                bytes_voter.copy_from_slice(&voter.to_vec());
+
+                bytes
+            }
+        }
     }
 }