refactor: rewrite BHW as a state machine

engine/src/witness/btc_e.rs

@@ -10,7 +10,9 @@ use cf_utilities::task_scope::{self, Scope};
 use futures::FutureExt;
 use pallet_cf_elections::{
 	electoral_system::ElectoralSystem,
-	electoral_systems::block_height_tracking::{primitives::Header, BlockHeightTrackingProperties},
+	electoral_systems::block_height_tracking::{
+		primitives::Header, BlockHeightTrackingProperties, InputHeaders,
+	},
 	vote_storage::VoteStorage,
 };
 use sp_core::bounded::alloc::collections::VecDeque;
@@ -132,7 +134,7 @@ impl VoterApi<BitcoinBlockHeightTracking> for BitcoinBlockHeightTrackingVoter {
 			Err(anyhow::anyhow!("btc: no new blocks found since best block height is {} for witness_from={witness_from_index}", best_block_header.block_height))
 		} else if witness_from_index == 0 {
 			headers.push_back(best_block_header);
-			Ok(headers)
+			Ok(InputHeaders(headers))
 		} else {
 			// fetch the headers we haven't got yet
 			for index in witness_from_index..best_block_header.block_height {
@@ -154,7 +156,7 @@ impl VoterApi<BitcoinBlockHeightTracking> for BitcoinBlockHeightTrackingVoter {
 					"bht: Submitting vote for (witness_from={witness_from_index})with {} headers",
 					headers.len()
 				);
-				Ok(headers)
+				Ok(InputHeaders(headers))
 			} else {
 				Err(anyhow::anyhow!("bht: Headers do not form a chain"))
 			}

state-chain/chains/src/lib.rs

@@ -69,7 +69,10 @@ pub mod instances;
 pub mod mocks;
 
 pub mod witness_period {
-	use core::ops::{Rem, Sub};
+	use core::{
+		iter::Step,
+		ops::{Rem, Sub},
+	};
 
 	use sp_runtime::traits::Block;
 	use sp_std::ops::RangeInclusive;
@@ -163,6 +166,63 @@ pub mod witness_period {
 		let floored_block_number = block_witness_floor(witness_period, block_number);
 		floored_block_number..=floored_block_number.saturating_add(witness_period - One::one())
 	}
+
+	impl<I: PartialOrd + Clone + Saturating> Step for BlockWitnessRange<I> {
+		fn steps_between(start: &Self, end: &Self) -> (usize, Option<usize>) {
+			// assert_eq!(start.period, end.period);
+			todo!()
+		}
+
+		fn forward_checked(mut start: Self, count: usize) -> Option<Self> {
+			(0..count)
+				.for_each(|_| start.root = start.root.clone().saturating_add(start.period.clone()));
+			Some(start)
+		}
+
+		fn backward_checked(mut start: Self, count: usize) -> Option<Self> {
+			(0..count)
+				.for_each(|_| start.root = start.root.clone().saturating_sub(start.period.clone()));
+			Some(start)
+		}
+	}
+
+	pub trait BlockZero {
+		fn zero() -> Self;
+		fn is_zero(&self) -> bool;
+	}
+
+	impl<I: BlockZero + From<u32>> BlockZero for BlockWitnessRange<I> {
+		fn zero() -> Self {
+			Self {
+				root: I::zero(),
+				period: 1.into(), // NOTE: of course this is horribly wrong
+			}
+		}
+
+		fn is_zero(&self) -> bool {
+			self.root.is_zero()
+		}
+	}
+
+	impl BlockZero for u64 {
+		fn zero() -> Self {
+			0
+		}
+
+		fn is_zero(&self) -> bool {
+			*self == 0
+		}
+	}
+
+	impl BlockZero for u32 {
+		fn zero() -> Self {
+			0
+		}
+
+		fn is_zero(&self) -> bool {
+			*self == 0
+		}
+	}
 }
 
 /// A trait representing all the types and constants that need to be implemented for supported

state-chain/pallets/cf-elections/Cargo.toml

@@ -45,6 +45,8 @@ sp-std = { workspace = true }
 [dev-dependencies]
 cf-test-utilities = { workspace = true, default-features = true }
 rand = { workspace = true, features = ["std"] }
+proptest = { version = "1.6"}
+proptest-derive = { version = "0.5.1" }
 
 [features]
 default = ["std"]