Use a SpliceContribution enum for passing splice-in params

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 2170f25..e23001e 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -58,6 +58,8 @@ use crate::ln::channelmanager::{
 };
 use crate::ln::funding::FundingTxInput;
 #[cfg(splicing)]
+use crate::ln::funding::SpliceContribution;
+#[cfg(splicing)]
 use crate::ln::interactivetxs::{
 	calculate_change_output_value, AbortReason, InteractiveTxMessageSend,
 };
@@ -10603,8 +10605,7 @@ where
 	///   generated by `SignerProvider::get_destination_script`.
 	#[cfg(splicing)]
 	pub fn splice_channel(
-		&mut self, our_funding_contribution_satoshis: i64, our_funding_inputs: Vec<FundingTxInput>,
-		change_script: Option<ScriptBuf>, funding_feerate_per_kw: u32, locktime: u32,
+		&mut self, contribution: SpliceContribution, funding_feerate_per_kw: u32, locktime: u32,
 	) -> Result<msgs::SpliceInit, APIError> {
 		// Check if a splice has been initiated already.
 		// Note: only a single outstanding splice is supported (per spec)
@@ -10628,7 +10629,7 @@ where
 
 		// TODO(splicing): check for quiescence
 
-		let our_funding_contribution = SignedAmount::from_sat(our_funding_contribution_satoshis);
+		let our_funding_contribution = contribution.value();
 		if our_funding_contribution > SignedAmount::MAX_MONEY {
 			return Err(APIError::APIMisuseError {
 				err: format!(
@@ -10657,7 +10658,7 @@ where
 		// Check that inputs are sufficient to cover our contribution.
 		let _fee = check_v2_funding_inputs_sufficient(
 			our_funding_contribution.to_sat(),
-			&our_funding_inputs,
+			contribution.inputs(),
 			true,
 			true,
 			funding_feerate_per_kw,
@@ -10670,7 +10671,7 @@ where
 			),
 		})?;
 
-		for FundingTxInput { utxo, prevtx, .. } in our_funding_inputs.iter() {
+		for FundingTxInput { utxo, prevtx, .. } in contribution.inputs().iter() {
 			const MESSAGE_TEMPLATE: msgs::TxAddInput = msgs::TxAddInput {
 				channel_id: ChannelId([0; 32]),
 				serial_id: 0,
@@ -10692,6 +10693,7 @@ where
 		}
 
 		let prev_funding_input = self.funding.to_splice_funding_input();
+		let (our_funding_inputs, change_script) = contribution.into_tx_parts();
 		let funding_negotiation_context = FundingNegotiationContext {
 			is_initiator: true,
 			our_funding_contribution,
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index abb1049..3d14937 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -30,8 +30,6 @@ use bitcoin::hashes::{Hash, HashEngine, HmacEngine};
 
 use bitcoin::secp256k1::Secp256k1;
 use bitcoin::secp256k1::{PublicKey, SecretKey};
-#[cfg(splicing)]
-use bitcoin::ScriptBuf;
 use bitcoin::{secp256k1, Sequence, SignedAmount};
 
 use crate::blinded_path::message::MessageForwardNode;
@@ -66,7 +64,7 @@ use crate::ln::channel::{
 };
 use crate::ln::channel_state::ChannelDetails;
 #[cfg(splicing)]
-use crate::ln::funding::FundingTxInput;
+use crate::ln::funding::SpliceContribution;
 use crate::ln::inbound_payment;
 use crate::ln::interactivetxs::{HandleTxCompleteResult, InteractiveTxMessageSendResult};
 use crate::ln::msgs;
@@ -4460,14 +4458,13 @@ where
 	#[cfg(splicing)]
 	#[rustfmt::skip]
 	pub fn splice_channel(
-		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey, our_funding_contribution_satoshis: i64,
-		our_funding_inputs: Vec<FundingTxInput>, change_script: Option<ScriptBuf>,
-		funding_feerate_per_kw: u32, locktime: Option<u32>,
+		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey,
+		contribution: SpliceContribution, funding_feerate_per_kw: u32, locktime: Option<u32>,
 	) -> Result<(), APIError> {
 		let mut res = Ok(());
 		PersistenceNotifierGuard::optionally_notify(self, || {
 			let result = self.internal_splice_channel(
-				channel_id, counterparty_node_id, our_funding_contribution_satoshis, our_funding_inputs, change_script, funding_feerate_per_kw, locktime
+				channel_id, counterparty_node_id, contribution, funding_feerate_per_kw, locktime
 			);
 			res = result;
 			match res {
@@ -4482,8 +4479,7 @@ where
 	#[cfg(splicing)]
 	fn internal_splice_channel(
 		&self, channel_id: &ChannelId, counterparty_node_id: &PublicKey,
-		our_funding_contribution_satoshis: i64, our_funding_inputs: Vec<FundingTxInput>,
-		change_script: Option<ScriptBuf>, funding_feerate_per_kw: u32, locktime: Option<u32>,
+		contribution: SpliceContribution, funding_feerate_per_kw: u32, locktime: Option<u32>,
 	) -> Result<(), APIError> {
 		let per_peer_state = self.per_peer_state.read().unwrap();
 
@@ -4504,13 +4500,8 @@ where
 			hash_map::Entry::Occupied(mut chan_phase_entry) => {
 				let locktime = locktime.unwrap_or_else(|| self.current_best_block().height);
 				if let Some(chan) = chan_phase_entry.get_mut().as_funded_mut() {
-					let msg = chan.splice_channel(
-						our_funding_contribution_satoshis,
-						our_funding_inputs,
-						change_script,
-						funding_feerate_per_kw,
-						locktime,
-					)?;
+					let msg =
+						chan.splice_channel(contribution, funding_feerate_per_kw, locktime)?;
 					peer_state.pending_msg_events.push(MessageSendEvent::SendSpliceInit {
 						node_id: *counterparty_node_id,
 						msg,
diff --git a/lightning/src/ln/funding.rs b/lightning/src/ln/funding.rs
index 7dc5910..21bc42b 100644
--- a/lightning/src/ln/funding.rs
+++ b/lightning/src/ln/funding.rs
@@ -9,11 +9,54 @@
 
 //! Types pertaining to funding channels.
 
+#[cfg(splicing)]
+use bitcoin::{Amount, ScriptBuf, SignedAmount};
 use bitcoin::{Script, Sequence, Transaction, Weight};
 
 use crate::events::bump_transaction::{Utxo, EMPTY_SCRIPT_SIG_WEIGHT};
 use crate::sign::{P2TR_KEY_PATH_WITNESS_WEIGHT, P2WPKH_WITNESS_WEIGHT};
 
+/// The components of a splice's funding transaction that are contributed by one party.
+#[cfg(splicing)]
+pub enum SpliceContribution {
+	/// When funds are added to a channel.
+	SpliceIn {
+		/// The amount to contribute to the splice.
+		value: Amount,
+
+		/// The inputs included in the splice's funding transaction to meet the contributed amount.
+		/// Any excess amount will be sent to a change output.
+		inputs: Vec<FundingTxInput>,
+
+		/// An optional change output script. This will be used if needed or, when not set,
+		/// generated using [`SignerProvider::get_destination_script`].
+		change_script: Option<ScriptBuf>,
+	},
+}
+
+#[cfg(splicing)]
+impl SpliceContribution {
+	pub(super) fn value(&self) -> SignedAmount {
+		match self {
+			SpliceContribution::SpliceIn { value, .. } => {
+				value.to_signed().unwrap_or(SignedAmount::MAX)
+			},
+		}
+	}
+
+	pub(super) fn inputs(&self) -> &[FundingTxInput] {
+		match self {
+			SpliceContribution::SpliceIn { inputs, .. } => &inputs[..],
+		}
+	}
+
+	pub(super) fn into_tx_parts(self) -> (Vec<FundingTxInput>, Option<ScriptBuf>) {
+		match self {
+			SpliceContribution::SpliceIn { inputs, change_script, .. } => (inputs, change_script),
+		}
+	}
+}
+
 /// An input to contribute to a channel's funding transaction either when using the v2 channel
 /// establishment protocol or when splicing.
 #[derive(Clone)]
diff --git a/lightning/src/ln/splicing_tests.rs b/lightning/src/ln/splicing_tests.rs
index c1340c8..6061632 100644
--- a/lightning/src/ln/splicing_tests.rs
+++ b/lightning/src/ln/splicing_tests.rs
@@ -8,9 +8,12 @@
 // licenses.
 
 use crate::ln::functional_test_utils::*;
+use crate::ln::funding::SpliceContribution;
 use crate::ln::msgs::{BaseMessageHandler, ChannelMessageHandler, MessageSendEvent};
 use crate::util::errors::APIError;
 
+use bitcoin::Amount;
+
 /// Splicing test, simple splice-in flow. Starts with opening a V1 channel first.
 /// Builds on test_channel_open_simple()
 #[test]
@@ -66,15 +69,20 @@ fn test_v1_splice_in() {
 		&initiator_node,
 		&[extra_splice_funding_input_sats],
 	);
+
+	let contribution = SpliceContribution::SpliceIn {
+		value: Amount::from_sat(splice_in_sats),
+		inputs: funding_inputs,
+		change_script: None,
+	};
+
 	// Initiate splice-in
 	let _res = initiator_node
 		.node
 		.splice_channel(
 			&channel_id,
 			&acceptor_node.node.get_our_node_id(),
-			splice_in_sats as i64,
-			funding_inputs,
-			None, // change_script
+			contribution,
 			funding_feerate_per_kw,
 			None, // locktime
 		)
@@ -295,13 +303,17 @@ fn test_v1_splice_in_negative_insufficient_inputs() {
 	let funding_inputs =
 		create_dual_funding_utxos_with_prev_txs(&nodes[0], &[extra_splice_funding_input_sats]);
 
+	let contribution = SpliceContribution::SpliceIn {
+		value: Amount::from_sat(splice_in_sats),
+		inputs: funding_inputs,
+		change_script: None,
+	};
+
 	// Initiate splice-in, with insufficient input contribution
 	let res = nodes[0].node.splice_channel(
 		&channel_id,
 		&nodes[1].node.get_our_node_id(),
-		splice_in_sats as i64,
-		funding_inputs,
-		None, // change_script
+		contribution,
 		1024, // funding_feerate_per_kw,
 		None, // locktime
 	);