Core/Channel: function to calc outpoints of commitTx

Currently, we need to use a couple of internal functions to
calculate commitment outpoints amounts so we want to have this
to not only make a public API for this but also simplify it and
reduce the amount of duplicated code

We use this to calculate the exact amount of toRemote and toLocal
output to then be able to calculate the watch tower reward based
on that.

Upstream PR: joemphilips#162

src/DotNetLightning.Core/Channel/Commitments.fs

@@ -10,6 +10,7 @@ open DotNetLightning.Serialization.Msgs
 
 open ResultUtils
 open ResultUtils.Portability
+open DotNetLightning.Transactions
 
 type LocalChanges = {
     Proposed: IUpdateMsg list
@@ -115,6 +116,12 @@ type RemoteNextCommitInfo =
                 | Waiting _ -> remoteNextCommitInfo
                 | Revoked _ -> Revoked commitmentPubKey)
 
+type Amounts = 
+    {
+        ToLocal: Money
+        ToRemote: Money
+    }
+
 type Commitments = {
     LocalParams: LocalParams
     RemoteParams: RemoteParams
@@ -222,3 +229,46 @@ type Commitments = {
             let untrimmedMax = LNMoney.Min(untrimmedSpendableBalance, dustLimit)
             let spendableBalance = LNMoney.Max(untrimmedMax, untrimmedSpendableBalance)
             spendableBalance
+
+        static member RemoteCommitAmount (remoteParams: RemoteParams)
+                                         (localParams: LocalParams)
+                                         (remoteCommit: RemoteCommit)
+                                             : Amounts =
+            let commitFee = Transactions.commitTxFee
+                                remoteParams.DustLimitSatoshis
+                                remoteCommit.Spec
+
+            let (toLocalAmount, toRemoteAmount) =
+                if (localParams.IsFunder) then
+                    (remoteCommit.Spec.ToLocal.Satoshi
+                     |> Money.Satoshis),
+                    (remoteCommit.Spec.ToRemote.Satoshi
+                     |> Money.Satoshis) - commitFee
+                else
+                    (remoteCommit.Spec.ToLocal.Satoshi
+                     |> Money.Satoshis) - commitFee,
+                    (remoteCommit.Spec.ToRemote.Satoshi
+                     |> Money.Satoshis)
+
+            {Amounts.ToLocal = toLocalAmount; ToRemote = toRemoteAmount}
+
+        static member LocalCommitAmount (localParams: LocalParams)
+                                        (localCommit: LocalCommit)
+                                            : Amounts =
+            let commitFee = Transactions.commitTxFee
+                                localParams.DustLimitSatoshis
+                                localCommit.Spec
+
+            let (toLocalAmount, toRemoteAmount) =
+                if (localParams.IsFunder) then
+                    (localCommit.Spec.ToLocal.Satoshi
+                     |> Money.Satoshis) - commitFee,
+                    (localCommit.Spec.ToRemote.Satoshi
+                     |> Money.Satoshis)
+                else
+                    (localCommit.Spec.ToLocal.Satoshi
+                     |> Money.Satoshis),
+                    (localCommit.Spec.ToRemote.Satoshi
+                     |> Money.Satoshis) - commitFee
+
+            {Amounts.ToLocal = toLocalAmount; ToRemote = toRemoteAmount}

src/DotNetLightning.Core/Channel/CommitmentsModule.fs

@@ -505,34 +505,19 @@ module ForceCloseFundsRecovery =
 
         let toLocalWitScriptPubKey = toLocalScriptPubKey.WitHash.ScriptPubKey
 
-        let commitFee =
-            commitTxFee 
-                remoteParams.DustLimitSatoshis 
-                remoteCommit.Spec
-
-        let (toLocalAmount, toRemoteAmount) =
-            if (localParams.IsFunder) then
-                (remoteCommit.Spec.ToLocal.Satoshi
-                 |> Money.Satoshis),
-                (remoteCommit.Spec.ToRemote.Satoshi
-                 |> Money.Satoshis) - commitFee
-            else
-                (remoteCommit.Spec.ToLocal.Satoshi
-                 |> Money.Satoshis) - commitFee,
-                (remoteCommit.Spec.ToRemote.Satoshi
-                 |> Money.Satoshis)
+        let amounts = Commitments.RemoteCommitAmount remoteParams localParams remoteCommit
 
         let toLocalTxOut = 
-            TxOut(toLocalAmount, toLocalWitScriptPubKey)
+            TxOut(amounts.ToLocal, toLocalWitScriptPubKey)
         let toRemoteTxOut = 
-            TxOut(toRemoteAmount, toRemoteScriptPubKey)
+            TxOut(amounts.ToRemote, toRemoteScriptPubKey)
 
         let outputs = 
             seq {
-                if toLocalAmount > remoteParams.DustLimitSatoshis then
+                if amounts.ToLocal > remoteParams.DustLimitSatoshis then
                     yield toLocalTxOut
 
-                if toRemoteAmount > remoteParams.DustLimitSatoshis then
+                if amounts.ToRemote > remoteParams.DustLimitSatoshis then
                     yield toRemoteTxOut
             }
             |> Seq.sortWith TxOut.LexicographicCompare