Add Channel.getSpendableBalance

Add a method to query a channel for the largest payment size that it
will allow us to make.

Author: canndrew

src/DotNetLightning.Core/Channel/Channel.fs

@@ -119,6 +119,43 @@ module Channel =
             }
         [ WeSentChannelReestablish ourChannelReestablish ] |> Ok
 
+    let getSpendableBalance (cs: Channel): Option<Money> =
+        match cs.State.Commitments with
+        | None -> None
+        | Some commitments ->
+            let remoteCommit =
+                match commitments.RemoteNextCommitInfo with
+                | RemoteNextCommitInfo.Waiting info -> info.NextRemoteCommit
+                | RemoteNextCommitInfo.Revoked _info -> commitments.RemoteCommit
+            let reducedRes =
+                remoteCommit.Spec.Reduce(
+                    commitments.RemoteChanges.ACKed,
+                    commitments.LocalChanges.Proposed
+                )
+            let reduced =
+                match reducedRes with
+                | Error err ->
+                    failwithf
+                        "reducing commit failed even though we have not proposed any changes\
+                        error: %A"
+                        err
+                | Ok reduced -> reduced
+            let dustLimit = commitments.RemoteParams.DustLimitSatoshis
+            let fees =
+                if commitments.LocalParams.IsFunder then
+                    Transactions.commitTxFee dustLimit reduced
+                else
+                    Money.Zero
+            let channelReserve = commitments.RemoteParams.ChannelReserveSatoshis
+            let totalBalance = reduced.ToRemote.ToMoney()
+            let untrimmedSpendableBalance = totalBalance - channelReserve - fees
+            let htlcSuccessFee = reduced.FeeRatePerKw.ToFee(Transactions.Constants.HTLC_SUCCESS_WEIGHT)
+            let htlcFee = reduced.FeeRatePerKw.ToFee(Transactions.Constants.COMMITMENT_TX_WEIGHT_PER_HTLC)
+            let untrimmedMax = Money.Min(untrimmedSpendableBalance, htlcSuccessFee + dustLimit)
+            let trimmedSpendableBalance = untrimmedSpendableBalance - htlcFee
+            let spendableBalance = Money.Max(untrimmedMax, trimmedSpendableBalance)
+            Some spendableBalance
+
     let executeCommand (cs: Channel) (command: ChannelCommand): Result<ChannelEvent list, ChannelError> =
         match cs.State, command with
 

src/DotNetLightning.Core/Channel/ChannelTypes.fs

@@ -397,3 +397,24 @@ type ChannelState =
             | ErrFundingLost _
             | ErrFundingTimeOut _
             | ErrInformationLeak _ -> Abnormal
+
+        member this.Commitments: Option<Commitments> =
+            match this with
+            | WaitForInitInternal
+            | WaitForOpenChannel _
+            | WaitForAcceptChannel _
+            | WaitForFundingCreated _
+            | WaitForFundingSigned _ -> None
+            | WaitForFundingConfirmed data -> Some (data :> IHasCommitments).Commitments
+            | WaitForFundingLocked data -> Some (data :> IHasCommitments).Commitments
+            | Normal data -> Some (data :> IHasCommitments).Commitments
+            | Shutdown data -> Some (data :> IHasCommitments).Commitments
+            | Negotiating data -> Some (data :> IHasCommitments).Commitments
+            | Closing data -> Some (data :> IHasCommitments).Commitments
+            | Closed _
+            | Offline _
+            | Syncing _
+            | ErrFundingLost _
+            | ErrFundingTimeOut _
+            | ErrInformationLeak _ -> None
+