looprpc: add simple pagination to the ListSwaps command

This commit adds the ability to set a max_swaps and an index_offset
flag to the ListSwaps command. These new fields are applied AFTER
the initial filtering step. The response also now passes additional
information about the index count and total swaps filtered.

loopd/swapclient_server.go

@@ -2,11 +2,13 @@ package loopd
 
 import (
 	"bytes"
+	"cmp"
 	"context"
 	"encoding/hex"
 	"errors"
 	"fmt"
 	"reflect"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -563,8 +565,11 @@ func (s *swapClientServer) ListSwaps(ctx context.Context,
 	req *looprpc.ListSwapsRequest) (*looprpc.ListSwapsResponse, error) {
 
 	var (
-		rpcSwaps = []*looprpc.SwapStatus{}
-		idx      = 0
+		rpcSwaps      = []*looprpc.SwapStatus{}
+		swapInfos     = []*loop.SwapInfo{}
+		maxSwaps      = int(req.MaxSwaps)
+		nextStartTime = int64(0)
+		canPage       = false
 	)
 
 	s.swapsLock.Lock()
@@ -580,14 +585,43 @@ func (s *swapClientServer) ListSwaps(ctx context.Context,
 			continue
 		}
 
-		rpcSwap, err := s.marshallSwap(ctx, &swp)
+		swapInfos = append(swapInfos, &swp)
+	}
+
+	// Sort the swaps by initiation time in ascending order (oldest first).
+	slices.SortFunc(swapInfos, func(a, b *loop.SwapInfo) int {
+		return cmp.Compare(
+			a.InitiationTime.UnixNano(),
+			b.InitiationTime.UnixNano(),
+		)
+	})
+
+	// Apply the maxSwaps limit if specified.
+	if maxSwaps > 0 && len(swapInfos) > maxSwaps {
+		canPage = true
+		swapInfos = swapInfos[:maxSwaps]
+	}
+
+	// Marshal the filtered and limited swaps.
+	for _, swp := range swapInfos {
+		rpcSwap, err := s.marshallSwap(ctx, swp)
 		if err != nil {
 			return nil, err
 		}
 		rpcSwaps = append(rpcSwaps, rpcSwap)
-		idx++
 	}
-	return &looprpc.ListSwapsResponse{Swaps: rpcSwaps}, nil
+
+	// Set the next start time for pagination if needed.
+	if canPage && len(rpcSwaps) > 0 {
+		// Use the initiation time of the last swap plus 1 nanosecond.
+		nextStartTime = rpcSwaps[len(rpcSwaps)-1].InitiationTime + 1
+	}
+
+	response := looprpc.ListSwapsResponse{
+		Swaps:         rpcSwaps,
+		NextStartTime: nextStartTime,
+	}
+	return &response, nil
 }
 
 // filterSwap filters the given swap based on the provided filter.
@@ -617,6 +651,13 @@ func filterSwap(swapInfo *loop.SwapInfo, filter *looprpc.ListSwapsFilter) bool {
 		return false
 	}
 
+	// If timestamp filters are set, only return swaps within the specified time range.
+	if filter.StartTimestampNs > 0 &&
+		swapInfo.InitiationTime.UnixNano() < filter.StartTimestampNs {
+
+		return false
+	}
+
 	// If the swap is of type loop out and the outgoing channel filter is
 	// set, we only return swaps that match the filter.
 	if swapInfo.SwapType == swap.TypeOut && filter.OutgoingChanSet != nil {