From 20f06d960840739f8054048f31dfee3e3e1a1338 Mon Sep 17 00:00:00 2001
From: fiatjaf <fiatjaf@gmail.com>
Date: Thu, 30 Oct 2025 11:42:04 -0300
Subject: [PATCH] eventstore: SortedMerge() helper for combining results from
 different eventstores.

---
 eventstore/combine.go      | 61 ++++++++++++++++++++++++++++++++++++++
 eventstore/combine_test.go | 52 ++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+)
 create mode 100644 eventstore/combine.go
 create mode 100644 eventstore/combine_test.go

diff --git a/eventstore/combine.go b/eventstore/combine.go
new file mode 100644
index 0000000..f10dec9
--- /dev/null
+++ b/eventstore/combine.go
@@ -0,0 +1,61 @@
+package eventstore
+
+import (
+	"iter"
+
+	"fiatjaf.com/nostr"
+)
+
+func SortedMerge(it1, it2 iter.Seq[nostr.Event]) iter.Seq[nostr.Event] {
+	next1, done1 := iter.Pull(it1)
+	next2, done2 := iter.Pull(it2)
+
+	return func(yield func(nostr.Event) bool) {
+		defer done1()
+		defer done2()
+
+		evt1, ok1 := next1()
+		evt2, ok2 := next2()
+
+	both:
+		if ok1 && ok2 {
+			if evt2.CreatedAt > evt1.CreatedAt {
+				if !yield(evt2) {
+					return
+				}
+				evt2, ok2 = next2()
+				goto both
+			} else {
+				if !yield(evt1) {
+					return
+				}
+				evt1, ok1 = next1()
+				goto both
+			}
+		}
+
+		if !ok2 {
+		only1:
+			if ok1 {
+				if !yield(evt1) {
+					return
+				}
+				evt1, ok1 = next1()
+				goto only1
+			}
+		}
+
+		if !ok1 {
+		only2:
+			if ok2 {
+				if !yield(evt2) {
+					return
+				}
+				evt2, ok2 = next2()
+				goto only2
+			}
+		}
+
+		return
+	}
+}
diff --git a/eventstore/combine_test.go b/eventstore/combine_test.go
new file mode 100644
index 0000000..78f7584
--- /dev/null
+++ b/eventstore/combine_test.go
@@ -0,0 +1,52 @@
+package eventstore
+
+import (
+	"cmp"
+	"slices"
+	"testing"
+
+	"fiatjaf.com/nostr"
+)
+
+func FuzzSortedMerge(f *testing.F) {
+	f.Add(uint(4), uint(4), uint(3), uint(7), uint8(2), uint8(1))
+	f.Add(uint(0), uint(4), uint(3), uint(7), uint8(2), uint8(1))
+	f.Fuzz(func(t *testing.T, len1, len2 uint, start1, start2 uint, diff1, diff2 uint8) {
+		maxxx := max(len1*uint(diff1), len2*uint(diff2))
+		start1 += maxxx
+		start2 += maxxx
+
+		merged := SortedMerge(
+			func(yield func(nostr.Event) bool) {
+				for range len1 {
+					if !yield(nostr.Event{CreatedAt: nostr.Timestamp(start1)}) {
+						return
+					}
+					start1 -= uint(diff1)
+				}
+			},
+			func(yield func(nostr.Event) bool) {
+				for range len2 {
+					if !yield(nostr.Event{CreatedAt: nostr.Timestamp(start2)}) {
+						return
+					}
+					start2 -= uint(diff2)
+				}
+			},
+		)
+		result := slices.Collect(merged)
+
+		// assert length
+		if len(result) != int(len1+len2) {
+			t.Fatalf("expected %d events, got %d", len1+len2, len(result))
+		}
+
+		// assert sorted descending
+		slices.IsSortedFunc(result, func(a, b nostr.Event) int { return -1 * cmp.Compare(a.CreatedAt, b.CreatedAt) })
+		for i := 1; i < len(result); i++ {
+			if result[i].CreatedAt > result[i-1].CreatedAt {
+				t.Fatalf("events not sorted descending at index %d: %d > %d", i, result[i].CreatedAt, result[i-1].CreatedAt)
+			}
+		}
+	})
+}