filter theoretical limits.

filter.go

@@ -170,3 +170,47 @@ func (ef Filter) Clone() Filter {
 
 	return clone
 }
+
+// GetTheoreticalLimit gets the maximum number of events that a normal filter would ever return, for example, if
+// there is a number of "ids" in the filter, the theoretical limit will be that number of ids.
+//
+// It returns -1 if there are no theoretical limits.
+//
+// The given .Limit present in the filter is ignored.
+func GetTheoreticalLimit(filter Filter) int {
+	if len(filter.IDs) > 0 {
+		return len(filter.IDs)
+	}
+
+	if len(filter.Kinds) == 0 {
+		return -1
+	}
+
+	if len(filter.Authors) > 0 {
+		allAreReplaceable := true
+		for _, kind := range filter.Kinds {
+			if !IsReplaceableKind(kind) {
+				allAreReplaceable = false
+				break
+			}
+		}
+		if allAreReplaceable {
+			return len(filter.Authors) * len(filter.Kinds)
+		}
+
+		if len(filter.Tags["d"]) > 0 {
+			allAreAddressable := true
+			for _, kind := range filter.Kinds {
+				if !IsAddressableKind(kind) {
+					allAreAddressable = false
+					break
+				}
+			}
+			if allAreAddressable {
+				return len(filter.Authors) * len(filter.Kinds) * len(filter.Tags["d"])
+			}
+		}
+	}
+
+	return -1
+}

filter_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestFilterUnmarshal(t *testing.T) {
@@ -141,3 +142,13 @@ func TestFilterClone(t *testing.T) {
 	*clone4.Since++
 	assert.False(t, FilterEqual(flt, clone4), "modifying the clone since should cause it to not be equal anymore")
 }
+
+func TestTheoreticalLimit(t *testing.T) {
+	require.Equal(t, 6, GetTheoreticalLimit(Filter{IDs: []string{"a", "b", "c", "d", "e", "f"}}))
+	require.Equal(t, 9, GetTheoreticalLimit(Filter{Authors: []string{"a", "b", "c"}, Kinds: []int{3, 0, 10002}}))
+	require.Equal(t, 4, GetTheoreticalLimit(Filter{Authors: []string{"a", "b", "c", "d"}, Kinds: []int{10050}}))
+	require.Equal(t, -1, GetTheoreticalLimit(Filter{Authors: []string{"a", "b", "c", "d"}}))
+	require.Equal(t, -1, GetTheoreticalLimit(Filter{Kinds: []int{3, 0, 10002}}))
+	require.Equal(t, 24, GetTheoreticalLimit(Filter{Authors: []string{"a", "b", "c", "d", "e", "f"}, Kinds: []int{30023, 30024}, Tags: TagMap{"d": []string{"aaa", "bbb"}}}))
+	require.Equal(t, -1, GetTheoreticalLimit(Filter{Authors: []string{"a", "b", "c", "d", "e", "f"}, Kinds: []int{30023, 30024}}))
+}