Add getFilterLimit function

filter.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect } from 'bun:test'
-import { matchFilter, matchFilters, mergeFilters } from './filter.ts'
+import { getFilterLimit, matchFilter, matchFilters, mergeFilters } from './filter.ts'
 import { buildEvent } from './test-helpers.ts'
 
 describe('Filter', () => {
@@ -241,4 +241,27 @@ describe('Filter', () => {
       ).toEqual({ kinds: [1, 7, 9, 10], since: 10, until: 30 })
     })
   })
+
+  describe('getFilterLimit', () => {
+    test('should handle ids', () => {
+      expect(getFilterLimit({ ids: ['123'] })).toEqual(1)
+      expect(getFilterLimit({ ids: ['123'], limit: 2 })).toEqual(1)
+      expect(getFilterLimit({ ids: ['123'], limit: 0 })).toEqual(0)
+      expect(getFilterLimit({ ids: ['123'], limit: -1 })).toEqual(0)
+    })
+
+    test('should count the authors times replaceable kinds', () => {
+      expect(getFilterLimit({ kinds: [0], authors: ['alex'] })).toEqual(1)
+      expect(getFilterLimit({ kinds: [0, 3], authors: ['alex'] })).toEqual(2)
+      expect(getFilterLimit({ kinds: [0, 3], authors: ['alex', 'fiatjaf'] })).toEqual(4)
+    })
+
+    test('should return Infinity for authors with regular kinds', () => {
+      expect(getFilterLimit({ kinds: [1], authors: ['alex'] })).toEqual(Infinity)
+    })
+
+    test('should return Infinity for empty filters', () => {
+      expect(getFilterLimit({})).toEqual(Infinity)
+    })
+  })
 })

filter.ts

@@ -1,4 +1,5 @@
 import { Event } from './core.ts'
+import { isReplaceableKind } from './kinds.ts'
 
 export type Filter = {
   ids?: string[]
@@ -70,3 +71,19 @@ export function mergeFilters(...filters: Filter[]): Filter {
 
   return result
 }
+
+/** Calculate the intrinsic limit of a filter. This function may return `Infinity`. */
+export function getFilterLimit(filter: Filter): number {
+  if (filter.ids && !filter.ids.length) return 0
+  if (filter.kinds && !filter.kinds.length) return 0
+  if (filter.authors && !filter.authors.length) return 0
+
+  return Math.min(
+    Math.max(0, filter.limit ?? Infinity),
+    filter.ids?.length ?? Infinity,
+    filter.authors?.length &&
+      filter.kinds?.every((kind) => isReplaceableKind(kind))
+      ? filter.authors.length * filter.kinds.length
+      : Infinity,
+  )
+}