perf: create faker schema just once (#30)

This change creates the faker schema just once and reuses it, rather
than unnecessarily creating it for each generated document.

On my machine, this speeds up `node --test` by about 350 milliseconds,
or about 8%. I suspect the gains will be a little bigger in
`@comapeo/core` where we call `generate` a lot more.

index.js

@@ -1,6 +1,7 @@
 import { docSchemas } from '@comapeo/schema'
 import { JSONSchemaFaker, createFakerSchema } from './lib/faker.js'
 import { extractSchemaVersion, isValidSchemaName } from './lib/schema.js'
+/** @typedef {import('@comapeo/schema/dist/types.js').SchemaName} SchemaName */
 
 export function listSchemas() {
   /** @type {{[name: string]: Array<string>}} */
@@ -13,16 +14,32 @@ export function listSchemas() {
   return result
 }
 
+/** @type {Map<SchemaName, ReturnType<typeof createFakerSchema>>} schemaCache */
+const fakerSchemaCache = new Map()
+
 /**
- * @template {import('@comapeo/schema/dist/types.js').SchemaName} TSchemaName
+ * @param {SchemaName} schemaName
+ */
+function getFakerSchema(schemaName) {
+  const cached = fakerSchemaCache.get(schemaName)
+  if (cached) return cached
+  const targetSchema = docSchemas[schemaName]
+  const result = createFakerSchema(targetSchema)
+  fakerSchemaCache.set(schemaName, result)
+  return result
+}
+
+/**
+ * @template {SchemaName} TSchemaName
  * @param {TSchemaName} schemaName
  * @param {{version?: string, count?: number}} [options]
  * @returns {Array<Extract<import('@comapeo/schema').MapeoDoc, { schemaName: TSchemaName }>>}
  */
 export function generate(schemaName, { count } = {}) {
   isValidSchemaName(schemaName)
 
-  const targetSchema = docSchemas[schemaName]
+  const schema = getFakerSchema(schemaName)
+
   const numberToGenerate = count || 1
 
   /** @type {Array<Extract<import('@comapeo/schema').MapeoDoc, { schemaName: TSchemaName }>>} */
@@ -31,7 +48,7 @@ export function generate(schemaName, { count } = {}) {
   for (let i = 0; i < numberToGenerate; i++) {
     result.push(
       /** @type {Extract<import('@comapeo/schema').MapeoDoc, { schemaName: TSchemaName }>} */ (
-        JSONSchemaFaker.generate(createFakerSchema(targetSchema))
+        JSONSchemaFaker.generate(schema)
       ),
     )
   }