fix(core): document an id that is actually a valid id

The example generator emitted twenty-five payload characters where an id has
twenty-six, so every documented id was twenty-nine characters — one short of
the width the column holds and, since last commit, one short of what the schema
publishing it will accept. Nothing caught it because an example is never
parsed: it is copied into documentation and read by people.

The width now comes from the generator's own constant instead of being typed
out, in the two places that had counted it by hand. Counting twenty-six of
anything by eye is a thing people get right once and never re-check.

A test pins the three together — a generated id, the schema for one, and the
documented example must all agree, for every prefix. It fails on the
off-by-one that prompted this, and on a prefix without its separator, which
would otherwise read as an id of that type because it starts with the same
three letters.

packages/core/src/examples.ts

@@ -1,8 +1,12 @@
 import { Identifier } from './id.js';
 
 export namespace Examples {
+	// The width is taken from the generator rather than typed out. Counting
+	// twenty-six of anything by eye is a thing people get wrong once and then
+	// never look at again — this was one short, which made every documented id
+	// a value the schema that published it would reject.
 	export const Id = (prefix: keyof typeof Identifier.prefixes) =>
-		`${Identifier.prefixes[prefix]}_XXXXXXXXXXXXXXXXXXXXXXXXX`;
+		`${Identifier.prefixes[prefix]}_${'X'.repeat(Identifier.LENGTH)}`;
 
 	export const User = {
 		id: Id('user'),

packages/core/src/id.test.ts

@@ -0,0 +1,51 @@
+import { describe, expect, test } from 'bun:test';
+
+import { Examples } from './examples.js';
+import { Identifier } from './id.js';
+
+const prefixes = Object.keys(Identifier.prefixes) as (keyof typeof Identifier.prefixes)[];
+
+describe('an id, the rule for one, and the documented example agree', () => {
+	// Three things have to say the same thing and only one of them is the
+	// generator. They drifted once already: the example was twenty-nine
+	// characters against a rule demanding thirty, so every id in the published
+	// documentation was a value the schema beside it would reject. Nothing
+	// noticed, because an example is never parsed.
+
+	test('every generated id satisfies its own schema', () => {
+		for (const prefix of prefixes) {
+			const parsed = Identifier.schema(prefix).safeParse(Identifier.ascending(prefix));
+			expect(parsed.success).toBe(true);
+		}
+	});
+
+	test('every documented example satisfies the schema that publishes it', () => {
+		for (const prefix of prefixes) {
+			const parsed = Identifier.schema(prefix).safeParse(Examples.Id(prefix));
+			expect(parsed.success).toBe(true);
+		}
+	});
+
+	test('an id is the width the column holds', () => {
+		// `ulid()` is `char(26 + 4)`, and a char column refuses an overlong
+		// value rather than truncating — so a generator that drifted wider
+		// would fail every insert, not merely look wrong.
+		for (const prefix of prefixes) {
+			expect(Identifier.ascending(prefix)).toHaveLength(30);
+			expect(Examples.Id(prefix)).toHaveLength(30);
+		}
+	});
+
+	test('the schema refuses the near misses, not just the obvious ones', () => {
+		const schema = Identifier.schema('user');
+		const body = 'a'.repeat(Identifier.LENGTH);
+		expect(schema.safeParse(`usr_${body}`).success).toBe(true);
+		// One short, one long, right length with the wrong prefix, and the
+		// prefix without its separator — which would otherwise read as a user
+		// id because it starts with the same three letters.
+		expect(schema.safeParse(`usr_${body.slice(1)}`).success).toBe(false);
+		expect(schema.safeParse(`usr_${body}a`).success).toBe(false);
+		expect(schema.safeParse(`mch_${body}`).success).toBe(false);
+		expect(schema.safeParse(`usr${body}a`).success).toBe(false);
+	});
+});

packages/core/src/id.ts

@@ -47,7 +47,14 @@ export namespace Identifier {
 			.length(prefixes[prefix].length + 1 + LENGTH);
 	}
 
-	const LENGTH = 26;
+	/**
+	 * How many characters follow the prefix and separator.
+	 *
+	 * Exported because three things have to agree on it and two of them are
+	 * not the generator: the column is fixed-width, {@link schema} refuses
+	 * anything else, and the documented examples have to be values that pass.
+	 */
+	export const LENGTH = 26;
 
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