refactor(api): simplify & improve username suggestion logic (#44)

fluxer_api/src/auth/AuthController.ts

@@ -412,7 +412,7 @@ export const AuthController = (app: HonoApp) => {
 		Validator('json', UsernameSuggestionsRequest),
 		async (ctx) => {
 			const {global_name} = ctx.req.valid('json');
-			const suggestions = generateUsernameSuggestions(global_name, 5);
+			const suggestions = generateUsernameSuggestions(global_name);
 			return ctx.json({suggestions});
 		},
 	);

fluxer_api/src/auth/services/AuthRegistrationService.ts

@@ -45,6 +45,7 @@ import * as AgeUtils from '~/utils/AgeUtils';
 import * as IpUtils from '~/utils/IpUtils';
 import {parseAcceptLanguage} from '~/utils/LocaleUtils';
 import {generateRandomUsername} from '~/utils/UsernameGenerator';
+import {deriveUsernameFromDisplayName} from '~/utils/UsernameSuggestionUtils';
 
 const MINIMUM_AGE_BY_COUNTRY: Record<string, number> = {
 	KR: 14,
@@ -202,8 +203,32 @@ export class AuthRegistrationService {
 			if (emailTaken) throw InputValidationError.create('email', 'Email already in use');
 		}
 
-		const username = data.username || generateRandomUsername();
-		const discriminator = await this.allocateDiscriminator(username);
+		let usernameCandidate: string | undefined = data.username ?? undefined;
+		let discriminator: number | null = null;
+
+		if (!usernameCandidate) {
+			const derivedUsername = deriveUsernameFromDisplayName(data.global_name ?? '');
+			if (derivedUsername) {
+				try {
+					discriminator = await this.allocateDiscriminator(derivedUsername);
+					usernameCandidate = derivedUsername;
+				} catch (error) {
+					if (!(error instanceof InputValidationError)) {
+						throw error;
+					}
+				}
+			}
+		}
+
+		if (!usernameCandidate) {
+			usernameCandidate = generateRandomUsername();
+			discriminator = await this.allocateDiscriminator(usernameCandidate);
+		} else if (discriminator === null) {
+			discriminator = await this.allocateDiscriminator(usernameCandidate);
+		}
+
+		const username = usernameCandidate!;
+
 		const userId = this.generateUserId(emailKey);
 
 		const acceptLanguage = request.headers.get('accept-language');

fluxer_api/src/utils/UsernameSuggestionUtils.ts

@@ -17,64 +17,36 @@
  * along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
  */
 
+import {UsernameType} from '~/Schema';
 import {transliterate as tr} from 'transliteration';
-import {generateRandomUsername} from '~/utils/UsernameGenerator';
 
-function sanitizeForFluxerTag(input: string): string {
-	let result = tr(input.trim());
+const MAX_USERNAME_LENGTH = 32;
 
-	result = result.replace(/[\s\-.]+/g, '_');
+function sanitizeDisplayName(globalName: string): string | null {
+	const trimmed = globalName.trim();
+	if (!trimmed) return null;
 
-	result = result.replace(/[^a-zA-Z0-9_]/g, '');
-
-	if (!result) {
-		result = 'user';
+	let sanitized = tr(trimmed);
+	sanitized = sanitized.replace(/[\s\-.]+/g, '_');
+	sanitized = sanitized.replace(/[^a-zA-Z0-9_]/g, '');
+	if (!sanitized) return null;
+	if (sanitized.length > MAX_USERNAME_LENGTH) {
+		sanitized = sanitized.substring(0, MAX_USERNAME_LENGTH);
 	}
 
-	if (result.length > 32) {
-		result = result.substring(0, 32);
+	const validation = UsernameType.safeParse(sanitized);
+	if (!validation.success) {
+		return null;
 	}
 
-	return result.toLowerCase();
+	return sanitized;
 }
 
-export function generateUsernameSuggestions(globalName: string, count: number = 5): Array<string> {
-	const suggestions: Array<string> = [];
-
-	const transliterated = tr(globalName.trim());
-	const hasMeaningfulContent = /[a-zA-Z]/.test(transliterated);
-
-	if (!hasMeaningfulContent) {
-		for (let i = 0; i < count; i++) {
-			const randomUsername = generateRandomUsername();
-			const sanitizedRandom = sanitizeForFluxerTag(randomUsername);
-			if (sanitizedRandom && sanitizedRandom.length <= 32) {
-				suggestions.push(sanitizedRandom.toLowerCase());
-			}
-		}
-		return Array.from(new Set(suggestions)).slice(0, count);
-	}
-
-	const baseUsername = sanitizeForFluxerTag(globalName);
-	suggestions.push(baseUsername);
-
-	const suffixes = ['_', '__', '___', '123', '_1', '_official', '_real'];
-	for (const suffix of suffixes) {
-		if (suggestions.length >= count) break;
-		const suggestion = baseUsername + suffix;
-		if (suggestion.length <= 32) {
-			suggestions.push(suggestion);
-		}
-	}
-
-	let counter = 2;
-	while (suggestions.length < count) {
-		const suggestion = `${baseUsername}${counter}`;
-		if (suggestion.length <= 32) {
-			suggestions.push(suggestion);
-		}
-		counter++;
-	}
-
-	return Array.from(new Set(suggestions)).slice(0, count);
+export function deriveUsernameFromDisplayName(globalName: string): string | null {
+	return sanitizeDisplayName(globalName);
+}
+
+export function generateUsernameSuggestions(globalName: string): Array<string> {
+	const candidate = deriveUsernameFromDisplayName(globalName);
+	return candidate ? [candidate] : [];
 }

tests/integration/auth_register_display_name_autogen_test.go

@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2026 Fluxer Contributors
+ *
+ * This file is part of Fluxer.
+ *
+ * Fluxer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Fluxer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package integration
+
+import (
+	"fmt"
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestAuthRegisterDerivesUsernameFromDisplayName(t *testing.T) {
+	client := newTestClient(t)
+
+	email := fmt.Sprintf("integration-derived-username-%d@example.com", time.Now().UnixNano())
+	password := uniquePassword()
+
+	payload := map[string]any{
+		"email":         email,
+		"password":      password,
+		"global_name":   "Magic Tester",
+		"date_of_birth": adultDateOfBirth(),
+		"consent":       true,
+	}
+
+	resp, err := client.postJSON("/auth/register", payload)
+	if err != nil {
+		t.Fatalf("failed to call register endpoint: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("register returned %d: %s", resp.StatusCode, readResponseBody(resp))
+	}
+
+	var parsed registerResponse
+	decodeJSONResponse(t, resp, &parsed)
+	if parsed.Token == "" {
+		t.Fatalf("expected register response to include a token")
+	}
+
+	meResp, err := client.getWithAuth("/users/@me", parsed.Token)
+	if err != nil {
+		t.Fatalf("failed to fetch current user: %v", err)
+	}
+	if meResp.StatusCode != http.StatusOK {
+		t.Fatalf("/users/@me returned %d: %s", meResp.StatusCode, readResponseBody(meResp))
+	}
+
+	var userResp userPrivateResponse
+	decodeJSONResponse(t, meResp, &userResp)
+	if userResp.Username != "Magic_Tester" {
+		t.Fatalf("expected derived username to be %q, got %q", "Magic_Tester", userResp.Username)
+	}
+}