Included model and controller specs

Unit test for username suggestion
Integration test for api response of username

config/routes.rb

@@ -109,6 +109,7 @@ Rails.application.routes.draw do
     Gitlab.ee do
       draw :smartcard
       draw :jira_connect
+      draw :username
     end
 
     Gitlab.ee do

ee/app/controllers/usernames_controller.rb

+# frozen_string_literal: true
+
+class UsernamesController < ApplicationController
+  skip_before_action :authenticate_user!, only: [:suggest]
+
+  def suggest
+    if validate_params
+      username = ::User.username_suggestion(params[:name])
+
+      render json: { username: username }, status: :ok
+    else
+      render json: { message: 'Invalid input provided' }, status: :bad_request
+    end
+  end
+
+  private
+
+  def validate_params
+    !params[:name].blank?
+  end
+end

ee/app/models/ee/user.rb

@@ -13,6 +13,7 @@ module EE
 
     DEFAULT_ROADMAP_LAYOUT = 'months'.freeze
     DEFAULT_GROUP_VIEW = 'details'.freeze
+    MAX_USERNAME_SUGGESTION_ATTEMPTS = 15
 
     prepended do
       EMAIL_OPT_IN_SOURCE_ID_GITLAB_COM = 1
@@ -126,6 +127,19 @@ module EE
         joins(:smartcard_identities)
           .find_by(smartcard_identities: { subject: certificate_subject, issuer: certificate_issuer })
       end
+
+      def username_suggestion(base_name)
+        suffix = nil
+        base_name = base_name.parameterize(separator: '_')
+        MAX_USERNAME_SUGGESTION_ATTEMPTS.times do |attempt|
+          username = "#{base_name}#{suffix}"
+          return username unless ::Namespace.find_by_path_or_name(username)
+
+          suffix = attempt + 1
+        end
+
+        ''
+      end
     end
 
     def cannot_be_admin_and_auditor

ee/changelogs/unreleased/username_suggests.yml

+---
+title: Adds an api to generate suggestions for username
+merge_request: 15048
+author:
+type: added

ee/config/routes/username.rb

+# frozen_string_literal: true
+
+scope :username do
+  get 'suggestion', to: 'usernames#suggest'
+end

ee/spec/controllers/usernames_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe UsernamesController do
+  describe 'GET #suggest' do
+    context 'namespace does not exist' do
+      it 'returns JSON with the suggested username' do
+        get :suggest, params: { name: 'Arthur' }
+
+        expected_json = { username: 'arthur' }.to_json
+        expect(response.body).to eq(expected_json)
+      end
+    end
+
+    context 'namespace exists' do
+      before do
+        create(:user, name: 'disney')
+      end
+
+      it 'returns JSON with the parameterized username and suffix as a suggestion' do
+        get :suggest, params: { name: 'Disney' }
+
+        expected_json = { username: 'disney1' }.to_json
+        expect(response.body).to eq(expected_json)
+      end
+    end
+
+    context 'no name provided' do
+      it 'returns bad request response' do
+        get :suggest
+
+        expect(response.status).to eq(400)
+      end
+    end
+  end
+end

ee/spec/models/user_spec.rb

@@ -597,4 +597,59 @@ describe User do
       end
     end
   end
+
+  describe '.username_suggestion' do
+    context 'namespace with input name does not exist' do
+      let(:name) { 'Arthur Morgan' }
+
+      it 'returns the parameterized name' do
+        username = described_class.username_suggestion(name)
+
+        expect(username).to eq('arthur_morgan')
+      end
+    end
+
+    context 'namespace with input name exists' do
+      let(:name) { 'Disney' }
+      before do
+        create(:user, name: 'disney')
+      end
+
+      it 'returns the parameterized name with a suffix' do
+        username = described_class.username_suggestion(name)
+
+        expect(username).to eq('disney1')
+      end
+    end
+
+    context 'namespace with input name and suffix exists' do
+      let(:name) { 'Disney' }
+      before do
+        create(:user, name: 'disney')
+        create(:user, name: 'disney1')
+      end
+
+      it 'loops through parameterized name with suffixes, until it finds one that does not exist' do
+        username = described_class.username_suggestion(name)
+
+        expect(username).to eq('disney2')
+      end
+    end
+
+    context 'when max attempts for suggestion is exceeded' do
+      let(:name) { 'Disney' }
+      let(:max_attempts) { described_class::MAX_USERNAME_SUGGESTION_ATTEMPTS }
+
+      before do
+        allow(::Namespace).to receive(:find_by_path_or_name).with("disney").and_return(true)
+        max_attempts.times { |count| allow(::Namespace).to receive(:find_by_path_or_name).with("disney#{count}").and_return(true) }
+      end
+
+      it 'returns an empty response' do
+        username = described_class.username_suggestion(name)
+
+        expect(username).to eq('')
+      end
+    end
+  end
 end