1 files changed, 66 insertions, 0 deletions
diff --git a/BackEnd/Timeline/Services/BasicUserService.cs b/BackEnd/Timeline/Services/BasicUserService.cs
new file mode 100644
index 00000000..fbbb6677
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+++ b/BackEnd/Timeline/Services/BasicUserService.cs
@@ -0,0 +1,66 @@
+using Microsoft.EntityFrameworkCore;
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Timeline.Entities;
+using Timeline.Models.Validation;
+using Timeline.Services.Exceptions;
+
+namespace Timeline.Services
+{
+    /// <summary>
+    /// This service provide some basic user features, which should be used internally for other services.
+    /// </summary>
+    public interface IBasicUserService
+    {
+        /// <summary>
+        /// Check if a user exists.
+        /// </summary>
+        /// <param name="id">The id of the user.</param>
+        /// <returns>True if exists. Otherwise false.</returns>
+        Task<bool> CheckUserExistence(long id);
+
+        /// <summary>
+        /// Get the user id of given username.
+        /// </summary>
+        /// <param name="username">Username of the user.</param>
+        /// <returns>The id of the user.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="username"/> is null.</exception>
+        /// <exception cref="ArgumentException">Thrown when <paramref name="username"/> is of bad format.</exception>
+        /// <exception cref="UserNotExistException">Thrown when the user with given username does not exist.</exception>
+        Task<long> GetUserIdByUsername(string username);
+    }
+
+    public class BasicUserService : IBasicUserService
+    {
+        private readonly DatabaseContext _database;
+
+        private readonly UsernameValidator _usernameValidator = new UsernameValidator();
+
+        public BasicUserService(DatabaseContext database)
+        {
+            _database = database;
+        }
+
+        public async Task<bool> CheckUserExistence(long id)
+        {
+            return await _database.Users.AnyAsync(u => u.Id == id);
+        }
+
+        public async Task<long> GetUserIdByUsername(string username)
+        {
+            if (username == null)
+                throw new ArgumentNullException(nameof(username));
+
+            if (!_usernameValidator.Validate(username, out var message))
+                throw new ArgumentException(message);
+
+            var entity = await _database.Users.Where(user => user.Username == username).Select(u => new { u.Id }).SingleOrDefaultAsync();
+
+            if (entity == null)
+                throw new UserNotExistException(username);
+
+            return entity.Id;
+        }
+    }
+}