implement `ModerationService`

add type ``NotBanned`` to ``UserBanType`` enum

src/Schema/UserBanType.cs

@@ -2,6 +2,7 @@ namespace sodoff.Schema;
 
 public enum UserBanType
 {
+    NotBanned = 0,
     IndefiniteOpenChatBan = 1,
     TemporaryOpenChatBan = 2,
     IndefiniteAccountBan = 3,

src/Services/ModerationService.cs

@@ -0,0 +1,84 @@
+using System;
+using sodoff.Model;
+using sodoff.Schema;
+
+namespace sodoff.Services;
+
+public class ModerationService
+{
+    public readonly DBContext ctx;
+
+    public ModerationService(DBContext ctx)
+    {
+        this.ctx = ctx;
+    }
+
+    public UserBan AddBanToViking(Viking viking, UserBanType userBanType, DateTime expirationDate = new DateTime())
+    {
+        // get UTC time stamp of function execution
+        DateTime timestamp = DateTime.UtcNow;
+
+        // construct user ban
+        UserBan userBan = new UserBan
+        {
+            UserBanType = userBanType,
+            ExpiresOn = expirationDate,
+            CreatedAt = timestamp
+        };
+
+        // add to viking userban list
+        viking.UserBans.Add(userBan);
+        ctx.SaveChanges();
+
+        // return ban
+        return userBan;
+    }
+    
+    public bool RemoveBanById(int id)
+    {
+        // find ban
+        UserBan? ban = ctx.Bans.FirstOrDefault(e => e.Id == id);
+
+        // remove it
+        if (ban != null) { ctx.Bans.Remove(ban); ctx.SaveChanges(); return true; }
+        else return false;
+    }
+
+    public bool RemoveBansFromVikingByType(Viking viking, UserBanType userBanType)
+    {
+        // get all bans of type
+        List<UserBan> userBans = viking.UserBans.Where(e => e.UserBanType == userBanType).ToList();
+
+        if (userBans.Count == 0) return false;
+
+        // delete all
+        foreach(var ban in userBans)
+            viking.UserBans.Remove(ban);
+
+        ctx.SaveChanges();
+
+        return true;
+    }
+
+    public UserBanType IsVikingBanned(Viking viking)
+    {
+        // get UTC time stamp of function execution
+        DateTime timestamp = DateTime.UtcNow;
+
+        // sort viking ban list by latest first
+        List<UserBan> bans = viking.UserBans.OrderByDescending(e => e.CreatedAt).ToList();
+
+        if (bans.Count == 0) return UserBanType.NotBanned; // no bans in list means viking is not banned
+
+        if (bans.First().UserBanType == UserBanType.IndefiniteAccountBan) return UserBanType.IndefiniteAccountBan;
+        else if (bans.First().UserBanType == UserBanType.IndefiniteOpenChatBan) return UserBanType.IndefiniteOpenChatBan;
+
+        if (DateTime.Compare(bans.First().ExpiresOn ?? new DateTime(9999, 99, 99), timestamp) > 0) return bans.First().UserBanType;
+        else
+        {
+            // ban should be removed
+            RemoveBanById(bans.First().Id);
+            return UserBanType.NotBanned;
+        }
+    }
+}