1 files changed, 122 insertions, 0 deletions
diff --git a/BackEnd/Timeline/Services/DataManager.cs b/BackEnd/Timeline/Services/DataManager.cs
new file mode 100644
index 00000000..d447b0d5
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+++ b/BackEnd/Timeline/Services/DataManager.cs
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+using Microsoft.EntityFrameworkCore;
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Timeline.Entities;
+
+namespace Timeline.Services
+{
+    /// <summary>
+    /// A data manager controlling data.
+    /// </summary>
+    /// <remarks>
+    /// Identical data will be saved as one copy and return the same tag.
+    /// Every data has a ref count. When data is retained, ref count increase.
+    /// When data is freed, ref count decease. If ref count is decreased
+    /// to 0, the data entry will be destroyed and no longer occupy space.
+    /// </remarks>
+    public interface IDataManager
+    {
+        /// <summary>
+        /// Saves the data to a new entry if it does not exist, 
+        /// increases its ref count and returns a tag to the entry.
+        /// </summary>
+        /// <param name="data">The data. Can't be null.</param>
+        /// <returns>The tag of the created entry.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="data"/> is null.</exception>
+        public Task<string> RetainEntry(byte[] data);
+
+        /// <summary>
+        /// Decrease the the ref count of the entry.
+        /// Remove it if ref count is zero.
+        /// </summary>
+        /// <param name="tag">The tag of the entry.</param>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="tag"/> is null.</exception>
+        /// <remarks>
+        /// It's no-op if entry with tag does not exist.
+        /// </remarks>
+        public Task FreeEntry(string tag);
+
+        /// <summary>
+        /// Retrieve the entry with given tag.
+        /// </summary>
+        /// <param name="tag">The tag of the entry.</param>
+        /// <returns>The data of the entry.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="tag"/> is null.</exception>
+        /// <exception cref="InvalidOperationException">Thrown when entry with given tag does not exist.</exception>
+        public Task<byte[]> GetEntry(string tag);
+    }
+
+    public class DataManager : IDataManager
+    {
+        private readonly DatabaseContext _database;
+        private readonly IETagGenerator _eTagGenerator;
+
+        public DataManager(DatabaseContext database, IETagGenerator eTagGenerator)
+        {
+            _database = database;
+            _eTagGenerator = eTagGenerator;
+        }
+
+        public async Task<string> RetainEntry(byte[] data)
+        {
+            if (data == null)
+                throw new ArgumentNullException(nameof(data));
+
+            var tag = await _eTagGenerator.Generate(data);
+
+            var entity = await _database.Data.Where(d => d.Tag == tag).SingleOrDefaultAsync();
+
+            if (entity == null)
+            {
+                entity = new DataEntity
+                {
+                    Tag = tag,
+                    Data = data,
+                    Ref = 1
+                };
+                _database.Data.Add(entity);
+            }
+            else
+            {
+                entity.Ref += 1;
+            }
+            await _database.SaveChangesAsync();
+            return tag;
+        }
+
+        public async Task FreeEntry(string tag)
+        {
+            if (tag == null)
+                throw new ArgumentNullException(nameof(tag));
+
+            var entity = await _database.Data.Where(d => d.Tag == tag).SingleOrDefaultAsync();
+
+            if (entity != null)
+            {
+                if (entity.Ref == 1)
+                {
+                    _database.Data.Remove(entity);
+                }
+                else
+                {
+                    entity.Ref -= 1;
+                }
+                await _database.SaveChangesAsync();
+            }
+        }
+
+        public async Task<byte[]> GetEntry(string tag)
+        {
+            if (tag == null)
+                throw new ArgumentNullException(nameof(tag));
+
+            var entity = await _database.Data.Where(d => d.Tag == tag).Select(d => new { d.Data }).SingleOrDefaultAsync();
+
+            if (entity == null)
+                throw new InvalidOperationException(Resources.Services.DataManager.ExceptionEntryNotExist);
+
+            return entity.Data;
+        }
+    }
+}