From 657fb589137099794e58fbd35beb7d942b376965 Mon Sep 17 00:00:00 2001
From: crupest <crupest@outlook.com>
Date: Sun, 25 Apr 2021 21:20:04 +0800
Subject: ...

---
 BackEnd/Timeline/Services/Data/DataManager.cs   | 138 ++++++++++++++++++++++++
 BackEnd/Timeline/Services/Data/ETagGenerator.cs |  45 ++++++++
 2 files changed, 183 insertions(+)
 create mode 100644 BackEnd/Timeline/Services/Data/DataManager.cs
 create mode 100644 BackEnd/Timeline/Services/Data/ETagGenerator.cs

(limited to 'BackEnd/Timeline/Services/Data')

diff --git a/BackEnd/Timeline/Services/Data/DataManager.cs b/BackEnd/Timeline/Services/Data/DataManager.cs
new file mode 100644
index 00000000..d9a4491d
--- /dev/null
+++ b/BackEnd/Timeline/Services/Data/DataManager.cs
@@ -0,0 +1,138 @@
+using Microsoft.EntityFrameworkCore;
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Timeline.Entities;
+
+namespace Timeline.Services.Data
+{
+    /// <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. If not exist, returns null.
+        /// </summary>
+        /// <param name="tag">The tag of the entry.</param>
+        /// <returns>The data of the entry. If not exist, returns null.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="tag"/> is null.</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 is null)
+                return null;
+
+            return entity.Data;
+        }
+    }
+
+    public static class DataManagerExtensions
+    {
+        /// <summary>
+        /// Try to get an entry and throw <see cref="DatabaseCorruptedException"/> if not exist.
+        /// </summary>
+        public static async Task<byte[]> GetEntryAndCheck(this IDataManager dataManager, string tag, string notExistMessage)
+        {
+            var data = await dataManager.GetEntry(tag);
+            if (data is null)
+                throw new DatabaseCorruptedException($"Can't get data of tag {tag}. {notExistMessage}");
+            return data;
+        }
+    }
+}
diff --git a/BackEnd/Timeline/Services/Data/ETagGenerator.cs b/BackEnd/Timeline/Services/Data/ETagGenerator.cs
new file mode 100644
index 00000000..847c120b
--- /dev/null
+++ b/BackEnd/Timeline/Services/Data/ETagGenerator.cs
@@ -0,0 +1,45 @@
+using System;
+using System.Security.Cryptography;
+using System.Threading.Tasks;
+
+namespace Timeline.Services.Data
+{
+    public interface IETagGenerator
+    {
+        /// <summary>
+        /// Generate a etag for given source.
+        /// </summary>
+        /// <param name="source">The source data.</param>
+        /// <returns>The generated etag.</returns>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="source"/> is null.</exception>
+        Task<string> Generate(byte[] source);
+    }
+
+    public sealed class ETagGenerator : IETagGenerator, IDisposable
+    {
+        private readonly SHA1 _sha1;
+
+        [System.Diagnostics.CodeAnalysis.SuppressMessage("Security", "CA5350:Do Not Use Weak Cryptographic Algorithms", Justification = "Sha1 is enough ??? I don't know.")]
+        public ETagGenerator()
+        {
+            _sha1 = SHA1.Create();
+        }
+
+        public Task<string> Generate(byte[] source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Task.Run(() => Convert.ToBase64String(_sha1.ComputeHash(source)));
+        }
+
+        private bool _disposed; // To detect redundant calls
+
+        public void Dispose()
+        {
+            if (_disposed) return;
+            _sha1.Dispose();
+            _disposed = true;
+        }
+    }
+}
-- 
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