Unified ImageFileCache implementations

MapControl/Shared/ImageFileCache.cs

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+// XAML Map Control - https://github.com/ClemensFischer/XAML-Map-Control
+// © 2021 Clemens Fischer
+// Licensed under the Microsoft Public License (Ms-PL)
+
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Text;
+
+namespace MapControl.Caching
+{
+    /// <summary>
+    /// Image Cache implementation based on local image files.
+    /// The only valid data type for cached values is MapControl.ImageCacheItem.
+    /// </summary>
+    public partial class ImageFileCache
+    {
+        private const string expiresTag = "EXPIRES:";
+
+        private readonly string rootDirectory;
+
+        public ImageFileCache(string directory)
+        {
+            if (string.IsNullOrEmpty(directory))
+            {
+                throw new ArgumentException("The directory argument must not be null or empty.", nameof(directory));
+            }
+
+            rootDirectory = directory;
+            Debug.WriteLine("Created ImageFileCache in " + rootDirectory);
+        }
+
+        private string GetPath(string key)
+        {
+            try
+            {
+                return Path.Combine(rootDirectory, Path.Combine(key.Split('/', ':', ';', ',')));
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine("ImageFileCache: Invalid key {0}/{1}: {2}", rootDirectory, key, ex.Message);
+            }
+
+            return null;
+        }
+
+        private static DateTime ReadExpiration(ref byte[] buffer)
+        {
+            DateTime? expiration = ReadExpiration(buffer);
+
+            if (expiration.HasValue)
+            {
+                Array.Resize(ref buffer, buffer.Length - 16);
+                return expiration.Value;
+            }
+
+            return DateTime.Today;
+        }
+
+        private static DateTime? ReadExpiration(byte[] buffer)
+        {
+            if (buffer.Length >= 16 &&
+                Encoding.ASCII.GetString(buffer, buffer.Length - 16, 8) == expiresTag)
+            {
+                return new DateTime(BitConverter.ToInt64(buffer, buffer.Length - 8), DateTimeKind.Utc);
+            }
+
+            return null;
+        }
+    }
+}