ElwigData: Check zip file integrity

Elwig/Helpers/Extensions.cs

@@ -1,5 +1,7 @@
 using System;
 using System.IO;
+using System.IO.Compression;
+using System.IO.Hashing;
 using System.Net.Http;
 using System.Runtime.InteropServices;
 using System.Threading;
@@ -95,5 +97,16 @@ namespace Elwig.Helpers {
             await download.CopyToAsync(destination, 81920, relativeProgress, cancellationToken);
             progress.Report(100.0);
         }
+
+        public static async Task CheckIntegrity(this ZipArchive zip) {
+            var crc = new Crc32();
+            foreach (var entry in zip.Entries) {
+                crc.Reset();
+                using var stream = entry.Open();
+                await crc.AppendAsync(stream);
+                if (crc.GetCurrentHashAsUInt32() != entry.Crc32)
+                    throw new InvalidDataException($"CRC-32 mismatch in '{entry.FullName}'");
+            }
+        }
     }
 }