From c93e32ebb35b58d878fc03bb1ba3ce378277c027 Mon Sep 17 00:00:00 2001
From: Benito Palacios Sanchez <pleonex@protonmail.com>
Date: Tue, 28 Nov 2023 06:33:05 +0100
Subject: [PATCH] :bug: WriteTo truncates output file

---
 src/Yarhl.UnitTests/IO/DataStreamTests.cs | 43 ++++++++++++++++++-----
 src/Yarhl/IO/DataStream.cs                |  2 +-
 2 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/src/Yarhl.UnitTests/IO/DataStreamTests.cs b/src/Yarhl.UnitTests/IO/DataStreamTests.cs
index d619cb79..e93f214c 100644
--- a/src/Yarhl.UnitTests/IO/DataStreamTests.cs
+++ b/src/Yarhl.UnitTests/IO/DataStreamTests.cs
@@ -1368,17 +1368,44 @@ public void WriteToWhenLengthZeroCreatesEmptyFile()
         {
             string tempFile = Path.Combine(
                 Path.GetTempPath(),
-                Path.GetRandomFileName(),
                 Path.GetRandomFileName());
 
-            DataStream stream = new DataStream();
-            stream.WriteTo(tempFile);
+            try {
+                var stream = new DataStream();
+                stream.WriteTo(tempFile);
 
-            Assert.That(File.Exists(tempFile), Is.True);
-            FileStream fs = new FileStream(tempFile, FileMode.Open, FileAccess.Read);
-            Assert.AreEqual(0, fs.Length);
-            fs.Dispose();
-            File.Delete(tempFile);
+                Assert.That(File.Exists(tempFile), Is.True);
+
+                using var fs = new FileStream(tempFile, FileMode.Open, FileAccess.Read);
+                Assert.That(fs.Length, Is.EqualTo(0));
+            } finally {
+                File.Delete(tempFile);
+            }
+        }
+
+        [Test]
+        public void WriteToWhenTruncatesExistingFile()
+        {
+            string tempFile = Path.Combine(
+                Path.GetTempPath(),
+                Path.GetRandomFileName());
+
+            byte[] expected = new byte[] { 0xCA, 0xFE };
+            using var stream = new DataStream();
+            stream.Write(expected);
+
+            try {
+                // Pre-fill the output file with some long content
+                File.WriteAllText(tempFile, "hello world!");
+
+                // Overwrite with 2 bytes stream content
+                stream.WriteTo(tempFile);
+
+                byte[] actual = File.ReadAllBytes(tempFile);
+                Assert.That(actual, Is.EquivalentTo(expected));
+            } finally {
+                File.Delete(tempFile);
+            }
         }
 
         [Test]
diff --git a/src/Yarhl/IO/DataStream.cs b/src/Yarhl/IO/DataStream.cs
index 4ce701eb..b57dccd6 100644
--- a/src/Yarhl/IO/DataStream.cs
+++ b/src/Yarhl/IO/DataStream.cs
@@ -616,7 +616,7 @@ public void WriteSegmentTo(long start, long length, string fileOut)
             }
 
             // We use FileStream so it creates a file even when the length is zero
-            using var segment = new FileStream(fileOut, FileMode.OpenOrCreate, FileAccess.Write);
+            using var segment = new FileStream(fileOut, FileMode.Create, FileAccess.Write);
             WriteSegmentTo(start, length, segment);
         }