Add test for binary STLs with 'solid' header and fix STLDocument

src/STLDocument.cs

@@ -125,22 +125,7 @@ public void AppendFacets(IEnumerable<Facet> facets)
         /// <returns>True if the <see cref="STLDocument"/> is text-based, otherwise false.</returns>
         public static bool IsText(Stream stream)
         {
-            if (stream == null) throw new NullReferenceException(nameof(stream));
-
-            const string solid = "solid";
-
-            byte[] buffer = new byte[5];
-            string header;
-
-            // Reset the stream to tbe beginning and read the first few bytes, then reset the stream to the beginning again.
-            stream.Seek(0, SeekOrigin.Begin);
-            stream.Read(buffer, 0, buffer.Length);
-            stream.Seek(0, SeekOrigin.Begin);
-
-            // Read the header as ASCII.
-            header = Encoding.ASCII.GetString(buffer);
-
-            return solid.Equals(header, StringComparison.InvariantCultureIgnoreCase);
+            return !IsBinary(stream);
         }
 
         /// <summary>Determines if the <see cref="STLDocument"/> contained within the <paramref name="stream"/> is binary-based.</summary>
@@ -149,7 +134,18 @@ public static bool IsText(Stream stream)
         /// <returns>True if the <see cref="STLDocument"/> is binary-based, otherwise false.</returns>
         public static bool IsBinary(Stream stream)
         {
-            return !IsText(stream);
+            if (stream == null) throw new NullReferenceException(nameof(stream));
+
+            using var reader = new BinaryReader(stream, Encoding.Default, true);
+
+            stream.Seek(80, SeekOrigin.Begin);
+
+            const numTriangles = reader.ReadUInt32();
+            const expectedSize = 84 + numTriangles * 50;
+
+            stream.Seek(0, SeekOrigin.Begin);
+
+            return stream.Length == expectedSize;
         }
 
         /// <summary>Reads the <see cref="STLDocument"/> contained within the <paramref name="stream"/> into a new <see cref="STLDocument"/>.</summary>

tests/STLDocumentTests.cs

@@ -33,6 +33,19 @@ public void Read__FromBinaryStream()
             ValidateSTL(stlBinary);
         }
 
+        [Fact]
+        public void Read__FromBinaryStreamWithSolidHeader()
+        {
+            STLDocument stlBinary;
+
+            using (var stream = GetData("BinaryWithSolidHeader.stl"))
+            {
+                stlBinary = STLDocument.Read(stream);
+            }
+
+            ValidateSTL(stlBinary);
+        }
+
         [Fact]
         public void Read__FromTextReader()
         {