Read full-width packed 16- and 32-bit integers little-endian

CHANGELOG.md

@@ -6,6 +6,11 @@ API between minor releases.
 
 ## [Unreleased]
 
+### Fixed
+- Packed 16-bit and 32-bit integers in their full-width form are read little-endian. They were read with their bytes
+  reversed, so entity records that store a colour, extended data size or reactor count in that form were decoded from
+  the wrong bit position and failed or gave NaN or absurd coordinates (#7).
+
 ## [0.4.0] - 2026-11-11
 
 ### Added

src/DwgIO/Internal/Container/BitReaderMsb.cs

@@ -75,7 +75,7 @@ public uint ReadPackedInt()
         uint tag = ReadTwoBits();
         return tag switch
         {
-            0 => (uint)ReadBits(32),
+            0 => ReadRawUInt32(),
             1 => (uint)ReadBits(8),
             _ => 0,
         };
@@ -89,7 +89,7 @@ public uint ReadPackedShort()
         uint tag = ReadTwoBits();
         return tag switch
         {
-            0 => (uint)ReadBits(16),
+            0 => ReadRawByte() | (uint)ReadRawByte() << 8, // operands run left to right: low byte first
             1 => (uint)ReadBits(8),
             2 => 0,
             _ => 256,

tests/DwgIO.Tests/BitReaderTests.cs

@@ -24,6 +24,52 @@ public void ReadRawUInt32_LittleEndian()
         Assert.Equal(0x12345678u, reader.ReadRawUInt32());
     }
 
+    [Fact]
+    public void ReadPackedShort_FullWidth_IsLittleEndian()
+    {
+        // 6 padding bits, then tag 00 and the bytes 0x34 0x12.
+        var reader = new BitReaderMsb([0x00, 0x34, 0x12]);
+        reader.ReadBits(6);
+        Assert.Equal(0x1234u, reader.ReadPackedShort());
+    }
+
+    [Fact]
+    public void ReadPackedInt_FullWidth_IsLittleEndian()
+    {
+        var reader = new BitReaderMsb([0x00, 0x78, 0x56, 0x34, 0x12]);
+        reader.ReadBits(6);
+        Assert.Equal(0x12345678u, reader.ReadPackedInt());
+    }
+
+    [Fact]
+    public void ReadPackedShort_FullWidth_Unaligned()
+    {
+        var reader = new BitReaderMsb([0x0D, 0x04, 0x80]);
+        Assert.Equal(0x1234u, reader.ReadPackedShort());
+    }
+
+    [Fact]
+    public void ReadPackedShortAndInt_ShortForms_AreUnchanged()
+    {
+        // Tags 01 (one byte 0xAB), 10 and 11.
+        var shorts = new BitReaderMsb([0x6A, 0xEC]);
+        Assert.Equal(0xABu, shorts.ReadPackedShort());
+        Assert.Equal(0u, shorts.ReadPackedShort());
+        Assert.Equal(256u, shorts.ReadPackedShort());
+
+        var ints = new BitReaderMsb([0x6A, 0xEC]);
+        Assert.Equal(0xABu, ints.ReadPackedInt());
+        Assert.Equal(0u, ints.ReadPackedInt());
+        Assert.Equal(0u, ints.ReadPackedInt());
+    }
+
+    [Fact]
+    public void ReadObjectType_BeforeR2010_FullWidthIsLittleEndian()
+    {
+        var reader = new BitReaderMsb([0x3D, 0x00, 0x40]);
+        Assert.Equal(0x1F4, reader.ReadObjectType(r2010Plus: false));
+    }
+
     [Fact]
     public void ReadVarUInt_MultiByteContinuation()
     {