From 68cf3ae8b7ee29c7bc9da5fbca4cf711d7927aa8 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Wed, 7 Oct 2026 14:11:08 -0400
Subject: [PATCH] fix(abcip): stride user-string arrays by the read buffer, not
 the built-in STRING size

Live read-only check against a production CompactLogix press: Program:UN01_Press._ModeNames
(STR0016[32], 4 elements requested) returned an 80-byte buffer with libplctag.NET 1.5.2's
ElementSize unset. The driver fell back to the static 88-byte STRING stride, read element
1 past the buffer, and the whole read failed (ErrorOutOfBounds -> BadCommunicationError).
When the buffer divides evenly by the element count, that quotient is now the stride;
the read returns [undefined, Production, Maintenance, Manual] as the controller holds.
---
 .../LibplctagTagRuntime.cs                    | 23 +++++++++-
 .../AbCipArrayStrideTests.cs                  | 45 +++++++++++++++++++
 2 files changed, 66 insertions(+), 2 deletions(-)
 create mode 100644 tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipArrayStrideTests.cs

diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs
index 11ef9b26..b11cbd49 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/LibplctagTagRuntime.cs
@@ -79,8 +79,7 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
         // accounts for STRING capacity + atomic widths. ElementSize is int? on the wrapper — fall
         // back to a static size table when it is null/<=0 (e.g. a zero-length read) so the stride
         // is never 0.
-        var elementSize = _tag.ElementSize ?? 0;
-        var stride = elementSize > 0 ? elementSize : ElementByteSize(type);
+        var stride = ResolveArrayStride(_tag.ElementSize, _tag.GetSize(), count, type);
 
         return type switch
         {
@@ -99,6 +98,26 @@ internal sealed class LibplctagTagRuntime : IAbCipTagRuntime
         };
     }
 
+    /// <summary>
+    ///     Byte stride between array elements. libplctag's <c>ElementSize</c> wins when populated, but
+    ///     after a read it is <c>null</c> (observed live, libplctag.NET 1.5.2) — and the static table
+    ///     assumes the built-in 88-byte STRING, which is wrong for a user string type: a <c>STR0016</c>
+    ///     element is 20 bytes, so element 1 was read at offset 88 of an 80-byte buffer and the whole
+    ///     read failed. When the read buffer divides evenly by the element count, that quotient IS the
+    ///     element size, so it is used before the static table.
+    /// </summary>
+    /// <param name="elementSize">libplctag's reported element size, if any.</param>
+    /// <param name="bufferSize">Bytes in the read buffer.</param>
+    /// <param name="count">Element count requested.</param>
+    /// <param name="type">Element type.</param>
+    /// <returns>The stride in bytes.</returns>
+    internal static int ResolveArrayStride(int? elementSize, int bufferSize, int count, AbCipDataType type)
+    {
+        if (elementSize is > 0) return elementSize.Value;
+        if (count > 0 && bufferSize > 0 && bufferSize % count == 0) return bufferSize / count;
+        return ElementByteSize(type);
+    }
+
     private bool[] BuildBoolArray(int count)
     {
         var arr = new bool[count];
diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipArrayStrideTests.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipArrayStrideTests.cs
new file mode 100644
index 00000000..86111824
--- /dev/null
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests/AbCipArrayStrideTests.cs
@@ -0,0 +1,45 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.AbCip;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.AbCip.Tests;
+
+/// <summary>
+///     Array element stride. Seen live on a production CompactLogix: reading
+///     <c>Program:UN01_Press._ModeNames</c> (<c>STR0016[32]</c>, 4 elements requested) returned an
+///     80-byte buffer with libplctag's <c>ElementSize</c> unset; the driver strode 88 bytes (built-in
+///     STRING) and the read failed <c>ErrorOutOfBounds</c>. The real values were
+///     "undefined" / "Production" / "Maintenance" / "Manual" at a 20-byte stride.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class AbCipArrayStrideTests
+{
+    /// <summary>A user string type's stride comes from the buffer, not the built-in STRING size.</summary>
+    [Fact]
+    public void User_string_array_strides_by_buffer_size_over_count()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 80, count: 4, AbCipDataType.String).ShouldBe(20);
+    }
+
+    /// <summary>The built-in STRING still strides 88.</summary>
+    [Fact]
+    public void Builtin_STRING_array_strides_88()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 3 * 88, count: 3, AbCipDataType.String).ShouldBe(88);
+    }
+
+    /// <summary>libplctag's own element size wins when it is populated.</summary>
+    [Fact]
+    public void Reported_element_size_wins()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(4, bufferSize: 80, count: 4, AbCipDataType.DInt).ShouldBe(4);
+    }
+
+    /// <summary>An empty or uneven buffer falls back to the static per-type size.</summary>
+    [Fact]
+    public void Empty_or_uneven_buffer_falls_back_to_the_type_size()
+    {
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 0, count: 4, AbCipDataType.Real).ShouldBe(4);
+        LibplctagTagRuntime.ResolveArrayStride(null, bufferSize: 81, count: 4, AbCipDataType.String).ShouldBe(88);
+    }
+}