Fix NIndex/NRange ToString truncation on 64-bit: cast to nuint instead of uint

[Copilot]

Description

NIndex.ToStringFromEnd() and NRange.ToString() cast nint values to uint before formatting, truncating values exceeding uint.MaxValue on 64-bit platforms:

var ni = NIndex.FromEnd(unchecked((nint)(1L + uint.MaxValue)));
Console.WriteLine(ni.ToString()); // Prints "^0" instead of "^4294967296"

Fix

• NIndex.cs: (uint)Value → (nuint)Value in ToStringFromEnd()
• NRange.cs: (uint)Start.Value → (nuint)Start.Value and (uint)End.Value → (nuint)End.Value in ToString()

Tests

• Replaced [Fact] ToStringTest with [Theory] + [InlineData] for both NIndexTests and NRangeTests
• Added [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] tests exercising values above uint.MaxValue

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[Copilot]

Pull request overview

Fixes incorrect string formatting for NIndex/NRange on 64-bit platforms where values larger than uint.MaxValue were being truncated due to casts to uint during formatting.

Changes:

• Update NIndex.ToStringFromEnd() to format using (nuint)Value rather than (uint)Value.
• Update NRange.ToString() to format start/end using (nuint)Start.Value / (nuint)End.Value rather than uint.
• Expand/adjust unit tests to cover 64-bit values above uint.MaxValue, converting prior [Fact] coverage into [Theory] + [InlineData] and adding 64-bit conditional cases.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Numerics.Tensors/src/System/Buffers/NIndex.cs	Fixes 64-bit truncation in ToStringFromEnd() by formatting via nuint.
src/libraries/System.Numerics.Tensors/src/System/Buffers/NRange.cs	Fixes 64-bit truncation in ToString() by formatting start/end via nuint.
src/libraries/System.Numerics.Tensors/tests/NIndexTests.cs	Adds theory-based coverage and conditional 64-bit test cases for large values.
src/libraries/System.Numerics.Tensors/tests/NRangeTests.cs	Adds theory-based coverage and conditional 64-bit test cases for large range endpoints.

[vcsjones]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR.

[Copilot]

@copilot please invoke the code-review skill and post your analysis/results as a comment on this PR.

🤖 Copilot Code Review

Holistic Assessment

Motivation: The bug is real and clearly reproducible — (uint) casts truncate nint values exceeding uint.MaxValue on 64-bit platforms, causing NIndex.FromEnd(1L + uint.MaxValue).ToString() to return ^0 instead of ^4294967296. The buffer comment on line 177 ("20 for longest possible nuint value") confirms the original intent was nuint.

Approach: Changing (uint) → (nuint) is the minimal correct fix. The tests use [Theory]/[InlineData] and [ConditionalTheory] with PlatformDetection.Is64BitProcess appropriately, consistent with patterns used in NFloatTests.cs and elsewhere in the test project.

Summary: ✅ LGTM. The three cast fixes are correct, minimal, and well-tested. All findings below are positive confirmations or out-of-scope observations.

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Detailed Findings

✅ Correctness — Cast fix is correct in all three locations

• NIndex.cs:178: (uint)Value → (nuint)Value in ToStringFromEnd()
• NRange.cs:73: (uint)Start.Value → (nuint)Start.Value in ToString()
• NRange.cs:84: (uint)End.Value → (nuint)End.Value in ToString()

The Value property returns nint (always non-negative for valid NIndex values), so the nuint cast is safe. The stackalloc buffer was already sized for nuint (21 chars = 1 for ^ + 20 digits for max nuint on 64-bit). The non-FromEnd path (NIndex.ToString() line 172) uses Value.ToString() which calls nint.ToString() directly — no truncation issue there.

✅ Test quality — Good coverage with appropriate conditional attributes

The tests cover both fromStart and fromEnd paths, use boundary values (int.MaxValue, uint.MaxValue + 1, long.MaxValue), and correctly gate 64-bit-specific cases with [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))]. PlatformDetection is already used in other tests in this project (TensorPrimitives.Generic.cs, TensorPrimitivesTests.cs).

💡 Out-of-scope observation — NRange.GetOffsetAndLength has a similar (uint) cast on nint

NRange.cs:113 uses (uint)end > (uint)length || (uint)start > (uint)end — this truncates nint values on 64-bit, which could cause incorrect bounds validation for large collections. However, this is mirrored from Range.GetOffsetAndLength (which uses int → uint, which is correct for that type) and is a separate issue from the ToString fix.

[tannergooding]

💡 Out-of-scope observation — NRange.GetOffsetAndLength has a similar (uint) cast on nint

@copilot can you open a separate PR to address this.

[Copilot]

💡 Out-of-scope observation — NRange.GetOffsetAndLength has a similar (uint) cast on nint

@copilot can you open a separate PR to address this.

I'm not able to open new PRs. Could you open an issue for tracking the (uint) truncation in NRange.GetOffsetAndLength (line 113) so it can be picked up separately? The fix there would be changing (uint) to (nuint) for the bounds checks on end, length, and start.

[vcsjones]

I will open a separate PR for the offset / length issue.

[tannergooding]

This one should be backported, yes?

[vcsjones]

@tannergooding Up to you, but there are a couple of things you may want to back port when things are merged.

1. This (Fix NIndex/NRange ToString truncation on 64-bit: cast to nuint instead of uint #124294)
2. Fix non-dense TensorSpan handling in SequenceEqual, Fill*, Resize*, and IndexOf* methods #124225
3. Fix Tensor.Reverse for tensors with length-1 dimensions #124180
4. Fix integer overflow in TensorPrimitives IndexOf methods #124274

[tannergooding]

I'm fine with taking them all together, but I do think we should take them for bar check at least. -- CC. @jeffhandley