Add n_columns() to PreProcessedTraceVariant

[ilyalesokhin-starkware]

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

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Note

Low Risk
Low risk: adds a small helper method and a unit test; no proving/verification logic or data handling is changed.

Overview
Adds PreProcessedTraceVariant::n_columns() to expose the expected number of preprocessed columns for each trace variant.

Adds a unit test (test_n_columns) that asserts variant.to_preprocessed_trace().columns.len() matches variant.n_columns() for all variants, keeping the hardcoded metadata in sync with trace construction.

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This change is 

[ilyalesokhin-starkware]

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• Add n_columns() to PreProcessedTraceVariant #1709 👈 (View in Graphite)
• Move PreProcessedTraceVariant to its own module with tests #1708
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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Hardcoded magic numbers for column counts are fragile

Low Severity

n_columns() uses inline magic numbers (161, 105, 156) with no named constants or derivation from existing definitions, unlike n_trace_cells() which references named constants (CANONICAL_SIZE, etc.) defined in the common crate alongside the column definitions. If column definitions change (e.g., adding a new range check or changing PEDERSEN_TABLE_N_COLUMNS), these hardcoded values silently become wrong until the test happens to run. Extracting these into named constants co-located with the column definitions would make the relationship explicit and match the existing pattern.

 

[ilyalesokhin-starkware]

that would be the only places where those constants are used.

[Gali-StarkWare]

@Gali-StarkWare reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: 1 of 2 files reviewed, 2 unresolved discussions (waiting on ilyalesokhin-starkware).

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stwo_cairo_prover/crates/common/src/preprocessed_columns/preprocessed_trace.rs line 42 at r2 (raw file):

    }

    pub fn n_columns(&self) -> usize {

What will you do with the ids? You can just move the list here..

[Gali-StarkWare]

@Gali-StarkWare reviewed 1 file.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on ilyalesokhin-starkware).

[ilyalesokhin-starkware]

stwo_cairo_prover/crates/common/src/preprocessed_columns/preprocessed_trace.rs line 42 at r2 (raw file):

Previously, Gali-StarkWare wrote…

What will you do with the ids? You can just move the list here..

I don't need the IDs when I'm asking about the number of columns.

[ilyalesokhin-starkware]

that would be the only places where those constants are used.

[ilyalesokhin-starkware]

@ilyalesokhin-starkware resolved 1 discussion.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on Gali-StarkWare).

[ilyalesokhin-starkware]

stwo_cairo_prover/crates/common/src/preprocessed_columns/preprocessed_trace.rs line 42 at r2 (raw file):

Previously, ilyalesokhin-starkware wrote…

I don't need the IDs when I'm asking about the number of columns.

What list needs to move here?

[Gali-StarkWare]

@Gali-StarkWare made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

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stwo_cairo_prover/crates/common/src/preprocessed_columns/preprocessed_trace.rs line 42 at r2 (raw file):

Previously, ilyalesokhin-starkware wrote…

What list needs to move here?

The one you have in stwo-circuits. You don't need it for the len but you need it for CairoStatement::get_preprocessed_column_ids()

[ilyalesokhin-starkware]

stwo_cairo_prover/crates/common/src/preprocessed_columns/preprocessed_trace.rs line 42 at r2 (raw file):

Previously, Gali-StarkWare wrote…

The one you have in stwo-circuits. You don't need it for the len but you need it for CairoStatement::get_preprocessed_column_ids()

I thought I'd get the IDs by calling to_preprocessed_trace().