Checkpoint 5: Specialize Common Column Shapes

The Checkpoint 3 fallback calls ColumnView::get(row), so Array, Constant, and Indexed inputs all behave correctly. Its flexibility has a cost: every input selects its representation again on every row. In this checkpoint, you will choose the common shapes once per batch and keep the typed fallback for everything else.

Start from your completed Checkpoint 4 workspace, copy the cumulative tests, and run the focused test before editing:

cargo x copy-test --chapter 5
cargo test -p type-exercise-starter-supplied-tests chapter_5 --locked

The first run should fail only because the three auto-vectorization adapters do not exist yet.

Give one loop a concrete input shape

In core/src/column.rs, let the core expression module inspect the private typed representation enum. Keep that enum crate-private: callers still construct checked ColumnViewImpl values and cannot bypass validation.

Add private Array and Constant accessors in core/src/expression.rs. Each accessor exposes the same typed len and nullable get operations, but its concrete type is selected before the loop begins. The loop remains generic over the accessor and no longer matches a representation at every row.

Build these public adapters around those loops:

• auto_vectorize_unary specializes Array and Constant; Indexed uses the existing typed fallback.
• auto_vectorize_binary specializes Array/Array, Array/Constant, Constant/Array, and Constant/Constant; any Indexed input uses the fallback.
• auto_vectorize_ternary specializes Array/Array/Array; every other combination uses the fallback.

Use the same generic scalar relationships as evaluate_unary, evaluate_binary, and evaluate_ternary. The input families may differ, and the output family belongs to the generic output scalar. Validate physical types and equal lengths before selecting a shape.

Preserve one behavior across every path

A concrete loop must keep the fallback’s semantics:

1. read borrowed typed values from each input;
2. call the scalar function only when every required input is non-null;
3. append null otherwise; and
4. return a new owned output array.

Keep the fallback because Indexed inputs still need indirect lookup. Specialize Array and Constant inputs for unary and binary expressions, along with the common ternary case where all three inputs are Arrays. Covering all 27 Array/Constant/Indexed ternary combinations would add a lot of code without introducing new behavior.

Run the focused and cumulative contracts:

cargo test -p type-exercise-starter-supplied-tests chapter_5 --locked
cargo test -p type-exercise-starter-supplied-tests --lib --locked

The three Checkpoint 5 tests compare nullable owned results across Array and Constant combinations, mixed scalar families, Indexed and non-dense ternary fallback, and invalid types and lengths. Together with Checkpoints 1–4, the cumulative suite has 17 tests. Because callers see results and errors rather than an internal route, both specialized and fallback paths must keep the same contract.

The next checkpoint keeps that contract while separating operations that are total, fallible, or nullable-aware.