Motivation
----------
`Scalar` was a tagged enum (I64/F64/Bool/Ts), so every scalar value
physically carried its own kind tag. But the kind is already known from
the schema/port/column the value flows through (C7: the type is a
property of the column, not of the value — the hot path is already
columnar `Column<T>`, and `AnyColumn::get` *reconstructs* the tag from
the column on the way out). The per-value tag was therefore redundant
with the kind the surrounding context already holds.
That redundancy had three costs:
* It baked an implicit `match` (a branch) into every function that read
a Scalar payload — even where the caller statically knew the type.
The tag could never be exploited away.
* Size: a tagged enum is tag + payload = 16 bytes (f64/i64 alignment),
twice the 8 bytes the value needs. A `Column<Scalar>` would be double
the memory and half the cache utilisation.
* It is the shared root of several downstream papercuts we keep hitting
— the lossy f64 manifest field, the `unreachable!` panic on a
non-numeric param, the serde-tag question — all symptoms of "the type
is baked into the value".
Change
------
Introduce `Cell`: a type-erased 64-bit word (`struct Cell(u64)`) that is
not readable without external type context. It is constructed per base
type (`from_i64/from_f64/from_bool/from_ts`) and read only by naming the
type at the call site (`i64()/f64()/bool()/ts()`) — each a branch-free
bit-cast. The hot path resolves the kind once at the boundary (from the
schema) and then reads natively, with no per-value branch. `Cell` knows
nothing of `Scalar` or `ScalarKind`; the dependency is strictly one-way,
and it lives in its own `cell.rs` (more is planned on top of it).
`Scalar` becomes `struct { kind: ScalarKind, cell: Cell }` — the
self-describing form for the dynamic boundaries (builder binding,
serialization, rendering), built on top of `Cell`. Its `as_*` accessors
now `debug_assert` the kind and return the native value (free in
release); calling the wrong accessor is a caller bug, not a checked
`Option`. The variant constructors `Scalar::I64(..)` become associated
fns `Scalar::i64(..)`.
`PartialEq` is hand-written (not derived) to preserve the former enum's
value semantics: kinds must match, then native payloads compare, so f64
keeps IEEE-754 behaviour (`NaN != NaN`, `+0.0 == -0.0`) and a kind
mismatch is never equal even when the raw words coincide. A fixture
(`scalar_eq_is_value_not_bitwise`) pins exactly the cases where bit- and
value-equality diverge, so it can't silently regress. `Cell`'s own
`Eq`/`Hash` stay bitwise — correct for a raw word.
The change is behaviour-preserving: Scalar's observable behaviour is
identical to the pre-Cell enum (the value-equality fixture proves it);
only the internal representation changed. The ~440 call sites across the
workspace are a mechanical constructor rename plus ~12 destructuring
sites (match-arms / `let`-patterns) rewritten to `kind()` + `as_*`.
Verified: cargo build --workspace --all-targets, cargo clippy --workspace
--all-targets -- -D warnings, cargo test --workspace — all green.
Collapse the three verbatim-duplicated #[cfg(test)] fixtures —
synthetic_prices, sma_cross, composite_sma_cross_harness — that lived
in both blueprint.rs's and sweep.rs's test modules into one shared
crate-root `#[cfg(test)] mod test_fixtures` (pub(crate) free fns). The
two consumers now import from `crate::test_fixtures`; sweep.rs's
test-mod `use` block sheds the imports that were only needed by the
moved fns.
Why: the topology had to move in lockstep across the two copies but no
test pinned them equal, so an edit to one silently diverged the other
(#53). One definition removes the hazard structurally — better than a
pin-them-equal test, which loses its premise once there is a single
definition. Test-only; no production code, public API, or runtime path
changes. C16 untouched (a #[cfg(test)] module ships in no artifact).
The CLI sample is no longer a third peer (cycle 0036 enriched it into a
trend+momentum blend), so this is an engine-internal two-copy dedup, not
the three-way the issue originally named. graph_model.rs keeps its own
intentionally-minimal golden copy, out of scope.
Plan glitch fixed in-flight: the plan's literal import list named
sma_cross, but neither consumer calls it directly (only transitively via
composite_sma_cross_harness), so importing it tripped the plan's own
Step-7 clippy -D warnings gate. Trimmed to {composite_sma_cross_harness,
synthetic_prices} — within the task's "drop now-unused imports" scope.
Verified: cargo test --workspace green (aura-engine 114 passed, count
unchanged — behaviour-preserving); the two load-bearing fixture
consumers composite_sma_cross_runs_bit_identical_to_hand_wired (C1) and
sweep_equals_n_independent_runs (C11/C12) both green; cargo clippy
--workspace --all-targets -- -D warnings clean.
closes#53
Brummel