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| 1 | +# ColsRef macro |
| 2 | + |
| 3 | +The `ColsRef` procedural macro is used in constraint generation to create column structs that have dynamic sizes. |
| 4 | + |
| 5 | +Note: this macro was originally created for use in the SHA-2 VM extension, where we reuse the same constraint generation code for three different circuits (SHA-256, SHA-512, and SHA-384). |
| 6 | +See the [SHA-2 VM extension](../../../../../../extensions/sha2/circuit/src/sha2_chip/air.rs) for an example of how to use the `ColsRef` macro to reuse constraint generation code over multiple circuits. |
| 7 | + |
| 8 | +## Overview |
| 9 | + |
| 10 | +As an illustrative example, consider the following columns struct: |
| 11 | +```rust |
| 12 | +struct ExampleCols<T, const N: usize> { |
| 13 | + arr: [T; N], |
| 14 | + sum: T, |
| 15 | +} |
| 16 | +``` |
| 17 | +Let's say we want to constrain `sum` to be the sum of the elements of `arr`, and `N` can be either 5 or 10. |
| 18 | +We can define a trait that stores the config parameters. |
| 19 | +```rust |
| 20 | +pub trait ExampleConfig { |
| 21 | + const N: usize; |
| 22 | +} |
| 23 | +``` |
| 24 | +and then implement it for the two different configs. |
| 25 | +```rust |
| 26 | +pub struct ExampleConfigImplA; |
| 27 | +impl ExampleConfig for ExampleConfigImplA { |
| 28 | + const N: usize = 5; |
| 29 | +} |
| 30 | +pub struct ExampleConfigImplB; |
| 31 | +impl ExampleConfig for ExampleConfigImplB { |
| 32 | + const N: usize = 10; |
| 33 | +} |
| 34 | +``` |
| 35 | +Then we can use the `ColsRef` macro like this |
| 36 | +```rust |
| 37 | +#[derive(ColsRef)] |
| 38 | +#[config(ExampleConfig)] |
| 39 | +struct ExampleCols<T, const N: usize> { |
| 40 | + arr: [T; N], |
| 41 | + sum: T, |
| 42 | +} |
| 43 | +``` |
| 44 | +which will generate a columns struct that uses references to the fields. |
| 45 | +```rust |
| 46 | +struct ExampleColsRef<'a, T, const N: usize> { |
| 47 | + arr: &'a [T; N], |
| 48 | + sum: &'a T, |
| 49 | +} |
| 50 | +``` |
| 51 | +The `ColsRef` macro will also generate a `from` method that takes a slice of the correct length and returns an instance of the columns struct. |
| 52 | +The `from` method is parameterized by a struct that implements the `ExampleConfig` trait, and it uses the associated constants to determine how to split the input slice into the fields of the columns struct. |
| 53 | + |
| 54 | +So, the constraint generation code can be written as |
| 55 | +```rust |
| 56 | +impl<AB: InteractionBuilder, C: ExampleConfig> Air<AB> for ExampleAir<C> { |
| 57 | + fn eval(&self, builder: &mut AB) { |
| 58 | + let main = builder.main(); |
| 59 | + let (local, _) = (main.row_slice(0), main.row_slice(1)); |
| 60 | + let local_cols = ExampleColsRef::<AB::Var>::from::<C>(&local[..C::N + 1]); |
| 61 | + let sum = local_cols.arr.iter().sum(); |
| 62 | + builder.assert_eq(local_cols.sum, sum); |
| 63 | + } |
| 64 | +} |
| 65 | +``` |
| 66 | +Notes: |
| 67 | +- the `arr` and `sum` fields of `ExampleColsRef` are references to the elements of the `local` slice. |
| 68 | +- the name, `N`, of the const generic parameter must match the name of the associated constant `N` in the `ExampleConfig` trait. |
| 69 | + |
| 70 | +The `ColsRef` macro also generates a `ExampleColsRefMut` struct that stores mutable references to the fields, for use in trace generation. |
| 71 | + |
| 72 | +The `ColsRef` macro supports more than just variable-length array fields. |
| 73 | +The field types can also be: |
| 74 | +- any type that derives `AlignedBorrow` via `#[derive(AlignedBorrow)]` |
| 75 | +- any type that derives `ColsRef` via `#[derive(ColsRef)]` |
| 76 | +- (possibly nested) arrays of `T` or (possibly nested) arrays of a type that derives `AlignedBorrow` |
| 77 | + |
| 78 | +Note that we currently do not support arrays of types that derive `ColsRef`. |
| 79 | + |
| 80 | +## Specification |
| 81 | + |
| 82 | +Annotating a struct named `ExampleCols` with `#[derive(ColsRef)]` and `#[config(ExampleConfig)]` produces two structs, `ExampleColsRef` and `ExampleColsRefMut`. |
| 83 | +- we assume `ExampleCols` has exactly one generic type parameter, typically named `T`, and any number of const generic parameters. Each const generic parameter must have a name that matches an associated constant in the `ExampleConfig` trait |
| 84 | + |
| 85 | +The fields of `ExampleColsRef` have the same names as the fields of `ExampleCols`, but their types are transformed as follows: |
| 86 | +- type `T` becomes `&T` |
| 87 | +- type `[T; LEN]` becomes `&ArrayView1<T>` (see [ndarray](https://docs.rs/ndarray/latest/ndarray/index.html)) where `LEN` is an associated constant in `ExampleConfig` |
| 88 | + - the `ExampleColsRef::from` method will correctly infer the length of the array from the config |
| 89 | +- fields with names that end in `Cols` are assumed to be a columns struct that derives `ColsRef` and are transformed into the appropriate `ColsRef` type recursively |
| 90 | + - one restriction is that any nested `ColsRef` type must have the same config as the outer `ColsRef` type |
| 91 | +- fields that are annotated with `#[aligned_borrow]` are assumed to derive `AlignedBorrow` and are borrowed from the input slice. The new type is a reference to the `AlignedBorrow` type |
| 92 | +- nested arrays of `U` become `&ArrayViewX<U>` where `X` is the number of dimensions in the nested array type |
| 93 | + - `U` can be either the generic type `T` or a type that derives `AlignedBorrow`. In the latter case, the field must be annotated with `#[aligned_borrow]` |
| 94 | + - the `ArrayViewX` type provides a `X`-dimensional view into the row slice |
| 95 | + |
| 96 | +The fields of `ExampleColsRefMut` are almost the same as the fields of `ExampleColsRef`, but they are mutable references. |
| 97 | +- the `ArrayViewMutX` type is used instead of `ArrayViewX` for the array fields. |
| 98 | +- fields that derive `ColsRef` are transformed into the appropriate `ColsRefMut` type recursively. |
| 99 | + |
| 100 | +Each of the `ExampleColsRef` and `ExampleColsRefMut` types has the following methods implemented: |
| 101 | +```rust |
| 102 | +// Takes a slice of the correct length and returns an instance of the columns struct. |
| 103 | +pub const fn from<C: ExampleConfig>(slice: &[T]) -> Self; |
| 104 | +// Returns the number of cells in the struct |
| 105 | +pub const fn width<C: ExampleConfig>() -> usize; |
| 106 | +``` |
| 107 | +Note that the `width` method on both structs returns the same value. |
| 108 | + |
| 109 | +Additionally, the `ExampleColsRef` struct has a `from_mut` method that takes a `ExampleColsRefMut` and returns a `ExampleColsRef`. |
| 110 | +This may be useful in trace generation to pass a `ExampleColsRefMut` to a function that expects a `ExampleColsRef`. |
| 111 | + |
| 112 | +See the [tests](../../tests/test_cols_ref.rs) for concrete examples of how the `ColsRef` macro handles each of the supported field types. |
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