Struct Context

pub struct Context<F: ScalarField> {
    pub advice: Vec<Assigned<F>>,
    pub selector: Vec<bool>,
    pub copy_manager: SharedCopyConstraintManager<F>,
    /* private fields */
}

Represents a single thread of an execution trace.

• We keep the naming Context for historical reasons.

Context is CPU thread-local.

Fields

advice: Vec<Assigned<F>>

Single column of advice cells.

selector: Vec<bool>

Vec representing the selector column of this Context accompanying each advice column

• Assumed to have the same length as advice

copy_manager: SharedCopyConstraintManager<F>

Global shared thread-safe manager for all copy (equality) constraints between virtual advice, constants, and raw external Halo2 cells.

Implementations

impl<F: ScalarField> Context<F>

pub fn witness_gen_only(&self) -> bool

Flag to determine whether only witness generation or proving and verification key generation is being performed.

• If witness gen is performed many operations can be skipped for optimization.

pub fn phase(&self) -> usize

The challenge phase that this Context will map to.

pub fn type_id(&self) -> &'static str

Identifier for what virtual region this context is in. Warning: the circuit writer must ensure that distinct virtual regions have distinct names as strings to prevent possible errors. We do not use std::any::TypeId because it is not stable across rust builds or dependencies.

impl<F: ScalarField> Context<F>

pub fn new( witness_gen_only: bool, phase: usize, type_id: &'static str, context_id: usize, copy_manager: SharedCopyConstraintManager<F>, ) -> Self

Creates a new Context with the given context_id and witness generation enabled/disabled by the witness_gen_only flag.

• witness_gen_only: flag to determine whether public key generation or only witness generation is being performed.
• context_id: identifier to reference advice cells from this Context later.

Warning: If you create your own Context in a new virtual region not provided by our libraries, you must ensure that the type_id: &str of the context is a globally unique identifier for the virtual region, distinct from the other type_id strings used to identify other virtual regions. We suggest that you either include your crate name as a prefix in the type_id or use module_path! to generate a prefix. In the future we will introduce a macro to check this uniqueness at compile time.

pub fn id(&self) -> usize

The context id, this can be used as a tag when CPU multi-threading

pub fn tag(&self) -> ContextTag

A unique tag that should identify this context across all virtual regions and phases.

pub fn assign_cell(&mut self, input: impl Into<QuantumCell<F>>)

Virtually assigns the input within the current Context, with different handling depending on the QuantumCell variant.

pub fn last(&self) -> Option<AssignedValue<F>>

Returns the AssignedValue of the last cell in the advice column of Context or None if advice is empty

pub fn get(&self, offset: isize) -> AssignedValue<F>

Returns the AssignedValue of the cell at the given offset in the advice column of Context

• offset: the offset of the cell to be fetched
  • offset may be negative indexing from the end of the column (e.g., -1 is the last cell)
• Assumes offset is a valid index in advice;
  • 0 <= offset < advice.len() (or advice.len() + offset >= 0 if offset is negative)

pub fn constrain_equal(&mut self, a: &AssignedValue<F>, b: &AssignedValue<F>)

Creates an equality constraint between two advice cells.

• a: the first advice cell to be constrained equal
• b: the second advice cell to be constrained equal
• Assumes both cells are advice cells

pub fn assign_region<Q>( &mut self, inputs: impl IntoIterator<Item = Q>, gate_offsets: impl IntoIterator<Item = isize>, )

where Q: Into<QuantumCell<F>>,

Pushes multiple advice cells to the advice column of Context and enables them by enabling the corresponding selector specified in gate_offset.

• inputs: Iterator that specifies the cells to be assigned
• gate_offsets: specifies relative offset from current position to enable selector for the gate (e.g., 0 is inputs[0]).
  • offset may be negative indexing from the end of the column (e.g., -1 is the last previously assigned cell)

pub fn assign_region_last<Q>( &mut self, inputs: impl IntoIterator<Item = Q>, gate_offsets: impl IntoIterator<Item = isize>, ) -> AssignedValue<F>

where Q: Into<QuantumCell<F>>,

Pushes multiple advice cells to the advice column of Context and enables them by enabling the corresponding selector specified in gate_offset and returns the last assigned cell.

Assumes gate_offsets is the same length as inputs

Returns the last assigned cell

• inputs: Iterator that specifies the cells to be assigned
• gate_offsets: specifies indices to enable selector for the gate; assume gate_offsets is sorted in increasing order
  • offset may be negative indexing from the end of the column (e.g., -1 is the last cell)

pub fn assign_region_smart<Q>( &mut self, inputs: impl IntoIterator<Item = Q>, gate_offsets: impl IntoIterator<Item = isize>, equality_offsets: impl IntoIterator<Item = (isize, isize)>, external_equality: impl IntoIterator<Item = (Option<ContextCell>, isize)>, )

where Q: Into<QuantumCell<F>>,

Pushes multiple advice cells to the advice column of Context and enables them by enabling the corresponding selector specified in gate_offset.

Allows for the specification of equality constraints between cells at equality_offsets within the advice column and external advice cells specified in external_equality (e.g, Fixed column).

• gate_offsets: specifies indices to enable selector for the gate;
  • offset may be negative indexing from the end of the column (e.g., -1 is the last cell)
• equality_offsets: specifies pairs of indices to constrain equality
• external_equality: specifies an existing cell to constrain equality with the cell at a certain index

pub fn assign_witnesses( &mut self, witnesses: impl IntoIterator<Item = F>, ) -> Vec<AssignedValue<F>>

Assigns a region of witness cells in an iterator and returns a Vec of assigned cells.

• witnesses: Iterator that specifies the cells to be assigned

pub fn load_witness(&mut self, witness: F) -> AssignedValue<F>

Assigns a witness value and returns the corresponding assigned cell.

• witness: the witness value to be assigned

pub fn load_constant(&mut self, c: F) -> AssignedValue<F>

Assigns a constant value and returns the corresponding assigned cell.

• c: the constant value to be assigned

pub fn load_constants(&mut self, c: &[F]) -> Vec<AssignedValue<F>>

Assigns a list of constant values and returns the corresponding assigned cells.

• c: the list of constant values to be assigned

pub fn load_zero(&mut self) -> AssignedValue<F>

Assigns the 0 value to a new cell or returns a previously assigned zero cell from zero_cell.

pub fn debug_assert_false(&mut self)

Helper function for debugging using MockProver. This adds a constraint that always fails. The MockProver will print out the row, column where it fails, so it serves as a debugging “break point” so you can add to your code to search for where the actual constraint failure occurs.

Trait Implementations

impl<F: Clone + ScalarField> Clone for Context<F>

fn clone(&self) -> Context<F>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<F: Debug + ScalarField> Debug for Context<F>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.