Figure 1: Prover workflow: Trace polynomials

In the previous section, we created a table (aka spreadsheet). In this section, we will convert the table into something called trace polynomials.

Figure 2: From spreadsheet to trace polynomials

As we can see in Figure 2, the cells in each column of the table can be seen as evaluations of a Lagrange polynomial. A characteristic of a Lagrange polynomial is that interpolating nn distinct points will result in a unique polynomial of at most n−1n-1 degrees. So if we consider each row value f(xi)f(x_i) as the evaluation of a distinct point xix_i, we can get a unique polynomial of degree num_rows−1num\_rows-1 for each column, which is also known as a trace polynomial. We will explain why using a polynomial representation is useful in the next section, but for now, let’s see how we can create trace polynomials for our code. Note that we are building upon the code from the previous section, so there’s not much new code here. 
use stwo_prover::core::{
    backend::{
        simd::{
            column::BaseColumn,
            m31::{LOG_N_LANES, N_LANES},
            SimdBackend,
        },
        Column,
    },
    fields::m31::M31,
    poly::{
        circle::{CanonicCoset, CircleEvaluation},
        BitReversedOrder,
    },
    ColumnVec,
};

fn main() {
    let num_rows = N_LANES;
    let log_num_rows = LOG_N_LANES;

    // Create the table
    let mut col_1 = BaseColumn::zeros(num_rows);
    col_1.set(0, M31::from(1));
    col_1.set(1, M31::from(7));

    let mut col_2 = BaseColumn::zeros(num_rows);
    col_2.set(0, M31::from(5));
    col_2.set(1, M31::from(11));

    // Convert table to trace polynomials
    let domain = CanonicCoset::new(log_num_rows).circle_domain();
    let _trace: ColumnVec<CircleEvaluation<SimdBackend, M31, BitReversedOrder>> =
        vec![col_1, col_2]
            .into_iter()
            .map(|col| CircleEvaluation::new(domain, col))
            .collect();
}
Here, domain refers to the xix_i values used to interpolate the trace polynomials. For example, x1,x2x_1, x_2 values are the domain values for our example (in reality, we need the size of the domain needs to be 16 as we have 16 rows). We can ignore terms like CanonicCoset and .circle_domain() for now, but should note that the log_num_rows in CanonicCoset::new(log_num_rows).circle_domain() needs to be equal to the log of the actual number of rows that are used in the table.