Compressed Sparse Row (CSR) Matrix

csr.js

// https://en.wikipedia.org/wiki/Sparse_matrix#Compressed_sparse_row_.28CSR.2C_CRS_or_Yale_format.29
function csr(matrix) {
    const A = [];
    const IA = [0];
    const JA = [];
    const nnz = [];

    for (const [ index, row ] of matrix.entries()) {
        nnz[index] = 0;
      
        for (const [ column, value ] of row.entries()) {
            if (value !== 0) {
                A.push(value);
                JA.push(column);
                nnz[index] += 1;
            }
        }
        
        IA[index + 1] = IA[index] + nnz[index];
    }

    return [ A, IA, JA ];
}

function uncsr([ A, IA, JA ]) {
    const result = [];
    let count = 0;

    for (const [ index, value ] of IA.entries()) {
        if (index < IA.length - 1) {
            result[index] = [];
            const row = JA.slice(value, IA[index + 1]);
            for (const pos of row) {
                result[index][pos] = A[count];
                count += 1;
            }
        }
    }

    return result;
}