matrix.js

/**
 * from https://rosettacode.org/wiki/Matrix_multiplication#ES6
 */
const matrixMultiply = (a, b) => {
  const bCols = transpose(b);
  return a.map(aRow => bCols.map(bCol => dotProduct(aRow, bCol)));
};

// dotProduct :: Num a => [[a]] -> [[a]] -> [[a]]
const dotProduct = (xs, ys) => sum(zipWith(product, xs, ys));

// GENERIC

// zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
const zipWith = (f, xs, ys) =>
  xs.length === ys.length ? xs.map((x, i) => f(x, ys[i])) : undefined;

// transpose :: [[a]] -> [[a]]
const transpose = xs => xs[0].map((_, iCol) => xs.map(row => row[iCol]));

// sum :: (Num a) => [a] -> a
const sum = xs => xs.reduce((a, x) => a + x, 0);

// product :: Num a => a -> a -> a
const product = (a, b) => a * b;

module.exports = {
  matrixMultiply,
};