reduce and mapcar

mapreduce.lisp

(in-package :cl-user)

(defun dot-product (lst1 lst2 &optional (acc 0))
  "Computes the dot product of two sequences, represented as lists."
  (if (not lst1)
      acc
      (let ((x (first lst1))
            (y (first lst2))
            (lst11 (rest lst1))
            (lst22 (rest lst2)))
        (dot-product lst11 lst22 (+ acc (* x y))))))

(declaim (inline mapreduce))
(defun mapreduce (reducer initial-value mapper list &rest more-lists)
  (declare (dynamic-extent more-lists))
  (let ((acc initial-value))
    (apply #'mapc
           (lambda (&rest args)
             (declare (dynamic-extent args))
             (setq acc (funcall reducer acc (apply mapper args))))
           list more-lists)
    acc))

(declaim (inline mapreduce2))
(defun mapreduce2 (reducer initial-value mapper list1 list2)
  (loop with acc = initial-value
        for x in list1
        for y in list2
        do (setq acc (funcall reducer acc (funcall mapper x y)))
        finally (return acc)))

(defun dot-product1 (list1 list2)
  (mapreduce2 #'+ 0 #'* list1 list2))