Add `initial-value` and `modulo` to `summation` and `product`
author |
Steve Losh <steve@stevelosh.com> |
date |
Wed, 07 Nov 2018 18:08:28 -0500 |
parents |
de9d10a9b4b5 |
children |
(none) |
(in-package :losh.weightlists)
(defstruct (weightlist (:constructor %make-weightlist))
weights sums items total)
(defun make-weightlist (weights-and-items)
"Make a weightlist of the given items and weights.
`weights-and-items` should be an alist of `(weight . item)` pairs.
Weights can be any `real` numbers. Weights of zero are fine, as long as at
least one of the weights is nonzero (otherwise there's nothing to choose).
"
(let ((weights (mapcar #'car weights-and-items))
(items (mapcar #'cdr weights-and-items)))
(%make-weightlist
:items items
:weights weights
:sums (prefix-sums weights)
:total (apply #'+ weights))))
(defun weightlist-random (weightlist)
"Return a random item from the weightlist, taking the weights into account."
(iterate
(with n = (random (weightlist-total weightlist)))
(for item :in (weightlist-items weightlist))
(for weight :in (weightlist-sums weightlist))
(finding item :such-that (< n weight))))