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;;;; Under Creative Commons Attribution-ShareAlike 4.0
;;;; International. See
;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
(define-module (net ricketyspace sicp two eleven)
#:use-module (net ricketyspace sicp two seven)
#:export (mul-interval-alt))
(define (mul-interval-alt x y)
(define lbx (lower-bound x))
(define ubx (upper-bound x))
(define lby (lower-bound y))
(define uby (upper-bound y))
(define (n a) (< a 0)) ; is negative
(define (p a) (>= a 0)) ; is positive
(define (signs slbx subx slby suby)
(and (slbx lbx)
(subx ubx)
(slby lby)
(suby uby)))
(cond ((signs n p p p) (make-interval (* lbx uby) (* ubx uby)))
((signs p p n p) (make-interval (* ubx lby) (* ubx uby)))
((signs p p p p) (make-interval (* lbx lby) (* ubx uby)))
((signs n n n n) (make-interval (* lbx lby) (* ubx uby)))
((signs n n p p) (make-interval (* ubx uby) (* lbx lby)))
((signs p p n n) (make-interval (* ubx uby) (* lbx lby)))
((signs n n n p) (make-interval (* ubx uby) (* ubx lby)))
((signs n p n n) (make-interval (* ubx uby) (* lbx uby)))
((signs n p n p) (make-interval (min (* lbx uby) (* ubx lby))
(* ubx uby)))
(else (error "Intervals not supported"))))
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