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;;;; License: CC0-1.0
(define-module (net ricketyspace sicp two twentynine)
#:export (make-mobile
make-branch
left-branch
right-branch
branch-length
branch-structure
total-weight
mobile-balanced?))
(define (make-mobile left right)
(list left right))
(define (make-branch length structure)
(list length structure))
(define (left-branch mobile)
(car mobile))
(define (right-branch mobile)
(car (cdr mobile)))
(define (branch-length branch)
(car branch))
(define (branch-structure branch)
(car (cdr branch)))
(define (branch-weight b)
(let ((bs (branch-structure b)))
(cond ((not (pair? bs)) bs)
(else (+ (branch-weight (left-branch bs))
(branch-weight (right-branch bs)))))))
(define (total-weight mobile)
(+ (branch-weight (left-branch mobile))
(branch-weight (right-branch mobile))))
(define (has-submobile? branch)
(not (number? (branch-structure branch))))
(define (torque branch)
(* (branch-length branch)
(branch-weight branch)))
(define (mobile-balanced? m)
(let ((lb (left-branch m))
(rb (right-branch m)))
(and
(cond ((has-submobile? lb)
(mobile-balanced? (branch-structure lb)))
(else #t))
(cond ((has-submobile? rb)
(mobile-balanced? (branch-structure rb)))
(else #t))
(= (torque lb) (torque rb)))))
;;; Guile REPL
;;;
;;; scheme@(guile-user)> (make-mobile
;;; (make-branch 3 45)
;;; (make-branch 4
;;; (make-mobile (make-branch 5 50)
;;; (make-branch 1 20))))
;;; $11 = ((3 45) (4 ((5 50) (1 20))))
;;; scheme@(guile-user)> (left-branch $11)
;;; $12 = (3 45)
;;; scheme@(guile-user)> (right-branch $11)
;;; $13 = (4 ((5 50) (1 20)))
;;; scheme@(guile-user)> (branch-length (left-branch $11))
;;; $14 = 3
;;; scheme@(guile-user)> (branch-structure (left-branch $11))
;;; $15 = 45
;;; scheme@(guile-user)> (branch-length (right-branch $11))
;;; $16 = 4
;;; scheme@(guile-user)> (branch-structure (right-branch $11))
;;; $17 = ((5 50) (1 20))
;;; scheme@(guile-user)> (branch-structure (right-branch $11))
;;; $18 = ((5 50) (1 20))
;;; scheme@(guile-user)> (left-branch (branch-structure (right-branch $11)))
;;; $19 = (5 50)
;;; scheme@(guile-user)> (right-branch (branch-structure (right-branch $11)))
;;; $20 = (1 20)
;;; scheme@(guile-user)> (total-weight $11)
;;; $21 = 115
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