functions: Add tbf.

functions.rkt

@@ -10,6 +10,9 @@
 (require "utils.rkt")
 
 (provide
+ ;; Structures
+ (contract-out
+  [struct tbf ((weights number?) (threshold (vectorof number?)))])
  ;; Functions
  (contract-out
   [tabulate (-> procedure? (listof generic-set?) (listof list?))]
@@ -23,7 +26,9 @@
   [enumerate-boolean-functions/list (-> number? (stream/c procedure?))]
   [random-boolean-table (-> number? (listof (*list/c boolean? boolean?)))]
   [random-boolean-function (-> number? procedure?)]
-  [random-boolean-function/list (-> number? procedure?)]))
+  [random-boolean-function/list (-> number? procedure?)]
+  [tbf-w (-> tbf? (vectorof number?))]
+  [tbf-θ (-> tbf? number?)]))
 
 (module+ test
   (require rackunit))
@@ -197,3 +202,16 @@
     (define f (random-boolean-function/list 2))
     (check-false (f '(#f #f))) (check-true (f '(#f #t)))
     (check-true (f '(#t #f))) (check-false (f '(#t #t)))))
+
+
+;;; ===========================
+;;; Threshold Boolean functions
+;;; ===========================
+
+;;; A threshold Boolean function (TBF) is a pair (w, θ), where w is a
+;;; vector of weights and θ is the threshold.
+(struct tbf (weights threshold) #:transparent)
+
+;;; Unicode shortcuts for accessing the elements of a TBF.
+(define tbf-w tbf-weights)
+(define tbf-θ tbf-threshold)