Generalize period-based definitions lookup.

period.scm

@@ -39,6 +39,8 @@
   month-in-periods?
   periods->string
   periods-match
+  make-period-lookup-table
+  lookup-by-period
   period-tests!
   )
 
@@ -171,6 +173,30 @@
 	       (car ps)
 	       (loop (cdr ps)))))))
 
+ ;; Creates lookup table from definition source
+ (define (make-period-lookup-table source)
+   (map
+    (lambda (src)
+      (let* ((since0 (car src))
+	     (since (apply make-month since0))
+	     (before0 (cadr src))
+	     (before (if before0
+			 (apply make-month before0)
+			 #f))
+	     (definitions (cddr src)))
+	(cons (make-period since before)
+	      definitions)))
+    source))
+
+ ;; Looks up current month and returns associated definitions
+ (define (lookup-by-period table)
+   (let loop ((lst table))
+     (if (null? lst)
+	 #f
+	 (if (month-in-period? (caar lst))
+	     (cdar lst)
+	     (loop (cdr lst))))))
+
  ;; Performs self-tests of the period module.
  (define (period-tests!)
    (run-tests