[commodity-utils] refactor gnc:pricelist-price-find-nearest

instead of O(3n), this implementation is O(n)

gnucash/report/report-system/commodity-utilities.scm

@@ -295,27 +295,21 @@ construct with gnc:make-gnc-monetary and gnc:monetary->string instead.")
 ;; pricelist comes from
 ;; e.g. gnc:get-commodity-totalavg-prices. Returns a <gnc-numeric> or,
 ;; if pricelist was empty, #f.
-(define (gnc:pricelist-price-find-nearest
-         pricelist date)
-  (let* ((later (find (lambda (p)
-                        (< date (car p)))
-                      pricelist))
-         (earlierlist (take-while
-                       (lambda (p)
-                         (>= date (car p)))
-                       pricelist))
-         (earlier (and (not (null? earlierlist))
-                       (last earlierlist))))
-
-    (if (and earlier later)
-        (if (< (abs (- date (car earlier)))
-               (abs (- date (car later))))
-            (cadr earlier)
-            (cadr later))
-        (or
-         (and earlier (cadr earlier))
-         (and later (cadr later))))))
-
+(define (gnc:pricelist-price-find-nearest pricelist date)
+  (let lp ((pricelist pricelist))
+    (cond
+     ((null? pricelist) #f)
+     ((null? (cdr pricelist)) (cadr (car pricelist)))
+     (else
+      (let ((earlier (car pricelist))
+            (later (cadr pricelist)))
+        (cond
+         ((< (car later) date)
+          (lp (cdr pricelist)))
+         ((< (- date (car earlier)) (- (car later) date))
+          (cadr earlier))
+         (else
+          (cadr later))))))))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Functions to get one price at a given time (i.e. not time-variant).