[API] gnc:list-flatten flattens lists recursively

and is a schemey algorithm rather than a report algorithm, so, belongs
centrally.

gnucash/report/standard-reports/budget.scm

@@ -317,17 +317,6 @@
                   (gnc-budget-get-account-period-actual-value budget acct period))
                 periodlist)))
 
-    (define (flatten lst)
-      (reverse!
-       (let loop ((lst lst) (result '()))
-         (if (null? lst)
-             result
-             (let ((elt (car lst))
-                   (rest (cdr lst)))
-               (if (pair? elt)
-                   (loop rest (append (loop elt '()) result))
-                   (loop rest (cons elt result))))))))
-
     ;; Adds a line to the budget report.
     ;;
     ;; Parameters:
@@ -342,7 +331,7 @@
              column-list exchange-fn)
       (let* ((comm (xaccAccountGetCommodity acct))
              (reverse-balance? (gnc-reverse-balance acct))
-             (allperiods (filter number? (flatten column-list)))
+             (allperiods (filter number? (gnc:list-flatten column-list)))
              (total-periods (if accumulate?
                                 (iota (1+ (apply max allperiods)))
                                 allperiods))

libgnucash/scm/test/test-libgnucash-scm-utilities.scm

@@ -10,6 +10,7 @@
   (test-traverse-vec)
   (test-substring-replace)
   (test-sort-and-delete-duplicates)
+  (test-gnc:list-flatten)
   (test-begin "test-libgnucash-scm-utilities.scm"))
 
 (define (test-traverse-vec)
@@ -87,3 +88,14 @@
     '(1 2 3)
     (sort-and-delete-duplicates '(3 1 2) <))
   (test-end "sort-and-delete-duplicates"))
+
+(define (test-gnc:list-flatten)
+  (test-equal "gnc:list-flatten null"
+    '()
+    (gnc:list-flatten '()))
+  (test-equal "gnc:list-flatten noop"
+    '(1 2 3)
+    (gnc:list-flatten '(1 2 3)))
+  (test-equal "gnc:list-flatten deep"
+    '(1 2 3 4 5 6)
+    (gnc:list-flatten '(1 (2) (() () (((((3))) ())) 4 () ((5) (6)))))))

libgnucash/scm/utilities.scm

@@ -47,6 +47,7 @@
 (export gnc:debug)
 (export addto!)
 (export sort-and-delete-duplicates)
+(export gnc:list-flatten)
 
 ;; Do this stuff very early -- but other than that, don't add any
 ;; executable code until the end of the file if you can help it.
@@ -191,6 +192,17 @@
   (define (kons a b) (if (and (pair? b) (= a (car b))) b (cons a b)))
   (reverse (fold kons '() (sort lst <))))
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; flattens an arbitrary deep nested list into simple list.  this is
+;; probably the most efficient algorithm available. '(1 2 (3 4)) -->
+;; '(1 2 3 4)
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+(define (gnc:list-flatten . lst)
+  (reverse
+   (let lp ((e lst) (accum '()))
+     (if (list? e)
+         (fold lp accum e)
+         (cons e accum)))))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; compatibility hack for fixing guile-2.0 string handling. this code