ui.gadgets.grid-lines: make it work with non-zero grid gap

basis/ui/gadgets/grid-lines/grid-lines-tests.factor

@@ -1,9 +1,6 @@
 IN: ui.gadgets.grid-lines.tests
-USING: tools.test ui.gadgets.grid-lines ui.gadgets.grid-lines.tests colors ;
-
-[ { 1 3 5 7 9 } ] [
-    8 { 0 2 4 6 } horizontal { 2 2 } grid-line-offsets
-] unit-test
+USING: tools.test ui.gadgets ui.gadgets.grid-lines ui.gadgets.grid-lines.private
+ui.gadgets.grids.private accessors arrays ui.gadgets.grids sorting kernel ;
 
 : 100x100 ( -- gadget ) <gadget> { 100 100 } >>dim ;
 
@@ -22,6 +19,22 @@ USING: tools.test ui.gadgets.grid-lines ui.gadgets.grid-lines.tests colors ;
     compute-grid-lines natural-sort
 ] unit-test
 
+[
+    {
+        { { 5 5 } { 5 115 } }
+        { { 5 5 } { 115 5 } }
+        { { 5 115 } { 115 115 } }
+        { { 115 5 } { 115 115 } }
+    }
+] [
+    100x100 1array
+    1array
+    <grid>
+    { 10 10 } >>gap
+    dup prefer
+    compute-grid-lines natural-sort
+] unit-test
+
 [
     {
         { { 0 0 } { 0 200 } }

basis/ui/gadgets/grid-lines/grid-lines.factor

@@ -3,7 +3,7 @@
 USING: kernel accessors math namespaces opengl opengl.gl
 sequences math.vectors ui.gadgets ui.gadgets.grids
 ui.gadgets.grids.private ui.render math.rectangles
-fry locals arrays ;
+fry locals arrays assocs ;
 IN: ui.gadgets.grid-lines
 
 TUPLE: grid-lines color ;
@@ -12,15 +12,11 @@ C: <grid-lines> grid-lines
 
 <PRIVATE
 
-: grid-line-offsets ( n ns orientation gap -- seq )
-    [ swap suffix ] [ v. 2/ ] 2bi* '[ _ + ] map ;
-
 :: (compute-grid-lines) ( grid n ns orientation -- seq )
-    n ns orientation grid gap>> grid-line-offsets [
-        [ orientation n*v ]
-        [ dup 2array grid dim>> swap orientation set-axis ]
-        bi 2array
-    ] map ;
+    grid gap>> :> gap
+    ns n suffix gap orientation v. '[ _ - orientation n*v ] map
+    dup grid dim>> gap v- orientation reverse v* '[ _ v+ ] map
+    [ [ gap [ 2/ ] map v+ ] map ] bi@ zip ;
 
 : compute-grid-lines ( grid -- lines )
     dup <grid-layout>