ui: register: reduce "warping", keep selection at nearest txn

When the transaction list changes, eg due to toggling a query filter,
and the previously selected item has disappeared, instead of moving
the selection to the end, we now select the nearest transaction by
date (and if several have the same date, by journal order).

hledger-ui/Hledger/UI/RegisterScreen.hs

@@ -17,7 +17,7 @@ import Data.List.Split (splitOn)
 import Data.Monoid
 import Data.Maybe
 import qualified Data.Text as T
-import Data.Time.Calendar (Day)
+import Data.Time.Calendar
 import qualified Data.Vector as V
 import Graphics.Vty (Event(..),Key(..),Modifier(..))
 import Brick
@@ -25,6 +25,7 @@ import Brick.Widgets.List
 import Brick.Widgets.Edit
 import Brick.Widgets.Border (borderAttr)
 import Lens.Micro.Platform
+import Safe
 import System.Console.ANSI
 
 
@@ -101,15 +102,27 @@ rsInit d reset ui@UIState{aopts=UIOpts{cliopts_=CliOpts{reportopts_=ropts}}, ajo
     -- build the List
     newitems = list RegisterList (V.fromList $ displayitems ++ blankitems) 1
 
-    -- keep the selection on the previously selected transaction if possible,
-    -- (eg after toggling nonzero mode), otherwise select the last element.
+    -- decide which transaction is selected.
+    -- if reset is true, select the last (latest) transaction;
+    -- otherwise, select the previously selected transaction if possible;
+    -- otherwise, select the transaction nearest in date to it;
+    -- or if there's several with the same date, the nearest in journal order;
+    -- otherwise, select the last (latest) transaction. 
     newitems' = listMoveTo newselidx newitems
       where
-        newselidx = case (reset, listSelectedElement rsList) of
-                      (True, _)    -> endidx
-                      (_, Nothing) -> endidx
-                      (_, Just (_,RegisterScreenItem{rsItemTransaction=Transaction{tindex=ti}}))
-                                   -> fromMaybe endidx $ findIndex ((==ti) . tindex . rsItemTransaction) displayitems
+        newselidx = 
+          case (reset, listSelectedElement rsList) of
+            (True, _)    -> endidx
+            (_, Nothing) -> endidx
+            (_, Just (_, RegisterScreenItem{rsItemTransaction=Transaction{tindex=prevselidx, tdate=prevseld}})) ->
+              headDef endidx $ catMaybes [
+                 findIndex ((==prevselidx) . tindex . rsItemTransaction) displayitems
+                ,findIndex ((==nearestidbydatethenid) . Just . tindex . rsItemTransaction) displayitems
+                ]
+              where
+                nearestidbydatethenid = third3 <$> (headMay $ sort
+                  [(abs $ diffDays (tdate t) prevseld, abs (tindex t - prevselidx), tindex t) | t <- ts])
+                ts = map rsItemTransaction displayitems
         endidx = length displayitems - 1
 
 rsInit _ _ _ = error "init function called with wrong screen type, should not happen"