ReaderLink: allow a forward location stack (#10228)

this allows going back and forth from links (think of undo / redo)

when going back and no forward locations and when we are not on the same page as the last saved location, add the current location to the forward stack, helping if one goes back by mistake they can jump back to their current location

when going back and no forward locations and when we are not on the same page as the last saved location, add the current location to the forward stack, helping if one goes back by mistake they can jump back to thier current location

frontend/apps/reader/modules/readerlink.lua

@@ -64,6 +64,7 @@ end
 
 local ReaderLink = InputContainer:extend{
     location_stack = nil, -- table, per-instance
+    forward_location_stack = nil, -- table, per-instance
     _external_link_buttons = nil,
 }
 
@@ -250,7 +251,7 @@ ReaderLink.onPhysicalKeyboardConnected = ReaderLink.registerKeyEvents
 
 function ReaderLink:onReadSettings(config)
     -- called when loading new document
-    self.location_stack = {}
+    self:onClearLocationStack()
 end
 
 local function isTapToFollowLinksOn()
@@ -306,6 +307,21 @@ function ReaderLink:addToMainMenu(menu_items)
             })
         end,
     }
+    menu_items.go_to_next_location = {
+        text = _("Go Forward to next location"),
+        enabled_func = function() return self.forward_location_stack and #self.forward_location_stack > 0 end,
+        callback = function() self:onGoForwardLink() end,
+        hold_callback = function(touchmenu_instance)
+            UIManager:show(ConfirmBox:new{
+                text = _("Clear Forward location history?"),
+                ok_text = _("Clear"),
+                ok_callback = function()
+                    self:onClearForwardLocationStack()
+                    touchmenu_instance:closeMenu()
+                end,
+            })
+        end,
+    }
     if not Device:isTouchDevice() then
         -- Menu items below aren't needed.
         return
@@ -662,19 +678,40 @@ function ReaderLink:onTap(_, ges)
     end
 end
 
---- Remember current location so we can go back to it
-function ReaderLink:addCurrentLocationToStack()
+function ReaderLink:getCurrentLocation()
+    local location
     if self.ui.document.info.has_pages then
-        table.insert(self.location_stack, self.ui.paging:getBookLocation())
+        location = self.ui.paging:getBookLocation()
     else
-        table.insert(self.location_stack, {
-            xpointer = self.ui.rolling:getBookLocation(),
-        })
+        location = {xpointer = self.ui.rolling:getBookLocation(),}
     end
+    return location
+end
+
+-- Returns true, current_location if the current location is the same as the
+-- saved_location on the top of the stack.
+-- Otherwise returns false, current_location
+function ReaderLink:compareLocationToCurrent(saved_location)
+    local current_location = self:getCurrentLocation()
+    if self.ui.rolling and saved_location.xpointer and saved_location.xpointer == current_location.xpointer then
+        return true, current_location
+    end
+    if self.ui.paging and saved_location[1] and current_location[1] and current_location[1].page == saved_location[1].page then
+        return true, current_location
+    end
+    return false, current_location
+end
+
+-- Remember current location so we can go back to it
+function ReaderLink:addCurrentLocationToStack(loc)
+    local location = loc and loc or self:getCurrentLocation()
+    self:onClearForwardLocationStack()
+    table.insert(self.location_stack, location)
 end
 
 function ReaderLink:onClearLocationStack(show_notification)
     self.location_stack = {}
+    self:onClearForwardLocationStack()
     if show_notification then
         UIManager:show(Notification:new{
             text = _("Location history cleared."),
@@ -683,6 +720,11 @@ function ReaderLink:onClearLocationStack(show_notification)
     return true
 end
 
+function ReaderLink:onClearForwardLocationStack()
+    self.forward_location_stack = {}
+    return true
+end
+
 function ReaderLink:getPreviousLocationPages()
     local previous_locations = {}
     if #self.location_stack > 0 then
@@ -751,7 +793,7 @@ function ReaderLink:onGotoLink(link, neglect_current_location, allow_footnote_po
                             marker_xpointer = link.from_xpointer,
                         }
                     end
-                    table.insert(self.location_stack, saved_location)
+                    self:addCurrentLocationToStack(saved_location)
                 else
                     self:addCurrentLocationToStack()
                 end
@@ -827,6 +869,24 @@ end
 function ReaderLink:onGoBackLink(show_notification_if_empty)
     local saved_location = table.remove(self.location_stack)
     if saved_location then
+        local same_page, current_location = self:compareLocationToCurrent(saved_location)
+        -- If there are no forward items
+        if #self.forward_location_stack == 0 then
+            -- If we are not on the same page as the current item,
+            -- then add our current location to the forward stack
+            if not same_page then
+                table.insert(self.forward_location_stack, current_location)
+            end
+        end
+        if same_page then
+            -- If we are on the same page pass through to the next location
+            table.insert(self.forward_location_stack, saved_location)
+            saved_location = table.remove(self.location_stack)
+        end
+    end
+
+    if saved_location then
+        table.insert(self.forward_location_stack, saved_location)
         logger.dbg("GoBack: restoring:", saved_location)
         self.ui:handleEvent(Event:new('RestoreBookLocation', saved_location))
         return true
@@ -837,6 +897,26 @@ function ReaderLink:onGoBackLink(show_notification_if_empty)
     end
 end
 
+--- Goes to next location.
+function ReaderLink:onGoForwardLink()
+    local saved_location = table.remove(self.forward_location_stack)
+    if saved_location then
+        local same_page = self:compareLocationToCurrent(saved_location)
+        if same_page then
+            -- If we are on the same page pass through to the next location
+            table.insert(self.location_stack, saved_location)
+            saved_location = table.remove(self.forward_location_stack)
+        end
+    end
+
+    if saved_location then
+        table.insert(self.location_stack, saved_location)
+        logger.dbg("GoForward: restoring:", saved_location)
+        self.ui:handleEvent(Event:new('RestoreBookLocation', saved_location))
+        return true
+    end
+end
+
 function ReaderLink:onSwipe(arg, ges)
     local direction = BD.flipDirectionIfMirroredUILayout(ges.direction)
     if direction == "east" then