# HG changeset patch
# User Rubidium <rubidium@openttd.org>
# Date 2021-04-18 18:29:46
# Node ID 136702761466f0e1a8af53a48038de0af5483264
# Parent  ff804fc9f5695985323a46ebc01bb6a27a8a43ce

Codechange: merge duplicated logic to scroll in lists by key into a single function

diff --git a/src/network/network_content_gui.cpp b/src/network/network_content_gui.cpp
--- a/src/network/network_content_gui.cpp
+++ b/src/network/network_content_gui.cpp
@@ -863,55 +863,31 @@ public:
 
 	EventState OnKeyPress(WChar key, uint16 keycode) override
 	{
-		switch (keycode) {
-			case WKC_UP:
-				/* scroll up by one */
-				if (this->list_pos > 0) this->list_pos--;
-				break;
-			case WKC_DOWN:
-				/* scroll down by one */
-				if (this->list_pos < (int)this->content.size() - 1) this->list_pos++;
-				break;
-			case WKC_PAGEUP:
-				/* scroll up a page */
-				this->list_pos = (this->list_pos < this->vscroll->GetCapacity()) ? 0 : this->list_pos - this->vscroll->GetCapacity();
-				break;
-			case WKC_PAGEDOWN:
-				/* scroll down a page */
-				this->list_pos = std::min(this->list_pos + this->vscroll->GetCapacity(), (int)this->content.size() - 1);
-				break;
-			case WKC_HOME:
-				/* jump to beginning */
-				this->list_pos = 0;
-				break;
-			case WKC_END:
-				/* jump to end */
-				this->list_pos = (int)this->content.size() - 1;
-				break;
+		if (this->vscroll->UpdateListPositionOnKeyPress(this->list_pos, keycode) == ES_NOT_HANDLED) {
+			switch (keycode) {
+				case WKC_SPACE:
+				case WKC_RETURN:
+					if (keycode == WKC_RETURN || !IsWidgetFocused(WID_NCL_FILTER)) {
+						if (this->selected != nullptr) {
+							_network_content_client.ToggleSelectedState(this->selected);
+							this->content.ForceResort();
+							this->InvalidateData();
+						}
+						if (this->filter_data.types.any()) {
+							this->content.ForceRebuild();
+							this->InvalidateData();
+						}
+						return ES_HANDLED;
+					}
+					/* space is pressed and filter is focused. */
+					FALLTHROUGH;
 
-			case WKC_SPACE:
-			case WKC_RETURN:
-				if (keycode == WKC_RETURN || !IsWidgetFocused(WID_NCL_FILTER)) {
-					if (this->selected != nullptr) {
-						_network_content_client.ToggleSelectedState(this->selected);
-						this->content.ForceResort();
-						this->InvalidateData();
-					}
-					if (this->filter_data.types.any()) {
-						this->content.ForceRebuild();
-						this->InvalidateData();
-					}
-					return ES_HANDLED;
-				}
-				/* space is pressed and filter is focused. */
-				FALLTHROUGH;
-
-			default:
-				return ES_NOT_HANDLED;
+				default:
+					return ES_NOT_HANDLED;
+			}
 		}
 
 		if (this->content.size() == 0) {
-			this->list_pos = 0; // above stuff may result in "-1".
 			if (this->UpdateFilterState()) {
 				this->content.ForceRebuild();
 				this->InvalidateData();
diff --git a/src/network/network_gui.cpp b/src/network/network_gui.cpp
--- a/src/network/network_gui.cpp
+++ b/src/network/network_gui.cpp
@@ -66,8 +66,8 @@ void UpdateNetworkGameWindow()
 }
 
 typedef GUIList<NetworkGameList*, StringFilter&> GUIGameServerList;
-typedef uint16 ServerListPosition;
-static const ServerListPosition SLP_INVALID = 0xFFFF;
+typedef int ServerListPosition;
+static const ServerListPosition SLP_INVALID = -1;
 
 /** Full blown container to make it behave exactly as we want :) */
 class NWidgetServerListHeader : public NWidgetContainer {
@@ -771,39 +771,8 @@ public:
 		EventState state = ES_NOT_HANDLED;
 
 		/* handle up, down, pageup, pagedown, home and end */
-		if (keycode == WKC_UP || keycode == WKC_DOWN || keycode == WKC_PAGEUP || keycode == WKC_PAGEDOWN || keycode == WKC_HOME || keycode == WKC_END) {
-			if (this->servers.size() == 0) return ES_HANDLED;
-			switch (keycode) {
-				case WKC_UP:
-					/* scroll up by one */
-					if (this->list_pos == SLP_INVALID) return ES_HANDLED;
-					if (this->list_pos > 0) this->list_pos--;
-					break;
-				case WKC_DOWN:
-					/* scroll down by one */
-					if (this->list_pos == SLP_INVALID) return ES_HANDLED;
-					if (this->list_pos < this->servers.size() - 1) this->list_pos++;
-					break;
-				case WKC_PAGEUP:
-					/* scroll up a page */
-					if (this->list_pos == SLP_INVALID) return ES_HANDLED;
-					this->list_pos = (this->list_pos < this->vscroll->GetCapacity()) ? 0 : this->list_pos - this->vscroll->GetCapacity();
-					break;
-				case WKC_PAGEDOWN:
-					/* scroll down a page */
-					if (this->list_pos == SLP_INVALID) return ES_HANDLED;
-					this->list_pos = std::min(this->list_pos + this->vscroll->GetCapacity(), (int)this->servers.size() - 1);
-					break;
-				case WKC_HOME:
-					/* jump to beginning */
-					this->list_pos = 0;
-					break;
-				case WKC_END:
-					/* jump to end */
-					this->list_pos = (ServerListPosition)this->servers.size() - 1;
-					break;
-				default: NOT_REACHED();
-			}
+		if (this->vscroll->UpdateListPositionOnKeyPress(this->list_pos, keycode) == ES_HANDLED) {
+			if (this->list_pos == SLP_INVALID) return ES_HANDLED;
 
 			this->server = this->servers[this->list_pos];
 
diff --git a/src/newgrf_gui.cpp b/src/newgrf_gui.cpp
--- a/src/newgrf_gui.cpp
+++ b/src/newgrf_gui.cpp
@@ -1318,42 +1318,8 @@ struct NewGRFWindow : public Window, New
 	{
 		if (!this->editable) return ES_NOT_HANDLED;
 
-		switch (keycode) {
-			case WKC_UP:
-				/* scroll up by one */
-				if (this->avail_pos > 0) this->avail_pos--;
-				break;
-
-			case WKC_DOWN:
-				/* scroll down by one */
-				if (this->avail_pos < (int)this->avails.size() - 1) this->avail_pos++;
-				break;
-
-			case WKC_PAGEUP:
-				/* scroll up a page */
-				this->avail_pos = (this->avail_pos < this->vscroll2->GetCapacity()) ? 0 : this->avail_pos - this->vscroll2->GetCapacity();
-				break;
+		if (this->vscroll2->UpdateListPositionOnKeyPress(this->avail_pos, keycode) == ES_NOT_HANDLED) return ES_NOT_HANDLED;
 
-			case WKC_PAGEDOWN:
-				/* scroll down a page */
-				this->avail_pos = std::min(this->avail_pos + this->vscroll2->GetCapacity(), (int)this->avails.size() - 1);
-				break;
-
-			case WKC_HOME:
-				/* jump to beginning */
-				this->avail_pos = 0;
-				break;
-
-			case WKC_END:
-				/* jump to end */
-				this->avail_pos = (uint)this->avails.size() - 1;
-				break;
-
-			default:
-				return ES_NOT_HANDLED;
-		}
-
-		if (this->avails.size() == 0) this->avail_pos = -1;
 		if (this->avail_pos >= 0) {
 			this->active_sel = nullptr;
 			DeleteWindowByClass(WC_GRF_PARAMETERS);
diff --git a/src/widget.cpp b/src/widget.cpp
--- a/src/widget.cpp
+++ b/src/widget.cpp
@@ -1975,6 +1975,66 @@ int Scrollbar::GetScrolledRowFromWidget(
 }
 
 /**
+ * Update the given list position as if it were on this scroll bar when the given keycode was pressed.
+ * This does not update the actual position of this scroll bar, that is left to the caller. It does,
+ * however use the capacity and count of the scroll bar for the bounds and amount to scroll.
+ *
+ * When the count is 0 or the return is ES_NOT_HANDLED, then the position is not updated.
+ * With WKC_UP and WKC_DOWN the position goes one up or down respectively.
+ * With WKC_PAGEUP and WKC_PAGEDOWN the position goes one capacity up or down respectively.
+ * With WKC_HOME the first position is selected and with WKC_END the last position is selected.
+ * This function ensures that pos is in the range [0..count).
+ * @param list_position The current position in the list.
+ * @param key_code      The pressed key code.
+ * @return ES_NOT_HANDLED when another key than the 6 specific keys was pressed, otherwise ES_HANDLED.
+ */
+EventState Scrollbar::UpdateListPositionOnKeyPress(int &list_position, uint16 keycode) const
+{
+	int new_pos = list_position;
+	switch (keycode) {
+		case WKC_UP:
+			/* scroll up by one */
+			new_pos--;
+			break;
+
+		case WKC_DOWN:
+			/* scroll down by one */
+			new_pos++;
+			break;
+
+		case WKC_PAGEUP:
+			/* scroll up a page */
+			new_pos -= this->GetCapacity();
+			break;
+
+		case WKC_PAGEDOWN:
+			/* scroll down a page */
+			new_pos += this->GetCapacity();
+			break;
+
+		case WKC_HOME:
+			/* jump to beginning */
+			new_pos = 0;
+			break;
+
+		case WKC_END:
+			/* jump to end */
+			new_pos = this->GetCount() - 1;
+			break;
+
+		default:
+			return ES_NOT_HANDLED;
+	}
+
+	/* If there are no elements, there is nothing to scroll/update. */
+	if (this->GetCount() != 0) {
+		list_position = Clamp(new_pos, 0, this->GetCount() - 1);
+	}
+	return ES_HANDLED;
+}
+
+
+/**
  * Set capacity of visible elements from the size and resize properties of a widget.
  * @param w       Window.
  * @param widget  Widget with size and resize properties.
diff --git a/src/widget_type.h b/src/widget_type.h
--- a/src/widget_type.h
+++ b/src/widget_type.h
@@ -749,6 +749,7 @@ public:
 	}
 
 	int GetScrolledRowFromWidget(int clickpos, const Window * const w, int widget, int padding = 0, int line_height = -1) const;
+	EventState UpdateListPositionOnKeyPress(int &list_position, uint16 keycode) const;
 };
 
 /**