Add range selection ability on list contexts

This adds range select ability in two ways:
1) Sticky: like what we already have with the staging view i.e. press v then use arrow keys
2) Non-sticky: where you just use shift+up/down to expand the range

The state machine works like this:
(no range, press 'v') -> sticky range
(no range, press arrow) -> no range
(no range, press shift+arrow) -> nonsticky range
(sticky range, press 'v') -> no range
(sticky range, press arrow) -> sticky range
(sticky range, press shift+arrow) -> nonsticky range
(nonsticky range, press 'v') -> no range
(nonsticky range, press arrow) -> no range
(nonsticky range, press shift+arrow) -> nonsticky range

3 files changed, 110 insertions, 11 deletions

diff --git a/pkg/gui/context/list_context_trait.go b/pkg/gui/context/list_context_trait.go
index ca3b3254f..c4f0e2549 100644
--- a/pkg/gui/context/list_context_trait.go
+++ b/pkg/gui/context/list_context_trait.go
@@ -32,6 +32,14 @@ func (self *ListContextTrait) FocusLine() {
 		self.GetViewTrait().FocusPoint(
 			self.ModelIndexToViewIndex(self.list.GetSelectedLineIdx()))
 
+		selectRangeIndex, isSelectingRange := self.list.GetRangeStartIdx()
+		if isSelectingRange {
+			selectRangeIndex = self.ModelIndexToViewIndex(selectRangeIndex)
+			self.GetViewTrait().SetRangeSelectStart(selectRangeIndex)
+		} else {
+			self.GetViewTrait().CancelRangeSelect()
+		}
+
 		// If FocusPoint() caused the view to scroll (because the selected line
 		// was out of view before), we need to rerender the view port again.
 		// This can happen when pressing , or . to scroll by pages, or < or > to
@@ -84,7 +92,7 @@ func (self *ListContextTrait) HandleFocusLost(opts types.OnFocusLostOpts) error
 
 // OnFocus assumes that the content of the context has already been rendered to the view. OnRender is the function which actually renders the content to the view
 func (self *ListContextTrait) HandleRender() error {
-	self.list.RefreshSelectedIdx()
+	self.list.ClampSelection()
 	content := self.renderLines(-1, -1)
 	self.GetViewTrait().SetContent(content)
 	self.c.Render()
diff --git a/pkg/gui/context/traits/list_cursor.go b/pkg/gui/context/traits/list_cursor.go
index 9e86d5139..647f2a36b 100644
--- a/pkg/gui/context/traits/list_cursor.go
+++ b/pkg/gui/context/traits/list_cursor.go
@@ -9,13 +9,34 @@ type HasLength interface {
 	Len() int
 }
 
+type RangeSelectMode int
+
+const (
+	// None means we are not selecting a range
+	RangeSelectModeNone RangeSelectMode = iota
+	// Sticky range select is started by pressing 'v', then the range is expanded
+	// when you move up or down. It is cancelled by pressing 'v' again.
+	RangeSelectModeSticky
+	// Nonsticky range select is started by pressing shift+arrow and cancelled
+	// when pressing up/down without shift, or by pressing 'v'
+	RangeSelectModeNonSticky
+)
+
 type ListCursor struct {
-	selectedIdx int
-	list        HasLength
+	selectedIdx     int
+	rangeSelectMode RangeSelectMode
+	// value is ignored when rangeSelectMode is RangeSelectModeNone
+	rangeStartIdx int
+	list          HasLength
 }
 
 func NewListCursor(list HasLength) *ListCursor {
-	return &ListCursor{selectedIdx: 0, list: list}
+	return &ListCursor{
+		selectedIdx:     0,
+		rangeStartIdx:   0,
+		rangeSelectMode: RangeSelectModeNone,
+		list:            list,
+	}
 }
 
 var _ types.IListCursor = (*ListCursor)(nil)
@@ -25,24 +46,86 @@ func (self *ListCursor) GetSelectedLineIdx() int {
 }
 
 func (self *ListCursor) SetSelectedLineIdx(value int) {
+	self.selectedIdx = self.clampValue(value)
+}
+
+func (self *ListCursor) clampValue(value int) int {
 	clampedValue := -1
 	if self.list.Len() > 0 {
 		clampedValue = utils.Clamp(value, 0, self.list.Len()-1)
 	}
 
-	self.selectedIdx = clampedValue
+	return clampedValue
+}
+
+// Moves the cursor up or down by the given amount.
+// If we are in non-sticky range select mode, this will cancel the range select
+func (self *ListCursor) MoveSelectedLine(change int) {
+	if self.rangeSelectMode == RangeSelectModeNonSticky {
+		self.CancelRangeSelect()
+	}
+
+	self.SetSelectedLineIdx(self.selectedIdx + change)
 }
 
-// moves the cursor up or down by the given amount
-func (self *ListCursor) MoveSelectedLine(delta int) {
-	self.SetSelectedLineIdx(self.selectedIdx + delta)
+// Moves the cursor up or down by the given amount, and also moves the range start
+// index by the same amount
+func (self *ListCursor) MoveSelection(delta int) {
+	self.selectedIdx = self.clampValue(self.selectedIdx + delta)
+	if self.IsSelectingRange() {
+		self.rangeStartIdx = self.clampValue(self.rangeStartIdx + delta)
+	}
 }
 
-// to be called when the model might have shrunk so that our selection is not not out of bounds
-func (self *ListCursor) RefreshSelectedIdx() {
-	self.SetSelectedLineIdx(self.selectedIdx)
+// To be called when the model might have shrunk so that our selection is not out of bounds
+func (self *ListCursor) ClampSelection() {
+	self.selectedIdx = self.clampValue(self.selectedIdx)
+	self.rangeStartIdx = self.clampValue(self.rangeStartIdx)
 }
 
 func (self *ListCursor) Len() int {
 	return self.list.Len()
 }
+
+func (self *ListCursor) GetRangeStartIdx() (int, bool) {
+	if self.IsSelectingRange() {
+		return self.rangeStartIdx, true
+	}
+
+	return 0, false
+}
+
+func (self *ListCursor) CancelRangeSelect() {
+	self.rangeSelectMode = RangeSelectModeNone
+}
+
+func (self *ListCursor) IsSelectingRange() bool {
+	return self.rangeSelectMode != RangeSelectModeNone
+}
+
+func (self *ListCursor) GetSelectionRange() (int, int) {
+	if self.IsSelectingRange() {
+		return utils.MinMax(self.selectedIdx, self.rangeStartIdx)
+	}
+
+	return self.selectedIdx, self.selectedIdx
+}
+
+func (self *ListCursor) ToggleStickyRange() {
+	if self.IsSelectingRange() {
+		self.CancelRangeSelect()
+	} else {
+		self.rangeStartIdx = self.selectedIdx
+		self.rangeSelectMode = RangeSelectModeSticky
+	}
+}
+
+func (self *ListCursor) ExpandNonStickyRange(change int) {
+	if !self.IsSelectingRange() {
+		self.rangeStartIdx = self.selectedIdx
+	}
+
+	self.rangeSelectMode = RangeSelectModeNonSticky
+
+	self.SetSelectedLineIdx(self.selectedIdx + change)
+}
diff --git a/pkg/gui/context/view_trait.go b/pkg/gui/context/view_trait.go
index bf8a49e43..1179a8b14 100644
--- a/pkg/gui/context/view_trait.go
+++ b/pkg/gui/context/view_trait.go
@@ -21,6 +21,14 @@ func (self *ViewTrait) FocusPoint(yIdx int) {
 	self.view.FocusPoint(self.view.OriginX(), yIdx)
 }
 
+func (self *ViewTrait) SetRangeSelectStart(yIdx int) {
+	self.view.SetRangeSelectStart(yIdx)
+}
+
+func (self *ViewTrait) CancelRangeSelect() {
+	self.view.CancelRangeSelect()
+}
+
 func (self *ViewTrait) SetViewPortContent(content string) {
 	_, y := self.view.Origin()
 	self.view.OverwriteLines(y, content)