scrollable: extract scrolling behavior for reuse

Extract the vertical scrolling ability into its own Scrollable struct
that can be embedded and reused across any ui element that relies on
scrolling.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 68 insertions, 0 deletions

diff --git a/widgets/scrollable.go b/widgets/scrollable.go
new file mode 100644
index 0000000..0e8bb5a
--- /dev/null
+++ b/widgets/scrollable.go
@@ -0,0 +1,68 @@
+package widgets
+
+// Scrollable implements vertical scrolling
+type Scrollable struct {
+	scroll int
+	height int
+	elems  int
+}
+
+func (s *Scrollable) Scroll() int {
+	return s.scroll
+}
+
+func (s *Scrollable) PercentVisible() float64 {
+	if s.elems <= 0 {
+		return 1.0
+	}
+	return float64(s.height) / float64(s.elems)
+}
+
+func (s *Scrollable) PercentScrolled() float64 {
+	if s.elems <= 0 {
+		return 1.0
+	}
+	return float64(s.scroll) / float64(s.elems)
+}
+
+func (s *Scrollable) NeedScrollbar() bool {
+	needScrollbar := true
+	if s.PercentVisible() >= 1.0 {
+		needScrollbar = false
+	}
+	return needScrollbar
+}
+
+func (s *Scrollable) UpdateScroller(height, elems int) {
+	s.height = height
+	s.elems = elems
+}
+
+func (s *Scrollable) EnsureScroll(selectingIdx int) {
+	if selectingIdx < 0 {
+		return
+	}
+
+	maxScroll := s.elems - s.height
+	if maxScroll < 0 {
+		maxScroll = 0
+	}
+
+	if selectingIdx >= s.scroll && selectingIdx < s.scroll+s.height {
+		if s.scroll > maxScroll {
+			s.scroll = maxScroll
+		}
+		return
+	}
+
+	if selectingIdx >= s.scroll+s.height {
+		s.scroll = selectingIdx - s.height + 1
+	} else if selectingIdx < s.scroll {
+		s.scroll = selectingIdx
+	}
+
+	if s.scroll > maxScroll {
+		s.scroll = maxScroll
+	}
+
+}