implamented Aroll cuter

transcode/timeline.go

@@ -0,0 +1,45 @@
+package transcode
+
+// HandlerZoom holds the current scroll center for timeline zoom calculations.
+type HandlerZoom struct {
+	Center float64 // center of the visible window in seconds
+}
+
+// ZoomResult is the result of a TimelineZoom calculation.
+type ZoomResult struct {
+	Type      string  `json:"type"`
+	Percent   float64 `json:"percent"`
+	ViewStart float64 `json:"viewStart"`
+	ViewEnd   float64 `json:"viewEnd"`
+}
+
+// TimelineZoom computes the visible time window for the given zoomPercentage
+// (1–100, where 100 = full duration visible) centered on h.Center.
+func (h *HandlerZoom) TimelineZoom(zoomPercentage, duration float64) *ZoomResult {
+	visibleDuration := duration * (zoomPercentage / 100)
+	half := visibleDuration / 2
+
+	viewStart := h.Center - half
+	viewEnd := h.Center + half
+
+	// Clamp to [0, duration], shifting the window rather than just truncating
+	// so the visible span stays the same size when near the edges.
+	if viewStart < 0 {
+		viewEnd -= viewStart
+		viewStart = 0
+	}
+	if viewEnd > duration {
+		viewStart -= viewEnd - duration
+		viewEnd = duration
+	}
+	if viewStart < 0 {
+		viewStart = 0
+	}
+
+	return &ZoomResult{
+		Type:      "zoom",
+		Percent:   zoomPercentage,
+		ViewStart: viewStart,
+		ViewEnd:   viewEnd,
+	}
+}