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,
	}
}