package world

// PivotZoomCameraNoWrap adjusts camera center so that the world point under the cursor remains fixed
// when zoom changes from oldZoomFp to newZoomFp.
//
// Coordinate conventions:
// - CameraXWorldFp/YWorldFp is the center of the viewport in world-fixed units.
// - cursorXPx/cursorYPx are pixel coordinates relative to the top-left of the viewport.
// - viewportW/H are viewport size in pixels.
//
// This function does not clamp the result; caller should clamp for no-wrap mode using ClampCameraNoWrapViewport.
func PivotZoomCameraNoWrap(
	cameraXWorldFp, cameraYWorldFp int,
	viewportW, viewportH int,
	cursorXPx, cursorYPx int,
	oldZoomFp, newZoomFp int,
) (newCamX, newCamY int) {
	if oldZoomFp <= 0 || newZoomFp <= 0 {
		panic("PivotZoomCameraNoWrap: invalid zoom")
	}
	if viewportW <= 0 || viewportH <= 0 {
		panic("PivotZoomCameraNoWrap: invalid viewport")
	}

	// Offset of cursor from viewport center in pixels.
	offXPx := cursorXPx - viewportW/2
	offYPx := cursorYPx - viewportH/2

	// World-fixed per 1 pixel at each zoom.
	// (Conservative: integer arithmetic, consistent with PixelSpanToWorldFixed.)
	oldWorldPerPx := PixelSpanToWorldFixed(1, oldZoomFp)
	newWorldPerPx := PixelSpanToWorldFixed(1, newZoomFp)

	// World point under cursor before zoom:
	// world = camera + offsetPx * worldPerPx
	worldX := cameraXWorldFp + offXPx*oldWorldPerPx
	worldY := cameraYWorldFp + offYPx*oldWorldPerPx

	// Choose new camera so that the same world point stays under cursor:
	// camera' = world - offsetPx * newWorldPerPx
	newCamX = worldX - offXPx*newWorldPerPx
	newCamY = worldY - offYPx*newWorldPerPx
	return
}