Prevent zoomout in both dimensions using double-pass system (#150)

* wip; start working on making it not flash when transferring new images

* don't delete the images, just clear them

* add changelog entry

* fmt

src/tui.rs

@@ -72,7 +72,7 @@ struct PageConstraints {
 	r_to_l: bool
 }
 
-#[derive(Default, Debug)]
+#[derive(Default, Debug, Clone, Copy)]
 struct Zoom {
 	// just how much 'zoom' you have. 0 means it fills the screen (instead of fits), such
 	// that one axis is fully on-screen
@@ -86,7 +86,7 @@ struct Zoom {
 }
 impl Zoom {
 	/// Returns the zoom factor, where 1 is the default and means fill-screen
-	fn factor(&self) -> f32 {
+	fn factor(self) -> f32 {
 		// TODO: Make these configurable once we have a good way to set options after startup
 		const ZOOM_RATE: f32 = 1.1;
 		const ZOOM_RATE_GRANULAR: f32 = 1.05;
@@ -262,7 +262,7 @@ impl Tui {
 			let img_page_w_ratio = img_section_w / initial_page_w;
 			let img_page_h_ratio = img_section_h / initial_page_h;
 
-			let shrink_move_page = |dim: &mut u16, pos: &mut u16, axis_zoom_factor: f32| {
+			fn shrink_move_page(dim: &mut u16, pos: &mut u16, axis_zoom_factor: f32) {
 				let old_dim = *dim;
 				// The axis zoom factor tells us what portion of the axis
 				// we need to show.
@@ -272,26 +272,56 @@ impl Tui {
 					.checked_sub(*dim)
 					.expect("zooming out should shrink the image")
 					/ 2;
-			};
-
-			// TODO: Detect max zoom-out in zoom levels
-			if img_page_w_ratio < img_page_h_ratio {
-				// vertical scroll / tall image. zooming out means decreasing the width of the page area
-				shrink_move_page(
-					&mut img_area.width,
-					&mut img_area.x,
-					// disallow zooming out past fit-screen
-					zoom_factor.max(1.0 / img_page_h_ratio)
-				);
-			} else {
-				// horizontal scroll / wide image. zooming out means decreasing the width of the page area
-				shrink_move_page(
-					&mut img_area.height,
-					&mut img_area.y,
-					// disallow zooming out past fit-screen
-					zoom_factor.max(1.0 / img_page_w_ratio)
-				);
 			}
+
+			fn shrink_move_two_pass(
+				zoom: &mut Zoom,
+				dim: &mut u16,
+				pos: &mut u16,
+				zoom_factor: f32,
+				axis_zoom_factor: f32
+			) {
+				// To detect if we're already at fit-screen, we perform a first
+				// pass on `img_area` that emulates the last call to
+				// `render_zoomed` by subtracting from the `zoom`'s level. This
+				// holds because the `img_area` that gets passed is always the
+				// same.
+				let mut first_pass_dim = *dim;
+				let mut first_pass_pos = *pos;
+				let mut first_pass_zoom = *zoom;
+
+				if first_pass_zoom.level.is_negative() {
+					first_pass_zoom.step_in();
+				}
+				let first_pass_zoom_factor = first_pass_zoom.factor();
+				shrink_move_page(
+					&mut first_pass_dim,
+					&mut first_pass_pos,
+					first_pass_zoom_factor.max(1.0 / axis_zoom_factor)
+				);
+
+				log::debug!("first_pass_dim: {first_pass_dim}, first_pass_pos: {first_pass_pos}");
+				// vertical scroll / tall image. zooming out means decreasing
+				// the width of the page area
+				// use `max` to disallow zooming out past fit-screen
+				shrink_move_page(dim, pos, zoom_factor.max(1.0 / axis_zoom_factor));
+
+				log::debug!("new dim: {dim}, new pos: {pos}");
+				// The moment the image area is left unmodified, we've hit
+				// fit-screen and the zoom level ought be normalized.
+				if first_pass_dim == *dim && first_pass_pos == *pos {
+					zoom.step_in();
+				}
+			}
+
+			let (dim, pos, axis_zoom_factor) = if img_page_w_ratio < img_page_h_ratio {
+				(&mut img_area.width, &mut img_area.x, img_page_h_ratio)
+			} else {
+				// horizontal scroll / wide image. zooming out means decreasing
+				// the height of the page area
+				(&mut img_area.height, &mut img_area.y, img_page_w_ratio)
+			};
+			shrink_move_two_pass(zoom, dim, pos, zoom_factor, axis_zoom_factor);
 		}
 		log::debug!("after adjustment, page area is {img_area:#?}");