Plan UI observer frame clipping

src/app_core/app_frame.h

@@ -3,7 +3,9 @@
 #include "foundation/result.h"
 
 #include <cmath>
+#include <cstdint>
 #include <limits>
+#include <span>
 
 namespace pp::app {
 
@@ -39,6 +41,33 @@ struct AppResizePlan {
     bool request_redraw = true;
 };
 
+struct AppUiObserverRect {
+    float x = 0.0F;
+    float y = 0.0F;
+    float width = 0.0F;
+    float height = 0.0F;
+};
+
+struct AppUiObserverParentClip {
+    AppUiObserverRect clip;
+    float padding_top = 0.0F;
+    float padding_right = 0.0F;
+    float padding_bottom = 0.0F;
+    float padding_left = 0.0F;
+};
+
+struct AppUiObserverPlan {
+    bool draw_node = false;
+    bool notify_enter_screen = false;
+    bool notify_leave_screen = false;
+    bool next_on_screen = false;
+    AppUiObserverRect visible_clip;
+    std::int32_t scissor_x = 0;
+    std::int32_t scissor_y = 0;
+    std::int32_t scissor_width = 0;
+    std::int32_t scissor_height = 0;
+};
+
 [[nodiscard]] constexpr AppInitialSurfacePlan plan_app_initial_surface() noexcept
 {
     return AppInitialSurfacePlan {
@@ -110,4 +139,102 @@ struct AppResizePlan {
     });
 }
 
+[[nodiscard]] constexpr AppUiObserverRect intersect_app_ui_observer_rect(
+    AppUiObserverRect a,
+    AppUiObserverRect b) noexcept
+{
+    const float x0 = a.x > b.x ? a.x : b.x;
+    const float y0 = a.y > b.y ? a.y : b.y;
+    const float x1 = (a.x + a.width) < (b.x + b.width) ? (a.x + a.width) : (b.x + b.width);
+    const float y1 = (a.y + a.height) < (b.y + b.height) ? (a.y + a.height) : (b.y + b.height);
+    return AppUiObserverRect {
+        .x = x0,
+        .y = y0,
+        .width = x1 - x0,
+        .height = y1 - y0,
+    };
+}
+
+[[nodiscard]] inline pp::foundation::Result<AppUiObserverPlan> plan_app_ui_observer(
+    bool has_node,
+    bool display,
+    bool was_on_screen,
+    AppUiObserverRect node_clip,
+    std::span<const AppUiObserverParentClip> parent_clips,
+    float surface_height,
+    float zoom,
+    float offset_x,
+    float offset_y)
+{
+    if (!has_node || !display) {
+        return pp::foundation::Result<AppUiObserverPlan>::success(AppUiObserverPlan {
+            .draw_node = false,
+            .next_on_screen = was_on_screen,
+            .visible_clip = node_clip,
+        });
+    }
+
+    const auto finite_rect = [](AppUiObserverRect rect) noexcept {
+        return std::isfinite(rect.x) && std::isfinite(rect.y)
+            && std::isfinite(rect.width) && std::isfinite(rect.height);
+    };
+
+    if (!finite_rect(node_clip) || !std::isfinite(surface_height)
+        || !std::isfinite(zoom) || !std::isfinite(offset_x) || !std::isfinite(offset_y)) {
+        return pp::foundation::Result<AppUiObserverPlan>::failure(
+            pp::foundation::Status::invalid_argument("UI observer geometry must be finite"));
+    }
+
+    if (surface_height < 1.0F || zoom <= 0.0F) {
+        return pp::foundation::Result<AppUiObserverPlan>::failure(
+            pp::foundation::Status::invalid_argument("UI observer surface height and zoom must be positive"));
+    }
+
+    AppUiObserverRect visible = node_clip;
+    for (const auto& parent : parent_clips) {
+        if (!finite_rect(parent.clip)
+            || !std::isfinite(parent.padding_top)
+            || !std::isfinite(parent.padding_right)
+            || !std::isfinite(parent.padding_bottom)
+            || !std::isfinite(parent.padding_left)) {
+            return pp::foundation::Result<AppUiObserverPlan>::failure(
+                pp::foundation::Status::invalid_argument("UI observer parent geometry must be finite"));
+        }
+
+        const AppUiObserverRect padded {
+            .x = parent.clip.x + parent.padding_left,
+            .y = parent.clip.y + parent.padding_top,
+            .width = parent.clip.width - parent.padding_right - parent.padding_left,
+            .height = parent.clip.height - parent.padding_bottom - parent.padding_top,
+        };
+        visible = intersect_app_ui_observer_rect(visible, padded);
+    }
+
+    if (visible.width <= 0.0F || visible.height <= 0.0F) {
+        return pp::foundation::Result<AppUiObserverPlan>::success(AppUiObserverPlan {
+            .draw_node = false,
+            .notify_leave_screen = was_on_screen,
+            .next_on_screen = false,
+            .visible_clip = visible,
+        });
+    }
+
+    const float projected_x = (visible.x - 1.0F) * zoom;
+    const float projected_y = (surface_height / zoom - visible.y - visible.height - 1.0F) * zoom;
+    const float projected_width = (visible.width + 2.0F) * zoom;
+    const float projected_height = (visible.height + 2.0F) * zoom;
+
+    return pp::foundation::Result<AppUiObserverPlan>::success(AppUiObserverPlan {
+        .draw_node = true,
+        .notify_enter_screen = !was_on_screen,
+        .notify_leave_screen = false,
+        .next_on_screen = true,
+        .visible_clip = visible,
+        .scissor_x = static_cast<std::int32_t>(std::floor(projected_x + offset_x)),
+        .scissor_y = static_cast<std::int32_t>(std::floor(projected_y + offset_y)),
+        .scissor_width = static_cast<std::int32_t>(std::ceil(projected_width)),
+        .scissor_height = static_cast<std::int32_t>(std::ceil(projected_height)),
+    });
+}
+
 } // namespace pp::app