fix(web): optimize and fix present_with_damage (#150)

Co-authored-by: daxpedda <daxpedda@gmail.com>

src/util.rs

@@ -1,6 +1,10 @@
 // Not needed on all platforms
 #![allow(dead_code)]
 
+use std::cmp;
+use std::num::NonZeroU32;
+
+use crate::Rect;
 use crate::SoftBufferError;
 
 /// Takes a mutable reference to a container and a function deriving a
@@ -44,6 +48,41 @@ impl<'a, T: 'static + ?Sized, U: 'static + ?Sized> BorrowStack<'a, T, U> {
     }
 }
 
+/// Calculates the smallest `Rect` necessary to represent all damaged `Rect`s.
+pub(crate) fn union_damage(damage: &[Rect]) -> Option<Rect> {
+    struct Region {
+        left: u32,
+        top: u32,
+        bottom: u32,
+        right: u32,
+    }
+
+    let region = damage
+        .iter()
+        .map(|rect| Region {
+            left: rect.x,
+            top: rect.y,
+            right: rect.x + rect.width.get(),
+            bottom: rect.y + rect.height.get(),
+        })
+        .reduce(|mut prev, next| {
+            prev.left = cmp::min(prev.left, next.left);
+            prev.top = cmp::min(prev.top, next.top);
+            prev.right = cmp::max(prev.right, next.right);
+            prev.bottom = cmp::max(prev.bottom, next.bottom);
+            prev
+        })?;
+
+    Some(Rect {
+        x: region.left,
+        y: region.top,
+        width: NonZeroU32::new(region.right - region.left)
+            .expect("`right` must always be bigger then `left`"),
+        height: NonZeroU32::new(region.bottom - region.top)
+            .expect("`bottom` must always be bigger then `top`"),
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;