Improve has_constant_alpha* methods

src/imageops/resize.rs

@@ -5,9 +5,10 @@
 //! Everything that references pic-scale-safe crate is contained to this file
 //! so that it could easily be made an optional dependency in the future.
 
-use std::ops::{AddAssign, BitXor};
+use std::ops::{BitOrAssign, BitXor};
 
 use crate::{imageops::FilterType, DynamicImage, ImageBuffer, Pixel};
+use num_traits::Zero;
 use pic_scale_safe::ResamplingFunction;
 
 pub(crate) fn resize_impl(
@@ -158,26 +159,25 @@ fn has_constant_alpha_integer<P, Container>(img: &ImageBuffer<P, Container>) ->
 where
     P: Pixel,
     Container: std::ops::Deref<Target = [P::Subpixel]>,
-    P::Subpixel:
-        Copy + PartialEq + BitXor<P::Subpixel, Output = P::Subpixel> + AddAssign + Into<u64>,
+    P::Subpixel: Copy + PartialEq + BitXor<P::Subpixel, Output = P::Subpixel> + BitOrAssign,
 {
-    let first_pixel_alpha = *match img.pixels().first() {
-        Some(pixel) => pixel.channels().last().unwrap(), // there doesn't seem to be a better way to retrieve the alpha channel
-        None => return true,                             // empty input image
+    let first_pixel_alpha = match img.pixels().first() {
+        Some(pixel) => pixel.alpha(),
+        None => return true, // empty input image
     };
     // A naive, slower implementation that branches on every pixel:
     //
-    // img.pixels().map(|pixel| pixel.channels().last().unwrap()).all(|alpha| alpha == first_pixel_alpha);
+    // img.pixels().iter().all(|pixel| pixel.alpha() == first_pixel_alpha);
     //
     // Instead of doing that we scan every row first with cheap arithmetic instructions
     // and only compare the sum of divergences on every row, which should be 0
-    let mut sum_of_diffs: u64 = 0;
+    let mut diff_acc = <P::Subpixel as Zero>::zero();
     for row in img.rows() {
         row.iter().for_each(|pixel| {
             let alpha = pixel.alpha();
-            sum_of_diffs += alpha.bitxor(first_pixel_alpha).into();
+            diff_acc |= alpha.bitxor(first_pixel_alpha);
         });
-        if sum_of_diffs != 0 {
+        if !diff_acc.is_zero() {
             return false;
         }
     }
@@ -193,8 +193,7 @@ fn has_constant_alpha_f32(img: &ImageBuffer<crate::Rgba<f32>, Vec<f32>>) -> bool
     };
     img.pixels()
         .iter()
-        .map(|pixel| pixel.channels().last().unwrap())
-        .all(|alpha| *alpha == first_pixel_alpha)
+        .all(|pixel| pixel.alpha() == first_pixel_alpha)
 }
 
 #[cfg(test)]