From bb2aa4ce9b0fc18af8070efa55aca5e80fd18734 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kornel=20Lesi=C5=84ski?= <kornel@geekhood.net>
Date: Mon, 16 Jul 2018 15:26:30 +0100
Subject: [PATCH] Avoid slow modulo

---
 src/reduction/alpha.rs | 16 +++++++++-------
 src/reduction/color.rs | 12 +++++++-----
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/src/reduction/alpha.rs b/src/reduction/alpha.rs
index de1499cd..18248271 100644
--- a/src/reduction/alpha.rs
+++ b/src/reduction/alpha.rs
@@ -1,12 +1,14 @@
 use png::PngData;
 
-pub fn reduce_alpha_channel(png: &mut PngData, channels: usize) -> Option<Vec<u8>> {
-    let byte_depth: u8 = png.ihdr_data.bit_depth.as_u8() >> 3;
-    let bpp: usize = channels * byte_depth as usize;
-    let colored_bytes = bpp - byte_depth as usize;
+pub fn reduce_alpha_channel(png: &mut PngData, channels: u8) -> Option<Vec<u8>> {
+    let byte_depth = png.ihdr_data.bit_depth.as_u8() >> 3;
+    let bpp = channels * byte_depth;
+    let bpp_mask = bpp - 1;
+    assert_eq!(0, bpp & bpp_mask);
+    let colored_bytes = bpp - byte_depth;
     for line in png.scan_lines() {
         for (i, &byte) in line.data.iter().enumerate() {
-            if i % bpp >= colored_bytes {
+            if i as u8 & bpp_mask >= colored_bytes {
                 if byte != 255 {
                     return None;
                 }
@@ -18,7 +20,7 @@ pub fn reduce_alpha_channel(png: &mut PngData, channels: usize) -> Option<Vec<u8
     for line in png.scan_lines() {
         reduced.push(line.filter);
         for (i, &byte) in line.data.iter().enumerate() {
-            if i % bpp >= colored_bytes {
+            if i as u8 & bpp_mask >= colored_bytes {
                 continue;
             } else {
                 reduced.push(byte);
@@ -29,7 +31,7 @@ pub fn reduce_alpha_channel(png: &mut PngData, channels: usize) -> Option<Vec<u8
     // sBIT contains information about alpha channel's original depth,
     // and alpha has just been removed
     if let Some(sbit_header) = png.aux_headers.get_mut(&"sBIT".to_string()) {
-        assert_eq!(sbit_header.len(), channels);
+        assert_eq!(sbit_header.len(), channels as usize);
         sbit_header.pop();
     }
 
diff --git a/src/reduction/color.rs b/src/reduction/color.rs
index a61de657..432c100d 100644
--- a/src/reduction/color.rs
+++ b/src/reduction/color.rs
@@ -16,16 +16,18 @@ pub fn reduce_rgba_to_rgb(png: &mut PngData) -> bool {
 
 pub fn reduce_rgba_to_grayscale_alpha(png: &mut PngData) -> bool {
     let mut reduced = Vec::with_capacity(png.raw_data.len());
-    let byte_depth: u8 = png.ihdr_data.bit_depth.as_u8() >> 3;
-    let bpp: usize = 4 * byte_depth as usize;
-    let colored_bytes = bpp - byte_depth as usize;
+    let byte_depth = png.ihdr_data.bit_depth.as_u8() >> 3;
+    let bpp = 4 * byte_depth;
+    let bpp_mask = bpp - 1;
+    assert_eq!(0, bpp & bpp_mask);
+    let colored_bytes = bpp - byte_depth;
     for line in png.scan_lines() {
         reduced.push(line.filter);
         let mut low_bytes = Vec::with_capacity(4);
         let mut high_bytes = Vec::with_capacity(4);
         let mut trans_bytes = Vec::with_capacity(byte_depth as usize);
         for (i, byte) in line.data.iter().enumerate() {
-            if i % bpp < colored_bytes {
+            if i as u8 & bpp_mask < colored_bytes {
                 if byte_depth == 1 || i % 2 == 1 {
                     low_bytes.push(*byte);
                 } else {
@@ -35,7 +37,7 @@ pub fn reduce_rgba_to_grayscale_alpha(png: &mut PngData) -> bool {
                 trans_bytes.push(*byte);
             }
 
-            if i % bpp == bpp - 1 {
+            if (i as u8 & bpp_mask) == bpp - 1 {
                 if low_bytes.iter().unique().count() > 1 {
                     return false;
                 }