From de61b7abf924780c60f9b5876da7f315789fe378 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kornel=20Lesin=CC=81ski?= <kornel@geekhood.net>
Date: Thu, 17 Jan 2019 16:05:00 +0000
Subject: [PATCH] Faster bit depth check

---
 src/reduction/bit_depth.rs | 45 ++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/src/reduction/bit_depth.rs b/src/reduction/bit_depth.rs
index ef1055ac..24493f0f 100644
--- a/src/reduction/bit_depth.rs
+++ b/src/reduction/bit_depth.rs
@@ -78,28 +78,39 @@ pub fn reduce_bit_depth(png: &PngImage) -> Option<PngImage> {
 pub fn reduce_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
     let mut reduced = BitVec::with_capacity(png.data.len() * 8);
     let bit_depth: usize = png.ihdr.bit_depth.as_u8() as usize;
-    let mut allowed_bits = 1;
+    let mut minimum_bits = 1;
+    if minimum_bits >= bit_depth {
+        return None;
+    }
     for line in png.scan_lines() {
-        let bit_vec = BitVec::from_bytes(&line.data);
         if png.ihdr.color_type == ColorType::Indexed {
-            for (i, bit) in bit_vec.iter().enumerate() {
-                let bit_index = bit_depth - (i % bit_depth);
-                if bit && bit_index > allowed_bits {
-                    allowed_bits = bit_index.next_power_of_two();
-                    if allowed_bits == bit_depth {
-                        // Not reducable
-                        return None;
-                    }
+            let line_max = line.data.iter().map(|&byte| match png.ihdr.bit_depth {
+                BitDepth::Two => (byte & 0x3).max((byte >> 2) & 0x3).max((byte >> 4) & 0x3).max(byte >> 6),
+                BitDepth::Four => (byte & 0xF).max(byte >> 4),
+                _ => byte,
+            }).max().unwrap_or(0);
+            let required_bits = match line_max {
+                x if x > 0x0F => 8,
+                x if x > 0x03 => 4,
+                x if x > 0x01 => 2,
+                _ => 1,
+            };
+            if required_bits > minimum_bits {
+                minimum_bits = required_bits;
+                if minimum_bits >= bit_depth {
+                    // Not reducable
+                    return None;
                 }
             }
         } else {
+            let bit_vec = BitVec::from_bytes(&line.data);
             for byte in bit_vec.to_bytes() {
-                while allowed_bits < bit_depth {
-                    let permutations: &[u8] = if allowed_bits == 1 {
+                while minimum_bits < bit_depth {
+                    let permutations: &[u8] = if minimum_bits == 1 {
                         &ONE_BIT_PERMUTATIONS
-                    } else if allowed_bits == 2 {
+                    } else if minimum_bits == 2 {
                         &TWO_BIT_PERMUTATIONS
-                    } else if allowed_bits == 4 {
+                    } else if minimum_bits == 4 {
                         &FOUR_BIT_PERMUTATIONS
                     } else {
                         return None;
@@ -107,7 +118,7 @@ pub fn reduce_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
                     if permutations.iter().any(|perm| *perm == byte) {
                         break;
                     } else {
-                        allowed_bits <<= 1;
+                        minimum_bits <<= 1;
                     }
                 }
             }
@@ -119,7 +130,7 @@ pub fn reduce_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
         let bit_vec = BitVec::from_bytes(&line.data);
         for (i, bit) in bit_vec.iter().enumerate() {
             let bit_index = bit_depth - (i % bit_depth);
-            if bit_index <= allowed_bits {
+            if bit_index <= minimum_bits {
                 reduced.push(bit);
             }
         }
@@ -132,7 +143,7 @@ pub fn reduce_bit_depth_8_or_less(png: &PngImage) -> Option<PngImage> {
     Some(PngImage {
         data: reduced.to_bytes(),
         ihdr: IhdrData {
-            bit_depth: BitDepth::from_u8(allowed_bits as u8),
+            bit_depth: BitDepth::from_u8(minimum_bits as u8),
             ..png.ihdr
         },
         aux_headers: png.aux_headers.clone(),