Remove the concept of size bias (#199)

Closes #198

src/evaluate.rs

@@ -24,8 +24,6 @@ use std::thread;
 
 struct Candidate {
     image: PngData,
-    // compressed size multiplier. Fudge factor to prefer more promising formats.
-    bias: f32,
     // if false, that's baseline file to throw away
     is_reduction: bool,
     filter: u8,
@@ -42,30 +40,16 @@ impl Comparator {
     fn evaluate(&mut self, new: Candidate) {
         // a tie-breaker is required to make evaluation deterministic
         let is_best = if let Some(ref old) = self.best_result {
-            // ordering is important - later file gets to use bias over earlier, but not the other way
-            // (this way bias=0 replaces, but doesn't forbid later optimizations)
-            let new_len = (new.image.idat_data.len() as f64
-                * if new.nth > old.nth {
-                    f64::from(new.bias)
-                } else {
-                    1.0
-                }) as usize;
-            let old_len = (old.image.idat_data.len() as f64
-                * if new.nth < old.nth {
-                    f64::from(old.bias)
-                } else {
-                    1.0
-                }) as usize;
             // choose smallest compressed, or if compresses the same, smallest uncompressed, or cheaper filter
             let new = (
-                new_len,
+                new.image.idat_data.len(),
                 new.image.raw.data.len(),
                 new.image.raw.ihdr.bit_depth,
                 new.filter,
                 new.nth,
             );
             let old = (
-                old_len,
+                old.image.idat_data.len(),
                 old.image.raw.data.len(),
                 old.image.raw.ihdr.bit_depth,
                 old.filter,
@@ -141,17 +125,15 @@ impl Evaluator {
 
     /// Set baseline image. It will be used only to measure minimum compression level required
     pub fn set_baseline(&self, image: Arc<PngImage>) {
-        self.try_image_inner(image, 1.0, false)
+        self.try_image_inner(image, false)
     }
 
     /// Check if the image is smaller than others
-    /// Bias is a value in 0..=1 range. Compressed size is multiplied by
-    /// this fraction when comparing to the best, so 0.95 allows 5% larger size.
-    pub fn try_image(&self, image: Arc<PngImage>, bias: f32) {
-        self.try_image_inner(image, bias, true)
+    pub fn try_image(&self, image: Arc<PngImage>) {
+        self.try_image_inner(image, true)
     }
 
-    fn try_image_inner(&self, image: Arc<PngImage>, bias: f32, is_reduction: bool) {
+    fn try_image_inner(&self, image: Arc<PngImage>, is_reduction: bool) {
         let nth = self.nth.fetch_add(1, SeqCst);
         // These clones are only cheap refcounts
         let deadline = self.deadline.clone();
@@ -186,7 +168,6 @@ impl Evaluator {
                             idat_data,
                             raw: Arc::clone(&image),
                         },
-                        bias,
                         filter,
                         is_reduction,
                         nth,

src/lib.rs

@@ -740,7 +740,7 @@ fn perform_reductions(
     if let Some(interlacing) = opts.interlace {
         if let Some(reduced) = png.change_interlacing(interlacing) {
             png = Arc::new(reduced);
-            eval.try_image(png.clone(), 0.);
+            eval.try_image(png.clone());
         }
         if deadline.passed() {
             return;
@@ -750,7 +750,7 @@ fn perform_reductions(
     if opts.palette_reduction {
         if let Some(reduced) = reduced_palette(&png) {
             png = Arc::new(reduced);
-            eval.try_image(png.clone(), 0.95);
+            eval.try_image(png.clone());
             if opts.verbosity == Some(1) {
                 report_reduction(&png);
             }
@@ -765,13 +765,13 @@ fn perform_reductions(
             let previous = png.clone();
             let bits = reduced.ihdr.bit_depth;
             png = Arc::new(reduced);
-            eval.try_image(png.clone(), 1.0);
+            eval.try_image(png.clone());
             if (bits == BitDepth::One || bits == BitDepth::Two)
                 && previous.ihdr.bit_depth != BitDepth::Four
             {
                 // Also try 16-color mode for all lower bits images, since that may compress better
                 if let Some(reduced) = reduce_bit_depth(&previous, 4) {
-                    eval.try_image(Arc::new(reduced), 0.98);
+                    eval.try_image(Arc::new(reduced));
                 }
             }
             if opts.verbosity == Some(1) {
@@ -786,7 +786,7 @@ fn perform_reductions(
     if opts.color_type_reduction {
         if let Some(reduced) = reduce_color_type(&png) {
             png = Arc::new(reduced);
-            eval.try_image(png.clone(), 0.96);
+            eval.try_image(png.clone());
             if opts.verbosity == Some(1) {
                 report_reduction(&png);
             }

src/reduction/alpha.rs

@@ -24,7 +24,7 @@ pub(crate) fn try_alpha_reductions(
     let alphas_iter = alphas.par_iter().with_max_len(1);
     alphas_iter
         .filter_map(|&alpha| filtered_alpha_channel(&png, *alpha))
-        .for_each(|image| eval.try_image(Arc::new(image), 0.99));
+        .for_each(|image| eval.try_image(Arc::new(image)));
 }
 
 pub fn filtered_alpha_channel(png: &PngImage, optim: AlphaOptim) -> Option<PngImage> {