Fake Rayon (#162)

Dummy implementation of Rayon's traits to avoid `#[cfg()]` and duplicated non-parallel code.

src/evaluate.rs

@@ -9,7 +9,6 @@ use png::STD_COMPRESSION;
 use png::STD_FILTERS;
 use png::STD_STRATEGY;
 use png::STD_WINDOW;
-#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 use std::sync::mpsc::*;
 use std::sync::Arc;
@@ -63,10 +62,7 @@ impl Evaluator {
         let best_result: Mutex<Option<(PngData, _, _)>> = Mutex::new(None);
         // ends when sender is dropped
         for (nth, (image, bias, is_reduction)) in from_channel.iter().enumerate() {
-            #[cfg(feature = "parallel")]
             let filters_iter = STD_FILTERS.par_iter().with_max_len(1);
-            #[cfg(not(feature = "parallel"))]
-            let filters_iter = STD_FILTERS.iter();
 
             filters_iter.for_each(|&f| {
                 if let Ok(idat_data) = deflate::deflate(

src/lib.rs

@@ -9,6 +9,8 @@ extern crate miniz_oxide;
 extern crate num_cpus;
 #[cfg(feature = "parallel")]
 extern crate rayon;
+#[cfg(not(feature = "parallel"))]
+mod rayon;
 extern crate rgb;
 extern crate zopfli;
 
@@ -21,7 +23,6 @@ use image::{DynamicImage, GenericImageView, ImageFormat, Pixel};
 use png::PngImage;
 use png::PngData;
 use colors::BitDepth;
-#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 use std::collections::{HashMap, HashSet};
 use std::fmt;
@@ -569,10 +570,7 @@ fn optimize_png(png: &mut PngData, original_data: &[u8], opts: &Options) -> PngR
             }
         }
 
-        #[cfg(feature = "parallel")]
         let filter_iter = filter.par_iter().with_max_len(1);
-        #[cfg(not(feature = "parallel"))]
-        let filter_iter = filter.iter();
         let filters: HashMap<u8, Vec<u8>> = filter_iter
             .map(|f| {
                 let png = png.clone();
@@ -583,10 +581,7 @@ fn optimize_png(png: &mut PngData, original_data: &[u8], opts: &Options) -> PngR
         let added_interlacing = opts.interlace == Some(1) && original_png.raw.ihdr.interlaced == 0;
 
         let best_size = AtomicMin::new(if opts.force { None } else { Some(original_len) });
-        #[cfg(feature = "parallel")]
         let results_iter = results.into_par_iter().with_max_len(1);
-        #[cfg(not(feature = "parallel"))]
-        let results_iter = results.into_iter();
         let best = results_iter.filter_map(|trial| {
             if deadline.passed() {
                 return None;
@@ -635,20 +630,8 @@ fn optimize_png(png: &mut PngData, original_data: &[u8], opts: &Options) -> PngR
                 None
             }
         });
-        #[cfg(feature = "parallel")]
+        let best: Option<TrialWithData> = best.reduce_with(|i, j| {
-        let best: Option<TrialWithData> =
+            if i.1.len() <= j.1.len() { i } else { j }
-            best.reduce_with(|i, j| if i.1.len() <= j.1.len() { i } else { j });
-        #[cfg(not(feature = "parallel"))]
-        let best: Option<TrialWithData> = best.fold(None, |i, j| {
-            if let Some(i) = i {
-                if i.1.len() <= j.1.len() {
-                    Some(i)
-                } else {
-                    Some(j)
-                }
-            } else {
-                Some(j)
-            }
         });
 
         if let Some(better) = best {
@@ -704,15 +687,10 @@ fn optimize_png(png: &mut PngData, original_data: &[u8], opts: &Options) -> PngR
         }
     }
 
-    let (old_png, new_png) = {
+    let (old_png, new_png) = rayon::join(
-        let old_png = || image::load_from_memory_with_format(original_data, ImageFormat::PNG);
+        || image::load_from_memory_with_format(original_data, ImageFormat::PNG),
-        let new_png = || image::load_from_memory_with_format(&output, ImageFormat::PNG);
+        || image::load_from_memory_with_format(&output, ImageFormat::PNG)
-        #[cfg(feature = "parallel")]
+    );
-        let res = rayon::join(old_png, new_png);
-        #[cfg(not(feature = "parallel"))]
-        let res = (old_png(), new_png());
-        res
-    };
 
     if let Ok(new_png) = new_png {
         if let Ok(old_png) = old_png {

src/rayon.rs

@@ -0,0 +1,53 @@
+pub mod prelude {
+    pub use super::*;
+}
+
+pub trait ParallelIterator: Iterator + Sized {
+    fn with_max_len(self, _l: usize) -> Self {self}
+    fn reduce_with<OP>(mut self, op: OP) -> Option<Self::Item> where OP: Fn(Self::Item, Self::Item) -> Self::Item + Sync {
+        if let Some(a) = self.next() {
+            Some(self.fold(a, op))
+        } else {
+            None
+        }
+    }
+}
+
+pub trait IntoParallelIterator {
+    type Iter: Iterator<Item=Self::Item>;
+    type Item: Send;
+    fn into_par_iter(self) -> Self::Iter;
+}
+
+pub trait IntoParallelRefIterator<'data> {
+    type Iter: Iterator<Item=Self::Item>;
+    type Item: Send + 'data;
+    fn par_iter(&'data self) -> Self::Iter;
+}
+
+impl<I: IntoIterator> IntoParallelIterator for I where I::Item: Send {
+    type Iter = I::IntoIter;
+    type Item = I::Item;
+
+    fn into_par_iter(self) -> Self::Iter {
+        self.into_iter()
+    }
+}
+
+impl<'data, I: 'data + ?Sized> IntoParallelRefIterator<'data> for I
+    where &'data I: IntoParallelIterator
+{
+    type Iter = <&'data I as IntoParallelIterator>::Iter;
+    type Item = <&'data I as IntoParallelIterator>::Item;
+
+    fn par_iter(&'data self) -> Self::Iter {
+        self.into_par_iter()
+    }
+}
+
+impl<I: Iterator> ParallelIterator for I {
+}
+
+pub fn join<A, B>(a: impl FnOnce() -> A, b: impl FnOnce() -> B) -> (A, B) {
+    (a(), b())
+}

src/reduction/alpha.rs

@@ -7,16 +7,12 @@ use colors::AlphaOptim;
 use headers::IhdrData;
 use png::PngImage;
 use colors::ColorType;
-#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 
 pub fn try_alpha_reductions(png: Arc<PngImage>, alphas: &HashSet<AlphaOptim>, eval: &Evaluator) {
     assert!(!alphas.is_empty());
     let alphas = alphas.iter().collect::<Vec<_>>();
-    #[cfg(feature = "parallel")]
     let alphas_iter = alphas.par_iter().with_max_len(1);
-    #[cfg(not(feature = "parallel"))]
-    let alphas_iter = alphas.iter();
     alphas_iter
         .filter_map(|&alpha| filtered_alpha_channel(&png, *alpha))
         .for_each(|image| eval.try_image(Arc::new(image), 0.99));