Make Rayon an optional dependency (#112)

On WebAssembly, Rayon does not work (thread pool fails to initialize since threads are stubbed out).

Cargo.toml

@@ -32,10 +32,13 @@ byteorder = "^1.0.0"
 crc = "^1.2.0"
 itertools = "^0.7.7"
 num_cpus = "^1.0.0"
-rayon = "^1.0.0"
 zopfli = "^0.3.4"
 miniz_oxide = "0.1.2"
 
+[dependencies.rayon]
+optional = true
+version = "^1.0.0"
+
 [dependencies.clap]
 optional = true
 version = "^2.10.0"
@@ -62,7 +65,8 @@ binary = [
     "clap",
     "wild",
 ]
-default = ["binary"]
+default = ["binary", "parallel"]
+parallel = ["rayon"]
 cfzlib = ["cloudflare-zlib-sys"]
 dev = [
     "nightly-binary",

src/lib.rs

@@ -8,6 +8,7 @@ extern crate image;
 extern crate itertools;
 extern crate miniz_oxide;
 extern crate num_cpus;
+#[cfg(feature = "parallel")]
 extern crate rayon;
 extern crate zopfli;
 #[cfg(feature = "cfzlib")]
@@ -15,6 +16,7 @@ extern crate cloudflare_zlib_sys;
 
 use image::{DynamicImage, GenericImage, ImageFormat, Pixel};
 use png::PngData;
+#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 use std::collections::{HashMap, HashSet};
 use std::fs::{copy, File};
@@ -277,7 +279,9 @@ impl Default for Options {
 /// Perform optimization on the input file using the options provided
 pub fn optimize(input_path: &Path, opts: &Options) -> Result<(), PngError> {
     // Initialize the thread pool with correct number of threads
+    #[cfg(feature = "parallel")]
     let thread_count = opts.threads;
+    #[cfg(feature = "parallel")]
     let _ = rayon::ThreadPoolBuilder::new()
         .num_threads(thread_count)
         .build_global();
@@ -356,7 +360,9 @@ pub fn optimize(input_path: &Path, opts: &Options) -> Result<(), PngError> {
 /// loaded in-memory
 pub fn optimize_from_memory(data: &[u8], opts: &Options) -> Result<Vec<u8>, PngError> {
     // Initialize the thread pool with correct number of threads
+    #[cfg(feature = "parallel")]
     let thread_count = opts.threads;
+    #[cfg(feature = "parallel")]
     let _ = rayon::ThreadPoolBuilder::new()
         .num_threads(thread_count)
         .build_global();
@@ -483,9 +489,11 @@ fn optimize_png(
             }
         }
 
-        let filters: HashMap<u8, Vec<u8>> = filter
+        #[cfg(feature = "parallel")]
-            .par_iter()
+        let filter_iter = filter.par_iter().with_max_len(1);
-            .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();
                 (*f, png.filter_image(*f))
@@ -496,9 +504,11 @@ fn optimize_png(
         let added_interlacing = opts.interlace == Some(1) && original_png.ihdr_data.interlaced == 0;
 
         let best_size = AtomicMin::new(if opts.force {None} else {Some(original_len)});
-        let best: Option<TrialWithData> = results
+        #[cfg(feature = "parallel")]
-            .into_par_iter()
+        let results_iter = results.into_par_iter().with_max_len(1);
-            .with_max_len(1)
+        #[cfg(not(feature = "parallel"))]
+        let results_iter = results.into_iter();
+        let best = results_iter
             .filter_map(|trial| {
                 let filtered = &filters[&trial.filter];
                 let new_idat = if opts.deflate == Deflaters::Zlib {
@@ -540,8 +550,18 @@ fn optimize_png(
                 } else {
                     None
                 }
-            })
+            });
+        #[cfg(feature = "parallel")]
+        let best: Option<TrialWithData> = 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 {
             png.idat_data = better.1;

src/png/mod.rs

@@ -15,6 +15,7 @@ use std::fs::File;
 use std::io::{Read, Seek, SeekFrom};
 use std::iter::Iterator;
 use std::path::Path;
+#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 use atomicmin::AtomicMin;
 
@@ -605,15 +606,19 @@ impl PngData {
         assert!(!alphas.is_empty());
         let alphas = alphas.iter().collect::<Vec<_>>();
         let best_size = AtomicMin::new(None);
-        let best = alphas
+        #[cfg(feature = "parallel")]
-            .par_iter()
+        let alphas_iter = alphas.par_iter().with_max_len(1);
-            .with_max_len(1)
+        #[cfg(not(feature = "parallel"))]
+        let alphas_iter = alphas.iter();
+        let best = alphas_iter
             .filter_map(|&alpha| {
                 let mut image = self.clone();
                 image.reduce_alpha_channel(*alpha);
-                STD_FILTERS
+                #[cfg(feature = "parallel")]
-                    .par_iter()
+                let filters_iter = STD_FILTERS.par_iter().with_max_len(1);
-                    .with_max_len(1)
+                #[cfg(not(feature = "parallel"))]
+                let filters_iter = STD_FILTERS.iter();
+                filters_iter
                     .filter_map(|f| {
                         deflate::deflate(
                             &image.filter_image(*f),