Allow usage on wasm32-unknown-unknown target (#194)

* Avoid using time API when we don't need it

This avoids a syscall to the time API when the result is ignored later anyway.

This allows to use the library with default options on wasm32-unknown-unknown, where the unimplemented syscall would panic otherwise.

* eval_send doesn't need to be an Option

We can drop the value manually, thus avoiding unwrap on each access.

* Keep single `use rayon::prelude::*`

If either `rayon` is already imported, then `rayon::prelude::*` should always resolve.

* Extract comparator

* Fully enable non-parallel mode

src/evaluate.rs

@@ -11,15 +11,13 @@ use crate::png::STD_STRATEGY;
 use crate::png::STD_WINDOW;
 #[cfg(not(feature = "parallel"))]
 use crate::rayon;
-#[cfg(not(feature = "parallel"))]
-use crate::rayon::prelude::*;
 use crate::Deadline;
 #[cfg(feature = "parallel")]
 use rayon;
-#[cfg(feature = "parallel")]
 use rayon::prelude::*;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::SeqCst;
+#[cfg(feature = "parallel")]
 use std::sync::mpsc::*;
 use std::sync::Arc;
 use std::thread;
@@ -35,36 +33,112 @@ struct Candidate {
     nth: usize,
 }
 
+#[derive(Default)]
+struct Comparator {
+    best_result: Option<Candidate>,
+}
+
+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.raw.data.len(),
+                new.image.raw.ihdr.bit_depth,
+                new.filter,
+                new.nth,
+            );
+            let old = (
+                old_len,
+                old.image.raw.data.len(),
+                old.image.raw.ihdr.bit_depth,
+                old.filter,
+                old.nth,
+            );
+            // <= instead of < is important, because best_candidate_size has been set already,
+            // so the current result may be comparing its size with itself
+            new <= old
+        } else {
+            true
+        };
+        if is_best {
+            self.best_result = if new.is_reduction { Some(new) } else { None };
+        }
+    }
+
+    fn get_result(self) -> Option<PngData> {
+        self.best_result.map(|res| res.image)
+    }
+}
+
 /// Collect image versions and pick one that compresses best
 pub(crate) struct Evaluator {
     deadline: Arc<Deadline>,
     nth: AtomicUsize,
     best_candidate_size: Arc<AtomicMin>,
     /// images are sent to the thread for evaluation
-    eval_send: Option<SyncSender<Candidate>>,
+    #[cfg(feature = "parallel")]
+    eval_send: SyncSender<Candidate>,
     // the thread helps evaluate images asynchronously
+    #[cfg(feature = "parallel")]
     eval_thread: thread::JoinHandle<Option<PngData>>,
+    // in non-parallel mode, images are evaluated synchronously
+    #[cfg(not(feature = "parallel"))]
+    eval_comparator: std::cell::RefCell<Comparator>,
 }
 
 impl Evaluator {
     pub fn new(deadline: Arc<Deadline>) -> Self {
+        #[cfg(feature = "parallel")]
         let (tx, rx) = sync_channel(4);
         Self {
             deadline,
             best_candidate_size: Arc::new(AtomicMin::new(None)),
             nth: AtomicUsize::new(0),
-            eval_send: Some(tx),
-            eval_thread: thread::spawn(move || Self::evaluate_images(rx)),
+            #[cfg(feature = "parallel")]
+            eval_send: tx,
+            #[cfg(feature = "parallel")]
+            eval_thread: thread::spawn(move || {
+                let mut comparator = Comparator::default();
+                for candidate in rx {
+                    comparator.evaluate(candidate);
+                }
+                comparator.get_result()
+            }),
+            #[cfg(not(feature = "parallel"))]
+            eval_comparator: Default::default(),
         }
     }
 
     /// Wait for all evaluations to finish and return smallest reduction
     /// Or `None` if all reductions were worse than baseline.
-    pub fn get_result(mut self) -> Option<PngData> {
-        let _ = self.eval_send.take(); // disconnect the sender, breaking the loop in the thread
+    #[cfg(feature = "parallel")]
+    pub fn get_result(self) -> Option<PngData> {
+        drop(self.eval_send); // disconnect the sender, breaking the loop in the thread
         self.eval_thread.join().expect("eval thread")
     }
 
+    #[cfg(not(feature = "parallel"))]
+    pub fn get_result(self) -> Option<PngData> {
+        self.eval_comparator.into_inner().get_result()
+    }
+
     /// 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)
@@ -84,6 +158,7 @@ impl Evaluator {
         let best_candidate_size = self.best_candidate_size.clone();
         // sends it off asynchronously for compression,
         // but results will be collected via the message queue
+        #[cfg(feature = "parallel")]
         let eval_send = self.eval_send.clone();
         rayon::spawn(move || {
             let filters_iter = STD_FILTERS.par_iter().with_max_len(1);
@@ -106,71 +181,28 @@ impl Evaluator {
                 ) {
                     best_candidate_size.set_min(idat_data.len());
                     // the rest is shipped to the evavluation/collection thread
-                    eval_send
-                        .as_ref()
-                        .expect("not finished yet")
-                        .send(Candidate {
-                            image: PngData {
-                                idat_data,
-                                raw: Arc::clone(&image),
-                            },
-                            bias,
-                            filter,
-                            is_reduction,
-                            nth,
-                        })
-                        .expect("send");
+                    let new = Candidate {
+                        image: PngData {
+                            idat_data,
+                            raw: Arc::clone(&image),
+                        },
+                        bias,
+                        filter,
+                        is_reduction,
+                        nth,
+                    };
+
+                    #[cfg(feature = "parallel")]
+                    {
+                        eval_send.send(new).expect("send");
+                    }
+
+                    #[cfg(not(feature = "parallel"))]
+                    {
+                        self.eval_comparator.borrow_mut().evaluate(new);
+                    }
                 }
             });
         });
     }
-
-    /// Main loop of evaluation thread
-    fn evaluate_images(from_channel: Receiver<Candidate>) -> Option<PngData> {
-        let mut best_result: Option<Candidate> = None;
-        // ends when the last sender is dropped
-        for new in from_channel.iter() {
-            // a tie-breaker is required to make evaluation deterministic
-            let is_best = if let Some(ref old) = 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.raw.data.len(),
-                    new.image.raw.ihdr.bit_depth,
-                    new.filter,
-                    new.nth,
-                );
-                let old = (
-                    old_len,
-                    old.image.raw.data.len(),
-                    old.image.raw.ihdr.bit_depth,
-                    old.filter,
-                    old.nth,
-                );
-                // <= instead of < is important, because best_candidate_size has been set already,
-                // so the current result may be comparing its size with itself
-                new <= old
-            } else {
-                true
-            };
-            if is_best {
-                best_result = if new.is_reduction { Some(new) } else { None };
-            }
-        }
-        best_result.map(|res| res.image)
-    }
 }

src/lib.rs

@@ -779,19 +779,25 @@ fn perform_reductions(
     try_alpha_reductions(png, &opts.alphas, eval);
 }
 
+struct DeadlineImp {
+    start: Instant,
+    timeout: Duration,
+    print_message: AtomicBool,
+}
+
 /// Keep track of processing timeout
 pub(crate) struct Deadline {
-    start: Instant,
-    timeout: Option<Duration>,
-    print_message: AtomicBool,
+    imp: Option<DeadlineImp>,
 }
 
 impl Deadline {
     pub fn new(timeout: Option<Duration>, verbose: bool) -> Self {
         Self {
-            start: Instant::now(),
-            timeout,
-            print_message: AtomicBool::new(verbose),
+            imp: timeout.map(|timeout| DeadlineImp {
+                start: Instant::now(),
+                timeout,
+                print_message: AtomicBool::new(verbose),
+            })
         }
     }
 
@@ -799,11 +805,11 @@ impl Deadline {
     ///
     /// If the verbose option is on, it also prints a timeout message once.
     pub fn passed(&self) -> bool {
-        if let Some(timeout) = self.timeout {
-            let elapsed = self.start.elapsed();
-            if elapsed > timeout {
-                if self.print_message.load(Ordering::Relaxed) {
-                    self.print_message.store(false, Ordering::Relaxed);
+        if let Some(imp) = &self.imp {
+            let elapsed = imp.start.elapsed();
+            if elapsed > imp.timeout {
+                if imp.print_message.load(Ordering::Relaxed) {
+                    imp.print_message.store(false, Ordering::Relaxed);
                     eprintln!("Timed out after {} second(s)", elapsed.as_secs());
                 }
                 return true;