Revert "Unbreak no-default-features"

This reverts commit 4ef92089d5.

src/evaluate.rs

@@ -17,6 +17,7 @@ use rayon;
 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;
@@ -46,30 +47,46 @@ pub(crate) struct Evaluator {
     nth: AtomicUsize,
     best_candidate_size: Arc<AtomicMin>,
     /// images are sent to the thread for evaluation
+    #[cfg(feature = "parallel")]
     eval_send: SyncSender<Candidate>,
     // the thread helps evaluate images asynchronously
+    #[cfg(feature = "parallel")]
     eval_thread: thread::JoinHandle<Option<Candidate>>,
+    // in non-parallel mode, images are evaluated synchronously
+    #[cfg(not(feature = "parallel"))]
+    eval_best_candidate: Option<Candidate>,
 }
 
 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),
+            #[cfg(feature = "parallel")]
             eval_send: tx,
+            #[cfg(feature = "parallel")]
             eval_thread: thread::spawn(move || rx.into_iter().min_by_key(Candidate::cmp_key)),
+            #[cfg(not(feature = "parallel"))]
+            eval_best_candidate: None,
         }
     }
 
     /// Wait for all evaluations to finish and return smallest reduction
     /// Or `None` if all reductions were worse than baseline.
+    #[cfg(feature = "parallel")]
     fn get_best_candidate(self) -> Option<Candidate> {
         drop(self.eval_send); // disconnect the sender, breaking the loop in the thread
         self.eval_thread.join().expect("eval thread")
     }
 
+    #[cfg(not(feature = "parallel"))]
+    fn get_best_candidate(self) -> Option<Candidate> {
+        self.eval_best_candidate
+    }
+
     pub fn get_result(self) -> Option<PngData> {
         self.get_best_candidate().map(|candidate| candidate.image)
     }
@@ -91,6 +108,7 @@ 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);
@@ -126,7 +144,18 @@ impl Evaluator {
                         nth,
                     };
 
-                    eval_send.send(new).expect("send");
+                    #[cfg(feature = "parallel")]
+                    {
+                        eval_send.send(new).expect("send");
+                    }
+
+                    #[cfg(not(feature = "parallel"))]
+                    {
+                        match self.eval_best_candidate {
+                            Some(prev) if prev.cmp_key() < new.cmp_key() => {}
+                            _ => self.eval_best_candidate = Some(new),
+                        }
+                    }
                 }
             });
         });