FIX: In ParallelSplits, count maximum number of splits

Instead of requiring to use the size in elements of the thing-to-split,
simply use a counter for the number of splits.

Author: bluss

src/parallel/impl_par_methods.rs

@@ -45,7 +45,7 @@ where
 
 // Zip
 
-const COLLECT_MAX_PARTS: usize = 256;
+const COLLECT_MAX_SPLITS: usize = 10;
 
 macro_rules! zip_impl {
     ($([$notlast:ident $($p:ident)*],)+) => {
@@ -92,7 +92,7 @@ macro_rules! zip_impl {
                     ParallelSplits {
                         iter: self.and(SendProducer::new(output.raw_view_mut().cast::<R>())),
                         // Keep it from splitting the Zip down too small
-                        min_size: total_len / COLLECT_MAX_PARTS,
+                        max_splits: COLLECT_MAX_SPLITS,
                     }
                 };
 

src/parallel/par.rs

@@ -288,7 +288,7 @@ zip_impl! {
 /// or producer `P`.
 pub(crate) struct ParallelSplits<P> {
     pub(crate) iter: P,
-    pub(crate) min_size: usize,
+    pub(crate) max_splits: usize,
 }
 
 impl<P> ParallelIterator for ParallelSplits<P>
@@ -313,17 +313,17 @@ impl<P> UnindexedProducer for ParallelSplits<P>
     type Item = P;
 
     fn split(self) -> (Self, Option<Self>) {
-        if self.iter.size() <= self.min_size || !self.iter.can_split() {
+        if self.max_splits == 0 || !self.iter.can_split() {
             return (self, None)
         }
         let (a, b) = self.iter.split();
         (ParallelSplits {
             iter: a,
-            min_size: self.min_size,
+            max_splits: self.max_splits - 1,
         },
         Some(ParallelSplits {
             iter: b,
-            min_size: self.min_size,
+            max_splits: self.max_splits - 1,
         }))
     }
 

src/split_at.rs

@@ -8,7 +8,6 @@ pub(crate) trait SplitAt  {
 
 pub(crate) trait SplitPreference : SplitAt {
     fn can_split(&self) -> bool;
-    fn size(&self) -> usize;
     fn split_preference(&self) -> (Axis, usize);
     fn split(self) -> (Self, Self) where Self: Sized {
         let (axis, index) = self.split_preference();

src/zip/mod.rs

@@ -1126,8 +1126,6 @@ macro_rules! map_impl {
         {
             fn can_split(&self) -> bool { self.size() > 1 }
 
-            fn size(&self) -> usize { self.size() }
-
             fn split_preference(&self) -> (Axis, usize) {
                 // Always split in a way that preserves layout (if any)
                 let axis = self.max_stride_axis();