Merge #450

450: Introduce module coalesce r=jswrenn a=phimuemue

Hi! As #440 (thanks for reviewing and merging, btw) presumably conflicts with any existing PRs touching `coalesce` et al, I thought we could just as well seize the opportunity and move these types into an own module (so to speak as a starting point for the problem mentioned in #423 (comment)).

I cut-pasted `Coalesce` et al into an own module, rearranged the code so that specialized types follow their generic variants, and ran `cargo fmt` on the new module.

I suggest to import the `coalesce` types into `adaptors` similarly to what we already have for `multi_product`.

Co-authored-by: philipp <descpl@yahoo.de>

src/adaptors/coalesce.rs

@@ -0,0 +1,232 @@
+use std::fmt;
+use std::iter::FusedIterator;
+
+use crate::size_hint;
+
+pub struct CoalesceBy<I, F, T>
+where
+    I: Iterator,
+{
+    iter: I,
+    last: Option<T>,
+    f: F,
+}
+
+impl<I: Clone, F: Clone, T: Clone> Clone for CoalesceBy<I, F, T>
+where
+    I: Iterator,
+{
+    clone_fields!(last, iter, f);
+}
+
+impl<I, F, T> fmt::Debug for CoalesceBy<I, F, T>
+where
+    I: Iterator + fmt::Debug,
+    T: fmt::Debug,
+{
+    debug_fmt_fields!(CoalesceBy, iter);
+}
+
+pub trait CoalescePredicate<Item, T> {
+    fn coalesce_pair(&mut self, t: T, item: Item) -> Result<T, (T, T)>;
+}
+
+impl<I, F, T> Iterator for CoalesceBy<I, F, T>
+where
+    I: Iterator,
+    F: CoalescePredicate<I::Item, T>,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // this fuses the iterator
+        let mut last = match self.last.take() {
+            None => return None,
+            Some(x) => x,
+        };
+        for next in &mut self.iter {
+            match self.f.coalesce_pair(last, next) {
+                Ok(joined) => last = joined,
+                Err((last_, next_)) => {
+                    self.last = Some(next_);
+                    return Some(last_);
+                }
+            }
+        }
+        Some(last)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (low, hi) = size_hint::add_scalar(self.iter.size_hint(), self.last.is_some() as usize);
+        ((low > 0) as usize, hi)
+    }
+
+    fn fold<Acc, FnAcc>(self, acc: Acc, mut fn_acc: FnAcc) -> Acc
+    where
+        FnAcc: FnMut(Acc, Self::Item) -> Acc,
+    {
+        if let Some(last) = self.last {
+            let mut f = self.f;
+            let (last, acc) = self.iter.fold((last, acc), |(last, acc), elt| {
+                match f.coalesce_pair(last, elt) {
+                    Ok(joined) => (joined, acc),
+                    Err((last_, next_)) => (next_, fn_acc(acc, last_)),
+                }
+            });
+            fn_acc(acc, last)
+        } else {
+            acc
+        }
+    }
+}
+
+impl<I: Iterator, F: CoalescePredicate<I::Item, T>, T> FusedIterator for CoalesceBy<I, F, T> {}
+
+/// An iterator adaptor that may join together adjacent elements.
+///
+/// See [`.coalesce()`](../trait.Itertools.html#method.coalesce) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type Coalesce<I, F> = CoalesceBy<I, F, <I as Iterator>::Item>;
+
+impl<F, Item, T> CoalescePredicate<Item, T> for F
+where
+    F: FnMut(T, Item) -> Result<T, (T, T)>,
+{
+    fn coalesce_pair(&mut self, t: T, item: Item) -> Result<T, (T, T)> {
+        self(t, item)
+    }
+}
+
+/// Create a new `Coalesce`.
+pub fn coalesce<I, F>(mut iter: I, f: F) -> Coalesce<I, F>
+where
+    I: Iterator,
+{
+    Coalesce {
+        last: iter.next(),
+        iter,
+        f,
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates, determining equality using a comparison function.
+///
+/// See [`.dedup_by()`](../trait.Itertools.html#method.dedup_by) or [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type DedupBy<I, Pred> = CoalesceBy<I, DedupPred2CoalescePred<Pred>, <I as Iterator>::Item>;
+
+#[derive(Clone)]
+pub struct DedupPred2CoalescePred<DP>(DP);
+
+pub trait DedupPredicate<T> {
+    // TODO replace by Fn(&T, &T)->bool once Rust supports it
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool;
+}
+
+impl<DP, T> CoalescePredicate<T, T> for DedupPred2CoalescePred<DP>
+where
+    DP: DedupPredicate<T>,
+{
+    fn coalesce_pair(&mut self, t: T, item: T) -> Result<T, (T, T)> {
+        if self.0.dedup_pair(&t, &item) {
+            Ok(t)
+        } else {
+            Err((t, item))
+        }
+    }
+}
+
+#[derive(Clone)]
+pub struct DedupEq;
+
+impl<T: PartialEq> DedupPredicate<T> for DedupEq {
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
+        a == b
+    }
+}
+
+impl<T, F: FnMut(&T, &T) -> bool> DedupPredicate<T> for F {
+    fn dedup_pair(&mut self, a: &T, b: &T) -> bool {
+        self(a, b)
+    }
+}
+
+/// Create a new `DedupBy`.
+pub fn dedup_by<I, Pred>(mut iter: I, dedup_pred: Pred) -> DedupBy<I, Pred>
+where
+    I: Iterator,
+{
+    DedupBy {
+        last: iter.next(),
+        iter,
+        f: DedupPred2CoalescePred(dedup_pred),
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates.
+///
+/// See [`.dedup()`](../trait.Itertools.html#method.dedup) for more information.
+pub type Dedup<I> = DedupBy<I, DedupEq>;
+
+/// Create a new `Dedup`.
+pub fn dedup<I>(iter: I) -> Dedup<I>
+where
+    I: Iterator,
+{
+    dedup_by(iter, DedupEq)
+}
+
+/// An iterator adaptor that removes repeated duplicates, while keeping a count of how many
+/// repeated elements were present. This will determine equality using a comparison function.
+///
+/// See [`.dedup_by_with_count()`](../trait.Itertools.html#method.dedup_by_with_count) or
+/// [`.dedup_with_count()`](../trait.Itertools.html#method.dedup_with_count) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub type DedupByWithCount<I, Pred> =
+    CoalesceBy<I, DedupPredWithCount2CoalescePred<Pred>, (usize, <I as Iterator>::Item)>;
+
+#[derive(Clone)]
+pub struct DedupPredWithCount2CoalescePred<DP>(DP);
+
+impl<DP, T> CoalescePredicate<T, (usize, T)> for DedupPredWithCount2CoalescePred<DP>
+where
+    DP: DedupPredicate<T>,
+{
+    fn coalesce_pair(
+        &mut self,
+        (c, t): (usize, T),
+        item: T,
+    ) -> Result<(usize, T), ((usize, T), (usize, T))> {
+        if self.0.dedup_pair(&t, &item) {
+            Ok((c + 1, t))
+        } else {
+            Err(((c, t), (1, item)))
+        }
+    }
+}
+
+/// An iterator adaptor that removes repeated duplicates, while keeping a count of how many
+/// repeated elements were present.
+///
+/// See [`.dedup_with_count()`](../trait.Itertools.html#method.dedup_with_count) for more information.
+pub type DedupWithCount<I> = DedupByWithCount<I, DedupEq>;
+
+/// Create a new `DedupByWithCount`.
+pub fn dedup_by_with_count<I, Pred>(mut iter: I, dedup_pred: Pred) -> DedupByWithCount<I, Pred>
+where
+    I: Iterator,
+{
+    DedupByWithCount {
+        last: iter.next().map(|v| (1, v)),
+        iter,
+        f: DedupPredWithCount2CoalescePred(dedup_pred),
+    }
+}
+
+/// Create a new `DedupWithCount`.
+pub fn dedup_with_count<I>(iter: I) -> DedupWithCount<I>
+where
+    I: Iterator,
+{
+    dedup_by_with_count(iter, DedupEq)
+}