initialize project

.gitignore

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+/target
+/Cargo.lock

Cargo.toml

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+[package]
+name = "iter_accumulate"
+version = "0.1.0"
+authors = ["Niklas Elsbrock <mail@nelsbrock.de>"]
+edition = "2021"
+description = "An iterator adaptor that accumulates the elements and yields the current accumulated value for each iteration"
+repository = "https://github.com/nelsbrock/iter_accumulate"
+license = "Apache-2.0"
+keywords = ["accumulate", "iterator", "fold"]

src/lib.rs

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+#![no_std]
+#![warn(missing_docs)]
+
+//! This crate provides the [`accumulate()`](IterAccumulate::accumulate) iterator adaptor.
+
+/// An iterator adaptor that accumulates the elements from the base iterator using the provided
+/// closure.
+///
+/// See [`IterAccumulate::accumulate()`] for more information.
+pub struct Accumulate<I, B, F> {
+    iter: I,
+    acc: B,
+    f: F,
+}
+
+impl<I, B, F> Accumulate<I, B, F> {
+    fn new(iter: I, acc: B, f: F) -> Accumulate<I, B, F> {
+        Accumulate { iter, acc, f }
+    }
+}
+
+impl<I, B, F> Iterator for Accumulate<I, B, F>
+where
+    I: Iterator,
+    B: Copy,
+    F: FnMut(B, I::Item) -> B,
+{
+    type Item = B;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.iter.next() {
+            Some(item) => {
+                self.acc = (self.f)(self.acc, item);
+                Some(self.acc)
+            }
+            None => None,
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+/// An [`Iterator`] blanket implementation that provides the [`accumulate()`](Self::accumulate)
+/// function.
+pub trait IterAccumulate: Iterator {
+    /// Creates an iterator adaptor that accumulates the elements from the base iterator using the
+    /// provided closure.
+    ///
+    /// `accumulate()` is similar to [`fold()`], but instead of returning the final accumulated
+    /// result, it returns an iterator that yields the current accumulated value for each iteration.
+    /// In other words, the last element yielded by `accumulate()` is what would have been returned
+    /// by [`fold()`] if it was used instead.
+    ///
+    /// The returned iterator is **not** fused and it is not specified what happens when the base
+    /// iterator returns [`None`].
+    /// If you want a fused iterator, use [`fuse()`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use iter_accumulate::IterAccumulate;
+    ///
+    /// let input = [1, 2, 3, 4, 5];
+    /// let result = input
+    ///     .into_iter()
+    ///     .accumulate(1, |acc, i| acc * i)
+    ///     .collect::<Vec<_>>();
+    ///
+    /// assert_eq!(result, vec![1, 2, 6, 24, 120]);
+    /// ```
+    ///
+    /// [`fold()`]: Iterator::fold
+    /// [`fuse()`]: Iterator::fuse
+    #[inline]
+    fn accumulate<B, F>(self, init: B, f: F) -> Accumulate<Self, B, F>
+    where
+        Self: Sized,
+        B: Copy,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        Accumulate::new(self, init, f)
+    }
+}
+
+impl<I: Iterator> IterAccumulate for I {}