1 files changed, 109 insertions, 0 deletions
diff --git a/src/iter.rs b/src/iter.rs
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+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::ops::ControlFlow;
+
+/// Iterator with a lazy fallible initialiser
+///
+/// It is a common pattern that instantiating an effectful iterator is fallible,
+/// while traversing it is fallible, too. This yields unwieldy signatures like:
+///
+/// ```no_run
+/// fn my_iterator() -> Result<impl Iterator<Item = Result<T, F>>, E>
+/// ```
+///
+/// Often, however, we can unify the error types (`E` and `F` above), which
+/// allows for the more pleasant pattern that constructing the iterator is
+/// infallible, but an initialiser error is returned upon the first call to
+/// `next()`. Ie.:
+///
+/// ```no_run
+/// fn my_iterator() -> impl Iterator<Item = Result<T, E>>
+/// ```
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+pub struct Iter<E, F, I, G> {
+    init: Option<F>,
+    iter: Option<Result<I, E>>,
+    next: G,
+}
+
+impl<E, F, I, G> Iter<E, F, I, G> {
+    pub fn new(init: F, next: G) -> Self {
+        Self {
+            init: Some(init),
+            iter: None,
+            next,
+        }
+    }
+}
+
+impl<E, F, I, G, T, U> Iterator for Iter<E, F, I, G>
+where
+    F: FnOnce() -> Result<I, E>,
+    I: Iterator<Item = Result<T, E>>,
+    G: FnMut(Result<T, E>) -> Option<Result<U, E>>,
+{
+    type Item = Result<U, E>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.iter.take() {
+            None => {
+                let init = self.init.take()?;
+                self.iter = Some(init());
+                self.next()
+            },
+            Some(Err(e)) => Some(Err(e)),
+            Some(Ok(mut iter)) => {
+                let item = iter.next()?;
+                let next = (self.next)(item);
+                self.iter = Some(Ok(iter));
+                next
+            },
+        }
+    }
+}
+
+impl<E, F, I, G, T, U> DoubleEndedIterator for Iter<E, F, I, G>
+where
+    F: FnOnce() -> Result<I, E>,
+    I: Iterator<Item = Result<T, E>> + DoubleEndedIterator,
+    G: FnMut(Result<T, E>) -> Option<Result<U, E>>,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match self.iter.take() {
+            None => {
+                let init = self.init.take()?;
+                self.iter = Some(init());
+                self.next_back()
+            },
+            Some(Err(e)) => Some(Err(e)),
+            Some(Ok(mut iter)) => {
+                let item = iter.next_back()?;
+                let next = (self.next)(item);
+                self.iter = Some(Ok(iter));
+                next
+            },
+        }
+    }
+}
+
+pub(crate) trait IteratorExt {
+    fn try_find_map<F, T, E>(&mut self, mut f: F) -> crate::Result<Option<T>>
+    where
+        Self: Iterator + Sized,
+        F: FnMut(Self::Item) -> Result<Option<T>, E>,
+        E: Into<crate::Error>,
+    {
+        let x = self.try_fold((), |(), i| match f(i) {
+            Err(e) => ControlFlow::Break(Err(e.into())),
+            Ok(v) if v.is_some() => ControlFlow::Break(Ok(v)),
+            Ok(_) => ControlFlow::Continue(()),
+        });
+        match x {
+            ControlFlow::Continue(()) => Ok(None),
+            ControlFlow::Break(v) => v,
+        }
+    }
+}
+
+impl<T: Iterator> IteratorExt for T {}