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//! Functions and types to do something special when repeating for the first or //! last time (or in between!). This crate offers two distinct features: //! //! - [`IterStatusExt::with_status`]: a new method for **iterators**, that //! creates a new iterator which yields the item paired with information to //! tell you if this is the first/last item. //! - [`SkipFirst`]: a simple struct to help you always do something, except on //! the first repetition. Works without iterators, too! use std::{ iter::{FusedIterator, Peekable}, }; /// Allows you to always do something, except the first time. /// /// Internally, this is simply a `bool`. It stores whether /// [`skip_first`][SkipFirst::skip_first] has already been called. This struct /// is really just a wrapper for a dead simple logic you could easily write /// yourself. However, if you need to write it multiple times, it's better to /// use this type to avoid duplicate code. /// /// # Example /// /// In this example, it's also possible to use [`IterStatusExt::with_status`]. /// /// ``` /// use splop::SkipFirst; /// /// let mut comma = SkipFirst::new(); /// for name in &["peter", "ingrid", "barbara"] { /// comma.skip_first(|| print!(", ")); /// print!("{}", name); /// } /// println!(); /// /// // Printed "peter, ingrid, barbara" /// ``` pub struct SkipFirst { first: bool, } impl SkipFirst { /// Creates a new instance of `SkipFirst`. pub fn new() -> Self { Self { first: true, } } /// Executes the given function, except the first time this method is /// called on this instance. /// /// # Example /// /// ``` /// use splop::SkipFirst; /// /// let mut v = Vec::new(); /// let mut skipper = SkipFirst::new(); /// skipper.skip_first(|| v.push(1)); // won't be executed /// skipper.skip_first(|| v.push(2)); // will be executed /// skipper.skip_first(|| v.push(3)); // will be executed /// /// assert_eq!(v, [2, 3]); /// ``` /// /// Note that the state "has been called already" is stored in the /// [`SkipFirst`] instance and not globally: /// /// ``` /// use splop::SkipFirst; /// /// let mut v = Vec::new(); /// let mut skipper_a = SkipFirst::new(); /// let mut skipper_b = SkipFirst::new(); /// skipper_a.skip_first(|| v.push("a")); // won't be executed /// skipper_b.skip_first(|| v.push("b")); // won't be executed /// skipper_b.skip_first(|| v.push("b2")); // will be executed /// skipper_a.skip_first(|| v.push("a2")); // will be executed /// /// assert_eq!(v, ["b2", "a2"]); /// ``` pub fn skip_first(&mut self, f: impl FnOnce()) { if self.first { self.first = false; } else { f(); } } } /// Iterator wrapper which keeps track of the status. See /// [`IterStatusExt::with_status`] for more information. pub struct WithStatus<I: Iterator> { iter: Peekable<I>, first: bool, } impl<I: Iterator> WithStatus<I> { fn new(iter: I) -> Self { Self { iter: iter.peekable(), first: true, } } } impl<I: Iterator> Iterator for WithStatus<I> { type Item = (I::Item, Status); fn next(&mut self) -> Option<Self::Item> { // Get the next item from the iterator. let item = self.iter.next(); let status = Status { first: self.first, // Since we already got the real item above, we can now peek if // there is still another item. last: self.iter.peek().is_none(), }; if self.first { self.first = false; } item.map(|elem| (elem, status)) } fn size_hint(&self) -> (usize, Option<usize>) { // We pass through the `size_hint` method, as the underlying iterator // might have size information. self.iter.size_hint() } } // Implement traits when the underlying iterator implements them. impl<I: FusedIterator> FusedIterator for WithStatus<I> {} impl<I: ExactSizeIterator> ExactSizeIterator for WithStatus<I> { fn len(&self) -> usize { self.iter.len() } } /// Adds the `with_status` method to all iterators. pub trait IterStatusExt: Iterator + Sized { /// Creates an iterator that yields the original items paired with a /// status, which tells you if the item is the first and/or last one. /// /// The new iterator's item has the type `(Self::Item, Status)`. See /// [`Status`] for detailed information. The new iterator uses `peekable()` /// internally, so if the `next()` call of the underlying iterator has /// side effects, those will be visible earlier than expected. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// /// let mut s = String::new(); /// let names = ["anna", "peter", "bob"]; /// /// for (name, status) in names.iter().with_status() { /// if !status.is_first() { /// s += ", "; /// } /// /// s += name; /// } /// /// assert_eq!(s, "anna, peter, bob"); /// ``` fn with_status(self) -> WithStatus<Self>; } impl<I: Iterator> IterStatusExt for I { fn with_status(self) -> WithStatus<Self> { WithStatus::new(self) } } /// The status of an item from an iterator (e.g. "is this the first item?"). #[derive(Copy, Clone, Debug, PartialEq, Eq)] pub struct Status { first: bool, last: bool, } impl Status { /// Returns `true` if this is the first item of the iterator. /// /// Note that an item might simultaniously be the first and last item (if /// the iterator only contains one item). To check if the item is the first /// and and not the last, use [`Status::is_first_only`]. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// let v: Vec<_> = (0..4) /// .with_status() /// .map(|(i, status)| (i, status.is_first())) /// .collect(); /// /// assert_eq!(v, [ /// (0, true), /// (1, false), /// (2, false), /// (3, false), /// ]); /// ``` /// /// If there is only one element, this function returns `true`, as does /// `is_last`: /// /// ``` /// use splop::IterStatusExt; /// /// let (_, status) = [27].iter() /// .with_status() /// .next() /// .unwrap(); /// /// assert!(status.is_first()); /// assert!(status.is_last()); /// ``` pub fn is_first(&self) -> bool { self.first } /// Returns `true` if this is the first item and it's not the only item in /// the iterator. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// let v: Vec<_> = (0..4) /// .with_status() /// .map(|(i, status)| (i, status.is_first_only())) /// .collect(); /// /// assert_eq!(v, [ /// (0, true), /// (1, false), /// (2, false), /// (3, false), /// ]); /// ``` /// /// If there is only one element, this function returns `false`: /// /// ``` /// use splop::IterStatusExt; /// /// let (_, status) = [27].iter() /// .with_status() /// .next() /// .unwrap(); /// /// assert!(!status.is_first_only()); /// ``` pub fn is_first_only(&self) -> bool { self.first && !self.last } /// Returns `true` if this is the last item of the iterator. /// /// Note that an item might simultaniously be the last and first item (if /// the iterator only contains one item). To check if the item is the last /// and and not the first, use [`Status::is_last_only`]. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// let v: Vec<_> = (0..4) /// .with_status() /// .map(|(i, status)| (i, status.is_last())) /// .collect(); /// /// assert_eq!(v, [ /// (0, false), /// (1, false), /// (2, false), /// (3, true), /// ]); /// ``` /// /// If there is only one element, this function returns `true`, as does /// `is_first`: /// /// ``` /// use splop::IterStatusExt; /// /// let (_, status) = [27].iter() /// .with_status() /// .next() /// .unwrap(); /// /// assert!(status.is_first()); /// assert!(status.is_last()); /// ``` pub fn is_last(&self) -> bool { self.last } /// Returns `true` if this is the last item and it's not the only item in /// the iterator. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// let v: Vec<_> = (0..4) /// .with_status() /// .map(|(i, status)| (i, status.is_last_only())) /// .collect(); /// /// assert_eq!(v, [ /// (0, false), /// (1, false), /// (2, false), /// (3, true), /// ]); /// ``` /// /// If there is only one element, this function returns `false`: /// /// ``` /// use splop::IterStatusExt; /// /// let (_, status) = [27].iter() /// .with_status() /// .next() /// .unwrap(); /// /// assert!(!status.is_last_only()); /// ``` pub fn is_last_only(&self) -> bool { self.last && !self.first } /// Returns `true` if this is neither the first nor the last item. /// /// # Example /// /// ``` /// use splop::IterStatusExt; /// /// let v: Vec<_> = (0..4) /// .with_status() /// .map(|(i, status)| (i, status.is_in_between())) /// .collect(); /// /// assert_eq!(v, [ /// (0, false), /// (1, true), /// (2, true), /// (3, false), /// ]); /// ``` pub fn is_in_between(&self) -> bool { !self.first && !self.last } }