#![no_std]

//! The [`reverse_grapheme_clusters_in_place`][0] function reverses a string slice in-place without
//! allocating any memory on the heap.  It correctly handles multi-byte UTF-8 sequences and
//! grapheme clusters, including combining marks and astral characters such as Emoji.
//!
//! ## Example
//!
//! ```rust
//! use unicode_reverse::reverse_grapheme_clusters_in_place;
//!
//! let mut x = "man\u{0303}ana".to_string();
//! println!("{}", x); // prints "mañana"
//!
//! reverse_grapheme_clusters_in_place(&mut x);
//! println!("{}", x); // prints "anañam"
//! ```
//!
//! ## Background
//!
//! As described in [this article by Mathias Bynens][1], naively reversing a Unicode string can go
//! wrong in several ways. For example, merely reversing the `chars` (Unicode Scalar Values) in a
//! string can cause combining marks to become attached to the wrong characters:
//!
//! ```rust
//! let x = "man\u{0303}ana";
//! println!("{}", x); // prints "mañana"
//!
//! let y: String = x.chars().rev().collect();
//! println!("{}", y); // prints "anãnam": Oops! The '~' is now applied to the 'a'.
//! ```
//!
//! Reversing the [grapheme clusters][2] of the string fixes this problem:
//!
//! ```rust
//! extern crate unicode_segmentation;
//! use unicode_segmentation::UnicodeSegmentation;
//!
//! # fn main() {
//! let x = "man\u{0303}ana";
//! let y: String = x.graphemes(true).rev().collect();
//! println!("{}", y); // prints "anañam"
//! # }
//! ```
//!
//! The `reverse_grapheme_clusters_in_place` function from this crate performs this same operation,
//! but performs the reversal in-place rather than allocating a new string.
//!
//! Note: Even grapheme-level reversal may produce unexpected output if the input string contains
//! certain non-printable control codes, such as directional formatting characters. Handling such
//! characters is outside the scope of this crate.
//!
//! ## Algorithm
//!
//! The implementation is very simple. It makes two passes over the string's contents:
//!
//! 1. For each grapheme cluster, reverse the bytes within the grapheme cluster in-place.
//! 2. Reverse the bytes of the entire string in-place.
//!
//! After the second pass, each grapheme cluster has been reversed twice, so its bytes are now back
//! in their original order, but the clusters are now in the opposite order within the string.
//!
//! ## no_std
//!
//! This crate does not depend on libstd, so it can be used in [`no_std` projects][3].
//!
//! [0]: fn.reverse_grapheme_clusters_in_place.html
//! [1]: https://mathiasbynens.be/notes/javascript-unicode
//! [2]: http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
//! [3]: https://doc.rust-lang.org/book/no-stdlib.html

#[cfg(test)]
mod tests;

use core::str;
use unicode_segmentation::UnicodeSegmentation;

/// Reverse a Unicode string in-place without allocating.
///
/// This function reverses a string slice in-place without allocating any memory on the heap.  It
/// correctly handles multi-byte UTF-8 sequences and grapheme clusters, including combining marks
/// and astral characters such as Emoji.
///
/// See the [crate-level documentation](index.html) for more details.
///
/// ## Example
///
/// ```rust
/// extern crate unicode_reverse;
/// use unicode_reverse::reverse_grapheme_clusters_in_place;
///
/// fn main() {
///     let mut x = "man\u{0303}ana".to_string();
///     println!("{}", x); // prints "mañana"
///
///     reverse_grapheme_clusters_in_place(&mut x);
///     println!("{}", x); // prints "anañam"
/// }
/// ```
pub fn reverse_grapheme_clusters_in_place(s: &mut str) {
    unsafe {
        let v = s.as_bytes_mut();

        // Part 1: Reverse the bytes within each grapheme cluster.
        // This does not preserve UTF-8 validity.
        {
            // Invariant: `tail` points to data we have not modified yet, so it is always valid UTF-8.
            let mut tail = &mut v[..];
            while let Some(len) = str::from_utf8_unchecked(tail)
                .graphemes(true)
                .next()
                .map(str::len)
            {
                let (grapheme, new_tail) = tail.split_at_mut(len);
                grapheme.reverse();
                tail = new_tail;
            }
        }

        // Part 2: Reverse all bytes. This restores multi-byte sequences to their original order.
        v.reverse();

        // The string is now valid UTF-8 again.
        debug_assert!(str::from_utf8(v).is_ok());
    }
}