rcsubstring/

lib.rs

/*!
A reference-counted substring

For returning part of a string held in an [Rc] that needs to live longer than the source of the string itself.
For more complete alternatives see [arcstr](https://crates.io/crates/arcstr) or [slice-rc](https://crates.io/crates/slice-rc).
This is intended as a simple lightweight alternative where you just want a reference counted substring in single-threaded situations.

It implements both `Deref` and `AsRef` so can be used just as a `str` in most contexts.

# Example
```rust
# use rcsubstring::RcSubstring;
# use std::rc::Rc;
let shared_text: Rc<String> = Rc::new(String::from("Some text"));
let shared_substring = RcSubstring::new(Rc::clone(&shared_text), 5..9);
drop(shared_text);
assert_eq!(shared_substring, "text");
```

# Use Case
For an intended use case, consider a function that generates text and then returns an iterator over that text.
How do we get the lifetimes to work? Even if we pass the ownership of the generated text to the iterator the
iterator will not be allowed to pass back refs to the text it holds as it is a requirement that the values
returned by `next()` can outlive the iterator. This is simple crate that offeres a simple solution.

```rust
# use rcsubstring::RcSubstring;
# use std::rc::Rc;
struct WordIterator {
    rcstring: Rc<String>,
    start_pos: usize,
}
impl Iterator for WordIterator {
    type Item = RcSubstring;
    fn next(&mut self) -> Option<Self::Item> {
        let pos = self.start_pos + self.rcstring[self.start_pos..].find(" ")?;
        let value = RcSubstring::new(Rc::clone(&self.rcstring), self.start_pos..pos);
        self.start_pos = pos + 1;
        return Some(value);
    }
}

fn generate_text(values: Vec<usize>) -> String {
    let words = vec!["zero", "one", "two", "three", "four", "five"];
    let mut result = String::new();
    for i in values {
        result.push_str(words[i]);
        result.push_str(" ");
    }
    result
}

fn give_me_an_iterator() -> WordIterator {
    let text = generate_text(vec![2, 3, 1, 0, 5]);
    WordIterator {
        rcstring: Rc::new(text),
        start_pos: 0,
    }
}

let mut it = give_me_an_iterator();
assert_eq!(it.next().unwrap(), "two");
assert_eq!(it.next().unwrap(), "three");
assert_eq!(it.next().unwrap(), "one");
assert_eq!(it.next().unwrap(), "zero");
let value = it.next().unwrap();
drop(it);
assert_eq!(value, "five");
```

*/
#![warn(missing_docs)]
use std::convert::AsRef;
use std::fmt::{Debug, Display};
use std::ops::{Deref, Range};
use std::rc::Rc;

/**
A reference counted substring

Stores an `Rc<String>` and a range
The deref behaviour means this can be used just like a &str
The advantage is the internal [Rc] handles the memory management so you don't have to worry about borrow lifetimes
Useful for returning parts of a string that should live longer than the struct that returned them
eg. from an iterator over a string stored in the iterator itself
*/

#[derive(Debug)]
pub struct RcSubstring {
    rcstring: Rc<String>,
    range: Range<usize>,
}

impl Display for RcSubstring {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.deref())
    }
}

impl PartialEq<&str> for RcSubstring {
    fn eq(&self, other: &&str) -> bool {
        self.deref() == *other
    }
}

impl RcSubstring {
    /// Construct a new RcSubstring
    ///
    /// Takes the `Rc<String>` to wrap and the range for the substring in this text
    ///
    /// # Panics (in debug)
    ///
    /// Panics if `range` is invalid
    ///  - begin < end
    ///  - either begin or end > length of `Rc<String>` wrapped
    ///
    /// If it didn't panic here it would panic during the slice when the RcSubstring is used
    /// so it is better to catch the issues at source.
    ///
    /// These panics come from debug_assert! macros that are removed in release build
    /// for efficiency. You will still get a panic when you try to get the slice.
    pub fn new(rcstring: Rc<String>, range: Range<usize>) -> Self {
        debug_assert!(
            range.end >= range.start,
            "begin < end ({} < {}) when creating RcSubstring",
            range.start,
            range.end
        );
        debug_assert!(
            range.start <= rcstring.len(),
            "start index {} out of bounds when creating RcSubstring",
            range.start
        );
        debug_assert!(
            range.end <= rcstring.len(),
            "end index {} out of bounds when creating RcSubstring",
            range.end
        );
        RcSubstring { rcstring, range }
    }
}

impl Deref for RcSubstring {
    type Target = str;

    fn deref(&self) -> &Self::Target {
        &self.rcstring[self.range.start..self.range.end]
    }
}

impl<T> AsRef<T> for RcSubstring
where
    T: ?Sized,
    <RcSubstring as Deref>::Target: AsRef<T>,
{
    fn as_ref(&self) -> &T {
        self.deref().as_ref()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_basic_usage() {
        let text = "Line 1\nLine 2\nLine 3";
        let rcstring = Rc::new(text.to_string());
        let pos = text.find("\n").unwrap();
        let rcsubstring = RcSubstring::new(rcstring.clone(), 0..pos);
        let string_rep = format!("{}", rcsubstring);
        assert_eq!(string_rep, "Line 1");
        let debug_rep = format!("{:?}", rcsubstring);
        assert_eq!(
            debug_rep,
            "RcSubstring { rcstring: \"Line 1\\nLine 2\\nLine 3\", range: 0..6 }"
        );
        let pretty_rep = format!("{:#?}", rcsubstring);
        assert_eq!(
            pretty_rep,
            "RcSubstring {\n    rcstring: \"Line 1\\nLine 2\\nLine 3\",\n    range: 0..6,\n}"
        );
        assert_eq!(&rcsubstring[1..2], "i");
    }

    #[test]
    fn test_empty() {
        let rcsubstring = RcSubstring::new(Rc::new(String::from("Random text")), 3..3);
        assert_eq!(rcsubstring.len(), 0);
        assert_eq!(rcsubstring, "");
    }

    #[test]
    fn test_as_ref() {
        fn is_hello<T: AsRef<str>>(s: T) {
            assert_eq!(s.as_ref(), "hello");
        }

        let text = String::from("hello world!");
        let rcss = RcSubstring::new(Rc::new(text), 0..5);
        is_hello(rcss);
    }

    // Test these bad uses panic with our own message - ie. not in some other downstream code

    #[test]
    #[should_panic(expected = "RcSubstring")]
    fn test_end_before_start() {
        let _ = RcSubstring::new(Rc::new(String::from("Random text")), 3..0);
    }

    #[test]
    #[should_panic(expected = "RcSubstring")]
    fn test_start_out_of_range() {
        let _ = RcSubstring::new(Rc::new(String::from("Random text")), 100..101);
    }

    #[test]
    #[should_panic(expected = "RcSubstring")]
    fn test_end_out_of_range() {
        let _ = RcSubstring::new(Rc::new(String::from("Random text")), 0..101);
    }
}