use std::ops::{Bound, RangeBounds};

trait StringUtils {
    fn substring(&self, start: usize, len: usize) -> &str;
    fn slice(&self, range: impl RangeBounds<usize>) -> &str;
    fn get_vec_chars(&self) -> Vec<char>;
}

impl StringUtils for str {
    fn substring(&self, start: usize, len: usize) -> &str {
        let mut char_pos = 0;
        let mut byte_start = 0;
        let mut it = self.chars();
        loop {
            if char_pos == start { break; }
            if let Some(c) = it.next() {
                char_pos += 1;
                byte_start += c.len_utf8();
            }
            else { break; }
        }
        char_pos = 0;
        let mut byte_end = byte_start;
        loop {
            if char_pos == len { break; }
            if let Some(c) = it.next() {
                char_pos += 1;
                byte_end += c.len_utf8();
            }
            else { break; }
        }
        &self[byte_start..byte_end]
    }
    fn slice(&self, range: impl RangeBounds<usize>) -> &str {
        let start = match range.start_bound() {
            Bound::Included(bound) | Bound::Excluded(bound) => *bound,
            Bound::Unbounded => 0,
        };
        let len = match range.end_bound() {
            Bound::Included(bound) => *bound + 1,
            Bound::Excluded(bound) => *bound,
            Bound::Unbounded => self.len(),
        } - start;
        self.substring(start, len)
    }
    fn get_vec_chars(&self) -> Vec<char> { self.chars().collect() }
}

trait StringUtilsVecChars {
    fn to_string(&self) -> String;
    fn to_string_buf<'a>(&self, buf: & 'a mut String) -> & 'a String;
}

impl StringUtilsVecChars for Vec<char> {
    fn to_string(&self) -> String { self.iter().collect() }
    fn to_string_buf<'a>(&self, buf: & 'a mut String) -> & 'a String {
        buf.clear();
        for c in self.iter() { buf.push(*c); }
        buf
    }
}

trait StringUtilsSlices {
    fn to_string(&self) -> String;
    fn to_string_buf<'a>(&self, buf: & 'a mut String) -> & 'a String;
}

impl StringUtilsSlices for [char] {
    fn to_string(&self) -> String { self.iter().collect() }
    fn to_string_buf<'a>(&self, buf: & 'a mut String) -> & 'a String {
        buf.clear();
        for c in self.iter() { buf.push(*c); }
        buf
    }
}

fn main() {
    let s = "abcdèfghij";
    // All three statements should print:
    // "abcdè, abcdèfghij, dèfgh, dèfghij."
    println!("{}, {}, {}, {}.",
        s.substring(0, 5),
        s.substring(0, 50),
        s.substring(3, 5),
        s.substring(3, 50));
    println!("{}, {}, {}, {}.",
        s.slice(..5),
        s.slice(..50),
        s.slice(3..8),
        s.slice(3..));
    println!("{}, {}, {}, {}.",
        s.slice(..=4),
        s.slice(..=49),
        s.slice(3..=7),
        s.slice(3..));

    // Allocating a string from Vec
      
       .
      
    let mut vc = s.get_vec_chars(); 
    println!("{}, {}, {}, {}.",
        vc[..5].to_string(),
        vc.to_string(),
        vc[3..8].to_string(),
        vc[3..].to_string());

    // Reusing a String buffer from a Vec
      
       .
      
    let mut buf = String::new();
    print!("{}, ", vc[..5].to_string_buf(& mut buf));
    print!("{}, ", vc[..].to_string_buf(& mut buf));
    print!("{}, ", vc[3..8].to_string_buf(& mut buf));
    print!("{}.\n", vc[3..].to_string_buf(& mut buf));
    
    // Random access to the Vec
      
       . 
      
    for i in 0..(vc.len() - 2) {
        print!("{} ", vc[i..i+3].to_string_buf(& mut buf));
    }
    println!("");
    
    // Random modifications to the Vec
      
       .
      
    for i in (0..(vc.len() / 3) + 1).rev() {
        vc.insert(i*3, '#');
    }
    println!("{} ", vc.to_string());
    println!("{} ", vc.to_string_buf(& mut buf));
}

// Output:
//    abcdè, abcdèfghij, dèfgh, dèfghij.
//    abcdè, abcdèfghij, dèfgh, dèfghij.
//    abcdè, abcdèfghij, dèfgh, dèfghij.
//
//    abcdè, abcdèfghij, dèfgh, dèfghij.
//    abcdè, abcdèfghij, dèfgh, dèfghij.
//    abc bcd cdè dèf èfg fgh ghi hij 
//    #abc#dèf#ghi#j
//    #abc#dèf#ghi#j