You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ops.rs
    path of file: ./repos/ruffle/wstr/src
    
    the code of the file until where you have to start completion: use alloc::borrow::{Borrow, Cow};
use alloc::string::String;
use alloc::vec::Vec;
use core::fmt::{self, Write};
use core::hash::Hasher;
use core::slice::Iter as SliceIter;

use super::pattern::{SearchStep, Searcher};
use super::{utils, Pattern, Units, WStr, WString};

pub struct Iter<'a> {
    i
    
    fn translate_index(&self, index: usize, is_utf8: bool) -> Option<(usize, usize)> {
        let ascii_prefix_len = self.head.len();
        if index <= ascii_prefix_len {
            return Some((index, index));
        }

        if self.tail.is_empty() {
            return None;
        }

        let mut utf8_tail_pos = 0;
        let mut utf16_tail_pos = 0;

        while if is_utf8 {
            utf8_tail_pos + ascii_prefix_len < index
        } else {
            utf16_tail_pos + ascii_prefix_len < index
        } {
            let c = self.tail[utf16_tail_pos..].chars().next()?.ok()?;
            utf8_tail_pos += c.len_utf8();
            utf16_tail_pos += c.len_utf16();
        }

        Some((
            ascii_prefix_len + utf8_tail_pos,
            ascii_prefix_len + utf16_tail_pos,
        ))
    }