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use std::str::Chars;

pub(crate) const EOF_CHAR: char = '\0';

/// Handles peeking and consuming characters from the source, while keeping track
/// of the number of characters consumed.
#[derive(Debug, Clone)]
pub struct Cursor<'a> {
    initial_len: usize,
    chars: Chars<'a>,
}

impl<'a> Cursor<'a> {
    pub fn new(input: &'a str) -> Self {
        Self {
            initial_len: input.len(),
            chars: input.chars(),
        }
    }

    pub fn first(&mut self) -> char {
        self.chars.clone().next().unwrap_or(EOF_CHAR)
    }

    pub fn second(&mut self) -> char {
        let mut it = self.chars.clone();
        it.next();
        it.next().unwrap_or(EOF_CHAR)
    }

    pub fn is_eof(&self) -> bool {
        self.chars.as_str().is_empty()
    }

    pub fn bump(&mut self) -> Option<char> {
        self.chars.next()
    }

    pub fn eat_while(&mut self, mut predicate: impl FnMut(char) -> bool) {
        while predicate(self.first()) && !self.is_eof() {
            self.bump();
        }
    }

    pub fn len_consumed(&self) -> usize {
        self.initial_len - self.chars.as_str().len()
    }

    pub fn reset_len_consumed(&mut self) {
        self.initial_len = self.chars.as_str().len();
    }
}