use std::str::CharIndices;

/// `CharacterReader` may be used for iterating characters from left-to-right
/// in a string with miscellaneous operation methods.
#[derive(Clone)]
pub struct CharacterReader<'a> {
    length: usize,
    start_offset: usize,
    char_indices: CharIndices<'a>,
}

impl<'a> CharacterReader<'a> {
    /// Constructs a `CharacterReader` from a string starting at the given offset.
    pub fn from_offset(value: &'a str, offset: usize) -> Self {
        CharacterReader { length: value.len(), start_offset: offset, char_indices: value[offset..].char_indices() }
    }

    /// Indicates if there are remaining code points to read.
    pub fn has_remaining(&self) -> bool {
        self.clone().char_indices.next().is_some()
    }

    /// Indicates if the reader has reached the end of the string.
    pub fn reached_end(&self) -> bool {
        self.clone().char_indices.next().is_none()
    }

    /// Skips a code point in the reader. This is equivalent to
    /// calling `next()`.
    pub fn skip_in_place(&mut self) {
        self.next();
    }

    /// Skips the given number of code points in the reader.
    pub fn skip_count_in_place(&mut self, count: usize) {
        for _ in 0..count {
            if self.next().is_none() {
                break;
            }
        }
    }

    /// Returns the current byte offset in the string.
    pub fn index(&self) -> usize {
        self.clone().char_indices.next().map_or(self.length, |(i, _)| self.start_offset + i)
    }

    /// Returns the next code point. If there are no code points
    /// available, returns U+00.
    pub fn next_or_zero(&mut self) -> char {
        self.char_indices.next().map_or('\x00', |(_, cp)| cp)
    }

    /// Peeks the next code point.
    pub fn peek(&self) -> Option<char> {
        self.clone().char_indices.next().map(|(_, cp)| cp)
    }

    /// Peeks the next code point. If there are no code points
    /// available, returns U+00.
    pub fn peek_or_zero(&self) -> char {
        self.clone().next_or_zero()
    }

    /// Peeks the next code point at the given zero based code point index.
    pub fn peek_at(&self, index: usize) -> Option<char> {
        let mut indices = self.clone().char_indices;
        for _ in 0..index {
            if indices.next().is_none() {
                break;
            }
        }
        indices.next().map(|(_, cp)| cp)
    }

    /// Peeks the next code point at the given zero based code point index.
    /// If there are no code points available, returns U+00.
    pub fn peek_at_or_zero(&self, index: usize) -> char {
        self.peek_at(index).unwrap_or('\x00')
    }

    /// Peeks a number of code points until the string's end.
    pub fn peek_seq(&self, num_code_points: u64) -> String {
        let mut r = String::new();
        let mut next_indices = self.char_indices.clone();
        for _ in 0..num_code_points {
            match next_indices.next() {
                None => {
                    break;
                },
                Some(cp) => {
                    r.push(cp.1);
                }
            }
        }
        r
    }
}

impl<'a> From<&'a str> for CharacterReader<'a> {
    /// Constructs a `CharacterReader` from a string.
    fn from(value: &'a str) -> Self {
        CharacterReader { length: value.len(), start_offset: 0, char_indices: value.char_indices() }
    }
}

impl<'a> From<&'a String> for CharacterReader<'a> {
    /// Constructs a `CharacterReader` from a string.
    fn from(value: &'a String) -> Self {
        CharacterReader { length: value.len(), start_offset: 0, char_indices: value.char_indices() }
    }
}

impl<'a> Iterator for CharacterReader<'a> {
    type Item = char;

    fn next(&mut self) -> Option<Self::Item> {
        self.char_indices.next().map(|(_, cp)| cp)
    }
}

#[cfg(test)]
mod test {
    use super::CharacterReader;
    #[test]
    fn test() {
        let mut reader = CharacterReader::from("foo");
        assert!(reader.has_remaining());
        assert_eq!(reader.peek_seq(5), "foo");
        assert_eq!(reader.peek_seq(1), "f");
        assert_eq!(reader.peek_or_zero(), 'f');
        for _ in 0..3 {
            reader.next();
        }
        assert_eq!(reader.peek_or_zero(), '\x00');
        assert!(reader.reached_end());
    }
}