pub(crate) struct Tokenizer {
    token_stack: Vec<String>,
    parse_string: String,
}

#[derive(Debug, PartialEq)]
pub(crate) enum ParseState {
    Empty,
    Alpha,
    AlphaDecimal,
    Numeric,
    NumericDecimal,
}

impl Tokenizer {
    pub(crate) fn new(parse_string: String) -> Self {
        Tokenizer {
            token_stack: Vec::new(),
            parse_string: parse_string.chars().rev().collect(),
        }
    }
}

impl Iterator for Tokenizer {
    type Item = String;

    fn next(&mut self) -> Option<Self::Item> {
        if !self.token_stack.is_empty() {
            return Some(self.token_stack.pop().unwrap());
        };
        if self.parse_string.is_empty() {
            return None;
        };

        let mut char_stack: Vec<char> = Vec::new();
        let mut seen_letters = false;
        let mut state = ParseState::Empty;

        while let Some(next) = self.parse_string.pop() {
            match state {
                ParseState::Empty => {
                    if next.is_numeric() {
                        state = ParseState::Numeric;
                        char_stack.push(next);
                    } else if next.is_alphabetic() {
                        state = ParseState::Alpha;
                        seen_letters = true;
                        char_stack.push(next);
                    } else if next.is_whitespace() {
                        char_stack.push(' ');
                        break;
                    } else {
                        char_stack.push(next);
                        break;
                    }
                }
                ParseState::Alpha => {
                    if next.is_alphabetic() {
                        char_stack.push(next);
                    } else if next == '.' {
                        state = ParseState::AlphaDecimal;
                        char_stack.push(next);
                    } else {
                        // We don't recognize the character, so push it back
                        // to be handled later.
                        self.parse_string.push(next);
                        break;
                    }
                }
                ParseState::AlphaDecimal => {
                    if next == '.' || next.is_alphabetic() {
                        char_stack.push(next);
                    } else if next.is_numeric() && char_stack.last().unwrap().clone() == '.' {
                        char_stack.push(next);
                        state = ParseState::NumericDecimal;
                    } else {
                        self.parse_string.push(next);
                        break;
                    }
                }
                ParseState::Numeric => {
                    if next.is_numeric() {
                        char_stack.push(next);
                    } else if next == '.' || (next == ',' && char_stack.len() >= 2) {
                        char_stack.push(next);
                        state = ParseState::NumericDecimal;
                    } else {
                        // We don't recognize the character, so push it back
                        // to be handled later
                        self.parse_string.push(next);
                        break;
                    }
                }
                ParseState::NumericDecimal => {
                    if next == '.' || next.is_numeric() {
                        char_stack.push(next);
                    } else if next.is_alphabetic() && char_stack.last().unwrap().clone() == '.' {
                        char_stack.push(next);
                        state = ParseState::AlphaDecimal;
                    } else {
                        self.parse_string.push(next);
                        break;
                    }
                }
            }
        }

        // I like Python's version of this much better:
        // needs_split = seen_letters or char_stack.count('.') > 1 or char_stack[-1] in '.,'
        let dot_count = char_stack.iter().fold(0, |count, character| {
            count + (if character == &'.' { 1 } else { 0 })
        });
        let needs_split = seen_letters || dot_count > 1 || char_stack.last().unwrap() == &'.'
            || char_stack.last().unwrap() == &',';
        let final_string: String = char_stack.into_iter().collect();

        let mut tokens = match state {
            ParseState::Empty => vec![final_string],
            ParseState::Alpha => vec![final_string],
            ParseState::Numeric => vec![final_string],
            ParseState::AlphaDecimal => {
                if needs_split {
                    decimal_split(&final_string, false)
                } else {
                    vec![final_string]
                }
            }
            ParseState::NumericDecimal => {
                if needs_split {
                    decimal_split(&final_string, dot_count == 0)
                } else {
                    vec![final_string]
                }
            }
        }.into_iter()
            .rev()
            .collect();

        self.token_stack.append(&mut tokens);
        // UNWRAP: Previous match guaranteed that at least one token was added
        let token = self.token_stack.pop().unwrap();
        if state == ParseState::NumericDecimal && !token.contains(".") {
            Some(token.replace(",", "."))
        } else {
            Some(token)
        }
    }
}

fn decimal_split(characters: &str, cast_period: bool) -> Vec<String> {
    let mut token_stack: Vec<String> = Vec::new();
    let mut char_stack: Vec<char> = Vec::new();
    let mut state = ParseState::Empty;

    for c in characters.chars() {
        match state {
            ParseState::Empty => {
                if c.is_alphabetic() {
                    char_stack.push(c);
                    state = ParseState::Alpha;
                } else if c.is_numeric() {
                    char_stack.push(c);
                    state = ParseState::Numeric;
                } else {
                    let character = if cast_period { '.' } else { c };
                    token_stack.push(character.to_string());
                }
            }
            ParseState::Alpha => {
                if c.is_alphabetic() {
                    char_stack.push(c);
                } else {
                    token_stack.push(char_stack.iter().collect());
                    char_stack.clear();
                    let character = if cast_period { '.' } else { c };
                    token_stack.push(character.to_string());
                    state = ParseState::Empty;
                }
            }
            ParseState::Numeric => {
                if c.is_numeric() {
                    char_stack.push(c);
                } else {
                    token_stack.push(char_stack.iter().collect());
                    char_stack.clear();
                    let character = if cast_period { '.' } else { c };
                    token_stack.push(character.to_string());
                    state = ParseState::Empty;
                }
            }
            _ => panic!("Invalid parse state during decimal_split()"),
        }
    }

    match state {
        ParseState::Alpha => token_stack.push(char_stack.iter().collect()),
        ParseState::Numeric => token_stack.push(char_stack.iter().collect()),
        ParseState::Empty => (),
        _ => panic!("Invalid parse state during decimal_split()"),
    }

    token_stack
}