use std::{fmt::Debug, iter::Peekable, str::Chars};

static DECIMAL_NUMERICS: &[char] = &['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];

pub fn tokenize<T: Into<String>>(to_tokenize: T) -> Result<Vec<FullToken>, String> {
    let to_tokenize = to_tokenize.into();
    let mut position = (0, 1);
    let mut cursor = Cursor {
        char_stream: to_tokenize.chars().peekable(),
        position,
    };

    let mut tokens = Vec::new();

    while let Some(character) = &cursor.next() {
        position.0 += 1;
        if *character == '\n' {
            position.1 += 1;
            position.0 = 0;
        }

        let peek = cursor.peek();

        let variant = match character {
            // Whitespace
            w if w.is_whitespace() => continue,
            // Comments
            '/' if peek == Some(&'/') => {
                while !matches!(&cursor.peek(), Some('\n')) {
                    cursor.next();
                }
                continue;
            }
            // "words"
            c if c.is_alphabetic() => {
                let mut value = character.to_string();
                while let Some(c) = &cursor.peek() {
                    if !c.is_ascii_alphanumeric() {
                        break;
                    }
                    value += &c.to_string();
                    cursor.next();
                }

                // Check for keywords
                let variant = match value.as_str() {
                    "let" => Token::LetKeyword,
                    "import" => Token::ImportKeyword,
                    _ => Token::Identifier(value),
                };
                variant
            }
            // Decimals
            c if DECIMAL_NUMERICS.contains(c) => {
                let mut value = character.to_string();
                while let Some(c) = &cursor.peek() {
                    if !DECIMAL_NUMERICS.contains(c) {
                        break;
                    }
                    value += &c.to_string();
                    cursor.next();
                }
                Token::DecimalValue(value)
            }
            // Single character tokens
            '=' => Token::Equals,
            ';' => Token::Semicolon,
            ':' => Token::Colon,
            '+' => Token::Plus,
            '*' => Token::Times,
            '(' => Token::ParenOpen,
            ')' => Token::ParenClose,
            // Invalid token
            _ => Err(format!(
                "Unknown token '{}' at {}, {}",
                character, position.0, position.1
            ))?,
        };

        tokens.push(FullToken {
            token: variant,
            position,
        });
    }

    position.0 += 1;

    tokens.push(FullToken {
        token: Token::Eof,
        position,
    });

    Ok(tokens)
}

pub struct FullToken {
    pub token: Token,
    position: Position,
}

impl Debug for FullToken {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_fmt(format_args!(
            "{:?} (Ln {}, Col {})",
            self.token, self.position.1, self.position.0
        ))
    }
}

pub type Position = (u32, u32);

#[derive(Debug, Eq, PartialEq, Clone)]
pub enum Token {
    // Values
    Identifier(String),
    /// Number with at most one decimal point
    DecimalValue(String),

    // Keywords
    LetKeyword,
    ImportKeyword,

    // Symbols
    Semicolon,
    Equals,
    Colon,
    Plus,
    Times,
    ParenOpen,  // (
    ParenClose, // )

    Eof,
}

pub struct Cursor<'a> {
    pub position: Position,
    char_stream: Peekable<Chars<'a>>,
}

impl<'a> Cursor<'a> {
    fn next(&mut self) -> Option<char> {
        let next = self.char_stream.next();
        self.position.0 += 1;
        if let Some('\n') = next {
            self.position.1 += 1;
            self.position.0 = 0;
        }
        next
    }

    fn peek(&mut self) -> Option<&char> {
        self.char_stream.peek()
    }
}