use alloc::{string::String, vec::Vec};

use crate::{Diagnostic, Span, diagnostic::Result};

#[derive(Debug, Clone, PartialEq)]
pub struct Token {
    pub kind: TokenKind,
    pub span: Span,
}

#[derive(Debug, Clone, PartialEq)]
pub enum TokenKind {
    Ident(String),
    Int(i64),
    Float(f64),
    String(String),
    Regex(String),
    True,
    False,
    Let,
    In,
    Match,
    Import,
    Default,
    Underscore,
    LBrace,
    RBrace,
    LBracket,
    RBracket,
    LParen,
    RParen,
    Semicolon,
    Comma,
    Dot,
    Colon,
    Equal,
    Arrow,
    Amp,
    SlashSlash,
    Gt,
    Gte,
    Lt,
    Lte,
    Eof,
}

pub struct Lexer<'a> {
    source: &'a str,
    bytes: &'a [u8],
    pos: usize,
}

impl<'a> Lexer<'a> {
    pub fn new(source: &'a str) -> Self {
        Self {
            source,
            bytes: source.as_bytes(),
            pos: 0,
        }
    }

    pub fn tokenize(mut self) -> Result<Vec<Token>> {
        let mut tokens = Vec::new();
        loop {
            let token = self.next_token()?;
            let is_eof = token.kind == TokenKind::Eof;
            tokens.push(token);
            if is_eof {
                return Ok(tokens);
            }
        }
    }

    fn next_token(&mut self) -> Result<Token> {
        self.skip_ws_and_comments();
        let start = self.pos;
        let Some(ch) = self.peek() else {
            return Ok(Token {
                kind: TokenKind::Eof,
                span: Span::empty(self.pos),
            });
        };

        let kind = match ch {
            b'{' => {
                self.pos += 1;
                TokenKind::LBrace
            }
            b'}' => {
                self.pos += 1;
                TokenKind::RBrace
            }
            b'[' => {
                self.pos += 1;
                TokenKind::LBracket
            }
            b']' => {
                self.pos += 1;
                TokenKind::RBracket
            }
            b'(' => {
                self.pos += 1;
                TokenKind::LParen
            }
            b')' => {
                self.pos += 1;
                TokenKind::RParen
            }
            b';' => {
                self.pos += 1;
                TokenKind::Semicolon
            }
            b',' => {
                self.pos += 1;
                TokenKind::Comma
            }
            b'.' => {
                self.pos += 1;
                TokenKind::Dot
            }
            b':' => {
                self.pos += 1;
                TokenKind::Colon
            }
            b'_' => {
                self.pos += 1;
                TokenKind::Underscore
            }
            b'&' => {
                self.pos += 1;
                TokenKind::Amp
            }
            b'=' => {
                self.pos += 1;
                if self.consume(b'>') {
                    TokenKind::Arrow
                } else {
                    TokenKind::Equal
                }
            }
            b'>' => {
                self.pos += 1;
                if self.consume(b'=') {
                    TokenKind::Gte
                } else {
                    TokenKind::Gt
                }
            }
            b'<' => {
                self.pos += 1;
                if self.consume(b'=') {
                    TokenKind::Lte
                } else {
                    TokenKind::Lt
                }
            }
            b'/' => {
                self.pos += 1;
                if self.consume(b'/') {
                    TokenKind::SlashSlash
                } else {
                    self.lex_regex(start)?
                }
            }
            b'"' => self.lex_string()?,
            b'0'..=b'9' => self.lex_number()?,
            c if is_ident_start(c) => self.lex_ident_or_keyword(),
            _ => {
                return Err(Diagnostic::syntax(
                    Span::new(start, start + 1),
                    "unexpected character",
                ));
            }
        };

        Ok(Token {
            kind,
            span: Span::new(start, self.pos),
        })
    }

    fn skip_ws_and_comments(&mut self) {
        loop {
            while matches!(self.peek(), Some(b' ' | b'\t' | b'\r' | b'\n')) {
                self.pos += 1;
            }
            if self.peek() == Some(b'#') {
                while let Some(c) = self.peek() {
                    self.pos += 1;
                    if c == b'\n' {
                        break;
                    }
                }
                continue;
            }
            break;
        }
    }

    fn lex_string(&mut self) -> Result<TokenKind> {
        let start = self.pos;
        self.pos += 1;
        let mut value = String::new();
        while let Some(c) = self.peek() {
            self.pos += 1;
            match c {
                b'"' => return Ok(TokenKind::String(value)),
                b'\\' => {
                    let Some(escaped) = self.peek() else {
                        return Err(Diagnostic::syntax(
                            Span::new(start, self.pos),
                            "unterminated escape",
                        ));
                    };
                    self.pos += 1;
                    let ch = match escaped {
                        b'"' => '"',
                        b'\\' => '\\',
                        b'n' => '\n',
                        b'r' => '\r',
                        b't' => '\t',
                        other => other as char,
                    };
                    value.push(ch);
                }
                other => value.push(other as char),
            }
        }
        Err(Diagnostic::syntax(
            Span::new(start, self.pos),
            "unterminated string",
        ))
    }

    fn lex_regex(&mut self, start: usize) -> Result<TokenKind> {
        let mut pattern = String::new();
        let mut escaped = false;
        while let Some(c) = self.peek() {
            self.pos += 1;
            if escaped {
                pattern.push(c as char);
                escaped = false;
                continue;
            }
            match c {
                b'\\' => {
                    pattern.push('\\');
                    escaped = true;
                }
                b'/' => return Ok(TokenKind::Regex(pattern)),
                other => pattern.push(other as char),
            }
        }
        Err(Diagnostic::syntax(
            Span::new(start, self.pos),
            "unterminated regex",
        ))
    }

    fn lex_number(&mut self) -> Result<TokenKind> {
        let start = self.pos;
        while matches!(self.peek(), Some(b'0'..=b'9')) {
            self.pos += 1;
        }
        let mut is_float = false;
        if self.peek() == Some(b'.') && matches!(self.peek_n(1), Some(b'0'..=b'9')) {
            is_float = true;
            self.pos += 1;
            while matches!(self.peek(), Some(b'0'..=b'9')) {
                self.pos += 1;
            }
        }
        let text = &self.source[start..self.pos];
        if is_float {
            text.parse::<f64>().map(TokenKind::Float).map_err(|_| {
                Diagnostic::syntax(Span::new(start, self.pos), "invalid float literal")
            })
        } else {
            text.parse::<i64>()
                .map(TokenKind::Int)
                .map_err(|_| Diagnostic::syntax(Span::new(start, self.pos), "invalid int literal"))
        }
    }

    fn lex_ident_or_keyword(&mut self) -> TokenKind {
        let start = self.pos;
        self.pos += 1;
        while matches!(self.peek(), Some(c) if is_ident_continue(c)) {
            self.pos += 1;
        }
        let text = &self.source[start..self.pos];
        match text {
            "true" => TokenKind::True,
            "false" => TokenKind::False,
            "let" => TokenKind::Let,
            "in" => TokenKind::In,
            "match" => TokenKind::Match,
            "import" => TokenKind::Import,
            "default" => TokenKind::Default,
            _ => TokenKind::Ident(String::from(text)),
        }
    }

    fn peek(&self) -> Option<u8> {
        self.bytes.get(self.pos).copied()
    }

    fn peek_n(&self, n: usize) -> Option<u8> {
        self.bytes.get(self.pos + n).copied()
    }

    fn consume(&mut self, expected: u8) -> bool {
        if self.peek() == Some(expected) {
            self.pos += 1;
            true
        } else {
            false
        }
    }
}

fn is_ident_start(c: u8) -> bool {
    c.is_ascii_alphabetic()
}

fn is_ident_continue(c: u8) -> bool {
    c.is_ascii_alphanumeric() || c == b'_'
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn tokenizes_basic_source() {
        let tokens = Lexer::new("port = Int & >= 1;").tokenize().unwrap();
        assert!(matches!(tokens[0].kind, TokenKind::Ident(_)));
        assert_eq!(tokens[1].kind, TokenKind::Equal);
        assert_eq!(tokens[3].kind, TokenKind::Amp);
        assert_eq!(tokens[4].kind, TokenKind::Gte);
    }
}