use {
    crate::{
        ident::{self, Ident},
        lexer::{self, Lexer, Token, TokenKind},
    },
    std::{
        cell::{Cell, UnsafeCell},
        ffi::OsStr,
        fmt, io,
        ops::{Deref, Not},
        path::PathBuf,
        ptr::NonNull,
        sync::atomic::AtomicUsize,
    },
};

pub type Pos = u32;
pub type IdentFlags = u32;
pub type Symbols = Vec<Symbol>;
pub type FileId = u32;
pub type IdentIndex = u16;
pub type Loader<'a> = &'a (dyn Fn(&str, &str) -> io::Result<FileId> + 'a);

pub mod idfl {
    use super::*;

    macro_rules! flags {
        ($($name:ident,)*) => {
            $(pub const $name: IdentFlags = 1 << (std::mem::size_of::<IdentFlags>() * 8 - 1 - ${index(0)});)*
            pub const ALL: IdentFlags = 0 $(| $name)*;
        };
    }

    flags! {
        MUTABLE,
        REFERENCED,
        COMPTIME,
    }
}

pub fn no_loader(_: &str, _: &str) -> io::Result<FileId> {
    Err(io::ErrorKind::NotFound.into())
}

#[derive(Debug)]
pub struct Symbol {
    pub name: Ident,
    pub flags: IdentFlags,
}

#[derive(Clone, Copy)]
struct ScopeIdent {
    ident: Ident,
    declared: bool,
    flags: IdentFlags,
}

pub struct Parser<'a, 'b> {
    path: &'b str,
    loader: Loader<'b>,
    lexer: Lexer<'b>,
    arena: &'b Arena<'a>,
    token: Token,
    symbols: &'b mut Symbols,
    ns_bound: usize,
    trailing_sep: bool,
    idents: Vec<ScopeIdent>,
    captured: Vec<Ident>,
}

impl<'a, 'b> Parser<'a, 'b> {
    pub fn new(arena: &'b Arena<'a>, symbols: &'b mut Symbols, loader: Loader<'b>) -> Self {
        let mut lexer = Lexer::new("");
        Self {
            loader,
            token: lexer.next(),
            lexer,
            path: "",
            arena,
            symbols,
            ns_bound: 0,
            trailing_sep: false,
            idents: Vec::new(),
            captured: Vec::new(),
        }
    }

    pub fn file(&mut self, input: &'b str, path: &'b str) -> &'a [Expr<'a>] {
        self.path = path;
        self.lexer = Lexer::new(input);
        self.token = self.lexer.next();

        let f = self.collect_list(TokenKind::Semi, TokenKind::Eof, |s| s.expr_low(true));

        self.pop_scope(0);
        let mut errors = String::new();
        for id in self.idents.drain(..) {
            report_to(
                self.lexer.source(),
                self.path,
                ident::pos(id.ident),
                format_args!("undeclared identifier: {}", self.lexer.slice(ident::range(id.ident))),
                &mut errors,
            );
        }

        if !errors.is_empty() {
            // TODO: we need error recovery
            eprintln!("{errors}");
            unreachable!();
        }

        f
    }

    fn next(&mut self) -> Token {
        std::mem::replace(&mut self.token, self.lexer.next())
    }

    fn ptr_expr(&mut self) -> &'a Expr<'a> {
        self.arena.alloc(self.expr())
    }

    fn expr_low(&mut self, top_level: bool) -> Expr<'a> {
        let left = self.unit_expr();
        self.bin_expr(left, 0, top_level)
    }

    fn expr(&mut self) -> Expr<'a> {
        self.expr_low(false)
    }

    fn bin_expr(&mut self, mut fold: Expr<'a>, min_prec: u8, top_level: bool) -> Expr<'a> {
        loop {
            let Some(prec) = self.token.kind.precedence() else {
                break;
            };

            if prec <= min_prec {
                break;
            }

            let checkpoint = self.token.start;
            let op = self.next().kind;

            if op == TokenKind::Decl {
                self.declare_rec(&fold, top_level);
            }

            let op_ass = op.ass_op().map(|op| {
                // this abomination reparses the left side, so that the desubaring adheres to the
                // parser invariants.
                let source = self.lexer.slice(0..checkpoint as usize);
                let prev_lexer =
                    std::mem::replace(&mut self.lexer, Lexer::restore(source, fold.pos()));
                let prev_token = std::mem::replace(&mut self.token, self.lexer.next());
                let clone = self.expr();
                self.lexer = prev_lexer;
                self.token = prev_token;

                (op, clone)
            });

            let right = self.unit_expr();
            let right = self.bin_expr(right, prec, false);
            let right = self.arena.alloc(right);
            let left = self.arena.alloc(fold);

            if let Some((op, clone)) = op_ass {
                self.flag_idents(*left, idfl::MUTABLE);

                let right = Expr::BinOp { left: self.arena.alloc(clone), op, right };
                fold = Expr::BinOp { left, op: TokenKind::Assign, right: self.arena.alloc(right) };
            } else {
                fold = Expr::BinOp { left, right, op };
                if op == TokenKind::Assign {
                    self.flag_idents(*left, idfl::MUTABLE);
                }
            }
        }

        fold
    }

    fn declare_rec(&mut self, expr: &Expr, top_level: bool) {
        match *expr {
            Expr::Ident { pos, id, is_first, .. } => self.declare(pos, id, is_first || top_level),
            Expr::Ctor { fields, .. } => {
                for CtorField { value, .. } in fields {
                    self.declare_rec(value, top_level)
                }
            }
            _ => self.report(expr.pos(), "cant declare this shit (yet)"),
        }
    }

    fn declare(&mut self, pos: Pos, id: Ident, valid_order: bool) {
        if !valid_order {
            self.report(
                pos,
                format_args!(
                    "out of order declaration not allowed: {}",
                    self.lexer.slice(ident::range(id))
                ),
            );
        }

        let index = self.idents.binary_search_by_key(&id, |s| s.ident).expect("fck up");
        if std::mem::replace(&mut self.idents[index].declared, true) {
            self.report(
                pos,
                format_args!("redeclaration of identifier: {}", self.lexer.slice(ident::range(id))),
            )
        }
    }

    fn resolve_ident(&mut self, token: Token) -> (Ident, bool) {
        let is_ct = token.kind == TokenKind::CtIdent;
        let name = self.lexer.slice(token.range());

        if let Some(builtin) = crate::ty::from_str(name) {
            return (builtin, false);
        }

        let (i, id, bl) = match self
            .idents
            .iter_mut()
            .enumerate()
            .rfind(|(_, elem)| self.lexer.slice(ident::range(elem.ident)) == name)
        {
            Some((i, elem)) => (i, elem, false),
            None => {
                let id = ident::new(token.start, name.len() as _);
                self.idents.push(ScopeIdent { ident: id, declared: false, flags: 0 });
                (self.idents.len() - 1, self.idents.last_mut().unwrap(), true)
            }
        };

        id.flags |= idfl::COMPTIME * is_ct as u32;
        if id.declared && self.ns_bound > i {
            id.flags |= idfl::COMPTIME;
            self.captured.push(id.ident);
        }

        (id.ident, bl)
    }

    fn move_str(&mut self, range: Token) -> &'a str {
        self.arena.alloc_str(self.lexer.slice(range.range()))
    }

    fn unit_expr(&mut self) -> Expr<'a> {
        use {Expr as E, TokenKind as T};
        let frame = self.idents.len();
        let token @ Token { start: pos, .. } = self.next();
        let prev_boundary = self.ns_bound;
        let prev_captured = self.captured.len();
        let mut expr = match token.kind {
            T::Ct => E::Ct { pos, value: self.ptr_expr() },
            T::Directive if self.lexer.slice(token.range()) == "use" => {
                self.expect_advance(TokenKind::LParen);
                let str = self.expect_advance(TokenKind::DQuote);
                self.expect_advance(TokenKind::RParen);
                let path = self.lexer.slice(str.range()).trim_matches('"');

                E::Mod {
                    pos,
                    path: self.arena.alloc_str(path),
                    id: match (self.loader)(path, self.path) {
                        Ok(id) => id,
                        Err(e) => {
                            self.report(str.start, format_args!("error loading dependency: {e:#}"))
                        }
                    },
                }
            }
            T::Directive => E::Directive {
                pos: pos - 1, // need to undo the directive shift
                name: self.move_str(token),
                args: {
                    self.expect_advance(T::LParen);
                    self.collect_list(T::Comma, T::RParen, Self::expr)
                },
            },
            T::True => E::Bool { pos, value: true },
            T::False => E::Bool { pos, value: false },
            T::Idk => E::Idk { pos },
            T::DQuote => E::String { pos, literal: self.move_str(token) },
            T::Struct => E::Struct {
                fields: {
                    self.ns_bound = self.idents.len();
                    self.expect_advance(T::LBrace);
                    self.collect_list(T::Comma, T::RBrace, |s| {
                        let tok = s.token;
                        if s.advance_if(T::Comment) {
                            CommentOr::Comment { literal: s.move_str(tok), pos: tok.start }
                        } else {
                            let name = s.expect_advance(T::Ident);
                            s.expect_advance(T::Colon);
                            CommentOr::Or(StructField {
                                pos: name.start,
                                name: s.move_str(name),
                                ty: s.expr(),
                            })
                        }
                    })
                },
                captured: {
                    self.ns_bound = prev_boundary;
                    self.captured[prev_captured..].sort_unstable();
                    let preserved = self.captured[prev_captured..].partition_dedup().0.len();
                    self.captured.truncate(prev_captured + preserved);
                    self.arena.alloc_slice(&self.captured[prev_captured..])
                },
                pos: {
                    if self.ns_bound == 0 {
                        // we might save some memory
                        self.captured.clear();
                    }
                    pos
                },
                trailing_comma: std::mem::take(&mut self.trailing_sep),
            },
            T::Ident | T::CtIdent => {
                let (id, is_first) = self.resolve_ident(token);
                let name = self.move_str(token);
                E::Ident { pos, is_ct: token.kind == T::CtIdent, name, id, is_first }
            }
            T::If => E::If {
                pos,
                cond: self.ptr_expr(),
                then: self.ptr_expr(),
                else_: self.advance_if(T::Else).then(|| self.ptr_expr()),
            },
            T::Loop => E::Loop { pos, body: self.ptr_expr() },
            T::Break => E::Break { pos },
            T::Continue => E::Continue { pos },
            T::Return => E::Return {
                pos,
                val: (!matches!(
                    self.token.kind,
                    T::Semi | T::RBrace | T::RBrack | T::RParen | T::Comma
                ))
                .then(|| self.ptr_expr()),
            },
            T::Fn => E::Closure {
                pos,
                args: {
                    self.expect_advance(T::LParen);
                    self.collect_list(T::Comma, T::RParen, |s| {
                        let name = s.advance_ident();
                        let (id, _) = s.resolve_ident(name);
                        s.declare(name.start, id, true);
                        s.expect_advance(T::Colon);
                        Arg {
                            pos: name.start,
                            name: s.move_str(name),
                            is_ct: name.kind == T::CtIdent,
                            id,
                            ty: s.expr(),
                        }
                    })
                },
                ret: {
                    self.expect_advance(T::Colon);
                    self.ptr_expr()
                },
                body: self.ptr_expr(),
            },
            T::Ctor => self.ctor(pos, None),
            T::Tupl => self.tupl(pos, None),
            T::LBrack => E::Slice {
                item: self.ptr_unit_expr(),
                size: self.advance_if(T::Semi).then(|| self.ptr_expr()),
                pos: {
                    self.expect_advance(T::RBrack);
                    pos
                },
            },
            T::Band | T::Mul | T::Xor | T::Sub => E::UnOp {
                pos,
                op: token.kind,
                val: {
                    let expr = self.ptr_unit_expr();
                    if token.kind == T::Band {
                        self.flag_idents(*expr, idfl::REFERENCED);
                    }
                    expr
                },
            },
            T::LBrace => E::Block { pos, stmts: self.collect_list(T::Semi, T::RBrace, Self::expr) },
            T::Number => {
                let slice = self.lexer.slice(token.range());
                let (slice, radix) = match &slice.get(0..2) {
                    Some("0x") => (slice.trim_start_matches("0x"), Radix::Hex),
                    Some("0b") => (slice.trim_start_matches("0b"), Radix::Binary),
                    Some("0o") => (slice.trim_start_matches("0o"), Radix::Octal),
                    _ => (slice, Radix::Decimal),
                };
                E::Number {
                    pos,
                    value: match u64::from_str_radix(slice, radix as u32) {
                        Ok(value) => value,
                        Err(e) => self.report(token.start, format_args!("invalid number: {e}")),
                    } as i64,
                    radix,
                }
            }
            T::LParen => {
                let expr = self.expr();
                self.expect_advance(T::RParen);
                expr
            }
            T::Comment => Expr::Comment { pos, literal: self.move_str(token) },
            tok => self.report(token.start, format_args!("unexpected token: {tok:?}")),
        };

        loop {
            let token = self.token;
            if matches!(token.kind, T::LParen | T::Ctor | T::Dot | T::Tupl | T::LBrack) {
                self.next();
            }

            expr = match token.kind {
                T::LParen => Expr::Call {
                    func: self.arena.alloc(expr),
                    args: self.collect_list(T::Comma, T::RParen, Self::expr),
                    trailing_comma: std::mem::take(&mut self.trailing_sep),
                },
                T::Ctor => self.ctor(token.start, Some(expr)),
                T::Tupl => self.tupl(token.start, Some(expr)),
                T::LBrack => E::Index {
                    base: self.arena.alloc(expr),
                    index: {
                        let index = self.expr();
                        self.expect_advance(T::RBrack);
                        self.arena.alloc(index)
                    },
                },
                T::Dot => E::Field {
                    target: self.arena.alloc(expr),
                    name: {
                        let token = self.expect_advance(T::Ident);
                        self.move_str(token)
                    },
                },
                _ => break,
            }
        }

        if matches!(token.kind, T::Loop | T::LBrace | T::Fn) {
            self.pop_scope(frame);
        }

        expr
    }

    fn tupl(&mut self, pos: Pos, ty: Option<Expr<'a>>) -> Expr<'a> {
        Expr::Tupl {
            pos,
            ty: ty.map(|ty| self.arena.alloc(ty)),
            fields: self.collect_list(TokenKind::Comma, TokenKind::RParen, Self::expr),
            trailing_comma: std::mem::take(&mut self.trailing_sep),
        }
    }

    fn ctor(&mut self, pos: Pos, ty: Option<Expr<'a>>) -> Expr<'a> {
        Expr::Ctor {
            pos,
            ty: ty.map(|ty| self.arena.alloc(ty)),
            fields: self.collect_list(TokenKind::Comma, TokenKind::RBrace, |s| {
                let name_tok = s.advance_ident();
                let name = s.move_str(name_tok);
                CtorField {
                    pos: name_tok.start,
                    name,
                    value: if s.advance_if(TokenKind::Colon) {
                        s.expr()
                    } else {
                        let (id, is_first) = s.resolve_ident(name_tok);
                        Expr::Ident { pos: name_tok.start, is_ct: false, id, name, is_first }
                    },
                }
            }),
            trailing_comma: std::mem::take(&mut self.trailing_sep),
        }
    }

    fn advance_ident(&mut self) -> Token {
        if matches!(self.token.kind, TokenKind::Ident | TokenKind::CtIdent) {
            self.next()
        } else {
            self.report(
                self.token.start,
                format_args!("expected identifier, found {:?}", self.token.kind),
            )
        }
    }

    fn pop_scope(&mut self, frame: usize) {
        let mut undeclared_count = frame;
        for i in frame..self.idents.len() {
            if !&self.idents[i].declared {
                self.idents.swap(i, undeclared_count);
                undeclared_count += 1;
            }
        }

        self.idents
            .drain(undeclared_count..)
            .map(|ident| Symbol { name: ident.ident, flags: ident.flags })
            .collect_into(self.symbols);
    }

    fn ptr_unit_expr(&mut self) -> &'a Expr<'a> {
        self.arena.alloc(self.unit_expr())
    }

    fn collect_list<T: Copy>(
        &mut self,
        delim: TokenKind,
        end: TokenKind,
        mut f: impl FnMut(&mut Self) -> T,
    ) -> &'a [T] {
        self.collect(|s| {
            s.advance_if(end).not().then(|| {
                let val = f(s);
                s.trailing_sep = s.advance_if(delim);
                val
            })
        })
    }

    fn collect<T: Copy>(&mut self, mut f: impl FnMut(&mut Self) -> Option<T>) -> &'a [T] {
        // TODO: avoid this allocation
        let vec = std::iter::from_fn(|| f(self)).collect::<Vec<_>>();
        self.arena.alloc_slice(&vec)
    }

    fn advance_if(&mut self, kind: TokenKind) -> bool {
        if self.token.kind == kind {
            self.next();
            true
        } else {
            false
        }
    }

    fn expect_advance(&mut self, kind: TokenKind) -> Token {
        if self.token.kind != kind {
            self.report(
                self.token.start,
                format_args!("expected {:?}, found {:?}", kind, self.token.kind),
            );
        }
        self.next()
    }

    #[track_caller]
    fn report(&self, pos: Pos, msg: impl fmt::Display) -> ! {
        let mut str = String::new();
        report_to(self.lexer.source(), self.path, pos, msg, &mut str);
        eprintln!("{str}");
        unreachable!();
    }

    fn flag_idents(&mut self, e: Expr<'a>, flags: IdentFlags) {
        match e {
            Expr::Ident { id, .. } => find_ident(&mut self.idents, id).flags |= flags,
            Expr::Field { target, .. } => self.flag_idents(*target, flags),
            _ => {}
        }
    }
}

fn find_ident(idents: &mut [ScopeIdent], id: Ident) -> &mut ScopeIdent {
    idents.binary_search_by_key(&id, |si| si.ident).map(|i| &mut idents[i]).unwrap()
}

pub fn find_symbol(symbols: &[Symbol], id: Ident) -> &Symbol {
    symbols.binary_search_by_key(&id, |s| s.name).map(|i| &symbols[i]).unwrap()
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Arg<'a> {
    pub pos: u32,
    pub name: &'a str,
    pub id: Ident,
    pub is_ct: bool,
    pub ty: Expr<'a>,
}

impl Poser for Arg<'_> {
    fn posi(&self) -> Pos {
        self.pos
    }
}

macro_rules! generate_expr {
    ($(#[$meta:meta])* $vis:vis enum $name:ident<$lt:lifetime> {$(
        $(#[$field_meta:meta])*
        $variant:ident {

            $($field:ident: $ty:ty,)*
        },
    )*}) => {
        #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        $vis enum $name<$lt> {$(
            $(#[$field_meta])*
            $variant {
                $($field: $ty,)*
            },
        )*}

        impl<$lt> $name<$lt> {
            pub fn pos(&self) -> Pos {
                #[allow(unused_variables)]
                match self {
                    $(Self::$variant { $($field),* } => generate_expr!(@first $(($field),)*).posi(),)*
                }
            }

            pub fn used_bytes(&self) -> usize {
                match self {$(
                    Self::$variant { $($field,)* } => {
                         #[allow(clippy::size_of_ref)]
                         let fields = [$(($field as *const _ as usize - self as *const _ as usize, std::mem::size_of_val($field)),)*];
                         let (last, size) = fields.iter().copied().max().unwrap();
                         last + size
                    },
               )*}
            }
        }
    };

    (@filed_names $variant:ident $ident1:ident) => { Self::$variant { $ident1: a } };

    (@first ($($first:tt)*), $($rest:tt)*) => { $($first)* };
    (@last ($($ign:tt)*), $($rest:tt)*) => { $($rest)* };
    (@last ($($last:tt)*),) => { $($last)* };
}

#[repr(u32)]
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum Radix {
    Hex = 16,
    Octal = 8,
    Binary = 2,
    Decimal = 10,
}

generate_expr! {
    /// `LIST(start, sep, end, elem) => start { elem sep } [elem] end`
    /// `OP := grep for `#define OP:`
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    pub enum Expr<'a> {
        /// `'ct' Expr`
        Ct {
            pos: Pos,
            value: &'a Self,
        },
        /// `'"([^"]|\\")"'`
        String {
            pos: Pos,
            literal: &'a str,
        },
        /// `'//[^\n]' | '/*' { '([^/*]|*/)*' | Comment } '*/'
        Comment {
            pos: Pos,
            literal: &'a str,
        },
        /// `'break'`
        Break {
            pos: Pos,
        },
        /// `'continue'`
        Continue {
            pos: Pos,
        },
        /// `'fn' LIST('(', ',', ')', Ident ':' Expr) ':' Expr Expr`
        Closure {
            pos:  Pos,
            args: &'a [Arg<'a>],
            ret:  &'a Self,
            body: &'a Self,
        },
        /// `Expr LIST('(', ',', ')', Expr)`
        Call {
            func: &'a Self,
            args: &'a [Self],
            trailing_comma: bool,
        },
        /// `'return' [Expr]`
        Return {
            pos: Pos,
            val: Option<&'a Self>,
        },
        /// note: ':unicode:' is any utf-8 character except ascii
        /// `'[a-zA-Z_:unicode:][a-zA-Z0-9_:unicode:]*'`
        Ident {
            pos: Pos,
            is_ct: bool,
            is_first: bool,
            id: Ident,
            name: &'a str,
        },
        /// `LIST('{', [';'], '}', Expr)`
        Block {
            pos:   Pos,
            stmts: &'a [Self],
        },
        /// `'0b[01]+' | '0o[0-7]+' | '[0-9]+' | '0b[01]+'`
        Number {
            pos:   Pos,
            value: i64,
            radix: Radix,
        },
        /// node: precedence defined in `OP` applies
        /// `Expr OP Expr`
        BinOp {
            left:  &'a Self,
            op:    TokenKind,
            right: &'a Self,
        },
        /// `'if' Expr Expr [else Expr]`
        If {
            pos:   Pos,
            cond:  &'a Self,
            then:  &'a Self,
            else_: Option<&'a Self>,
        },
        /// `'loop' Expr`
        Loop {
            pos:  Pos,
            body: &'a Self,
        },
        /// `('&' | '*' | '^') Expr`
        UnOp {
            pos: Pos,
            op:  TokenKind,
            val: &'a Self,
        },
        /// `'struct' LIST('{', ',', '}', Ident ':' Expr)`
        Struct {
            pos:      Pos,
            fields:   &'a [CommentOr<'a, StructField<'a>>],
            captured: &'a [Ident],
            trailing_comma: bool,
        },
        /// `[Expr] LIST('.{', ',', '}', Ident [':' Expr])`
        Ctor {
            pos:    Pos,
            ty:     Option<&'a Self>,
            fields: &'a [CtorField<'a>],
            trailing_comma: bool,
        },
        /// `[Expr] LIST('.(', ',', ')', Ident [':' Expr])`
        Tupl {
            pos:    Pos,
            ty:     Option<&'a Self>,
            fields: &'a [Self],
            trailing_comma: bool,
        },
        /// `'[' Expr [';' Expr] ']'`
        Slice {
            pos: Pos,
            size: Option<&'a Self>,
            item: &'a Self,
        },
        /// `Expr '[' Expr ']'`
        Index {
            base: &'a Self,
            index: &'a Self,
        },
        /// `Expr '.' Ident`
        Field {
            target: &'a Self,
            name:  &'a str,
        },
        /// `'true' | 'false'`
        Bool {
            pos:   Pos,
            value: bool,
        },
        /// `'idk'`
        Idk {
            pos: Pos,
        },
        /// `'@' Ident List('(', ',', ')', Expr)`
        Directive {
            pos:  u32,
            name: &'a str,
            args: &'a [Self],
        },
        /// `'@use' '(' String ')'`
        Mod {
            pos:  Pos,
            id:   FileId,
            path: &'a str,
        },
    }
}

impl<'a> Expr<'a> {
    pub fn declares(&self, iden: Result<Ident, &str>) -> Option<Ident> {
        match *self {
            Self::Ident { id, name, .. } if iden == Ok(id) || iden == Err(name) => Some(id),
            Self::Ctor { fields, .. } => fields.iter().find_map(|f| f.value.declares(iden)),
            _ => None,
        }
    }

    pub fn has_ct(&self, symbols: &[Symbol]) -> bool {
        match *self {
            Self::Ident { id, .. } => find_symbol(symbols, id).flags & idfl::COMPTIME != 0,
            Self::Ctor { fields, .. } => fields.iter().any(|f| f.value.has_ct(symbols)),
            _ => false,
        }
    }

    pub fn find_pattern_path<F: FnOnce(&Expr)>(
        &self,
        ident: Ident,
        target: &Expr,
        mut with_final: F,
    ) -> Result<(), F> {
        match *self {
            Self::Ident { id, .. } if id == ident => {
                with_final(target);
                Ok(())
            }
            Self::Ctor { fields, .. } => {
                for CtorField { name, value, .. } in fields {
                    match value.find_pattern_path(ident, &Expr::Field { target, name }, with_final)
                    {
                        Ok(()) => return Ok(()),
                        Err(e) => with_final = e,
                    }
                }
                Err(with_final)
            }
            _ => Err(with_final),
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct StructField<'a> {
    pub pos: Pos,
    pub name: &'a str,
    pub ty: Expr<'a>,
}

impl Poser for StructField<'_> {
    fn posi(&self) -> Pos {
        self.pos
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct CtorField<'a> {
    pub pos: Pos,
    pub name: &'a str,
    pub value: Expr<'a>,
}

impl Poser for CtorField<'_> {
    fn posi(&self) -> Pos {
        self.pos
    }
}

trait Poser {
    fn posi(&self) -> Pos;
}

impl Poser for Pos {
    fn posi(&self) -> Pos {
        *self
    }
}

impl<'a> Poser for Expr<'a> {
    fn posi(&self) -> Pos {
        self.pos()
    }
}

impl<'a, T: Poser> Poser for CommentOr<'a, T> {
    fn posi(&self) -> Pos {
        match self {
            CommentOr::Or(expr) => expr.posi(),
            CommentOr::Comment { pos, .. } => *pos,
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum CommentOr<'a, T> {
    Or(T),
    Comment { literal: &'a str, pos: Pos },
}

impl<'a, T: Copy> CommentOr<'a, T> {
    pub fn or(&self) -> Option<T> {
        match *self {
            CommentOr::Or(v) => Some(v),
            CommentOr::Comment { .. } => None,
        }
    }
}

thread_local! {
    static FMT_SOURCE: Cell<*const str> = const { Cell::new("") };
}

pub fn with_fmt_source<T>(source: &str, f: impl FnOnce() -> T) -> T {
    FMT_SOURCE.with(|s| s.set(source));
    let r = f();
    FMT_SOURCE.with(|s| s.set(""));
    r
}

impl<'a> fmt::Display for Expr<'a> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        thread_local! {
            static INDENT: Cell<usize> = const { Cell::new(0) };
            static DISPLAY_BUFFER: Cell<String> = const { Cell::new(String::new()) };
        }

        fn fmt_list<T: Poser>(
            f: &mut fmt::Formatter,
            trailing: bool,
            end: &str,
            sep: &str,
            list: &[T],
            fmt: impl Fn(&T, &mut fmt::Formatter) -> fmt::Result,
        ) -> fmt::Result {
            fmt_list_low(f, trailing, end, sep, list, |v, f| {
                fmt(v, f)?;
                Ok(true)
            })
        }

        fn fmt_list_low<T: Poser>(
            f: &mut fmt::Formatter,
            trailing: bool,
            end: &str,
            sep: &str,
            list: &[T],
            fmt: impl Fn(&T, &mut fmt::Formatter) -> Result<bool, fmt::Error>,
        ) -> fmt::Result {
            if !trailing {
                let mut first = true;
                for expr in list {
                    if !std::mem::take(&mut first) {
                        write!(f, "{sep} ")?;
                    }
                    first = !fmt(expr, f)?;
                }
                return write!(f, "{end}");
            }

            writeln!(f)?;
            INDENT.with(|i| i.set(i.get() + 1));
            let res = (|| {
                for (i, stmt) in list.iter().enumerate() {
                    for _ in 0..INDENT.with(|i| i.get()) {
                        write!(f, "\t")?;
                    }
                    let add_sep = fmt(stmt, f)?;
                    if add_sep {
                        write!(f, "{sep}")?;
                    }
                    let source: &str = unsafe { &*FMT_SOURCE.with(|s| s.get()) };
                    if let Some(expr) = list.get(i + 1)
                        && let Some(rest) = source.get(expr.posi() as usize..)
                    {
                        if insert_needed_semicolon(rest) {
                            write!(f, ";")?;
                        }
                        if preserve_newlines(&source[..expr.posi() as usize]) > 1 {
                            writeln!(f)?;
                        }
                    }
                    if add_sep {
                        writeln!(f)?;
                    }
                }
                Ok(())
            })();
            INDENT.with(|i| i.set(i.get() - 1));

            for _ in 0..INDENT.with(|i| i.get()) {
                write!(f, "\t")?;
            }
            write!(f, "{end}")?;
            res
        }

        macro_rules! impl_parenter {
            ($($name:ident => $pat:pat,)*) => {
                $(
                    struct $name<'a>(&'a Expr<'a>);

                    impl<'a> std::fmt::Display for $name<'a> {
                        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                            if matches!(self.0, $pat) {
                                write!(f, "({})", self.0)
                            } else {
                                write!(f, "{}", self.0)
                            }
                        }
                    }
                )*
            };
        }

        impl_parenter! {
            Unary => Expr::BinOp { .. },
            Postfix => Expr::UnOp { .. } | Expr::BinOp { .. },
            Consecutive => Expr::UnOp { .. },
        }

        match *self {
            Self::Ct { value, .. } => write!(f, "$: {}", value),
            Self::String { literal, .. } => write!(f, "{}", literal),
            Self::Comment { literal, .. } => write!(f, "{}", literal.trim_end()),
            Self::Mod { path, .. } => write!(f, "@use(\"{path}\")"),
            Self::Field { target, name: field } => write!(f, "{}.{field}", Postfix(target)),
            Self::Directive { name, args, .. } => {
                write!(f, "@{name}(")?;
                fmt_list(f, false, ")", ",", args, fmt::Display::fmt)
            }
            Self::Struct { fields, trailing_comma, .. } => {
                write!(f, "struct {{")?;
                fmt_list_low(f, trailing_comma, "}", ",", fields, |field, f| {
                    match field {
                        CommentOr::Or(StructField { name, ty, .. }) => write!(f, "{name}: {ty}")?,
                        CommentOr::Comment { literal, .. } => write!(f, "{literal}")?,
                    }
                    Ok(field.or().is_some())
                })
            }
            Self::Ctor { ty, fields, trailing_comma, .. } => {
                if let Some(ty) = ty {
                    write!(f, "{}", Unary(ty))?;
                }
                write!(f, ".{{")?;
                let fmt_field = |CtorField { name, value, .. }: &_, f: &mut fmt::Formatter| {
                    if matches!(value, Expr::Ident { name: n, .. } if name == n) {
                        write!(f, "{name}")
                    } else {
                        write!(f, "{name}: {value}")
                    }
                };
                fmt_list(f, trailing_comma, "}", ",", fields, fmt_field)
            }
            Self::Tupl { ty, fields, trailing_comma, .. } => {
                if let Some(ty) = ty {
                    write!(f, "{}", Unary(ty))?;
                }
                write!(f, ".(")?;
                fmt_list(f, trailing_comma, ")", ",", fields, fmt::Display::fmt)
            }
            Self::Slice { item, size, .. } => match size {
                Some(size) => write!(f, "[{item}; {size}]"),
                None => write!(f, "[{item}]"),
            },
            Self::Index { base, index } => write!(f, "{base}[{index}]"),
            Self::UnOp { op, val, .. } => write!(f, "{op}{}", Unary(val)),
            Self::Break { .. } => write!(f, "break"),
            Self::Continue { .. } => write!(f, "continue"),
            Self::If { cond, then, else_, .. } => {
                write!(f, "if {cond} {}", Consecutive(then))?;
                if let Some(else_) = else_ {
                    write!(f, " else {else_}")?;
                }
                Ok(())
            }
            Self::Loop { body, .. } => write!(f, "loop {body}"),
            Self::Closure { ret, body, args, .. } => {
                write!(f, "fn(")?;
                fmt_list(f, false, "", ",", args, |arg, f| {
                    if arg.is_ct {
                        write!(f, "$")?;
                    }
                    write!(f, "{}: {}", arg.name, arg.ty)
                })?;
                write!(f, "): {ret} {body}")?;
                Ok(())
            }
            Self::Call { func, args, trailing_comma } => {
                write!(f, "{}(", Postfix(func))?;
                fmt_list(f, trailing_comma, ")", ",", args, fmt::Display::fmt)
            }
            Self::Return { val: Some(val), .. } => write!(f, "return {val}"),
            Self::Return { val: None, .. } => write!(f, "return"),
            Self::Ident { name, is_ct: true, .. } => write!(f, "${name}"),
            Self::Ident { name, is_ct: false, .. } => write!(f, "{name}"),
            Self::Block { stmts, .. } => {
                write!(f, "{{")?;
                fmt_list(f, true, "}", "", stmts, fmt::Display::fmt)
            }
            Self::Number { value, radix, .. } => match radix {
                Radix::Decimal => write!(f, "{value}"),
                Radix::Hex => write!(f, "{value:#X}"),
                Radix::Octal => write!(f, "{value:#o}"),
                Radix::Binary => write!(f, "{value:#b}"),
            },
            Self::Bool { value, .. } => write!(f, "{value}"),
            Self::Idk { .. } => write!(f, "idk"),
            Self::BinOp {
                left,
                op: TokenKind::Assign,
                right: Self::BinOp { left: lleft, op, right },
            } if DISPLAY_BUFFER.with(|cb| {
                use std::fmt::Write;
                let mut b = cb.take();
                write!(b, "{lleft}").unwrap();
                let len = b.len();
                write!(b, "{left}").unwrap();
                let (lleft, left) = b.split_at(len);
                let res = lleft == left;
                b.clear();
                cb.set(b);
                res
            }) =>
            {
                write!(f, "{left} {op}= {right}")
            }
            Self::BinOp { right, op, left } => {
                let display_branch = |f: &mut fmt::Formatter, expr: &Self| {
                    if let Self::BinOp { op: lop, .. } = expr
                        && op.precedence() > lop.precedence()
                    {
                        write!(f, "({expr})")
                    } else {
                        write!(f, "{expr}")
                    }
                };

                let source: &str = unsafe { &*FMT_SOURCE.with(|s| s.get()) };
                display_branch(f, left)?;
                if let Some(mut prev) = source.get(..right.pos() as usize) {
                    prev = prev.trim_end();
                    let estimate_bound =
                        prev.rfind(|c: char| c.is_ascii_whitespace()).map_or(prev.len(), |i| i + 1);
                    let exact_bound = lexer::Lexer::new(&prev[estimate_bound..]).last().start;
                    prev = &prev[..exact_bound as usize + estimate_bound];
                    if preserve_newlines(prev) > 0 {
                        writeln!(f)?;
                        for _ in 0..INDENT.with(|i| i.get()) + 1 {
                            write!(f, "\t")?;
                        }
                        write!(f, "{op} ")?;
                    } else {
                        write!(f, " {op} ")?;
                    }
                } else {
                    write!(f, " {op} ")?;
                }
                display_branch(f, right)
            }
        }
    }
}

pub fn preserve_newlines(source: &str) -> usize {
    source[source.trim_end().len()..].chars().filter(|&c| c == '\n').count()
}

pub fn insert_needed_semicolon(source: &str) -> bool {
    let kind = lexer::Lexer::new(source).next().kind;
    kind.precedence().is_some() || matches!(kind, TokenKind::Ctor | TokenKind::Tupl)
}

#[repr(C)]
pub struct AstInner<T: ?Sized> {
    ref_count: AtomicUsize,
    mem: ArenaChunk,
    exprs: *const [Expr<'static>],

    pub path: Box<str>,
    pub file: Box<str>,
    pub symbols: T,
}

impl AstInner<[Symbol]> {
    fn layout(syms: usize) -> std::alloc::Layout {
        std::alloc::Layout::new::<AstInner<()>>()
            .extend(std::alloc::Layout::array::<Symbol>(syms).unwrap())
            .unwrap()
            .0
    }

    fn new(content: String, path: &str, loader: Loader) -> NonNull<Self> {
        let arena = Arena::default();
        let mut syms = Vec::new();
        let mut parser = Parser::new(&arena, &mut syms, loader);
        let exprs = parser.file(&content, path) as *const [Expr<'static>];

        syms.sort_unstable_by_key(|s| s.name);

        let layout = Self::layout(syms.len());

        unsafe {
            let ptr = std::alloc::alloc(layout);
            let inner: *mut Self = std::ptr::from_raw_parts_mut(ptr as *mut _, syms.len());

            std::ptr::write(inner as *mut AstInner<()>, AstInner {
                ref_count: AtomicUsize::new(1),
                mem: arena.chunk.into_inner(),
                exprs,
                path: path.into(),
                file: content.into(),
                symbols: (),
            });
            std::ptr::addr_of_mut!((*inner).symbols)
                .as_mut_ptr()
                .copy_from_nonoverlapping(syms.as_ptr(), syms.len());

            NonNull::new_unchecked(inner)
        }
    }

    pub fn report_to(&self, pos: Pos, msg: impl fmt::Display, out: &mut impl fmt::Write) {
        report_to(&self.file, &self.path, pos, msg, out);
    }
}

pub fn display_rel_path(path: &(impl AsRef<OsStr> + ?Sized)) -> std::path::Display {
    static CWD: std::sync::LazyLock<PathBuf> =
        std::sync::LazyLock::new(|| std::env::current_dir().unwrap_or_default());
    std::path::Path::new(path).strip_prefix(&*CWD).unwrap_or(std::path::Path::new(path)).display()
}

pub fn report_to(
    file: &str,
    path: &str,
    pos: Pos,
    msg: impl fmt::Display,
    out: &mut impl fmt::Write,
) {
    let (line, mut col) = lexer::line_col(file.as_bytes(), pos);
    _ = writeln!(out, "{}:{}:{}: {}", display_rel_path(path), line, col, msg);

    let line = &file[file[..pos as usize].rfind('\n').map_or(0, |i| i + 1)
        ..file[pos as usize..].find('\n').unwrap_or(file.len()) + pos as usize];
    col += line.matches('\t').count() * 3;

    _ = writeln!(out, "{}", line.replace("\t", "    "));
    _ = writeln!(out, "{}^", " ".repeat(col - 1));
}

#[derive(PartialEq, Eq, Hash)]
pub struct Ast(NonNull<AstInner<[Symbol]>>);

impl Ast {
    pub fn new(path: &str, content: String, loader: Loader) -> Self {
        Self(AstInner::new(content, path, loader))
    }

    pub fn exprs(&self) -> &[Expr] {
        unsafe { &*self.inner().exprs }
    }

    fn inner(&self) -> &AstInner<[Symbol]> {
        unsafe { self.0.as_ref() }
    }

    pub fn find_decl(&self, id: Result<Ident, &str>) -> Option<(&Expr, Ident)> {
        self.exprs().iter().find_map(|expr| match expr {
            Expr::BinOp { left, op: TokenKind::Decl, .. } => left.declares(id).map(|id| (expr, id)),
            _ => None,
        })
    }

    pub fn ident_str(&self, ident: Ident) -> &str {
        &self.file[ident::range(ident)]
    }
}

impl fmt::Display for Ast {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for expr in self.exprs() {
            writeln!(f, "{expr}\n")?;
        }
        Ok(())
    }
}

impl Default for Ast {
    fn default() -> Self {
        Self(AstInner::new(String::new(), "", &no_loader))
    }
}

#[derive(Clone, Copy)]
#[repr(packed)]
pub struct ExprRef(NonNull<Expr<'static>>);

impl ExprRef {
    pub fn new(expr: &Expr) -> Self {
        Self(NonNull::from(expr).cast())
    }

    pub fn get<'a>(&self, from: &'a Ast) -> Option<&'a Expr<'a>> {
        ArenaChunk::contains(from.mem.base, self.0.as_ptr() as _).then_some(())?;
        // SAFETY: the pointer is or was a valid reference in the past, if it points within one of
        // arenas regions, it muts be walid, since arena does not give invalid pointers to its
        // allocations
        Some(unsafe { { self.0 }.as_ref() })
    }

    pub fn dangling() -> Self {
        Self(NonNull::dangling())
    }
}

impl Default for ExprRef {
    fn default() -> Self {
        Self::dangling()
    }
}

unsafe impl Send for Ast {}
unsafe impl Sync for Ast {}

impl Clone for Ast {
    fn clone(&self) -> Self {
        unsafe { self.0.as_ref() }.ref_count.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
        Self(self.0)
    }
}

impl Drop for Ast {
    fn drop(&mut self) {
        let inner = unsafe { self.0.as_ref() };
        if inner.ref_count.fetch_sub(1, std::sync::atomic::Ordering::Relaxed) == 1 {
            let layout = AstInner::layout(inner.symbols.len());
            unsafe {
                std::alloc::dealloc(self.0.as_ptr() as _, layout);
            }
        }
    }
}

impl Deref for Ast {
    type Target = AstInner<[Symbol]>;

    fn deref(&self) -> &Self::Target {
        self.inner()
    }
}

#[derive(Default)]
pub struct Arena<'a> {
    chunk: UnsafeCell<ArenaChunk>,
    ph: std::marker::PhantomData<&'a ()>,
}

impl<'a> Arena<'a> {
    pub fn alloc_str(&self, token: &str) -> &'a str {
        let ptr = self.alloc_slice(token.as_bytes());
        unsafe { std::str::from_utf8_unchecked(ptr) }
    }

    pub fn alloc(&self, expr: Expr<'a>) -> &'a Expr<'a> {
        let align = std::mem::align_of::<Expr<'a>>();
        let size = expr.used_bytes();
        let layout = unsafe { std::alloc::Layout::from_size_align_unchecked(size, align) };
        let ptr = self.alloc_low(layout);
        unsafe {
            ptr.cast::<u64>().copy_from_nonoverlapping(NonNull::from(&expr).cast(), size / 8)
        };
        unsafe { ptr.cast::<Expr<'a>>().as_ref() }
    }

    pub fn alloc_slice<T: Copy>(&self, slice: &[T]) -> &'a [T] {
        if slice.is_empty() || std::mem::size_of::<T>() == 0 {
            return &mut [];
        }

        let layout = std::alloc::Layout::array::<T>(slice.len()).unwrap();
        let ptr = self.alloc_low(layout);
        unsafe { ptr.as_ptr().cast::<T>().copy_from_nonoverlapping(slice.as_ptr(), slice.len()) };
        unsafe { std::slice::from_raw_parts(ptr.as_ptr() as _, slice.len()) }
    }

    fn alloc_low(&self, layout: std::alloc::Layout) -> NonNull<u8> {
        assert!(layout.align() <= ArenaChunk::ALIGN);
        assert!(layout.size() <= ArenaChunk::CHUNK_SIZE);

        let chunk = unsafe { &mut *self.chunk.get() };

        if let Some(ptr) = chunk.alloc(layout) {
            return ptr;
        }

        unsafe {
            std::ptr::write(chunk, ArenaChunk::new(chunk.base));
        }

        chunk.alloc(layout).unwrap()
    }
}

struct ArenaChunk {
    base: *mut u8,
    end: *mut u8,
}

impl Default for ArenaChunk {
    fn default() -> Self {
        Self { base: std::ptr::null_mut(), end: std::ptr::null_mut() }
    }
}

impl ArenaChunk {
    const ALIGN: usize = std::mem::align_of::<Self>();
    const CHUNK_SIZE: usize = 1 << 16;
    const LAYOUT: std::alloc::Layout =
        unsafe { std::alloc::Layout::from_size_align_unchecked(Self::CHUNK_SIZE, Self::ALIGN) };
    const NEXT_OFFSET: usize = Self::CHUNK_SIZE - std::mem::size_of::<*mut u8>();

    fn new(next: *mut u8) -> Self {
        let base = unsafe { std::alloc::alloc(Self::LAYOUT) };
        let end = unsafe { base.add(Self::NEXT_OFFSET) };
        Self::set_next(base, next);
        Self { base, end }
    }

    fn set_next(curr: *mut u8, next: *mut u8) {
        unsafe { std::ptr::write(curr.add(Self::NEXT_OFFSET) as *mut _, next) };
    }

    fn next(curr: *mut u8) -> *mut u8 {
        unsafe { std::ptr::read(curr.add(Self::NEXT_OFFSET) as *mut _) }
    }

    fn alloc(&mut self, layout: std::alloc::Layout) -> Option<NonNull<u8>> {
        let padding = self.end as usize - (self.end as usize & !(layout.align() - 1));
        let size = layout.size() + padding;
        if size > self.end as usize - self.base as usize {
            return None;
        }
        unsafe { self.end = self.end.sub(size) };
        unsafe { Some(NonNull::new_unchecked(self.end)) }
    }

    fn contains(base: *mut u8, arg: *mut u8) -> bool {
        !base.is_null()
            && ((unsafe { base.add(Self::CHUNK_SIZE) } > arg && base <= arg)
                || Self::contains(Self::next(base), arg))
    }
}

impl Drop for ArenaChunk {
    fn drop(&mut self) {
        //log::inf!(
        //    "dropping chunk of size: {}",
        //    (Self::LAYOUT.size() - (self.end as usize - self.base as usize))
        //        * !self.end.is_null() as usize
        //);
        let mut current = self.base;
        while !current.is_null() {
            let next = Self::next(current);
            unsafe { std::alloc::dealloc(current, Self::LAYOUT) };
            current = next;
            //log::dbg!("deallocating full chunk");
        }
    }
}

#[cfg(test)]
pub mod test {
    pub fn format(ident: &str, input: &str) {
        let ast = super::Ast::new(ident, input.to_owned(), &|_, _| Ok(0));
        let mut output = Vec::new();
        crate::format_to(&ast, input, &mut output).unwrap();

        let input_path = format!("formatter_{ident}.expected");
        let output_path = format!("formatter_{ident}.actual");
        std::fs::write(&input_path, input).unwrap();
        std::fs::write(&output_path, output).unwrap();

        let success = std::process::Command::new("diff")
            .arg("-u")
            .arg("--color")
            .arg(&input_path)
            .arg(&output_path)
            .status()
            .unwrap()
            .success();
        std::fs::remove_file(&input_path).unwrap();
        std::fs::remove_file(&output_path).unwrap();
        assert!(success, "test failed");
    }

    macro_rules! test {
        ($($name:ident => $input:expr;)*) => {$(
            #[test]
            fn $name() {
                format(stringify!($name), $input);
            }
        )*};
    }

    test! {
        comments => "// comment\n// comment\n\n// comment\n\n\
            /* comment */\n/* comment */\n\n/* comment */";
        some_ordinary_code => "loft := fn(): int return loft(1, 2, 3)";
        some_arg_per_line_code => "loft := fn(): int return loft(\
            \n\t1,\n\t2,\n\t3,\n)";
        some_ordinary_struct => "loft := fn(): int return loft.{a: 1, b: 2}";
        some_ordinary_fild_per_lin_struct => "loft := fn(): int return loft.{\
            \n\ta: 1,\n\tb: 2,\n}";
        code_block => "loft := fn(): int {\n\tloft()\n\treturn 1\n}";
    }
}