use crate::{BString, bstr};

#[cfg(test)]
mod test;

pub trait Stage: PartialEq {
    type Str: std::fmt::Debug + Clone + PartialEq;
}

pub trait CmdDisplay {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()>;
}

#[derive(Debug, Clone, PartialEq)]
pub struct PreExpansion;
#[derive(Debug, Clone, PartialEq)]
pub struct PostExpansion;

impl Stage for PreExpansion {
    type Str = ExpString;
}

impl Stage for PostExpansion {
    type Str = BString;
}

type Res<T, E> = std::result::Result<T, E>;

pub trait Expander {
    type Error;
    fn expand_var(&mut self, v: BString, default: Option<BString>) -> Res<BString, Self::Error>;
    fn expand_cmd(&mut self, c: Ast<PostExpansion>) -> Res<BString, Self::Error>;

    type AliasAge;
    fn expand_alias(
        &mut self,
        cmd: &bstr,
        age: Option<Self::AliasAge>,
    ) -> Res<Option<(Self::AliasAge, Vec<ExpString>)>, Self::Error> {
        let _ = cmd;
        let _ = age;
        Ok(None)
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct Block {
    pub commands: Vec<Ast<PreExpansion>>,
}

#[derive(Debug, Clone, PartialEq)]
pub struct Script {
    pub stmts: Vec<Ast<PreExpansion>>,
}

impl Parse for Script {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        let mut stmts = Vec::new();

        loop {
            b.spaces();
            if b.is_empty() {
                break;
            }

            match Ast::parse(b) {
                Ok(s) => stmts.push(s),
                Err(ParseError::Eof) => break,
                Err(e) => Err(e)?,
            }
        }

        Ok(Script { stmts })
    }
}

#[derive(Debug, Clone, PartialEq)]
pub enum Ast<T: Stage> {
    FunDecl(FunDecl<T>),
    VarAssign(VarAssign<T>),
    Pipes(Pipes<T>),
}

pub fn decl(name: ExpString, body: Block) -> Ast<PreExpansion> {
    Ast::FunDecl(FunDecl { name, body })
}

pub fn assign(var: ExpString, val: ExpString) -> Ast<PreExpansion> {
    Ast::VarAssign(VarAssign { var, val })
}

pub fn pipes<const N: usize>(cmds: [Command<PreExpansion>; N]) -> Ast<PreExpansion> {
    Ast::Pipes(Pipes {
        cmds: cmds.to_vec(),
    })
}

pub fn estr(x: &[u8]) -> ExpString {
    ExpString {
        parts: vec![StringPart::Boring(x.to_vec())],
        delim: StringDelimiter::None,
    }
}

pub fn str<const N: usize>(parts: [StringPart; N]) -> ExpString {
    ExpString {
        parts: parts.to_vec(),
        delim: StringDelimiter::None,
    }
}

pub fn plain(x: &[u8]) -> StringPart {
    StringPart::Boring(x.to_vec())
}

pub fn var(x: &[u8]) -> StringPart {
    StringPart::Var(Var {
        name: VarName { name: x.to_vec() },
        default: None,
        already_complete: true,
    })
}

pub fn var_default(x: &[u8], default: ExpString) -> StringPart {
    StringPart::Var(Var {
        name: VarName { name: x.to_vec() },
        default: Some(default),
        already_complete: true,
    })
}

pub fn cmdp(x: Ast<PreExpansion>) -> StringPart {
    StringPart::Cmd(CmdInterp {
        cmd: x,
        already_complete: true,
    })
}

pub fn cmd<const N: usize>(x: [ExpString; N]) -> Command<PreExpansion> {
    Command {
        cmd: x[0].clone(),
        args: x[1..].to_vec(),
    }
}

pub fn block<const N: usize>(x: [Ast<PreExpansion>; N]) -> Block {
    Block {
        commands: x.to_vec(),
    }
}

impl CmdDisplay for Block {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        write!(w, "block([")?;
        let mut prev = false;
        for cmd in self.commands.iter() {
            if prev {
                write!(w, ",")?;
            }
            prev = true;
            cmd.cdisplay(w)?;
        }
        write!(w, "])")
    }
}

impl CmdDisplay for Ast<PreExpansion> {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        match self {
            Ast::FunDecl(fun_decl) => {
                write!(w, "decl(")?;
                fun_decl.name.cdisplay(w)?;
                write!(w, ", ")?;
                fun_decl.body.cdisplay(w)?;
                write!(w, ")")?;
            }
            Ast::VarAssign(var_assign) => {
                write!(w, "assign(")?;
                var_assign.var.cdisplay(w)?;
                write!(w, ", ")?;
                var_assign.val.cdisplay(w)?;
                write!(w, ")")?;
            }
            Ast::Pipes(pipes) => {
                write!(w, "pipes([")?;
                for cmd in pipes.cmds.iter() {
                    cmd.cdisplay(w)?;
                    write!(w, ",")?;
                }
                write!(w, "])")?;
            }
        }
        Ok(())
    }
}

impl CmdDisplay for ExpString {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        if self.parts.len() == 1 && self.parts[0].is_boring() {
            write!(
                w,
                "estr(b\"{}\")",
                self.parts[0].clone().unwrap_boring().escape_ascii()
            )
        } else {
            write!(w, "str([")?;
            let mut first = true;
            for part in self.parts.iter() {
                if !first {
                    write!(w, ",")?;
                }
                first = false;
                part.cdisplay(w)?;
            }
            write!(w, "])")
        }
    }
}

impl CmdDisplay for StringPart {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        match self {
            StringPart::Boring(items) => {
                write!(w, "plain(")?;
                items.as_slice().cdisplay(w)?;
                write!(w, ")")
            }
            StringPart::Var(var) => {
                if let Some(default) = &var.default {
                    write!(w, "var_default(")?;
                    var.name.name.as_slice().cdisplay(w)?;
                    write!(w, ",")?;
                    default.cdisplay(w)?;
                    write!(w, ")")
                } else {
                    write!(w, "var(")?;
                    var.name.name.as_slice().cdisplay(w)?;
                    write!(w, ")")
                }
            }
            StringPart::Cmd(ast) => {
                write!(w, "cmdp(")?;
                ast.cmd.cdisplay(w)?;
                write!(w, ")")
            }
        }
    }
}

impl CmdDisplay for Command<PreExpansion> {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        write!(w, "cmd([")?;
        self.cmd.cdisplay(w)?;
        for arg in self.args.iter() {
            write!(w, ", ")?;
            arg.cdisplay(w)?;
        }
        write!(w, "])")
    }
}

impl CmdDisplay for &[u8] {
    fn cdisplay(&self, w: &mut dyn std::io::Write) -> std::io::Result<()> {
        write!(w, "b\"")?;
        write!(w, "{}", self.escape_ascii())?;
        write!(w, "\"")
    }
}

impl Ast<PreExpansion> {
    pub fn expand<E: Expander>(self, e: &mut E) -> Res<Ast<PostExpansion>, E::Error> {
        match self {
            Ast::VarAssign(va) => Ok(Ast::VarAssign(va.expand(e)?)),
            Ast::Pipes(pipes) => Ok(Ast::Pipes(pipes.expand(e)?)),
            Ast::FunDecl(fd) => Ok(Ast::FunDecl(fd.expand(e)?)),
        }
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct FunBody {
    pub body: Box<Ast<PreExpansion>>,
}

impl Parse for FunBody {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        b.spaces();

        if b.is_empty() {
            return Err(ParseError::Eof);
        }

        if b.peek() != b'{' {
            return Err(ParseError::Expected('{'));
        }

        b.adv();
        let body = Box::new(Ast::parse(b)?);
        b.spaces();
        if b.is_empty() {
            if b.is_completion() {
                Ok(Self { body })
            } else {
                Err(ParseError::Eof)
            }
        } else if b.peek() == b'}' {
            Ok(Self { body })
        } else {
            Err(ParseError::Expected('}'))
        }
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct FunDecl<S: Stage> {
    pub name: S::Str,
    pub body: Block,
}

impl Parse for Block {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        let mut commands = Vec::new();
        b.spaces();

        if b.has() && b.peek() == b'{' {
            b.adv();
        } else {
            return Err(ParseError::NotABlock);
        }

        loop {
            while {
                b.spaces();
                b.has() && b.peek() == b';'
            } {
                b.adv();
            }

            if b.has() && b"})".contains(&b.peek()) {
                break;
            }

            let cmd = Ast::parse(b)?;
            commands.push(cmd);
        }

        if b.has() {
            if b.peek() != b'}' {
                return Err(ParseError::Expected('}'));
            }
        } else if !b.is_completion() {
            return Err(ParseError::Expected('}'));
        }

        Ok(Self { commands })
    }
}

impl Parse for FunDecl<PreExpansion> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        if !b.buf.starts_with(b"fun ") && !b.buf.starts_with(b"fun\t") {
            return Err(ParseError::NotAFunDecl);
        }
        b.advance(4);
        b.spaces();
        let name = ExpString::parse(b)?;
        let body = Block::parse(b)?;
        Ok(Self { name, body })
    }
}

impl FunDecl<PreExpansion> {
    fn expand<E: Expander>(self, e: &mut E) -> Res<FunDecl<PostExpansion>, E::Error> {
        Ok(FunDecl {
            name: self.name.expand(e)?,
            body: self.body,
        })
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct VarAssign<S: Stage> {
    pub var: S::Str,
    pub val: S::Str,
}

impl Parse for VarAssign<PreExpansion> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        if !b.buf.starts_with(b"set ") && !b.buf.starts_with(b"set\t") {
            return Err(ParseError::NotAVarAssign);
        }
        b.advance(4);
        b.spaces();
        let var = ExpString::parse(b)?;
        b.spaces();

        if b.is_empty() {
            return Err(ParseError::Eof);
        }
        let eq = b.adv();
        if eq != b'=' {
            return Err(ParseError::Expected('='));
        }
        let val = ExpString::parse(b)?;

        Ok(Self { var, val })
    }
}

impl VarAssign<PreExpansion> {
    fn expand<E: Expander>(self, e: &mut E) -> Res<VarAssign<PostExpansion>, E::Error> {
        Ok(VarAssign {
            var: self.var.expand(e)?,
            val: self.val.expand(e)?,
        })
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct Pipes<T: Stage> {
    pub cmds: Vec<Command<T>>,
}

impl Pipes<PreExpansion> {
    fn expand<E: Expander>(self, e: &mut E) -> Res<Pipes<PostExpansion>, E::Error> {
        let mut cmds = Vec::with_capacity(self.cmds.len());
        for cmd in self.cmds.into_iter() {
            cmds.push(cmd.expand(e)?);
        }
        Ok(Pipes { cmds })
    }
}

#[derive(Debug, Clone, PartialEq)]
pub enum StringPart {
    Boring(BString),
    Var(Var),
    Cmd(CmdInterp),
}

#[derive(Debug, Clone, PartialEq)]
pub struct CmdInterp {
    pub cmd: Ast<PreExpansion>,
    pub already_complete: bool,
}

#[derive(Debug, Clone, PartialEq)]
pub struct Var {
    name: VarName,
    default: Option<ExpString>,
    /// if pressing tab right after the parsed variable should not try to complete the variable
    ///
    /// i.e. `${HOM}` -> true, `$HOM` -> false, `${HOM` -> false
    already_complete: bool,
}

impl Var {
    pub fn new(name: VarName) -> Self {
        Self {
            name,
            default: None,
            already_complete: false,
        }
    }
}

impl StringPart {
    pub fn is_boring(&self) -> bool {
        matches!(self, StringPart::Boring(..))
    }
    pub fn unwrap_boring(self) -> BString {
        match self {
            StringPart::Boring(items) => items,
            _ => panic!("unwrap on non-boring value"),
        }
    }
}

#[derive(Debug, Clone, PartialEq)]
/// `"hi ${var} $(cmd) "` gets mapped to `[Boring("hi "), Var("var"), String(" "), Cmd(...), Boring(" ")]`
pub struct ExpString {
    parts: Vec<StringPart>,
    delim: StringDelimiter,
}

impl ExpString {
    fn expand<E: Expander>(self, e: &mut E) -> Res<BString, E::Error> {
        let mut out = BString::new();
        for part in self.parts.into_iter() {
            let mut x = match part {
                StringPart::Boring(items) => items,
                StringPart::Var(v) => {
                    let default = match v.default {
                        Some(default) => Some(default.expand(e)?),
                        None => None,
                    };
                    e.expand_var(v.name.name, default)?
                }
                StringPart::Cmd(ast) => {
                    let exp = ast.cmd.expand(e)?;
                    e.expand_cmd(exp)?
                }
            };
            out.append(&mut x);
        }
        Ok(out)
    }
}

fn is_symbol(x: u8) -> bool {
    matches!(
        x,
        b';' | b'|' | b'{' | b'}' | b'$' | b'(' | b')' | b'\'' | b'"'
    )
}

fn is_var_begin(x: u8) -> bool {
    x.is_ascii_alphanumeric()
}
fn is_var_name(x: u8) -> bool {
    x.is_ascii_alphanumeric() || x == b'_'
}

#[derive(Debug, Clone, PartialEq)]
pub struct VarName {
    name: BString,
}

impl Parse for VarName {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        if b.is_empty() {
            return Err(ParseError::Eof);
        }

        let mut name = BString::new();

        if b.peek().is_ascii_digit() {
            while b.has() && b.peek().is_ascii_digit() {
                name.push(b.adv());
            }
            return Ok(Self { name });
        }

        if !is_var_begin(b.peek()) {
            return Err(ParseError::ExpectedAlphabetic);
        }

        while b.has() {
            let x = b.peek();
            if is_var_name(x) {
                b.adv();
                name.push(x)
            } else {
                break;
            }
        }

        Ok(Self { name })
    }
}

#[derive(Clone, Debug, PartialEq)]
pub enum StringDelimiter {
    /// no delimiter, i.e. when parsing a simple command like `echo foo`
    None,

    /// double quotes, allows interpolation, `echo "foo $var"`
    Interp,

    /// single quotes, does not allow interpolation `echo 'foo $vardoesnotexpand'`
    Strict,

    /// triple quotes with custom prefix/suffix ```
    /// echo DELIM"""
    /// basically
    /// a
    /// heredoc
    /// with
    /// $variables
    /// """DELIM
    /// ```
    InterpCustom(BString),

    /// triple single quotes with custom prefix/suffix ```
    /// echo FOO'''
    /// basically
    /// a
    /// heredoc
    /// without
    /// variables
    /// '''FOO
    /// ```
    StrictCustom(BString),
}

trait PushAll {
    fn push_all(&mut self, other: &bstr);
}

impl PushAll for BString {
    fn push_all(&mut self, other: &bstr) {
        for &c in other {
            self.push(c);
        }
    }
}

/// gets the largest ident this slice starts with, might be empty
fn peek_ident(b: &[u8]) -> &[u8] {
    if b.is_empty() || !b[0].is_ascii_alphabetic() {
        return &[];
    }

    let mut out = &b[..1];

    for i in 1..b.len() {
        if b[i].is_ascii_alphanumeric() {
            out = &b[..=i];
        } else {
            break;
        }
    }

    out
}

impl StringDelimiter {
    fn try_begin(b: &mut Cursor<'_>) -> Option<Self> {
        if !b.has() {
            return None;
        }

        let ident = peek_ident(&b.buf);
        if b.buf[ident.len()..].starts_with(b"\"\"\"") {
            b.advance(ident.len() + 3);
            if b.has() && b.peek() == b'\n' {
                b.adv();
            }
            return Some(Self::InterpCustom(ident.to_vec()));
        }
        if b.buf[ident.len()..].starts_with(b"'''") {
            b.advance(ident.len() + 3);
            if b.has() && b.peek() == b'\n' {
                b.adv();
            }
            return Some(Self::StrictCustom(ident.to_vec()));
        }

        // at this point we know it's not a custom identifier with triple quotes

        let x = b.peek();

        if !x.is_ascii_whitespace() && (!is_symbol(x) || x == b'$') {
            return Some(Self::None);
        }

        if x == b'"' {
            b.adv();
            return Some(Self::Interp);
        }

        if x == b'\'' {
            b.adv();
            return Some(Self::Strict);
        }

        None
    }

    /// if the current string ends right at the cursor, consumes the string closing tokens and returns true
    /// otherwise, consumes no tokens and returns false
    fn try_end(&self, b: &mut Cursor<'_>) -> bool {
        if !b.has() {
            return matches!(self, Self::None);
        }

        let x = b.peek();
        let buf = &mut b.buf;

        match self {
            StringDelimiter::None if x.is_ascii_whitespace() || is_symbol(x) && x != b'$' => true,
            StringDelimiter::Interp if x == b'"' => {
                b.adv();
                true
            }
            StringDelimiter::Strict if x == b'\'' => {
                b.adv();
                true
            }
            StringDelimiter::InterpCustom(delim)
                if buf.len() >= 3 && &buf[..3] == b"\"\"\"" && buf[3..].starts_with(&delim) =>
            {
                b.advance(3 + delim.len());
                true
            }
            StringDelimiter::StrictCustom(delim)
                if buf.len() >= 3 && &buf[..3] == b"'''" && buf[3..].starts_with(&delim) =>
            {
                b.advance(3 + delim.len());
                true
            }
            _ => false,
        }
    }

    fn is_strict(&self) -> bool {
        matches!(self, Self::Strict | Self::StrictCustom(_))
    }

    fn is_none(&self) -> bool {
        matches!(self, Self::None)
    }

    /// assuming that `s` will be placed in the middle of some specifically delimited string
    pub fn escape(&self, mut s: &bstr, out: &mut BString) {
        while !s.is_empty() {
            let first = s[0];
            match self {
                StringDelimiter::None => {
                    if matches!(first, b' ' | b'$' | b'\\' | b'\'' | b'"') {
                        out.push(b'\\');
                    }
                }
                StringDelimiter::Interp | StringDelimiter::InterpCustom(_) => {
                    if matches!(first, b'$' | b'\\' | b'"') {
                        out.push(b'\\');
                    }
                }
                StringDelimiter::Strict => {
                    if first == b'\'' {
                        out.push_all(b"'\\'");
                    }
                }
                StringDelimiter::StrictCustom(delim) => {
                    if s.starts_with(b"'''") && s[3..].starts_with(delim) {
                        out.push_all(b"'''");
                        out.push_all(delim);
                        out.push_all(b"\\'\\'\\'");
                        out.push_all(delim);
                        out.push_all(b"''");
                        out.push_all(delim);
                        out.push_all(b"'''");
                        s = &s[3 + delim.len()..];
                        continue;
                    }
                }
            }
            out.push(first);
            s = &s[1..];
        }
    }

    pub fn write_closing_delimiter(&self, out: &mut BString) {
        match self {
            StringDelimiter::None => (),
            StringDelimiter::Interp => out.push(b'"'),
            StringDelimiter::Strict => out.push(b'\''),
            StringDelimiter::InterpCustom(delim) => {
                out.push_all(b"\"\"\"");
                out.push_all(&delim);
            }
            StringDelimiter::StrictCustom(delim) => {
                out.push_all(b"'''");
                out.push_all(&delim);
            }
        }
    }
}

fn parse_escape_code(b: &mut Cursor<'_>) -> Result<Option<u8>> {
    if !b.has() {
        return Err(ParseError::Eof);
    }

    let x = b.adv();

    let y = match x {
        b'\n' => return Ok(None),
        b'n' => b'\n',
        b'r' => b'\r',
        b't' => b'\t',
        b'e' => 0x1b, // escape
        b'x' => {
            // parse two hex digits
            if b.buf.len() < 2 {
                Err(ParseError::Eof)?;
            }
            let x1 = b.adv();
            let x2 = b.adv();

            if !x1.is_ascii_hexdigit() || !x2.is_ascii_hexdigit() {
                Err(ParseError::NotHexDigit)?;
            }

            let x1 = (x1 as char).to_digit(16).unwrap_or(0);
            let x2 = (x2 as char).to_digit(16).unwrap_or(0);

            ((x1 << 4) | x2) as u8
        }
        _ => x,
    };

    Ok(Some(y))
}

impl Parse for ExpString {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        b.spaces();
        if b.is_empty() {
            return Err(ParseError::NotAString);
        }

        let mut parts = Vec::new();
        let p = &mut parts;
        let add_char = |p: &mut Vec<StringPart>, x: u8| match p.last_mut() {
            Some(StringPart::Boring(v)) => v.push(x),
            _ => p.push(StringPart::Boring(vec![x])),
        };

        let mut already_parsed = false;

        let mut last_delim = StringDelimiter::None;

        'outer: loop {
            let Some(delim) = StringDelimiter::try_begin(b) else {
                break;
            };

            last_delim = delim.clone();

            already_parsed = true;

            while !delim.try_end(b) {
                if !b.has() {
                    if b.is_completion() {
                        break 'outer;
                    } else {
                        return Err(ParseError::Eof);
                    }
                }

                let x = b.adv();

                if x == b'\\' {
                    if let Some(x) = parse_escape_code(b)? {
                        add_char(p, x);
                    }
                } else if x == b'$' && !delim.is_strict() {
                    if !b.has() {
                        add_char(p, b'$');
                        continue;
                    }

                    let x = b.peek();

                    if is_var_begin(x) {
                        let v = VarName::parse(b)?;
                        p.push(StringPart::Var(Var::new(v)));
                        continue;
                    }

                    b.adv();

                    match x {
                        b'?' | b'!' => p.push(StringPart::Var(Var::new(VarName { name: vec![x] }))),
                        b'{' => {
                            let v = VarName::parse(b)?;
                            let mut default = None;

                            if !b.has() {
                                if !b.is_completion() {
                                    return Err(ParseError::Eof);
                                }
                            } else if b.peek() == b':' {
                                b.adv();
                                if !b.has() {
                                    return Err(ParseError::Eof);
                                }
                                if b.peek() == b'-' {
                                    b.adv();
                                    default = Some(ExpString::parse(b)?);
                                } else {
                                    todo!(": in var expansion")
                                }
                            }

                            if !b.has() {
                                if !b.is_completion() {
                                    return Err(ParseError::Eof);
                                }
                            } else if b.peek() != b'}' {
                                return Err(ParseError::Expected('}'));
                            }

                            let already_complete = b.has();

                            if already_complete {
                                b.adv();
                            }

                            p.push(StringPart::Var(Var {
                                name: v,
                                default,
                                already_complete,
                            }));
                        }
                        b'(' => {
                            let cmd = Ast::parse(b)?;
                            b.spaces();

                            if b.is_empty() && !b.is_completion() {
                                return Err(ParseError::Expected(')'));
                            }

                            if b.has() && b.peek() == b')' {
                                b.adv();
                                p.push(StringPart::Cmd(CmdInterp {
                                    cmd,
                                    already_complete: true,
                                }));
                            } else if b.is_completion() {
                                p.push(StringPart::Cmd(CmdInterp {
                                    cmd,
                                    already_complete: false,
                                }))
                            } else {
                                return Err(ParseError::Expected(')'));
                            }
                        }
                        x => {
                            add_char(p, b'$');
                            add_char(p, x);
                        }
                    }
                } else if delim.is_none()
                    && x == b'~'
                    && p.is_empty()
                    && (!b.has() || b.peek().is_ascii_whitespace() || b.peek() == b'/')
                {
                    p.push(StringPart::Var(Var {
                        name: VarName {
                            name: b"HOME".to_vec(),
                        },
                        default: None,
                        already_complete: true,
                    }));
                } else {
                    add_char(p, x);
                }
            }
        }

        if already_parsed {
            Ok(Self {
                parts,
                delim: last_delim,
            })
        } else {
            Err(ParseError::NotAString)
        }
    }
}

impl Parse for Vec<ExpString> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        let mut strings = Vec::new();
        loop {
            match ExpString::parse(b) {
                Ok(s) => strings.push(s),
                Err(ParseError::Eof) => break,
                Err(ParseError::NotAString) => break,
                Err(e) => Err(e)?,
            }
        }
        Ok(strings)
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct Command<T: Stage> {
    pub cmd: T::Str,
    pub args: Vec<T::Str>,
}

impl Command<PreExpansion> {
    fn full_alias_expansion<E: Expander>(&mut self, e: &mut E) -> Res<(), E::Error> {
        self.args.reverse();
        let mut age = None;

        while self.cmd.parts.len() == 1
            && let StringPart::Boring(s) = &self.cmd.parts[0]
        {
            if let Some((new_age, exp)) = e.expand_alias(&s, age.take())? {
                age = Some(new_age);
                self.cmd = exp.first().unwrap().clone();
                for e in exp.into_iter().skip(1).rev() {
                    self.args.push(e);
                }
            } else {
                break;
            }
        }
        self.args.reverse();

        Ok(())
    }

    fn expand<E: Expander>(mut self, e: &mut E) -> Res<Command<PostExpansion>, E::Error> {
        self.full_alias_expansion(e)?;
        let cmd = self.cmd.expand(e)?;
        let mut args = Vec::with_capacity(self.args.len());
        for arg in self.args.into_iter() {
            args.push(arg.expand(e)?);
        }
        Ok(Command { cmd, args })
    }
}

#[allow(unused)]
#[derive(Debug, PartialEq)]
pub enum ParseError {
    /// "clean" EOF, i.e. not in the middle of something
    Eof,

    /// "unclean" EOF, i.e. EOF after beginning a quoted string
    Incomplete,

    ExpectedAlphabetic,

    Unknown(u8),

    Expected(char),

    NotAString,

    NotAFunDecl,

    NotAVarAssign,

    NotHexDigit,

    NotABlock,
}

type Result<T> = std::result::Result<T, ParseError>;

pub fn do_parse(x: &[u8]) -> Res<Ast<PreExpansion>, (ParseError, &[u8])> {
    let mut c = Cursor::new(x, ParseMode::Command);
    match Ast::parse(&mut c) {
        Ok(ast) => Ok(ast),
        Err(e) => Err((e, c.buf)),
    }
}

#[derive(Debug, PartialEq, Clone)]
pub enum CompletionKind {
    Command,
    PathCommand,
    Argument,
    Variable,
    None,
}

pub struct CompletionContext {
    pub kind: CompletionKind,
    pub partial: BString,
    pub delim: StringDelimiter,
}

impl CompletionContext {
    pub fn none() -> Self {
        Self {
            kind: CompletionKind::None,
            partial: BString::new(),
            delim: StringDelimiter::None,
        }
    }
}

impl Block {
    fn completion<E: Expander>(&self, e: &mut E) -> CompletionContext {
        if let Some(cmd) = self.commands.last() {
            cmd.completion(e)
        } else {
            CompletionContext::none()
        }
    }
}

impl Ast<PreExpansion> {
    fn completion<E: Expander>(&self, e: &mut E) -> CompletionContext {
        match self {
            Ast::FunDecl(fd) => fd.body.completion(e),
            Ast::VarAssign(va) => va.val.completion(e, CompletionKind::Argument),
            Ast::Pipes(p) => p.completion(e),
        }
    }
}

impl ExpString {
    fn completion<E: Expander>(&self, e: &mut E, mut kind: CompletionKind) -> CompletionContext {
        if let Some(StringPart::Var(var)) = self.parts.last()
            && !var.already_complete
        {
            CompletionContext {
                kind: CompletionKind::Variable,
                partial: var.name.name.clone(),
                delim: self.delim.clone(),
            }
        } else if let Some(StringPart::Cmd(cmd)) = self.parts.last()
            && !cmd.already_complete
        {
            cmd.cmd.completion(e)
        } else if let Ok(s) = self.clone().expand(e) {
            if s.contains(&b'/') && kind == CompletionKind::Command {
                kind = CompletionKind::PathCommand;
            }
            CompletionContext {
                kind,
                partial: s,
                delim: self.delim.clone(),
            }
        } else {
            CompletionContext::none()
        }
    }
}

impl Pipes<PreExpansion> {
    fn completion<E: Expander>(&self, e: &mut E) -> CompletionContext {
        let Some(cmd) = self.cmds.last() else {
            return CompletionContext::none();
        };

        if let Some(arg) = cmd.args.last() {
            arg.completion(e, CompletionKind::Argument)
        } else {
            cmd.cmd.completion(e, CompletionKind::Command)
        }
    }
}

pub fn completion_context<E: Expander>(x: &[u8], e: &mut E) -> CompletionContext {
    let mut cursor = Cursor::new(x, ParseMode::Completion);
    let ast = Ast::parse(&mut cursor);

    let Ok(ast) = ast else {
        return CompletionContext::none();
    };

    if cursor.spaced {
        return CompletionContext::none();
    }

    ast.completion(e)
}

pub trait Parse: Sized {
    fn parse(b: &mut Cursor<'_>) -> Result<Self>;
    fn parse_from_bytes(x: &[u8]) -> Result<Self> {
        let mut c = Cursor::new(x, ParseMode::Command);
        let parsed = Self::parse(&mut c)?;
        if c.has() {
            return Err(ParseError::Unknown(c.buf[0]));
        }
        Ok(parsed)
    }
}

enum ParseMode {
    Command,
    Completion,
}

pub struct Cursor<'a> {
    buf: &'a [u8],
    mode: ParseMode,

    /// if the last byte that was consumed was whitespace or part of a word
    spaced: bool,

    backtrace: bool,
}

#[derive(Default)]
struct SpaceStats {
    space: u32,
    tab: u32,
    lf: u32,
    cr: u32,
}

impl<'a> Cursor<'a> {
    fn new(buf: &'a [u8], mode: ParseMode) -> Self {
        Self {
            buf,
            mode,
            spaced: false,
            backtrace: false,
        }
    }

    // non empty
    fn has(&self) -> bool {
        !self.buf.is_empty()
    }

    fn is_empty(&self) -> bool {
        self.buf.is_empty()
    }

    fn bt(&self, word: &str) {
        if self.backtrace {
            let bt = std::backtrace::Backtrace::capture();
            let bt = format!("{bt}");
            println!("{word} {}\r", self.buf[0] as char);
            for l in bt.lines().skip(4).take(2) {
                println!("{l}\r");
            }
            println!("\r");
        }
    }

    fn peek(&self) -> u8 {
        self.bt("peek");
        self.buf[0]
    }

    fn adv(&mut self) -> u8 {
        self.bt("adv");
        let out = self.buf[0];
        self.buf = &self.buf[1..];
        self.spaced = false;
        out
    }

    fn advance(&mut self, amt: usize) -> &[u8] {
        self.bt(&format!("adv({amt})"));
        let out = &self.buf[..amt];
        self.buf = &self.buf[amt..];
        self.spaced = false;
        out
    }

    fn peek_space(&self) -> bool {
        if self.buf.is_empty() {
            return false;
        }
        matches!(self.buf[0], b' ' | b'\t' | b'\n' | b'\r')
    }

    fn spaces(&mut self) {
        while self.peek_space() {
            self.adv();
            self.spaced = true;
        }
    }

    fn spaces_stats(&mut self) -> SpaceStats {
        let mut stats = SpaceStats::default();
        while self.has() && b" \t\n\r".contains(&self.buf[0]) {
            match self.buf[0] {
                b' ' => stats.space += 1,
                b'\t' => stats.tab += 1,
                b'\n' => stats.lf += 1,
                b'\r' => stats.cr += 1,
                _ => unreachable!(),
            }
            self.adv();
        }
        stats
    }

    /// returns true if the next thing in the buffer is whitespace (including at least one newline)
    ///
    /// does not modify the buffer
    fn whitespace_newline(&mut self) -> bool {
        let x = self.buf;
        let s = self.spaces_stats();
        self.buf = x;
        s.lf > 0
    }

    fn is_completion(&self) -> bool {
        matches!(self.mode, ParseMode::Completion)
    }

    fn parse<T: Parse>(&mut self) -> Result<T> {
        T::parse(self)
    }
}

impl Parse for Ast<PreExpansion> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        b.spaces();

        let orig_len = b.buf.len();
        let x = VarAssign::parse(b);
        if let Ok(va) = x {
            return Ok(Self::VarAssign(va));
        } else if b.buf.len() != orig_len {
            x?;
        }

        let orig_len = b.buf.len();
        let x = FunDecl::parse(b);
        if let Ok(fd) = x {
            return Ok(Self::FunDecl(fd));
        } else if b.buf.len() != orig_len {
            x?;
        }

        Ok(Self::Pipes(b.parse()?))
    }
}

impl Parse for Command<PreExpansion> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        let path: ExpString = b.parse()?;
        let mut args = Vec::new();
        while !b.whitespace_newline() {
            match ExpString::parse(b) {
                Ok(arg) => args.push(arg),
                Err(ParseError::NotAString) => break,
                Err(e) => Err(e)?,
            }
        }
        Ok(Self { cmd: path, args })
    }
}

impl Parse for Pipes<PreExpansion> {
    fn parse(b: &mut Cursor<'_>) -> Result<Self> {
        let mut cmds: Vec<Command<PreExpansion>> = vec![b.parse()?];

        loop {
            let space_stats = b.spaces_stats();

            if b.is_empty() {
                return Ok(Pipes { cmds });
            }

            let c = b.peek();
            if c == b'|' {
                b.adv();
                cmds.push(b.parse()?);
            } else if space_stats.lf > 0 || is_symbol(c) {
                return Ok(Pipes { cmds });
            } else {
                Err(ParseError::Unknown(c))?;
            }
        }
    }
}