retex_engine/

eqtb.rs

//! The table of equivalents (TeX's `eqtb`) and its storage.
//!
//! TeX keeps every quantity that obeys grouping — meanings of control
//! sequences, registers, parameters, character codes, fonts — in one big
//! array. Each entry carries the group *level* at which it was last defined.
//! retex keeps the quantities in typed vectors instead. [`Loc`] addresses any
//! one of them, so that the save stack ([`crate::save`]) can restore values
//! generically.

use std::collections::HashMap;
use std::rc::Rc;

use crate::cmd::Cmd;
use crate::glue::Glue;
use crate::hash::CsId;
use crate::nodes::NodeRef;
use crate::primitives_gen as pg;
use crate::token::{Macro, TokenList, empty_list};

/// The level of the outermost group (TeX's `level_one`).
pub const LEVEL_ONE: u16 = 1;
/// The level of undefined entries (TeX's `level_zero`).
pub const LEVEL_ZERO: u16 = 0;

/// Number of registers of each kind (LuaTeX: 65536).
pub const NUM_REGISTERS: usize = 65536;
/// Number of math families (LuaTeX: 256).
pub const NUM_FAMILIES: usize = 256;
/// The value of an unset attribute.
pub const UNUSED_ATTRIBUTE: i32 = -0x7FFF_FFFF;

/// The meaning of a control sequence: command code, modifier and, for
/// macros, the definition.
#[derive(Clone, Debug)]
pub struct Meaning {
    /// The command code (`eq_type`).
    pub cmd: Cmd,
    /// The modifier (`equiv` for non-macro commands).
    pub chr: u32,
    /// The definition, for `call` commands.
    pub mac: Option<Rc<Macro>>,
}

impl Meaning {
    /// The meaning of an undefined control sequence.
    pub const UNDEFINED: Meaning = Meaning {
        cmd: Cmd::UndefinedCs,
        chr: 0,
        mac: None,
    };

    /// A primitive or other non-macro meaning.
    pub const fn prim(cmd: Cmd, chr: u32) -> Meaning {
        Meaning {
            cmd,
            chr,
            mac: None,
        }
    }

    /// Identity of meanings as TeX's `eq_define` tests it (same command
    /// and same equivalent; macros compare by reference).
    pub fn same_as(&self, other: &Meaning) -> bool {
        self.cmd == other.cmd
            && self.chr == other.chr
            && match (&self.mac, &other.mac) {
                (None, None) => true,
                (Some(a), Some(b)) => Rc::ptr_eq(a, b),
                _ => false,
            }
    }
}

/// A value together with the level at which it was defined.
#[derive(Clone, Debug)]
pub struct Slot<T> {
    /// The value.
    pub val: T,
    /// The group level of its definition.
    pub level: u16,
}

impl<T> Slot<T> {
    fn new(val: T) -> Self {
        Slot {
            val,
            level: LEVEL_ONE,
        }
    }
}

/// Kinds of per-character code tables, other than category codes.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum CodeKind {
    /// `\lccode`.
    Lc,
    /// `\uccode`.
    Uc,
    /// `\sfcode`.
    Sf,
    /// `\mathcode` / `\Umathcode` (packed, see [`crate::codes`]).
    Math,
    /// `\delcode` / `\Udelcode` (packed, see [`crate::codes`]).
    Del,
}

/// A location in the table of equivalents.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum Loc {
    /// The meaning of a control sequence.
    Cs(CsId),
    /// An integer parameter (`assign_int` index).
    IntPar(u32),
    /// A `\count` register.
    Count(u32),
    /// An `\attribute` register.
    Attr(u32),
    /// A dimension parameter (`assign_dimen` index).
    DimenPar(u32),
    /// A `\dimen` register.
    Dimen(u32),
    /// A glue parameter (`assign_glue` index) or, offset by
    /// [`MU_GLUE_OFFSET`], a muglue parameter.
    GluePar(u32),
    /// A `\skip` register.
    Skip(u32),
    /// A `\muskip` register.
    MuSkip(u32),
    /// A token-list parameter (`assign_toks` index).
    ToksPar(u32),
    /// A `\toks` register.
    Toks(u32),
    /// A `\box` register.
    Box(u32),
    /// The current font.
    CurFont,
    /// A math font: `size * NUM_FAMILIES + family` (size 0 text, 1 script,
    /// 2 scriptscript).
    MathFont(u32),
    /// `\parshape`.
    ParShape,
    /// An e-TeX penalty array (`set_etex_shape` index).
    EtexShape(u32),
    /// A category code in catcode table `table`.
    Cat {
        /// The catcode table number.
        table: u32,
        /// The character.
        ch: u32,
    },
    /// Another character code.
    Code(CodeKind, u32),
    /// A math parameter (`\Umath…`): `param * 8 + style`.
    MathParam(u32),
    /// `\localleftbox` (0) or `\localrightbox` (1).
    LocalBox(u32),
    /// A direction parameter (`assign_dir` index: `\pagedir` …
    /// `\linedir`).
    DirPar(u32),
    /// LuaTeX's group-local counters `\nolocalwhatsits` (0) and
    /// `\nolocaldirs` (1), seen only in tracing (see `dirs`).
    LocalCount(u32),
}

/// Number of integer parameters, including LuaTeX's internal backend
/// integers (`\pdfvariable compresslevel` is index 137 … 164).
pub const INT_PARS: usize = 165;
/// Number of dimension parameters, including backend dimensions (28–33).
pub const DIMEN_PARS: usize = 34;
/// Number of token-list parameters, including backend token lists (11–17).
pub const TOKS_PARS: usize = 18;

/// Offset added to `assign_mu_glue` indices so that glue and muglue
/// parameters share one vector.
pub const MU_GLUE_OFFSET: u32 = pg::assign_glue::COUNT;

/// A saved value, as stored on the save stack.
#[derive(Clone, Debug)]
pub enum Val {
    /// A meaning.
    Meaning(Meaning),
    /// An integer, dimension or packed code.
    Int(i64),
    /// Glue.
    Glue(Glue),
    /// A token list.
    Toks(TokenList),
    /// A box (node list) or null.
    Node(NodeRef),
    /// A shape array (`\parshape` or penalties), or none.
    Shape(Option<Rc<[i32]>>),
}

/// A table of per-character codes with a default function.
///
/// Storage is a two-level page table: 256-character pages, allocated on
/// first assignment and filled with the defaults. Lookups are two array
/// indexings, with no hashing, which matters because the scanner consults
/// these tables for every character.
#[derive(Clone, Debug)]
pub struct CodeTable {
    /// Pages are shared between tables (catcode tables are mostly copies
    /// of one another) and copied on first write.
    pages: Vec<Option<Rc<Page>>>,
    default: fn(u32) -> i64,
}

/// Characters per page.
const PAGE: usize = 256;
/// One page of a code table.
pub type Page = [Slot<i64>; PAGE];

impl CodeTable {
    /// A table where every character has value `default(c)`.
    pub fn new(default: fn(u32) -> i64) -> Self {
        let mut t = CodeTable {
            pages: Vec::new(),
            default,
        };
        // The ASCII/Latin-1 page is always present.
        t.page_mut(0);
        t
    }

    fn page_mut(&mut self, c: u32) -> &mut Page {
        let k = c as usize / PAGE;
        if self.pages.len() <= k {
            self.pages.resize_with(k + 1, || None);
        }
        let default = self.default;
        let page = self.pages[k].get_or_insert_with(|| {
            let base = (k * PAGE) as u32;
            Rc::new(std::array::from_fn(|i| Slot::new(default(base + i as u32))))
        });
        Rc::make_mut(page)
    }

    #[inline]
    fn slot(&self, c: u32) -> Option<&Slot<i64>> {
        match self.pages.get(c as usize / PAGE) {
            Some(Some(p)) => Some(&p[c as usize % PAGE]),
            _ => None,
        }
    }

    /// The value for `c`.
    #[inline]
    pub fn get(&self, c: u32) -> i64 {
        self.slot(c).map_or_else(|| (self.default)(c), |s| s.val)
    }

    /// The level of the definition for `c`.
    pub fn level(&self, c: u32) -> u16 {
        self.slot(c).map_or(LEVEL_ONE, |s| s.level)
    }

    /// Assigns `val` to `c` at level `cur` when that needs no save-stack
    /// entry (the value is unchanged, or was defined at level `cur`, or
    /// `cur` is level one). Returns `false`, changing nothing, otherwise.
    #[inline]
    pub fn define_in_place(&mut self, c: u32, val: i64, cur: u16) -> bool {
        let (old, level) = match self.slot(c) {
            Some(s) => (s.val, s.level),
            None => ((self.default)(c), LEVEL_ONE),
        };
        if old == val {
            return true;
        }
        if level != cur && cur > LEVEL_ONE {
            return false;
        }
        let s = &mut self.page_mut(c)[c as usize % PAGE];
        s.val = val;
        s.level = cur;
        true
    }

    /// Sets value and level for `c`.
    pub fn set(&mut self, c: u32, val: i64, level: u16) {
        self.page_mut(c)[c as usize % PAGE] = Slot { val, level };
    }

    /// The default function of this table.
    pub fn default_fn(&self) -> fn(u32) -> i64 {
        self.default
    }

    /// The values of every allocated page that differs from the defaults,
    /// as (page number, values), for format dumping.
    pub fn dump_pages(&self) -> Vec<(usize, [i64; PAGE])> {
        let mut out = Vec::new();
        for (k, p) in self.pages.iter().enumerate() {
            let Some(p) = p else { continue };
            let base = (k * PAGE) as u32;
            let vals: [i64; PAGE] = std::array::from_fn(|i| p[i].val);
            if vals
                .iter()
                .enumerate()
                .any(|(i, v)| *v != (self.default)(base + i as u32))
            {
                out.push((k, vals));
            }
        }
        out
    }

    /// Installs a page (format loading); the page may be shared.
    pub fn load_page(&mut self, k: usize, page: Rc<Page>) {
        if self.pages.len() <= k {
            self.pages.resize_with(k + 1, || None);
        }
        self.pages[k] = Some(page);
    }

    /// Iterates, in code-point order, over the characters of all allocated
    /// pages: the first 256 characters and every page containing an
    /// assignment.
    pub fn explicit(&self) -> impl Iterator<Item = (u32, i64, u16)> + '_ {
        self.pages.iter().enumerate().flat_map(|(k, p)| {
            p.iter().flat_map(move |p| {
                p.iter()
                    .enumerate()
                    .map(move |(i, s)| ((k * PAGE + i) as u32, s.val, s.level))
            })
        })
    }
}

/// Default category codes of IniTeX: escape for `\`, letters for ASCII
/// letters, `%` comment, end of line for ^^M, space, ignored ^^@, invalid
/// ^^?, everything else other.
pub fn initex_catcode(c: u32) -> i64 {
    match c {
        0x5C => 0,                       // escape
        0x25 => 14,                      // comment
        0x41..=0x5A | 0x61..=0x7A => 11, // letter
        0x0D => 5,                       // car_ret
        0x20 => 10,                      // spacer
        0x00 => 9,                       // ignore
        0x7F => 15,                      // invalid
        _ => 12,                         // other
    }
}

/// The full table of equivalents.
#[derive(Clone, Debug)]
pub struct Eqtb {
    /// Meanings of control sequences, indexed by [`CsId`].
    pub cs: Vec<Slot<Meaning>>,
    /// Integer parameters.
    pub int_par: Vec<Slot<i32>>,
    /// `\count` registers.
    pub count: Vec<Slot<i32>>,
    /// `\attribute` registers.
    pub attr: Vec<Slot<i32>>,
    /// Dimension parameters.
    pub dimen_par: Vec<Slot<i32>>,
    /// `\dimen` registers.
    pub dimen: Vec<Slot<i32>>,
    /// Glue and muglue parameters.
    pub glue_par: Vec<Slot<Glue>>,
    /// `\skip` registers.
    pub skip: Vec<Slot<Glue>>,
    /// `\muskip` registers.
    pub muskip: Vec<Slot<Glue>>,
    /// Token-list parameters.
    pub toks_par: Vec<Slot<TokenList>>,
    /// `\toks` registers.
    pub toks: Vec<Slot<TokenList>>,
    /// `\box` registers.
    pub boxes: Vec<Slot<NodeRef>>,
    /// The current font.
    pub cur_font: Slot<u32>,
    /// Math fonts, `size * NUM_FAMILIES + family`.
    pub math_font: Vec<Slot<u32>>,
    /// `\parshape`: pairs (indent, length) flattened.
    pub par_shape: Slot<Option<Rc<[i32]>>>,
    /// e-TeX penalty arrays (`\interlinepenalties`, …).
    pub etex_shape: Vec<Slot<Option<Rc<[i32]>>>>,
    /// Catcode tables, indexed by table number.
    pub catcode_tables: HashMap<u32, CodeTable, crate::hash::FastHash>,
    /// `\lccode`.
    pub lc: CodeTable,
    /// `\uccode`.
    pub uc: CodeTable,
    /// `\sfcode`.
    pub sf: CodeTable,
    /// `\mathcode`.
    pub math: CodeTable,
    /// `\delcode`.
    pub del: CodeTable,
    /// `\Umath…` parameters: `param * 8 + style`.
    pub math_param: HashMap<u32, Slot<i32>>,
    /// `\localleftbox`, `\localrightbox`.
    pub local_box: [Slot<NodeRef>; 2],
    /// Direction parameters (`\pagedir` …, as numbers 0–3).
    pub dir_par: [Slot<i32>; pg::assign_dir::COUNT as usize],
    /// `\nolocalwhatsits`, `\nolocaldirs`.
    pub local_count: [Slot<i32>; 2],
    /// Incremented whenever an attribute register changes.
    pub attr_generation: u64,
    /// Whether any attribute register was ever set.
    pub attrs_in_use: bool,
}

impl Eqtb {
    /// The initial table of IniTeX: everything undefined or zero, with
    /// TeX's few non-zero defaults.
    pub fn initex() -> Self {
        use pg::assign_int as ip;
        let regs_i = || vec![Slot::new(0); NUM_REGISTERS];
        let mut int_par = vec![Slot::new(0i32); INT_PARS];
        int_par[ip::TOLERANCE as usize].val = 10000;
        int_par[ip::MAG as usize].val = 1000;
        int_par[ip::MAXDEADCYCLES as usize].val = 25;
        int_par[ip::ESCAPECHAR as usize].val = '\\' as i32;
        int_par[crate::backend::MAJOR_VERSION as usize].val = 1;
        int_par[ip::ENDLINECHAR as usize].val = 13;
        let mut catcode_tables: HashMap<u32, CodeTable, crate::hash::FastHash> = HashMap::default();
        catcode_tables.insert(0, CodeTable::new(initex_catcode));
        Eqtb {
            cs: Vec::new(),
            int_par,
            count: regs_i(),
            attr: vec![Slot::new(UNUSED_ATTRIBUTE); NUM_REGISTERS],
            dimen_par: vec![Slot::new(0); DIMEN_PARS],
            dimen: regs_i(),
            glue_par: vec![
                Slot::new(Glue::ZERO);
                (pg::assign_glue::COUNT + pg::assign_mu_glue::COUNT) as usize
            ],
            skip: vec![Slot::new(Glue::ZERO); NUM_REGISTERS],
            muskip: vec![Slot::new(Glue::ZERO); NUM_REGISTERS],
            toks_par: vec![Slot::new(empty_list()); TOKS_PARS],
            toks: vec![Slot::new(empty_list()); NUM_REGISTERS],
            boxes: vec![Slot::new(NodeRef::NULL); NUM_REGISTERS],
            cur_font: Slot::new(0),
            math_font: vec![Slot::new(0); 3 * NUM_FAMILIES],
            par_shape: Slot::new(None),
            etex_shape: vec![Slot::new(None); pg::set_etex_shape::COUNT as usize],
            catcode_tables,
            lc: CodeTable::new(crate::codes::initex_lccode),
            uc: CodeTable::new(crate::codes::initex_uccode),
            sf: CodeTable::new(crate::codes::initex_sfcode),
            math: CodeTable::new(crate::codes::initex_mathcode),
            del: CodeTable::new(crate::codes::initex_delcode),
            math_param: HashMap::new(),
            local_box: [Slot::new(NodeRef::NULL), Slot::new(NodeRef::NULL)],
            dir_par: std::array::from_fn(|_| Slot::new(0)),
            local_count: [Slot::new(0), Slot::new(0)],
            attr_generation: 0,
            attrs_in_use: false,
        }
    }

    /// Makes sure `cs` has an entry.
    #[inline]
    pub fn ensure_cs(&mut self, cs: CsId) {
        let n = cs.0 as usize + 1;
        if self.cs.len() < n {
            self.cs.resize(
                n,
                Slot {
                    val: Meaning::UNDEFINED,
                    level: LEVEL_ZERO,
                },
            );
        }
    }

    /// The meaning of a control sequence.
    #[inline]
    pub fn meaning(&self, cs: CsId) -> &Meaning {
        match self.cs.get(cs.0 as usize) {
            Some(slot) => &slot.val,
            None => &Meaning::UNDEFINED,
        }
    }

    /// The integer parameter with index `i`.
    #[inline]
    pub fn int(&self, i: u32) -> i32 {
        self.int_par[i as usize].val
    }

    /// The dimension parameter with index `i`.
    #[inline]
    pub fn dimen(&self, i: u32) -> i32 {
        self.dimen_par[i as usize].val
    }

    /// The glue parameter with index `i`.
    #[inline]
    pub fn glue(&self, i: u32) -> Glue {
        self.glue_par[i as usize].val
    }

    /// The token-list parameter with index `i`.
    #[inline]
    pub fn toks_param(&self, i: u32) -> &TokenList {
        &self.toks_par[i as usize].val
    }

    /// The current catcode table number.
    #[inline]
    pub fn cur_cat_table(&self) -> u32 {
        self.int(pg::assign_int::CATCODETABLE) as u32
    }

    /// The category code of `c` in the current catcode table.
    #[inline]
    pub fn catcode(&self, c: u32) -> u8 {
        self.catcode_in(self.cur_cat_table(), c)
    }

    /// The category code of `c` in table `table`. Missing tables read as
    /// IniTeX defaults.
    pub fn catcode_in(&self, table: u32, c: u32) -> u8 {
        match self.catcode_tables.get(&table) {
            Some(t) => t.get(c) as u8,
            None => initex_catcode(c) as u8,
        }
    }

    /// Reads the value at `loc`.
    pub fn get(&self, loc: Loc) -> Val {
        match loc {
            Loc::Cs(cs) => Val::Meaning(self.meaning(cs).clone()),
            Loc::IntPar(i) => Val::Int(self.int_par[i as usize].val.into()),
            Loc::Count(i) => Val::Int(self.count[i as usize].val.into()),
            Loc::Attr(i) => Val::Int(self.attr[i as usize].val.into()),
            Loc::DimenPar(i) => Val::Int(self.dimen_par[i as usize].val.into()),
            Loc::Dimen(i) => Val::Int(self.dimen[i as usize].val.into()),
            Loc::GluePar(i) => Val::Glue(self.glue_par[i as usize].val),
            Loc::Skip(i) => Val::Glue(self.skip[i as usize].val),
            Loc::MuSkip(i) => Val::Glue(self.muskip[i as usize].val),
            Loc::ToksPar(i) => Val::Toks(self.toks_par[i as usize].val.clone()),
            Loc::Toks(i) => Val::Toks(self.toks[i as usize].val.clone()),
            Loc::Box(i) => Val::Node(self.boxes[i as usize].val),
            Loc::CurFont => Val::Int(self.cur_font.val.into()),
            Loc::MathFont(i) => Val::Int(self.math_font[i as usize].val.into()),
            Loc::ParShape => Val::Shape(self.par_shape.val.clone()),
            Loc::EtexShape(i) => Val::Shape(self.etex_shape[i as usize].val.clone()),
            Loc::Cat { table, ch } => Val::Int(self.catcode_in(table, ch).into()),
            Loc::Code(kind, ch) => Val::Int(self.code_table(kind).get(ch)),
            Loc::MathParam(i) => Val::Int(
                self.math_param
                    .get(&i)
                    .map_or(crate::codes::UNDEFINED_MATH_PARAM.into(), |s| s.val.into()),
            ),
            Loc::LocalBox(i) => Val::Node(self.local_box[i as usize].val),
            Loc::DirPar(i) => Val::Int(self.dir_par[i as usize].val.into()),
            Loc::LocalCount(i) => Val::Int(self.local_count[i as usize].val.into()),
        }
    }

    /// Whether `loc` holds `val` already (TeX's test for "reassigning"),
    /// without cloning the current value.
    pub fn holds(&self, loc: Loc, val: &Val) -> bool {
        match (loc, val) {
            (Loc::Cs(cs), Val::Meaning(m)) => self.meaning(cs).same_as(m),
            (Loc::IntPar(i), Val::Int(v)) => i64::from(self.int_par[i as usize].val) == *v,
            (Loc::Count(i), Val::Int(v)) => i64::from(self.count[i as usize].val) == *v,
            (Loc::Dimen(i), Val::Int(v)) => i64::from(self.dimen[i as usize].val) == *v,
            (Loc::DimenPar(i), Val::Int(v)) => i64::from(self.dimen_par[i as usize].val) == *v,
            (Loc::Code(kind, ch), Val::Int(v)) => self.code_table(kind).get(ch) == *v,
            (Loc::ToksPar(i), Val::Toks(t)) => Rc::ptr_eq(&self.toks_par[i as usize].val, t),
            (Loc::Toks(i), Val::Toks(t)) => Rc::ptr_eq(&self.toks[i as usize].val, t),
            _ => same_val(&self.get(loc), val),
        }
    }

    /// The level at which the value at `loc` was defined.
    pub fn level(&self, loc: Loc) -> u16 {
        match loc {
            Loc::Cs(cs) => self.cs.get(cs.0 as usize).map_or(LEVEL_ZERO, |s| s.level),
            Loc::IntPar(i) => self.int_par[i as usize].level,
            Loc::Count(i) => self.count[i as usize].level,
            Loc::Attr(i) => self.attr[i as usize].level,
            Loc::DimenPar(i) => self.dimen_par[i as usize].level,
            Loc::Dimen(i) => self.dimen[i as usize].level,
            Loc::GluePar(i) => self.glue_par[i as usize].level,
            Loc::Skip(i) => self.skip[i as usize].level,
            Loc::MuSkip(i) => self.muskip[i as usize].level,
            Loc::ToksPar(i) => self.toks_par[i as usize].level,
            Loc::Toks(i) => self.toks[i as usize].level,
            Loc::Box(i) => self.boxes[i as usize].level,
            Loc::CurFont => self.cur_font.level,
            Loc::MathFont(i) => self.math_font[i as usize].level,
            Loc::ParShape => self.par_shape.level,
            Loc::EtexShape(i) => self.etex_shape[i as usize].level,
            Loc::Cat { table, ch } => self
                .catcode_tables
                .get(&table)
                .map_or(LEVEL_ONE, |t| t.level(ch)),
            Loc::Code(kind, ch) => self.code_table(kind).level(ch),
            Loc::MathParam(i) => self.math_param.get(&i).map_or(LEVEL_ONE, |s| s.level),
            Loc::LocalBox(i) => self.local_box[i as usize].level,
            Loc::DirPar(i) => self.dir_par[i as usize].level,
            Loc::LocalCount(i) => self.local_count[i as usize].level,
        }
    }

    /// Stores `val` at `loc` with level `level`. Mismatched value kinds are
    /// ignored (they indicate a bug and are caught in debug builds).
    pub fn set(&mut self, loc: Loc, val: Val, level: u16) {
        fn int(v: &Val) -> i64 {
            match v {
                Val::Int(i) => *i,
                _ => {
                    debug_assert!(false, "expected an integer value");
                    0
                }
            }
        }
        match loc {
            Loc::Cs(cs) => {
                self.ensure_cs(cs);
                if let Val::Meaning(m) = val {
                    self.cs[cs.0 as usize] = Slot { val: m, level };
                }
            }
            Loc::IntPar(i) => {
                self.int_par[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
            Loc::Count(i) => {
                self.count[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
            Loc::Attr(i) => {
                self.attr[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                };
                self.attr_generation += 1;
                self.attrs_in_use = true;
            }
            Loc::DimenPar(i) => {
                self.dimen_par[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
            Loc::Dimen(i) => {
                self.dimen[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
            Loc::GluePar(i) | Loc::Skip(i) | Loc::MuSkip(i) => {
                if let Val::Glue(g) = val {
                    let slot = Slot { val: g, level };
                    match loc {
                        Loc::GluePar(_) => self.glue_par[i as usize] = slot,
                        Loc::Skip(_) => self.skip[i as usize] = slot,
                        _ => self.muskip[i as usize] = slot,
                    }
                }
            }
            Loc::ToksPar(i) | Loc::Toks(i) => {
                if let Val::Toks(t) = val {
                    let slot = Slot { val: t, level };
                    if matches!(loc, Loc::ToksPar(_)) {
                        self.toks_par[i as usize] = slot;
                    } else {
                        self.toks[i as usize] = slot;
                    }
                }
            }
            Loc::Box(i) => {
                if let Val::Node(n) = val {
                    self.boxes[i as usize] = Slot { val: n, level };
                }
            }
            Loc::CurFont => {
                self.cur_font = Slot {
                    val: int(&val) as u32,
                    level,
                }
            }
            Loc::MathFont(i) => {
                self.math_font[i as usize] = Slot {
                    val: int(&val) as u32,
                    level,
                }
            }
            Loc::ParShape => {
                if let Val::Shape(s) = val {
                    self.par_shape = Slot { val: s, level };
                }
            }
            Loc::EtexShape(i) => {
                if let Val::Shape(s) = val {
                    self.etex_shape[i as usize] = Slot { val: s, level };
                }
            }
            Loc::Cat { table, ch } => {
                self.catcode_tables
                    .entry(table)
                    .or_insert_with(|| CodeTable::new(initex_catcode))
                    .set(ch, int(&val), level);
            }
            Loc::Code(kind, ch) => self.code_table_mut(kind).set(ch, int(&val), level),
            Loc::MathParam(i) => {
                self.math_param.insert(
                    i,
                    Slot {
                        val: int(&val) as i32,
                        level,
                    },
                );
            }
            Loc::LocalBox(i) => {
                if let Val::Node(n) = val {
                    self.local_box[i as usize] = Slot { val: n, level };
                }
            }
            Loc::DirPar(i) => {
                self.dir_par[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
            Loc::LocalCount(i) => {
                self.local_count[i as usize] = Slot {
                    val: int(&val) as i32,
                    level,
                }
            }
        }
    }

    /// The code table of the given kind.
    pub fn code_table(&self, kind: CodeKind) -> &CodeTable {
        match kind {
            CodeKind::Lc => &self.lc,
            CodeKind::Uc => &self.uc,
            CodeKind::Sf => &self.sf,
            CodeKind::Math => &self.math,
            CodeKind::Del => &self.del,
        }
    }

    pub(crate) fn code_table_mut(&mut self, kind: CodeKind) -> &mut CodeTable {
        match kind {
            CodeKind::Lc => &mut self.lc,
            CodeKind::Uc => &mut self.uc,
            CodeKind::Sf => &mut self.sf,
            CodeKind::Math => &mut self.math,
            CodeKind::Del => &mut self.del,
        }
    }

    /// Whether the location holds a "word" quantity (TeX regions 5–6:
    /// integers and dimensions, which use `xeq_level` semantics) as opposed
    /// to an "equivalent" (regions 1–4).
    pub fn is_word(loc: Loc) -> bool {
        matches!(
            loc,
            Loc::IntPar(_)
                | Loc::Count(_)
                | Loc::Attr(_)
                | Loc::DimenPar(_)
                | Loc::Dimen(_)
                | Loc::MathParam(_)
        )
    }
}

/// Compares two saved values for TeX's "reassigning" test.
pub fn same_val(a: &Val, b: &Val) -> bool {
    match (a, b) {
        (Val::Meaning(x), Val::Meaning(y)) => x.same_as(y),
        (Val::Int(x), Val::Int(y)) => x == y,
        (Val::Glue(x), Val::Glue(y)) => x == y,
        (Val::Toks(x), Val::Toks(y)) => Rc::ptr_eq(x, y),
        (Val::Node(x), Val::Node(y)) => x == y,
        (Val::Shape(x), Val::Shape(y)) => match (x, y) {
            (None, None) => true,
            (Some(p), Some(q)) => Rc::ptr_eq(p, q),
            _ => false,
        },
        _ => false,
    }
}