pest 2.9.0

// pest. The Elegant Parser
// Copyright (c) 2018 Dragoș Tiselice
//
// Licensed under the Apache License, Version 2.0
// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
// license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. All files in the project carrying such notice may not be copied,
// modified, or distributed except according to those terms.

//! Constructs useful in prefix, postfix, and infix operator parsing with the
//! Pratt parsing method.

use core::iter::Peekable;
use core::marker::PhantomData;
use core::mem::ManuallyDrop;
use core::ops::BitOr;

use alloc::boxed::Box;
use alloc::collections::BTreeMap;

use crate::iterators::Pair;
use crate::RuleType;

pub use crate::pratt_precedence;

/// Associativity of an infix binary operator, used by [`Op::infix(Assoc)`].
///
/// [`Op::infix(Assoc)`]: struct.Op.html
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Assoc {
    /// Left operator associativity. Evaluate expressions from left-to-right.
    Left,
    /// Right operator associativity. Evaluate expressions from right-to-left.
    Right,
}

/// Operator precedence level.
pub type Prec = u32;
const PREC_STEP: Prec = 10;

/// An operator that corresponds to a rule.
pub struct Op<R: RuleType> {
    rule: R,
    affix: Affix,
    next: Option<Box<Op<R>>>,
}

/// The position of an operator relative to its operand.
///
/// This is an implementation detail used by the Pratt parser internals.
#[doc(hidden)]
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Affix {
    /// Prefix operator, appearing before its operand.
    Prefix,
    /// Postfix operator, appearing after its operand.
    Postfix,
    /// Infix binary operator with the given associativity.
    Infix(Assoc),
}

impl<R: RuleType> Op<R> {
    /// Defines `rule` as a prefix unary operator.
    pub const fn prefix(rule: R) -> Self {
        Self {
            rule,
            affix: Affix::Prefix,
            next: None,
        }
    }

    /// Defines `rule` as a postfix unary operator.
    pub const fn postfix(rule: R) -> Self {
        Self {
            rule,
            affix: Affix::Postfix,
            next: None,
        }
    }

    /// Defines `rule` as an infix binary operator with associativity `assoc`.
    pub const fn infix(rule: R, assoc: Assoc) -> Self {
        Self {
            rule,
            affix: Affix::Infix(assoc),
            next: None,
        }
    }
}

impl<R: RuleType> BitOr for Op<R> {
    type Output = Self;

    fn bitor(mut self, rhs: Self) -> Self {
        fn assign_next<R: RuleType>(op: &mut Op<R>, next: Op<R>) {
            if let Some(ref mut child) = op.next {
                assign_next(child, next);
            } else {
                op.next = Some(Box::new(next));
            }
        }

        assign_next(&mut self, rhs);
        self
    }
}

/// Struct containing operators and precedences, which can perform [Pratt parsing][1] on
/// primary, prefix, postfix and infix expressions over [`Pairs`]. The tokens in [`Pairs`]
/// should alternate in the order:
/// `prefix* ~ primary ~ postfix* ~ (infix ~ prefix* ~ primary ~ postfix*)*`
///
/// # Panics
///
/// Panics will occur when:
/// * `pairs` is empty
/// * The tokens in `pairs` does not alternate in the expected order.
/// * No `map_*` function is specified for a certain kind of operator encountered in `pairs`.
///
/// # Example
///
/// The following pest grammar defines a calculator which can be used for Pratt parsing.
///
/// ```pest
/// WHITESPACE   =  _{ " " | "\t" | NEWLINE }
///
/// program      =   { SOI ~ expr ~ EOI }
///   expr       =   { prefix* ~ primary ~ postfix* ~ (infix ~ prefix* ~ primary ~ postfix* )* }
///     infix    =  _{ add | sub | mul | div | pow }
///       add    =   { "+" } // Addition
///       sub    =   { "-" } // Subtraction
///       mul    =   { "*" } // Multiplication
///       div    =   { "/" } // Division
///       pow    =   { "^" } // Exponentiation
///     prefix   =  _{ neg }
///       neg    =   { "-" } // Negation
///     postfix  =  _{ fac }
///       fac    =   { "!" } // Factorial
///     primary  =  _{ int | "(" ~ expr ~ ")" }
///       int    =  @{ (ASCII_NONZERO_DIGIT ~ ASCII_DIGIT+ | ASCII_DIGIT) }
/// ```
///
/// Below is a [`PrattParser`] that is able to parse an `expr` in the above grammar. The order
/// of precedence corresponds to the order in which [`op`] is called. Thus, `mul` will
/// have higher precedence than `add`. Operators can also be chained with `|` to give them equal
/// precedence.
///
/// ```
/// # use pest::pratt_parser::{Assoc, Op, PrattParser};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { program, expr, int, add, mul, sub, div, pow, fac, neg }
/// let pratt =
///     PrattParser::new()
///         .op(Op::infix(Rule::add, Assoc::Left) | Op::infix(Rule::sub, Assoc::Left))
///         .op(Op::infix(Rule::mul, Assoc::Left) | Op::infix(Rule::div, Assoc::Left))
///         .op(Op::infix(Rule::pow, Assoc::Right))
///         .op(Op::prefix(Rule::neg))
///         .op(Op::postfix(Rule::fac));
/// ```
///
/// To parse an expression, call the [`map_primary`], [`map_prefix`], [`map_postfix`],
/// [`map_infix`] and [`parse`] methods as follows:
///
/// ```
/// # use pest::{iterators::Pairs, pratt_parser::PrattParser};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { program, expr, int, add, mul, sub, div, pow, fac, neg }
/// fn parse_expr(pairs: Pairs<Rule>, pratt: &PrattParser<Rule>) -> i32 {
///     pratt
///         .map_primary(|primary| match primary.as_rule() {
///             Rule::int  => primary.as_str().parse().unwrap(),
///             Rule::expr => parse_expr(primary.into_inner(), pratt), // from "(" ~ expr ~ ")"
///             _          => unreachable!(),
///         })
///         .map_prefix(|op, rhs| match op.as_rule() {
///             Rule::neg  => -rhs,
///             _          => unreachable!(),
///         })
///         .map_postfix(|lhs, op| match op.as_rule() {
///             Rule::fac  => (1..lhs+1).product(),
///             _          => unreachable!(),
///         })
///         .map_infix(|lhs, op, rhs| match op.as_rule() {
///             Rule::add  => lhs + rhs,
///             Rule::sub  => lhs - rhs,
///             Rule::mul  => lhs * rhs,
///             Rule::div  => lhs / rhs,
///             Rule::pow  => (1..rhs+1).map(|_| lhs).product(),
///             _          => unreachable!(),
///         })
///         .parse(pairs)
/// }
/// ```
///
/// Note that [`map_prefix`], [`map_postfix`] and [`map_infix`] only need to be specified if the
/// grammar contains the corresponding operators.
///
/// [1]: https://en.wikipedia.org/wiki/Pratt_parser
/// [`Pairs`]: ../iterators/struct.Pairs.html
/// [`PrattParser`]: struct.PrattParser.html
/// [`map_primary`]: struct.PrattParser.html#method.map_primary
/// [`map_prefix`]: struct.PrattParserMap.html#method.map_prefix
/// [`map_postfix`]: struct.PrattParserMap.html#method.map_postfix
/// [`map_infix`]: struct.PrattParserMap.html#method.map_infix
/// [`parse`]: struct.PrattParserMap.html#method.parse
/// [`op`]: struct.PrattParserMap.html#method.op
pub struct PrattParser<R: RuleType> {
    prec: Prec,
    ops: BTreeMap<R, (Affix, Prec)>,
    has_prefix: bool,
    has_postfix: bool,
    has_infix: bool,
}

impl<R: RuleType> Default for PrattParser<R> {
    fn default() -> Self {
        Self::new()
    }
}

impl<R: RuleType> PrattParser<R> {
    /// Instantiate a new `PrattParser`.
    pub fn new() -> Self {
        Self {
            prec: PREC_STEP,
            ops: BTreeMap::new(),
            has_prefix: false,
            has_postfix: false,
            has_infix: false,
        }
    }

    /// Add `op` to `PrattParser`.
    pub fn op(mut self, op: Op<R>) -> Self {
        self.prec += PREC_STEP;
        let mut iter = Some(op);
        while let Some(Op { rule, affix, next }) = iter.take() {
            match affix {
                Affix::Prefix => self.has_prefix = true,
                Affix::Postfix => self.has_postfix = true,
                Affix::Infix(_) => self.has_infix = true,
            }
            self.ops.insert(rule, (affix, self.prec));
            iter = next.map(|op| *op);
        }
        self
    }

    /// Maps primary expressions with a closure `primary`.
    pub fn map_primary<'pratt, 'a, 'i, X, T>(
        &'pratt self,
        primary: X,
    ) -> PrattParserMap<'pratt, 'a, 'i, R, X, T>
    where
        X: FnMut(Pair<'i, R>) -> T,
        R: 'pratt,
    {
        PrattParserMap {
            pratt: self,
            primary,
            prefix: None,
            postfix: None,
            infix: None,
            phantom: PhantomData,
        }
    }

    fn get(&self, rule: &R) -> Option<(Affix, Prec)> {
        self.ops.get(rule).copied()
    }
}

/// Internal trait for types that provide operator metadata to [`PrattParserMap`].
///
/// This trait is an implementation detail and not intended to be implemented
/// by downstream code.
///
/// [`PrattParserMap`]: struct.PrattParserMap.html
#[doc(hidden)]
pub trait PrattParserOps<R: RuleType> {
    /// Look up the affix and precedence of `rule`.
    fn get(&self, rule: &R) -> Option<(Affix, Prec)>;
}

impl<R: RuleType> PrattParserOps<R> for PrattParser<R> {
    fn get(&self, rule: &R) -> Option<(Affix, Prec)> {
        PrattParser::get(self, rule)
    }
}

/// A Pratt parser that can be built in a `const` context and stored in static
/// memory.
///
/// It is functionally equivalent to [`PrattParser`], but it is constructed from
/// a static array of [`Op`]s rather than the chained `.op(...)` builder.
///
/// [`PrattParser`]: struct.PrattParser.html
pub struct ConstPrattParser<R: RuleType + 'static, const N: usize> {
    ops: [(R, Affix, Prec); N],
}

impl<R: RuleType + 'static, const N: usize> ConstPrattParser<R, N> {
    /// Create a `ConstPrattParser` from a static array of (`[`Op`]`, `bool`)
    /// pairs.
    ///
    /// Just like [`PrattParser::op`], but for use in a `const` context. The
    /// `bool` in each pair tells whether the operator starts a new precedence
    /// level (`true`) or shares the previous operator's level (`false`).
    /// Levels are ordered from lowest to highest precedence; the first
    /// operator must start a new level (`true`).
    ///
    /// # Example
    ///
    /// ```
    /// # use pest::pratt_parser::{Assoc, ConstPrattParser, Op};
    /// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
    /// # enum Rule { expr, int, add, sub, mul, div, pow, neg, fac }
    /// static PRATT: ConstPrattParser<Rule, 7> = ConstPrattParser::new_const([
    ///     (Op::infix(Rule::add, Assoc::Left), true), // lowest precedence
    ///     (Op::infix(Rule::sub, Assoc::Left), false), // same precedence as add
    ///     (Op::infix(Rule::mul, Assoc::Left), true), // next precedence level
    ///     (Op::infix(Rule::div, Assoc::Left), false), // same precedence as mul
    ///     (Op::infix(Rule::pow, Assoc::Right), true),
    ///     (Op::prefix(Rule::neg), true),
    ///     (Op::postfix(Rule::fac), true), // highest precedence
    /// ]);
    /// ```
    pub const fn new_const(ops: [(Op<R>, bool); N]) -> Self {
        const {
            assert!(N > 0, "ConstPrattParser requires at least one operator");
        }
        assert!(
            ops[0].1,
            "the first operator must start a new precedence level (`true`)"
        );

        // The initial values are dummies; every entry is overwritten below.
        let mut internal_ops: [(R, Affix, Prec); N] = [(ops[0].0.rule, Affix::Prefix, 0); N];
        let mut prec = 0;
        let mut index = 0;
        while index < N {
            let (op, new_level) = &ops[index];
            assert!(
                op.next.is_none(),
                "chained operators (created with `|`) are not supported in ConstPrattParser"
            );
            if *new_level {
                prec += PREC_STEP;
            }
            internal_ops[index] = (op.rule, op.affix, prec);
            index += 1;
        }
        let _ = ManuallyDrop::new(ops);
        Self { ops: internal_ops }
    }

    /// Maps primary expressions with a closure `primary`.
    pub fn map_primary<'pratt, 'a, 'i, X, T>(
        &'pratt self,
        primary: X,
    ) -> PrattParserMap<'pratt, 'a, 'i, R, X, T, Self>
    where
        X: FnMut(Pair<'i, R>) -> T,
        R: 'pratt,
    {
        PrattParserMap {
            pratt: self,
            primary,
            prefix: None,
            postfix: None,
            infix: None,
            phantom: PhantomData,
        }
    }

    #[inline]
    fn get(&self, rule: &R) -> Option<(Affix, Prec)> {
        let mut i = N;
        while i > 0 {
            i -= 1;
            if self.ops[i].0 == *rule {
                return Some((self.ops[i].1, self.ops[i].2));
            }
        }
        None
    }
}

impl<R: RuleType + 'static, const N: usize> PrattParserOps<R> for ConstPrattParser<R, N> {
    fn get(&self, rule: &R) -> Option<(Affix, Prec)> {
        ConstPrattParser::get(self, rule)
    }
}

type PrefixFn<'a, 'i, R, T> = Box<dyn FnMut(Pair<'i, R>, T) -> T + 'a>;
type PostfixFn<'a, 'i, R, T> = Box<dyn FnMut(T, Pair<'i, R>) -> T + 'a>;
type InfixFn<'a, 'i, R, T> = Box<dyn FnMut(T, Pair<'i, R>, T) -> T + 'a>;

/// Product of calling [`map_primary`] on a [`PrattParser`] or [`ConstPrattParser`],
/// defines how expressions should be mapped.
///
/// [`map_primary`]: struct.PrattParser.html#method.map_primary
/// [`PrattParser`]: struct.PrattParser.html
/// [`ConstPrattParser`]: struct.ConstPrattParser.html
pub struct PrattParserMap<'pratt, 'a, 'i, R, F, T, P = PrattParser<R>>
where
    R: RuleType,
    F: FnMut(Pair<'i, R>) -> T,
    P: PrattParserOps<R>,
{
    pratt: &'pratt P,
    primary: F,
    prefix: Option<PrefixFn<'a, 'i, R, T>>,
    postfix: Option<PostfixFn<'a, 'i, R, T>>,
    infix: Option<InfixFn<'a, 'i, R, T>>,
    phantom: PhantomData<T>,
}

impl<'pratt, 'a, 'i, R, F, T, P> PrattParserMap<'pratt, 'a, 'i, R, F, T, P>
where
    R: RuleType + 'pratt,
    F: FnMut(Pair<'i, R>) -> T,
    P: PrattParserOps<R> + 'pratt,
{
    /// Maps prefix operators with closure `prefix`.
    pub fn map_prefix<X>(mut self, prefix: X) -> Self
    where
        X: FnMut(Pair<'i, R>, T) -> T + 'a,
    {
        self.prefix = Some(Box::new(prefix));
        self
    }

    /// Maps postfix operators with closure `postfix`.
    pub fn map_postfix<X>(mut self, postfix: X) -> Self
    where
        X: FnMut(T, Pair<'i, R>) -> T + 'a,
    {
        self.postfix = Some(Box::new(postfix));
        self
    }

    /// Maps infix operators with a closure `infix`.
    pub fn map_infix<X>(mut self, infix: X) -> Self
    where
        X: FnMut(T, Pair<'i, R>, T) -> T + 'a,
    {
        self.infix = Some(Box::new(infix));
        self
    }

    /// The last method to call on the provided pairs to execute the Pratt
    /// parser (previously defined using [`map_primary`], [`map_prefix`], [`map_postfix`],
    /// and [`map_infix`] methods).
    ///
    /// # Panics
    ///
    /// Panics if the input pairs contain a prefix operator with no [`map_prefix`],
    /// an infix operator with no [`map_infix`], a postfix operator with no [`map_postfix`],
    /// or an unexpected token that is not a valid prefix, primary, infix, or postfix expression.
    ///
    /// [`map_primary`]: struct.PrattParser.html#method.map_primary
    /// [`map_prefix`]: struct.PrattParserMap.html#method.map_prefix
    /// [`map_postfix`]: struct.PrattParserMap.html#method.map_postfix
    /// [`map_infix`]: struct.PrattParserMap.html#method.map_infix
    pub fn parse<I: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: I) -> T {
        self.expr(&mut pairs.peekable(), 0)
    }

    fn expr<I: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<I>, rbp: Prec) -> T {
        let mut lhs = self.nud(pairs);
        while rbp < self.lbp(pairs) {
            lhs = self.led(pairs, lhs);
        }
        lhs
    }

    /// Null-Denotation
    ///
    /// "the action that should happen when the symbol is encountered
    ///  as start of an expression (most notably, prefix operators)
    fn nud<I: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<I>) -> T {
        let pair = pairs.next().expect("Pratt parsing expects non-empty Pairs");
        match self.pratt.get(&pair.as_rule()) {
            Some((Affix::Prefix, prec)) => {
                let rhs = self.expr(pairs, prec - 1);
                match self.prefix.as_mut() {
                    Some(prefix) => prefix(pair, rhs),
                    None => panic!("Could not map {}, no `.map_prefix(...)` specified", pair),
                }
            }
            None => (self.primary)(pair),
            _ => panic!("Expected prefix or primary expression, found {}", pair),
        }
    }

    /// Left-Denotation
    ///
    /// "the action that should happen when the symbol is encountered
    /// after the start of an expression (most notably, infix and postfix operators)"
    fn led<I: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<I>, lhs: T) -> T {
        let pair = pairs.next().unwrap();
        match self.pratt.get(&pair.as_rule()) {
            Some((Affix::Infix(assoc), prec)) => {
                let rhs = match assoc {
                    Assoc::Left => self.expr(pairs, prec),
                    Assoc::Right => self.expr(pairs, prec - 1),
                };
                match self.infix.as_mut() {
                    Some(infix) => infix(lhs, pair, rhs),
                    None => panic!("Could not map {}, no `.map_infix(...)` specified", pair),
                }
            }
            Some((Affix::Postfix, _)) => match self.postfix.as_mut() {
                Some(postfix) => postfix(lhs, pair),
                None => panic!("Could not map {}, no `.map_postfix(...)` specified", pair),
            },
            _ => panic!("Expected postfix or infix expression, found {}", pair),
        }
    }

    /// Left-Binding-Power
    ///
    /// "describes the symbol's precedence in infix form (most notably, operator precedence)"
    fn lbp<I: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<I>) -> Prec {
        match pairs.peek() {
            Some(pair) => match self.pratt.get(&pair.as_rule()) {
                Some((_, prec)) => prec,
                None => panic!("Expected operator, found {}", pair),
            },
            None => 0,
        }
    }
}

/// Convenience macro for building a const Pratt parser precedence table.
///
/// Each argument is a precedence level: a list of [`Op`] constructors sharing
/// the same precedence, separated by `|`. Levels are separated by `,`; later
/// levels bind more tightly than earlier ones.
///
/// Each operator must be written as a two-segment call, for example
/// `Op::infix(Rule::add, Assoc::Left)`. Fully qualified paths and turbofish
/// forms are not accepted, because a macro matcher cannot follow an
/// expression fragment with `|`. Import `Op` and use the short form.
///
/// # Example
///
/// ```
/// # use pest::pratt_parser::{Assoc, ConstPrattParser, Op, pratt_precedence};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { expr, int, add, sub, mul, div, pow, neg, fac }
/// static PRATT: ConstPrattParser<Rule, 7> = ConstPrattParser::new_const(pratt_precedence![
///     Op::infix(Rule::add, Assoc::Left) | Op::infix(Rule::sub, Assoc::Left),
///     Op::infix(Rule::mul, Assoc::Left) | Op::infix(Rule::div, Assoc::Left),
///     Op::infix(Rule::pow, Assoc::Right),
///     Op::prefix(Rule::neg),
///     Op::postfix(Rule::fac),
/// ]);
/// ```
///
/// Fully qualified paths are rejected with a compile error:
///
/// ```compile_fail
/// # use pest::pratt_parser::{Assoc, ConstPrattParser, pratt_precedence};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { expr, int, add, sub }
/// static PRATT: ConstPrattParser<Rule, 2> = ConstPrattParser::new_const(pratt_precedence![
///     pest::pratt_parser::Op::infix(Rule::add, Assoc::Left)
///         | pest::pratt_parser::Op::infix(Rule::sub, Assoc::Left),
/// ]);
/// ```
#[macro_export]
macro_rules! pratt_precedence {
    // Operators must use a two-segment path: `Op::infix(args)`.
    (
        $(
            $first_head:ident :: $first_tail:ident $first_args:tt
            $( | $head:ident :: $tail:ident $args:tt )*
        ),* $(,)?
    ) => {
        [$(
            ( $first_head :: $first_tail $first_args, true )
            $(, ( $head :: $tail $args, false ) )*
        ),*]
    };
    ($($t:tt)*) => {
        compile_error!(
            "unsupported operator syntax in `pratt_precedence!`: \
             each operator must be a two-segment call like `Op::infix(Rule::add, Assoc::Left)`; \
             fully qualified paths and turbofish forms are not accepted"
        )
    };
}