pupil 1.0.0

Arithmetic expression evaluator.
Documentation 
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use super::*;

/// Operator precedence.
#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
#[repr(i8)]
pub(crate) enum Order {
	/// Function barrier hack.
	///
	/// Prevents precedence rules from pushing past a function application.
	/// Only an explicit closing `)` can push past it.
	FnBarrier,
	/// Addition and subtraction precedence.
	AddSub,
	/// Multiplication and division precedence.
	MulDiv,
	/// Implicit multiplication precedence.
	///
	/// This makes implicit mul bind tightly under division allowing `1/2ans` to be evaulated as `1/(2*ans)`.
	/// But not high enough to overpower exponentiation so `2ans^3` will be evaluated as `2*(ans^3)`.
	IMul,
	/// Unary operator precedence.
	Unary,
	/// Exponentiation precedence.
	Pow,
}

/// Operator associativity.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
#[repr(u8)]
pub(crate) enum Assoc {
	/// Operator is left associative.
	Left,
	/// Operator is right associative.
	Right,
	// /// Operator has no associativity, unimplemented.
	// None,
}

/// Supported operator types.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
#[repr(u8)]
pub enum Operator {
	/// `+`
	Add,
	/// `-`
	///
	/// Also doubles as unary negation, disambiguated at parser level.
	Sub,
	/// `*`
	Mul,
	/// `/`
	Div,
	/// `%`
	Rem,
	/// Implicit multiplication.
	///
	/// Created on the fly as needed by the parser.
	IMul,
	/// `^`
	Pow,
}

/// Descriptor for an operator’s function, precedence, associativity and if available as unary operator.
pub(crate) struct OpDesc {
	pub pfn: Function,
	pub pre: Order,
	pub assoc: Assoc,
	pub unary: bool,
}

static OP_DESC: [OpDesc; 7] = [
	OpDesc { pfn: native::add, pre: Order::AddSub, assoc: Assoc::Left, unary: true },
	OpDesc { pfn: native::sub, pre: Order::AddSub, assoc: Assoc::Left, unary: true },
	OpDesc { pfn: native::mul, pre: Order::MulDiv, assoc: Assoc::Left, unary: false },
	OpDesc { pfn: native::div, pre: Order::MulDiv, assoc: Assoc::Left, unary: false },
	OpDesc { pfn: native::rem, pre: Order::MulDiv, assoc: Assoc::Left, unary: false },
	OpDesc { pfn: native::mul, pre: Order::IMul, assoc: Assoc::Left, unary: false },
	OpDesc { pfn: native::pow, pre: Order::Pow, assoc: Assoc::Right, unary: false },
];

impl Operator {
	/// Returns the operator’s descriptor.
	#[inline]
	pub(crate) fn desc(self) -> &'static OpDesc {
		&OP_DESC[self as usize]
	}
}