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use std::vec;
use crate::ast::AST;
use crate::lexer::Lexer;
use crate::token::Token;
// Top level parser.
// parses linear array of tokens into AST.
pub struct Parser<'a> {
lexer: &'a mut Lexer,
}
impl<'a> Parser<'a> {
// constructor for parser.
pub fn new(lexer: &'a mut Lexer) -> Self {
log::debug!("[expr-solver] Creating new parser instance.");
Self { lexer }
}
// public parse method.
pub fn parse(&mut self) -> Result<AST, String> {
log::debug!("[expr-solver] starting parsing.");
// we start with binding power of 0.
match self.expr(0) {
Ok(ast) => {
log::debug!("Parsed to ast:\n{}", ast);
Ok(ast)
}
Err(e) => {
log::debug!("Failed to parse AST.");
Err(e)
}
}
}
/// Parses an expression using Operator-Precedence parse (Pratt Parsing)
/// ref : https://en.wikipedia.org/wiki/Operator-precedence_parser
/// # Arguments
/// * min_binding_power - minimum binding power till recursivel parse the expression.
/// # Returns
/// * AST - ast of the expression.
fn expr(&mut self, min_binding_power: u8) -> Result<AST, String> {
// Parsing left hand side of the expression.
let mut left_hand_side = match self.lexer.next_token() {
// if the token is a number we simply create a node out of it.
Token::Number(f) => AST::Node(Token::Number(f)),
// if we reached the end we panic.
Token::Eof => return Err("Unexpected token : EOF".to_string()),
// if grouping, the AST can be treated as primary expression.
Token::LeftParen => {
let lhs = match self.expr(0) {
Ok(lhs) => lhs,
Err(e) => return Err(e),
};
if !matches!(self.lexer.next_token(), Token::RightParen) {
return Err("Expected ')' after expression.".to_string());
}
lhs
}
// if its a operator, then it means the operator is a unary.
operator => {
// we get the right binding power of the unary operator.
let ((), right_binding_power) = Parser::prefix_binding_power(operator);
// then recursively parse it.
match self.expr(right_binding_power) {
// save if its safe.
Ok(right_hand_side) => AST::Con(operator, vec![right_hand_side]),
// return if err.
Err(err) => return Err(err),
}
}
};
loop {
// Operator: Infix operator.
let operator = match self.lexer.peek() {
// shouldn't be a number, obviously.
Token::Number(n) => return Err(format!("Expected operator recieved number : {n}")),
// also shouldn't end.
Token::Eof => break,
// take the operator.
op => op,
};
// get the left binding power of the postfix operator.
if let Some((left_bp, ())) = Parser::postfix_binding_power(operator) {
// we break the loop when precendence of the current left binding
// power of the postfix operator is less than minimum binding power.
if left_bp < min_binding_power {
break;
}
self.lexer.next_token();
left_hand_side = AST::Con(operator, vec![left_hand_side]);
// we need to skip the current iteration.
continue;
}
// get the left binding power and right binding power of this infix operator.
if let Some((left_bp, right_bp)) = Parser::infix_binding_power(operator) {
// ends recursion when the minimum binding power for this
// expr function call is less then left binding power of the current operator.
if left_bp < min_binding_power {
break;
}
// consume operator token.
self.lexer.next_token();
// recurisvely call expr to parse right hand side of the expression.
match self.expr(right_bp) {
Ok(right_hand_side) => {
// create ast.
left_hand_side = AST::Con(operator, vec![left_hand_side, right_hand_side]);
}
Err(err) => return Err(err),
}
continue;
}
// break the loop if none of the above cases are met.
break;
}
// return the created AST.
Ok(left_hand_side)
}
/// Gets the infix binding power of a operator.
/// # Arguments
/// * token - the operator token.
/// # Returns
/// * (left, right) - left and right infix binding power of the operator.
fn infix_binding_power(token: Token) -> Option<(u8, u8)> {
let power = match token {
Token::Plus => (1, 2),
Token::Minus => (1, 2),
Token::Star => (3, 4),
Token::Slash => (3, 4),
// basically unreachable.
_ => return None,
};
Some(power)
}
/// Gets the postfix binding power of a operator.
/// # Arguments
/// * token - the operator token.
/// # Returns
/// * (left, ())) - left postfix binding power of the operator.
fn postfix_binding_power(token: Token) -> Option<(u8, ())> {
let power = match token {
Token::Bang => (6, ()),
// basically unreachable.
_ => return None,
};
Some(power)
}
/// Gets the prefix binding power of a unary operators.
/// # Arguments
/// * token - the operator token.
/// # Returns
/// * ((), right) - right prefix binding power of the operator.
fn prefix_binding_power(token: Token) -> ((), u8) {
match token {
Token::Minus | Token::Plus => ((), 5),
// basically unreachable.
t => panic!("Cannot get infix binding power of {t}"),
}
}
}