TIL the faintness sensation come from hyperventilation is a result of respiratory alkalosis, which is caused by reduced CO2 (acidic) level in blood. CO2 is equilibrated with HCO3- (basic) in blood as a pH buffering solution. The lung controls the amount of CO2 whereas the kidney controls the amount of HCO3-. So breathing rate is determined by the level of CO2. Although the CO2 exhaled contributes to the majority of weight loss in human, hyperventilation is not a feasible way to lose weight because the benefit is marginal compared to the downside of respiratory alkalosis. A better way to lose weight is through producing more CO2, which is achieved by increasing metabolism rate.

Pratt parsing is a parsing algorithm that solves the awkward handling of left-recursion in a recursive descent parser. It elegantly handles both operator precedence and associativity using a single "binding power" concept. The core algorithm is as simple as follows:

fn expr(input: &str) -> S {
    let mut lexer = Lexer::new(input);
    expr_bp(&mut lexer, 0) 
}

fn expr_bp(lexer: &mut Lexer, min_bp: u8) -> S { 
    let mut lhs = match lexer.next() {
        Token::Atom(it) => S::Atom(it),
        t => panic!("bad token: {:?}", t),
    };

    loop {
        let op = match lexer.peek() {
            Token::Eof => break,
            Token::Op(op) => op,
            t => panic!("bad token: {:?}", t),
        };

        let (l_bp, r_bp) = infix_binding_power(op);
        if l_bp < min_bp { 
            break;
        }

        lexer.next(); 
        let rhs = expr_bp(lexer, r_bp);

        lhs = S::Cons(op, vec![lhs, rhs]); 
    }

    lhs
}

fn infix_binding_power(op: char) -> (u8, u8) {
    match op {
        '+' | '-' => (1, 2),
        '*' | '/' => (3, 4),
        _ => panic!("bad op: {:?}"),
    }
}

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