Fix arithmetic and boolean binary expression order

Previously the expression operations happened right-to-left so `1 - 2 - 3 - 4 - 5` was actually calculated as `(1 - (2 - (3 - (4 - (5)))))`.  Now the expressions are calculated according to BODMAS, with boolean expressions calculated last.

lib/src/sql/expression.rs

@@ -27,6 +27,35 @@ impl Default for Expression {
 	}
 }
 
+impl Expression {
+	// Create a new expression
+	pub fn new(l: Value, o: Operator, r: Value) -> Expression {
+		Expression {
+			l,
+			o,
+			r,
+		}
+	}
+	// Augment an existing expression
+	pub fn augment(mut self, l: Value, o: Operator) -> Expression {
+		if o.precedence() >= self.o.precedence() {
+			match self.l {
+				Value::Expression(x) => {
+					self.l = x.augment(l, o).into();
+					self
+				}
+				_ => {
+					self.l = Expression::new(l, o, self.l).into();
+					self
+				}
+			}
+		} else {
+			let r = Value::from(self);
+			Expression::new(l, o, r)
+		}
+	}
+}
+
 impl Expression {
 	pub async fn compute(
 		&self,
@@ -96,14 +125,11 @@ pub fn expression(i: &str) -> IResult<&str, Expression> {
 	let (i, l) = single(i)?;
 	let (i, o) = operator(i)?;
 	let (i, r) = value(i)?;
-	Ok((
+	let v = match r {
-		i,
+		Value::Expression(r) => r.augment(l, o),
-		Expression {
+		_ => Expression::new(l, o, r),
-			l,
+	};
-			o,
+	Ok((i, v))
-			r,
-		},
-	))
 }
 
 #[cfg(test)]

lib/src/sql/operator.rs

@@ -56,6 +56,21 @@ impl Default for Operator {
 	}
 }
 
+impl Operator {
+	#[inline]
+	pub fn precedence(&self) -> u8 {
+		match self {
+			Operator::Or => 1,
+			Operator::And => 2,
+			Operator::Sub => 4,
+			Operator::Add => 5,
+			Operator::Mul => 6,
+			Operator::Div => 7,
+			_ => 3,
+		}
+	}
+}
+
 impl fmt::Display for Operator {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		match self {

lib/src/sql/value/value.rs

@@ -201,6 +201,12 @@ impl From<Expression> for Value {
 	}
 }
 
+impl From<Box<Expression>> for Value {
+	fn from(v: Box<Expression>) -> Self {
+		Value::Expression(v)
+	}
+}
+
 impl From<i8> for Value {
 	fn from(v: i8) -> Self {
 		Value::Number(Number::from(v))