From e59ec59b027f921a6bfbe5014b15c2a802445ada Mon Sep 17 00:00:00 2001
From: Aleksandr Nogikh <nogikh@google.com>
Date: Wed, 29 Nov 2023 16:01:41 +0100
Subject: pkg/ast: support expressions with ast.Type

So far they have the following grammar:

OP = "==", "!=", "&"

value-expr = value-expr OP value-expr
           | factor

factor = "(" and-expr ")"
         | integer
	 | identifier
         | string

Operators are left associative, e.g. A & B & C is the same as
(A & B) & C.

Further restrictions will be imposed in pkg/compiler.

This will help implement conditionally included fields.
---
 pkg/ast/parser.go | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

(limited to 'pkg/ast/parser.go')

diff --git a/pkg/ast/parser.go b/pkg/ast/parser.go
index c87d43f5e..2f2e62055 100644
--- a/pkg/ast/parser.go
+++ b/pkg/ast/parser.go
@@ -433,7 +433,58 @@ func (p *parser) parseField(parseAttrs bool) *Field {
 	return field
 }
 
+type operatorInfo struct {
+	op   Operator
+	prio int
+}
+
+const maxOperatorPrio = 1
+
+// The highest priority is 0.
+var binaryOperators = map[token]operatorInfo{
+	tokCmpEq:  {op: OperatorCompareEq, prio: 0},
+	tokCmpNeq: {op: OperatorCompareNeq, prio: 0},
+	tokBinAnd: {op: OperatorBinaryAnd, prio: 1},
+}
+
+// Parse out a single Type object, which can either be a plain object or an expression.
+// For now, only expressions constructed via '(', ')', "==", "!=", '&' are supported.
 func (p *parser) parseType() *Type {
+	return p.parseBinaryExpr(0)
+}
+
+func (p *parser) parseBinaryExpr(expectPrio int) *Type {
+	if expectPrio > maxOperatorPrio {
+		return p.parseExprFactor()
+	}
+	lastPos := p.pos
+	curr := p.parseBinaryExpr(expectPrio + 1)
+	for {
+		info, ok := binaryOperators[p.tok]
+		if !ok || info.prio != expectPrio {
+			return curr
+		}
+		p.consume(p.tok)
+		curr = &Type{
+			Pos: lastPos,
+			Expression: &BinaryExpression{
+				Pos:      p.pos,
+				Operator: info.op,
+				Left:     curr,
+				Right:    p.parseBinaryExpr(expectPrio + 1),
+			},
+		}
+		lastPos = p.pos
+	}
+}
+
+func (p *parser) parseExprFactor() *Type {
+	if p.tok == tokLParen {
+		p.consume(tokLParen)
+		ret := p.parseBinaryExpr(0)
+		p.consume(tokRParen)
+		return ret
+	}
 	arg := &Type{
 		Pos: p.pos,
 	}
-- 
cgit mrf-deployment