Fix nested CallExpr cost estimation (google#571)

* Fix nested CallExpr cost estimation.
* Add TODO about merging isScalar/ComputedSize.

(cherry picked from commit 1aae83a)

checker/cost.go

@@ -476,6 +476,15 @@ func (c *coster) sizeEstimate(t AstNode) SizeEstimate {
 	if l := c.estimator.EstimateSize(t); l != nil {
 		return *l
 	}
+	// return an estimate of 1 for return types of set
+	// lengths, since strings/bytes/more complex objects could be of
+	// variable length
+	if isScalar(t.Type()) {
+		// TODO: since the logic for size estimation is split between
+		// ComputedSize and isScalar, changing one will likely require changing
+		// the other, so they should be merged in the future if possible
+		return SizeEstimate{Min: 1, Max: 1}
+	}
 	return SizeEstimate{Min: 0, Max: math.MaxUint64}
 }
 
@@ -599,3 +608,20 @@ func (c *coster) newAstNode(e *exprpb.Expr) *astNode {
 	}
 	return &astNode{path: path, t: c.getType(e), expr: e, derivedSize: derivedSize}
 }
+
+// isScalar returns true if the given type is known to be of a constant size at
+// compile time. isScalar will return false for strings (they are variable-width)
+// in addition to protobuf.Any and protobuf.Value (their size is not knowable at compile time).
+func isScalar(t *exprpb.Type) bool {
+	switch kindOf(t) {
+	case kindPrimitive:
+		if t.GetPrimitive() != exprpb.Type_STRING && t.GetPrimitive() != exprpb.Type_BYTES {
+			return true
+		}
+	case kindWellKnown:
+		if t.GetWellKnown() == exprpb.Type_DURATION || t.GetWellKnown() == exprpb.Type_TIMESTAMP {
+			return true
+		}
+	}
+	return false
+}

checker/cost_test.go

@@ -354,6 +354,67 @@ func TestCost(t *testing.T) {
 			hints:  map[string]int64{"str1": 10, "str2": 10},
 			wanted: CostEstimate{Min: 2, Max: 6},
 		},
+		{
+			name: "list size comparison",
+			expr: `list1.size() == list2.size()`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("list1", decls.NewListType(decls.Int)),
+				decls.NewVar("list2", decls.NewListType(decls.Int)),
+			},
+			wanted: CostEstimate{Min: 5, Max: 5},
+		},
+		{
+			name: "list size from ternary",
+			expr: `x > y ? list1.size() : list2.size()`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("x", decls.Int),
+				decls.NewVar("y", decls.Int),
+				decls.NewVar("list1", decls.NewListType(decls.Int)),
+				decls.NewVar("list2", decls.NewListType(decls.Int)),
+			},
+			wanted: CostEstimate{Min: 5, Max: 5},
+		},
+		{
+			name: "str endsWith equality",
+			expr: `str1.endsWith("abcdefghijklmnopqrstuvwxyz") == str2.endsWith("abcdefghijklmnopqrstuvwxyz")`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("str1", decls.String),
+				decls.NewVar("str2", decls.String),
+			},
+			wanted: CostEstimate{Min: 9, Max: 9},
+		},
+		{
+			name:   "nested subexpression operators",
+			expr:   `((5 != 6) == (1 == 2)) == ((3 <= 4) == (9 != 9))`,
+			wanted: CostEstimate{Min: 7, Max: 7},
+		},
+		{
+			name: "str size estimate",
+			expr: `string(timestamp1) == string(timestamp2)`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("timestamp1", decls.Timestamp),
+				decls.NewVar("timestamp2", decls.Timestamp),
+			},
+			wanted: CostEstimate{Min: 5, Max: 1844674407370955268},
+		},
+		{
+			name: "timestamp equality check",
+			expr: `timestamp1 == timestamp2`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("timestamp1", decls.Timestamp),
+				decls.NewVar("timestamp2", decls.Timestamp),
+			},
+			wanted: CostEstimate{Min: 3, Max: 3},
+		},
+		{
+			name: "duration inequality check",
+			expr: `duration1 != duration2`,
+			decls: []*exprpb.Decl{
+				decls.NewVar("duration1", decls.Duration),
+				decls.NewVar("duration2", decls.Duration),
+			},
+			wanted: CostEstimate{Min: 3, Max: 3},
+		},
 	}
 
 	for _, tc := range cases {