Add a function to return a policy whose output is the explanation. (#977

)

Implements #974

policy/parser.go

@@ -103,6 +103,22 @@ func (p *Policy) SetMetadata(name string, value any) {
 	p.metadata[name] = value
 }
 
+// GetExplanationOutputPolicy returns a copy of the policy, except the output of each match block
+// is replaced by the expression in the explanation field.
+func (p *Policy) GetExplanationOutputPolicy() *Policy {
+	ep := Policy{
+		name:     p.name,
+		semantic: p.semantic,
+		info:     &*p.info,
+		metadata: p.metadata,
+		source:   &*p.source,
+	}
+	if p.rule != nil {
+		ep.rule = p.rule.getExplanationOutputRule()
+	}
+	return &ep
+}
+
 // NewRule creates a Rule instance.
 func NewRule() *Rule {
 	return &Rule{
@@ -165,6 +181,28 @@ func (r *Rule) AddVariable(v *Variable) {
 	r.variables = append(r.variables, v)
 }
 
+func (r *Rule) getExplanationOutputRule() *Rule {
+	if r == nil {
+		return nil
+	}
+	er := Rule{
+		id:          r.id,
+		description: r.description,
+	}
+	for _, variable := range r.variables {
+		er.variables = append(er.variables, &*variable)
+	}
+	for _, match := range r.matches {
+		em := Match{
+			condition: match.condition,
+			output:    match.explanation,
+			rule:      match.rule.getExplanationOutputRule(),
+		}
+		er.matches = append(er.matches, &em)
+	}
+	return &er
+}
+
 // NewVariable creates a variable instance.
 func NewVariable() *Variable {
 	return &Variable{}

policy/parser_test.go

@@ -166,3 +166,55 @@ rule:
 		}
 	}
 }
+
+func TestGetExplanationOutputPolicy(t *testing.T) {
+	tst := `
+rule:
+  match:
+    - condition: "false"
+      rule:
+        match:
+          - condition: "1 > 2"
+            output: "false"
+            explanation: "'bad_inner'"
+          - output: "true"
+            explanation: "'good_inner'"
+    - output: "true"
+      explanation: "'good_outer'"
+    `
+
+	parser, err := NewParser()
+	if err != nil {
+		t.Fatalf("NewParser() failed: %v", err)
+	}
+	policy, iss := parser.Parse(StringSource(tst, "<input>"))
+	if iss != nil {
+		t.Fatalf("Parse() failed: %v", err)
+	}
+
+	explanationPolicy := policy.GetExplanationOutputPolicy()
+
+	want := "'bad_inner'"
+	got := explanationPolicy.Rule().Matches()[0].rule.Matches()[0].output.Value
+	if got != want {
+		t.Errorf("First inner output = %v, wanted %v", got, want)
+	}
+
+	want = "1 > 2"
+	got = explanationPolicy.Rule().Matches()[0].rule.Matches()[0].condition.Value
+	if got != want {
+		t.Errorf("First inner condition = %v, wanted %v", got, want)
+	}
+
+	want = "'good_inner'"
+	got = explanationPolicy.Rule().Matches()[0].rule.Matches()[1].output.Value
+	if got != want {
+		t.Errorf("Second inner output = %v, wanted %v", got, want)
+	}
+
+	want = "'good_outer'"
+	got = explanationPolicy.Rule().Matches()[1].output.Value
+	if got != want {
+		t.Errorf("Second outer output = %v, wanted %v", got, want)
+	}
+}