Add fixer for prefer-equals-comparison rule

docs/fixing.md

@@ -22,6 +22,7 @@ Currently, the following rules are automatically fixable:
 - [use-assignment-operator](https://www.openpolicyagent.org/projects/regal/rules/style/use-assignment-operator)
 - [no-whitespace-comment](https://www.openpolicyagent.org/projects/regal/rules/style/no-whitespace-comment)
 - [directory-package-mismatch](https://www.openpolicyagent.org/projects/regal/rules/idiomatic/directory-package-mismatch)
+- [prefer-equals-comparison](https://www.openpolicyagent.org/projects/regal/rules/idiomatic/prefer-equals-comparison)
 - [use-rego-v1](https://www.openpolicyagent.org/projects/regal/rules/imports/use-rego-v1) (v0 Rego only)
 
 So, how do you go on about automatically fixing reported violations?

docs/rules/idiomatic/prefer-equals-comparison.md

@@ -4,6 +4,8 @@
 
 **Category**: Idiomatic
 
+**Automatically fixable**: [Yes](/regal/fixing)
+
 **Avoid**
 ```rego
 package policy

pkg/fixer/fixes/fixes.go

@@ -22,6 +22,7 @@ func NewDefaultFixes() []Fix {
 		&NoWhitespaceComment{},
 		&DirectoryPackageMismatch{},
 		&NonRawRegexPattern{},
+		&PreferEqualsComparison{},
 	}
 }
 
@@ -33,6 +34,7 @@ func NewDefaultFormatterFixes() []Fix {
 		&UseAssignmentOperator{},
 		&NoWhitespaceComment{},
 		&NonRawRegexPattern{},
+		&PreferEqualsComparison{},
 	}
 }
 

pkg/fixer/fixes/preferequalscomparison.go

@@ -0,0 +1,44 @@
+package fixes
+
+import (
+	"errors"
+	"strings"
+)
+
+type PreferEqualsComparison struct{}
+
+func (*PreferEqualsComparison) Name() string {
+	return "prefer-equals-comparison"
+}
+
+func (p *PreferEqualsComparison) Fix(fc *FixCandidate, opts *RuntimeOptions) ([]FixResult, error) {
+	if opts == nil {
+		return nil, errors.New("missing runtime options")
+	}
+
+	lines := strings.Split(fc.Contents, "\n")
+	fixed := false
+
+	for _, loc := range opts.Locations {
+		line := lines[loc.Row-1]
+		if loc.Row > len(lines) || loc.Column-1 < 0 || loc.Column-1 >= len(line) {
+			continue
+		}
+
+		eqIndex := strings.Index(line, "=")
+
+		// unification operator not found, skipping
+		if eqIndex == -1 {
+			continue
+		}
+
+		lines[loc.Row-1] = line[0:eqIndex] + "=" + line[eqIndex:]
+		fixed = true
+	}
+
+	if !fixed {
+		return nil, nil
+	}
+
+	return []FixResult{{Title: p.Name(), Root: opts.BaseDir, Contents: strings.Join(lines, "\n")}}, nil
+}

pkg/fixer/fixes/preferequalscomparison_test.go

@@ -0,0 +1,156 @@
+package fixes
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+
+	"github.com/open-policy-agent/regal/pkg/report"
+)
+
+func TestPreferEqualsComparison(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		contentAfterFix string
+		fc              *FixCandidate
+		fixExpected     bool
+		runtimeOptions  *RuntimeOptions
+	}{
+		"no change": {
+			fc:              &FixCandidate{Filename: "test.rego", Contents: "package test\n\nallow = true\n"},
+			contentAfterFix: "package test\n\nallow = true\n",
+			fixExpected:     false,
+			runtimeOptions:  &RuntimeOptions{},
+		},
+		"no change because no location": {
+			fc: &FixCandidate{
+				Filename: "test.rego",
+				Contents: `package test
+allow := true
+
+test_rule {
+	allow = true
+}`,
+			},
+			contentAfterFix: `package test
+allow := true
+
+test_rule {
+	allow = true
+}`,
+			fixExpected:    false,
+			runtimeOptions: &RuntimeOptions{},
+		},
+		"single change": {
+			fc: &FixCandidate{
+				Filename: "test.rego",
+				Contents: `package test
+allow := true
+
+test_rule {
+	allow = true
+}`,
+			},
+			contentAfterFix: `package test
+allow := true
+
+test_rule {
+	allow == true
+}`,
+			fixExpected:    true,
+			runtimeOptions: &RuntimeOptions{Locations: []report.Location{{Row: 5, Column: 2}}},
+		},
+		"bad change": {
+			fc: &FixCandidate{
+				Filename: "test.rego",
+				Contents: `package test
+allow := true
+allow = true
+
+test_rule {
+	allow = false
+}`,
+			},
+			contentAfterFix: `package test
+allow := true
+allow = true
+
+test_rule {
+	allow = false
+}`,
+			fixExpected:    false,
+			runtimeOptions: &RuntimeOptions{Locations: []report.Location{{Row: 1, Column: 1}}},
+		},
+		"many changes": {
+			fc: &FixCandidate{
+				Filename: "test.rego",
+				Contents: `package test
+allow := true
+
+test_rule {
+	allow = true
+	allow = false
+}`,
+			},
+			contentAfterFix: `package test
+allow := true
+
+test_rule {
+	allow == true
+	allow == false
+}`,
+			fixExpected:    true,
+			runtimeOptions: &RuntimeOptions{Locations: []report.Location{{Row: 5, Column: 2}, {Row: 6, Column: 2}}},
+		},
+		"different columns": {
+			fc: &FixCandidate{
+				Filename: "test.rego",
+				Contents: `package test
+allow := true
+
+test_rule {
+	allow = true
+		allow = false
+}`,
+			},
+			contentAfterFix: `package test
+allow := true
+
+test_rule {
+	allow == true
+		allow == false
+}`,
+			fixExpected:    true,
+			runtimeOptions: &RuntimeOptions{Locations: []report.Location{{Row: 5, Column: 2}, {Row: 6, Column: 2}}},
+		},
+	}
+	for testName, tc := range testCases {
+		t.Run(testName, func(t *testing.T) {
+			t.Parallel()
+
+			pec := PreferEqualsComparison{}
+
+			fixResults, err := pec.Fix(tc.fc, tc.runtimeOptions)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if !tc.fixExpected && len(fixResults) != 0 {
+				t.Fatalf("unexpected fix applied")
+			}
+
+			if !tc.fixExpected {
+				return
+			}
+
+			if diff := cmp.Diff(fixResults[0].Contents, tc.contentAfterFix); tc.fixExpected && diff != "" {
+				t.Fatalf(
+					"unexpected content, got:\n%s---\nexpected:\n%s---",
+					fixResults[0].Contents,
+					tc.contentAfterFix,
+				)
+			}
+		})
+	}
+}