support suggested fixes by analyzing a diff

Added `GetSuggestedFix` function creates unified diff for `unmodifiedFile` and `formattedFile`.
Then analyzes the diff and creates `analysis.SuggestedFix` if needed.

The Analyzer checks the result of `GetSuggestedFix` function and reports as `analysis.Diagnostic`.

Signed-off-by: Sergey Vilgelm <sergey@vilgelm.com>

pkg/analyzer/analyzer.go

@@ -1,7 +1,6 @@
 package analyzer
 
 import (
-	"bytes"
 	"fmt"
 	"go/token"
 	"strings"
@@ -13,6 +12,7 @@ import (
 	"github.com/daixiang0/gci/pkg/gci"
 	"github.com/daixiang0/gci/pkg/io"
 	"github.com/daixiang0/gci/pkg/log"
+	"github.com/daixiang0/gci/pkg/utils"
 )
 
 const (
@@ -77,39 +77,23 @@ func runAnalysis(pass *analysis.Pass) (interface{}, error) {
 		if err != nil {
 			return nil, err
 		}
-		// search for a difference
-		fileRunes := bytes.Runes(unmodifiedFile)
-		formattedRunes := bytes.Runes(formattedFile)
-		diffIdx := compareRunes(fileRunes, formattedRunes)
-		switch diffIdx {
-		case -1:
+		fix, err := utils.GetSuggestedFix(file, unmodifiedFile, formattedFile)
+		if err != nil {
+			return nil, err
+		}
+		if fix == nil {
 			// no difference
-		default:
-			pass.Reportf(file.Pos(diffIdx), "fix by `%s %s`", generateCmdLine(*gciCfg), filePath)
+			continue
 		}
+		pass.Report(analysis.Diagnostic{
+			Pos:            fix.TextEdits[0].Pos,
+			Message:        fmt.Sprintf("fix by `%s %s`", generateCmdLine(*gciCfg), filePath),
+			SuggestedFixes: []analysis.SuggestedFix{*fix},
+		})
 	}
 	return nil, nil
 }
 
-func compareRunes(a, b []rune) (differencePos int) {
-	// check shorter rune slice first to prevent invalid array access
-	shorterRune := a
-	if len(b) < len(a) {
-		shorterRune = b
-	}
-	// check for differences up to where the length is identical
-	for idx := 0; idx < len(shorterRune); idx++ {
-		if a[idx] != b[idx] {
-			return idx
-		}
-	}
-	// check that we have compared two equally long rune arrays
-	if len(a) != len(b) {
-		return len(shorterRune) + 1
-	}
-	return -1
-}
-
 func parseGciConfiguration() (*config.Config, error) {
 	fmtCfg := config.BoolConfig{
 		NoInlineComments: noInlineComments,

pkg/utils/diff.go

@@ -0,0 +1,78 @@
+package utils
+
+import (
+	"bytes"
+	"go/token"
+	"regexp"
+	"strconv"
+	"strings"
+
+	"github.com/pmezard/go-difflib/difflib"
+	"golang.org/x/tools/go/analysis"
+)
+
+var hunkRE = regexp.MustCompile(`@@ -(\d+),(\d+) \+\d+,\d+ @@`)
+
+func GetSuggestedFix(file *token.File, a, b []byte) (*analysis.SuggestedFix, error) {
+	d := difflib.UnifiedDiff{
+		A:       difflib.SplitLines(string(a)),
+		B:       difflib.SplitLines(string(b)),
+		Context: 1,
+	}
+	diff, err := difflib.GetUnifiedDiffString(d)
+	if err != nil {
+		return nil, err
+	}
+	if diff == "" {
+		return nil, nil
+	}
+	var (
+		fix   analysis.SuggestedFix
+		first = true
+		edit  analysis.TextEdit
+		buf   bytes.Buffer
+	)
+	for _, line := range strings.Split(diff, "\n") {
+		if line == "" {
+			continue
+		}
+		hunk := hunkRE.FindStringSubmatch(line)
+		switch {
+		case len(hunk) > 0:
+			if !first {
+				edit.NewText = buf.Bytes()
+				buf = bytes.Buffer{}
+				fix.TextEdits = append(fix.TextEdits, edit)
+				edit = analysis.TextEdit{}
+			}
+			first = false
+			start, err := strconv.Atoi(hunk[1])
+			if err != nil {
+				return nil, err
+			}
+			lines, err := strconv.Atoi(hunk[2])
+			if err != nil {
+				return nil, err
+			}
+			edit.Pos = file.LineStart(start)
+			end := start + lines
+			if end > file.LineCount() {
+				edit.End = token.Pos(file.Size())
+			} else {
+				edit.End = file.LineStart(end)
+			}
+		case line[0] == '+':
+			buf.WriteString(line[1:])
+			buf.WriteRune('\n')
+		case line[0] == '-':
+			// just skip
+		default:
+			buf.WriteString(line)
+			buf.WriteRune('\n')
+		}
+	}
+	edit.NewText = buf.Bytes()
+	fix.TextEdits = append(fix.TextEdits, edit)
+
+	return &fix, nil
+}