Leave no blank line at the end of a file after a move

internal/analysis/methodical/fix.go

@@ -219,6 +219,7 @@ func makeSuggestedFix(candidates []fixMethod, unsafe map[types.Object]bool) *ana
 	}
 	var edits []analysis.TextEdit
 	for anchor, group := range groups {
+		trimMovedTail(group)
 		slices.SortFunc(group, func(a, b fixMethod) int {
 			return cmp.Or(cmp.Compare(a.target.order, b.target.order), cmp.Compare(genericRank(a.method.decl), genericRank(b.method.decl)), strings.Compare(a.method.methodName, b.method.methodName))
 		})
@@ -236,6 +237,63 @@ func makeSuggestedFix(candidates []fixMethod, unsafe map[types.Object]bool) *ana
 	return &analysis.SuggestedFix{Message: "Move and order methods after their types", TextEdits: edits}
 }
 
+// trimMovedTail takes the blank line before the declarations that a move
+// removes from the end of a file. They kept the blank line that separated them
+// from the declaration above, which would end the file as a blank line.
+func trimMovedTail(group []fixMethod) {
+	slices.SortFunc(group, func(a, b fixMethod) int {
+		return cmp.Compare(a.start, b.start)
+	})
+	last := len(group) - 1
+	fix := group[last].file
+	if fix.token.Offset(group[last].end) != len(fix.source) {
+		return
+	}
+	first := last
+	for isAdjacentToPrevious(group, first) {
+		first--
+	}
+	limit := 0
+	if first > 0 {
+		limit = fix.token.Offset(group[first-1].end)
+	}
+	start := trimBlankTail(fix.source, fix.token.Offset(group[first].start))
+	if start > limit {
+		group[first].start = fix.token.Pos(start)
+	}
+}
+
+// trimBlankTail moves an offset back over the blank lines before it and leaves
+// the newline that ends the line above.
+func trimBlankTail(src []byte, offset int) int {
+	end := offset
+	for end > 0 && isBlankByte(src[end-1]) {
+		end--
+	}
+	if n := bytes.IndexByte(src[end:offset], '\n'); n >= 0 {
+		return end + n + 1
+	}
+	return offset
+}
+
+func isBlankByte(b byte) bool {
+	switch b {
+	case '\n', '\r', ' ', '\t':
+		return true
+	default:
+		return false
+	}
+}
+
+// isAdjacentToPrevious reports whether a removal starts where the one before it
+// ends, so both reach back to the same blank line.
+func isAdjacentToPrevious(group []fixMethod, i int) bool {
+	if i == 0 {
+		return false
+	}
+	return group[i-1].end == group[i].start
+}
+
 func genericRank(decl *ast.FuncDecl) int {
 	if decl.Type.TypeParams != nil {
 		return 0

internal/driver/iterate_test.go

@@ -3,6 +3,7 @@ package driver_test
 import (
 	"fmt"
 	"go/ast"
+	"go/format"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -59,6 +60,22 @@ func runFix(t *testing.T, analyzer *analysis.Analyzer, diff bool) (int, string,
 	return code, string(output), string(diagnostics)
 }
 
+func TestMethodicalFixTrimsTrailingBlankLine(t *testing.T) {
+	source := "package fix\n\ntype Widget struct{}\n\nfunc (Widget) Z() {}\n\nfunc (Widget) A() {}\n"
+	fixRepo(t, source)
+	code, output, diagnostics := runFix(t, methodical.Analyzer, false)
+	require.Equal(t, 0, code)
+	require.Empty(t, output)
+	require.Empty(t, diagnostics)
+	want := "package fix\n\ntype Widget struct{}\n\nfunc (Widget) A() {}\n\nfunc (Widget) Z() {}\n"
+	data, err := os.ReadFile("fix.go")
+	require.NoError(t, err)
+	require.Equal(t, want, string(data))
+	formatted, err := format.Source(data)
+	require.NoError(t, err)
+	require.Equal(t, string(formatted), string(data), "the fixed file is not formatted")
+}
+
 func TestMethodicalCumulativeFix(t *testing.T) {
 	source := "package fix\n\ntype First struct{}\n\nfunc (First) Z() {}\n\nfunc (First) A() {}\n\ntype Second struct{}\n\nfunc (Second) Z() {}\n\nfunc (Second) A() {}\n"
 	dir := fixRepo(t, source)