Allow for newlines to have a hanging indent if the next non-empty line

has a hanging indent.

This is necessary for a nested keep-sorted test case that uses group=yes
to capture the nested keep-sorted blocks as well as newline_separated=yes
in one of those nested blocks.

Without this, those newlines that the nested block introduces break up
the hanging indent grouping, resulting in pretty bad output.

keepsorted/keep_sorted_test.go

@@ -973,6 +973,29 @@ func TestLineGrouping(t *testing.T) {
 				}},
 			},
 		},
+		{
+			name: "Group_HangingNewline",
+			opts: defaultOptionsWith(func(opts *blockOptions) {
+				opts.Group = true
+			}),
+
+			want: []lineGroup{
+				{nil, []string{
+					"  foo",
+					"",
+					"    bar",
+				}},
+				{nil, []string{
+					"", // Next line doesn't have hanging indent for foo.
+				}},
+				{nil, []string{
+					"  baz",
+				}},
+				{nil, []string{
+					"", // There is no next non-empty line to test against.
+				}},
+			},
+		},
 		{
 			name: "block -- brackets",
 			opts: defaultOptionsWith(func(opts *blockOptions) {

keepsorted/line_group.go

@@ -33,9 +33,14 @@ func groupLines(lines []string, opts blockOptions) []lineGroup {
 	var groups []lineGroup
 	var commentRange indexRange
 	var lineRange indexRange
-	indent := -1
+	var indents []int
+	var indent *int
 	var block codeBlock
 
+	if opts.Group {
+		indents = calculateIndents(lines)
+	}
+
 	// append a line to both lineRange, and block, if necessary.
 	appendLine := func(i int, l string) {
 		lineRange.append(i)
@@ -53,7 +58,7 @@ func groupLines(lines []string, opts blockOptions) []lineGroup {
 	for i, l := range lines {
 		if opts.Block && !lineRange.empty() && block.expectsContinuation() {
 			appendLine(i, l)
-		} else if opts.Group && !lineRange.empty() && indent >= 0 && countIndent(l) > indent {
+		} else if opts.Group && !lineRange.empty() && indent != nil && indents[i] > *indent {
 			appendLine(i, l)
 		} else if opts.hasStickyPrefix(l) {
 			if !lineRange.empty() {
@@ -65,8 +70,8 @@ func groupLines(lines []string, opts blockOptions) []lineGroup {
 			if !lineRange.empty() {
 				finishGroup()
 			}
-			if opts.Group && indent < 0 {
-				indent = countIndent(l)
+			if opts.Group && indent == nil {
+				indent = &indents[i]
 			}
 			appendLine(i, l)
 		}
@@ -77,8 +82,38 @@ func groupLines(lines []string, opts blockOptions) []lineGroup {
 	return groups
 }
 
+// calculateIndents precalculates the indentation for each line.
+// We do this precalculation so that we don't get bad worst-case behavior if
+// someone had a bunch of newlines in a group=yes block.
+func calculateIndents(lines []string) []int {
+	ret := make([]int, len(lines))
+	for i, l := range lines {
+		indent, ok := countIndent(l)
+		if !ok {
+			indent = -1
+		}
+		ret[i] = indent
+	}
+
+	// Allow for newlines to have an indent if the next non-empty line has hanging
+	// indent.
+	// Go backwards through the indent list so that it's harder to accidentally
+	// get O(n^2) behavior for a long section of newlines.
+	indent := -1
+	for i := len(ret) - 1; i >= 0; i-- {
+		if ret[i] == -1 {
+			ret[i] = indent
+			continue
+		}
+
+		indent = ret[i]
+	}
+
+	return ret
+}
+
 // countIndent counts how many space characters occur at the beginning of s.
-func countIndent(s string) int {
+func countIndent(s string) (indent int, hasNonSpaceCharacter bool) {
 	c := 0
 	for _, ch := range s {
 		if unicode.IsSpace(ch) {
@@ -87,11 +122,10 @@ func countIndent(s string) int {
 		}
 		break
 	}
-	// The entire line is whitespace, it's a newline not an indent.
 	if c == len(s) {
-		return -1
+		return 0, false
 	}
-	return c
+	return c, true
 }
 
 // indexRange is a helper struct that let us gradually figure out how big a