Don't trim all space characters in SequenceNode.blockStyleString (#361)

* Don't trim all space characters in SequenceNode.blockStyleString

This fixes #356.
The code would have removed more spaces than it added.

A better solution long-term would probably be to add a specialized
method to StringNode to not add the prefix in the first place.

* Add issue 356 test case for block with first empty line

The indentation level in a block scalar is determined by the first
non-empty line.

ast/ast.go

@@ -1570,8 +1570,9 @@ func (n *SequenceNode) blockStyleString() string {
 		diffLength := len(splittedValues[0]) - len(trimmedFirstValue)
 		if len(splittedValues) > 1 && value.Type() == StringType || value.Type() == LiteralType {
 			// If multi-line string, the space characters for indent have already been added, so delete them.
+			prefix := space + "  "
 			for i := 1; i < len(splittedValues); i++ {
-				splittedValues[i] = strings.TrimLeft(splittedValues[i], " ")
+				splittedValues[i] = strings.TrimPrefix(splittedValues[i], prefix)
 			}
 		}
 		newValues := []string{trimmedFirstValue}

encode_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"github.com/goccy/go-yaml/parser"
 	"math"
 	"reflect"
 	"strconv"
@@ -1399,6 +1400,46 @@ func Example_MarshalYAML() {
 	// field: 13
 }
 
+func TestIssue356(t *testing.T) {
+	tests := map[string]struct {
+		in string
+	}{
+		"content on first line": {
+			in: `args:
+  - |
+
+    key:
+      nest1: something
+      nest2:
+        nest2a: b
+`,
+		},
+		"empty first line": {
+			in: `args:
+  - |
+
+    key:
+      nest1: something
+      nest2:
+        nest2a: b
+`,
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			f, err := parser.ParseBytes([]byte(test.in), 0)
+			if err != nil {
+				t.Fatalf("parse: %v", err)
+			}
+			got := f.String()
+			if test.in != got {
+				t.Fatalf("failed to encode.\nexpected:\n%s\nbut got:\n%s\n", test.in, got)
+			}
+		})
+	}
+}
+
 func TestMarshalIndentWithMultipleText(t *testing.T) {
 	t.Run("depth1", func(t *testing.T) {
 		b, err := yaml.MarshalWithOptions(map[string]interface{}{