tpl/transform: Add transform.Unmarshal func

Fixes #5428

tpl/transform/remarshal.go

@@ -2,9 +2,10 @@ package transform
 
 import (
 	"bytes"
-	"errors"
 	"strings"
 
+	"github.com/pkg/errors"
+
 	"github.com/gohugoio/hugo/parser"
 	"github.com/gohugoio/hugo/parser/metadecoders"
 	"github.com/spf13/cast"
@@ -34,9 +35,9 @@ func (ns *Namespace) Remarshal(format string, data interface{}) (string, error)
 		return "", err
 	}
 
-	fromFormat, err := detectFormat(from)
-	if err != nil {
-		return "", err
+	fromFormat := metadecoders.FormatFromContentString(from)
+	if fromFormat == "" {
+		return "", errors.New("failed to detect format from content")
 	}
 
 	meta, err := metadecoders.UnmarshalToMap([]byte(from), fromFormat)
@@ -56,24 +57,3 @@ func toFormatMark(format string) (metadecoders.Format, error) {
 
 	return "", errors.New("failed to detect target data serialization format")
 }
-
-func detectFormat(data string) (metadecoders.Format, error) {
-	jsonIdx := strings.Index(data, "{")
-	yamlIdx := strings.Index(data, ":")
-	tomlIdx := strings.Index(data, "=")
-
-	if jsonIdx != -1 && (yamlIdx == -1 || jsonIdx < yamlIdx) && (tomlIdx == -1 || jsonIdx < tomlIdx) {
-		return metadecoders.JSON, nil
-	}
-
-	if yamlIdx != -1 && (tomlIdx == -1 || yamlIdx < tomlIdx) {
-		return metadecoders.YAML, nil
-	}
-
-	if tomlIdx != -1 {
-		return metadecoders.TOML, nil
-	}
-
-	return "", errors.New("failed to detect data serialization format")
-
-}