Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

4 files changed, 100 insertions, 33 deletions

diff --git a/parser/lowercase_camel_json.go b/parser/lowercase_camel_json.go
index e6605c803..d48aa40c4 100644
--- a/parser/lowercase_camel_json.go
+++ b/parser/lowercase_camel_json.go
@@ -19,6 +19,8 @@ import (
 	"regexp"
 	"unicode"
 	"unicode/utf8"
+
+	"github.com/gohugoio/hugo/common/hreflect"
 )
 
 // Regexp definitions
@@ -57,3 +59,58 @@ func (c LowerCaseCamelJSONMarshaller) MarshalJSON() ([]byte, error) {
 
 	return converted, err
 }
+
+type ReplacingJSONMarshaller struct {
+	Value any
+
+	KeysToLower bool
+	OmitEmpty   bool
+}
+
+func (c ReplacingJSONMarshaller) MarshalJSON() ([]byte, error) {
+	converted, err := json.Marshal(c.Value)
+
+	if c.KeysToLower {
+		converted = keyMatchRegex.ReplaceAllFunc(
+			converted,
+			func(match []byte) []byte {
+				return bytes.ToLower(match)
+			},
+		)
+	}
+
+	if c.OmitEmpty {
+		// It's tricky to do this with a regexp, so convert it to a map, remove zero values and convert back.
+		var m map[string]interface{}
+		err = json.Unmarshal(converted, &m)
+		if err != nil {
+			return nil, err
+		}
+		var removeZeroVAlues func(m map[string]any)
+		removeZeroVAlues = func(m map[string]any) {
+			for k, v := range m {
+				if !hreflect.IsTruthful(v) {
+					delete(m, k)
+				} else {
+					switch v.(type) {
+					case map[string]interface{}:
+						removeZeroVAlues(v.(map[string]any))
+					case []interface{}:
+						for _, vv := range v.([]interface{}) {
+							if m, ok := vv.(map[string]any); ok {
+								removeZeroVAlues(m)
+							}
+						}
+					}
+
+				}
+
+			}
+		}
+		removeZeroVAlues(m)
+		converted, err = json.Marshal(m)
+
+	}
+
+	return converted, err
+}
diff --git a/parser/lowercase_camel_json_test.go b/parser/lowercase_camel_json_test.go
new file mode 100644
index 000000000..ffbc80295
--- /dev/null
+++ b/parser/lowercase_camel_json_test.go
@@ -0,0 +1,33 @@
+package parser
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestReplacingJSONMarshaller(t *testing.T) {
+	c := qt.New(t)
+
+	m := map[string]any{
+		"foo":        "bar",
+		"baz":        42,
+		"zeroInt1":   0,
+		"zeroInt2":   0,
+		"zeroFloat":  0.0,
+		"zeroString": "",
+		"zeroBool":   false,
+		"nil":        nil,
+	}
+
+	marshaller := ReplacingJSONMarshaller{
+		Value:       m,
+		KeysToLower: true,
+		OmitEmpty:   true,
+	}
+
+	b, err := marshaller.MarshalJSON()
+	c.Assert(err, qt.IsNil)
+
+	c.Assert(string(b), qt.Equals, `{"baz":42,"foo":"bar"}`)
+}
diff --git a/parser/metadecoders/format.go b/parser/metadecoders/format.go
index 17e13f467..2e7e6964c 100644
--- a/parser/metadecoders/format.go
+++ b/parser/metadecoders/format.go
@@ -16,8 +16,6 @@ package metadecoders
 import (
 	"path/filepath"
 	"strings"
-
-	"github.com/gohugoio/hugo/media"
 )
 
 type Format string
@@ -33,6 +31,16 @@ const (
 	XML  Format = "xml"
 )
 
+// FormatFromStrings returns the first non-empty Format from the given strings.
+func FormatFromStrings(ss ...string) Format {
+	for _, s := range ss {
+		if f := FormatFromString(s); f != "" {
+			return f
+		}
+	}
+	return ""
+}
+
 // FormatFromString turns formatStr, typically a file extension without any ".",
 // into a Format. It returns an empty string for unknown formats.
 func FormatFromString(formatStr string) Format {
@@ -59,18 +67,6 @@ func FormatFromString(formatStr string) Format {
 	return ""
 }
 
-// FormatFromMediaType gets the Format given a MIME type, empty string
-// if unknown.
-func FormatFromMediaType(m media.Type) Format {
-	for _, suffix := range m.Suffixes() {
-		if f := FormatFromString(suffix); f != "" {
-			return f
-		}
-	}
-
-	return ""
-}
-
 // FormatFromContentString tries to detect the format (JSON, YAML, TOML or XML)
 // in the given string.
 // It return an empty string if no format could be detected.
diff --git a/parser/metadecoders/format_test.go b/parser/metadecoders/format_test.go
index db33a7d8c..c70db3fb3 100644
--- a/parser/metadecoders/format_test.go
+++ b/parser/metadecoders/format_test.go
@@ -16,8 +16,6 @@ package metadecoders
 import (
 	"testing"
 
-	"github.com/gohugoio/hugo/media"
-
 	qt "github.com/frankban/quicktest"
 )
 
@@ -41,23 +39,6 @@ func TestFormatFromString(t *testing.T) {
 	}
 }
 
-func TestFormatFromMediaType(t *testing.T) {
-	c := qt.New(t)
-	for _, test := range []struct {
-		m      media.Type
-		expect Format
-	}{
-		{media.JSONType, JSON},
-		{media.YAMLType, YAML},
-		{media.XMLType, XML},
-		{media.RSSType, XML},
-		{media.TOMLType, TOML},
-		{media.CalendarType, ""},
-	} {
-		c.Assert(FormatFromMediaType(test.m), qt.Equals, test.expect)
-	}
-}
-
 func TestFormatFromContentString(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)