feat: Add JSON configuration parser

conv.go

@@ -16,6 +16,9 @@ import (
 //
 // ToString is used internally by zerocfg for representing option values as strings, including when passing ToString to custom parsers and for rendering configuration output.
 func ToString(v any) string {
+	if v == nil {
+		return "null"
+	}
 	if nv, ok := dereference(v); ok {
 		return ToString(nv)
 	}

json/parse.go

@@ -0,0 +1,142 @@
+package json
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+type Parser struct {
+	path *string
+
+	conv    func(any) string
+	awaited map[string]bool
+}
+
+func New(path *string) *Parser {
+	return &Parser{path: path}
+}
+
+func (p *Parser) Type() string {
+	if p.path == nil || *p.path == "" {
+		return "json"
+	}
+	return fmt.Sprintf("json[%s]", *p.path)
+}
+
+func (p *Parser) Parse(keys map[string]bool, conv func(any) string) (found, unknown map[string]string, err error) {
+	found = make(map[string]string)
+	unknown = make(map[string]string)
+
+	if p.path == nil || *p.path == "" {
+		return found, unknown, nil
+	}
+
+	data, err := os.ReadFile(*p.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return found, unknown, nil
+		}
+		return nil, nil, fmt.Errorf("read json file %q: %w", *p.path, err)
+	}
+
+	if len(data) == 0 {
+		return found, unknown, fmt.Errorf("json file %q is empty", *p.path)
+	}
+
+	p.conv = conv
+	p.awaited = keys
+
+	return p.parse(data)
+}
+
+func (p *Parser) parse(data []byte) (found, unknown map[string]string, err error) {
+	var settings any
+
+	tempFile, err := os.CreateTemp("", "json-input-*.json")
+	if err != nil {
+		return nil, nil, fmt.Errorf("create temp file for json: %w", err)
+	}
+	defer os.Remove(tempFile.Name())
+
+	if _, err := tempFile.Write(data); err != nil {
+		tempFile.Close()
+		return nil, nil, fmt.Errorf("write to temp file for json: %w", err)
+	}
+	if _, err := tempFile.Seek(0, 0); err != nil {
+		tempFile.Close()
+		return nil, nil, fmt.Errorf("seek temp file for json: %w", err)
+	}
+
+	decoder := json.NewDecoder(tempFile)
+	decoder.UseNumber()
+	decodeErr := decoder.Decode(&settings)
+	closeErr := tempFile.Close()
+
+	if decodeErr != nil {
+		return nil, nil, fmt.Errorf("unmarshal json: %w", decodeErr)
+	}
+	if closeErr != nil {
+		return nil, nil, fmt.Errorf("close temp file for json: %w", closeErr)
+	}
+
+	settingsMap, ok := settings.(map[string]any)
+	if !ok {
+		return nil, nil, fmt.Errorf("json root is not an object")
+	}
+
+	found, unknown = p.flatten(settingsMap)
+
+	return found, unknown, nil
+}
+
+func (p *Parser) flatten(settings map[string]any) (found, unknown map[string]string) {
+	found, unknown = make(map[string]string), make(map[string]string)
+	p.flattenDFS(settings, "", found, unknown)
+	return found, unknown
+}
+
+func (p *Parser) flattenDFS(m map[string]any, prefix string, found, unknown map[string]string) {
+	for k, v := range m {
+		newKey := k
+		if prefix != "" {
+			newKey = prefix + "." + k
+		}
+
+		_, isAwaited := p.awaited[newKey]
+
+		if subMap, ok := v.(map[string]any); ok {
+			if isAwaited {
+				found[newKey] = p.conv(v)
+
+			} else {
+
+				isParentOfAwaited := false
+				for awaitedKey := range p.awaited {
+					if len(awaitedKey) > len(newKey) && awaitedKey[len(newKey)] == '.' && awaitedKey[:len(newKey)] == newKey {
+						isParentOfAwaited = true
+						break
+					}
+				}
+
+				if !isParentOfAwaited {
+					unknown[newKey] = p.conv(v)
+				}
+
+				p.flattenDFS(subMap, newKey, found, unknown)
+			}
+		} else if _, ok := v.([]any); ok {
+			if isAwaited {
+				found[newKey] = p.conv(v)
+			} else {
+				unknown[newKey] = p.conv(v)
+			}
+		} else {
+			if isAwaited {
+				found[newKey] = p.conv(v)
+			} else {
+				unknown[newKey] = p.conv(v)
+			}
+		}
+	}
+}