Add semantic paths and safe diagnostics

path.go

@@ -0,0 +1,148 @@
+package configkit
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+)
+
+// Path identifies a configuration value using YAML field names. Its textual form
+// is a JSON Pointer: the root is empty, ~ escapes as ~0, and / escapes as ~1.
+// Paths are semantic rather than filesystem or URL paths: empty, dot, dot-dot,
+// percent-containing, and numeric keys retain their literal meaning. Parsed
+// numeric tokens are resolved only when traversal knows their containing type.
+//
+// Paths are immutable. The zero value is the root path.
+type Path struct {
+	segments []pathSegment
+	invalid  bool
+}
+
+type pathSegmentKind uint8
+
+const (
+	pathSegmentKey pathSegmentKind = iota
+	pathSegmentIndex
+	pathSegmentParsed
+)
+
+type pathSegment struct {
+	kind  pathSegmentKind
+	key   string
+	index int
+}
+
+// Keys constructs a path of mapping keys. Keys with no arguments returns the
+// root path.
+func Keys(keys ...string) Path {
+	p := Path{}
+	for _, key := range keys {
+		p = p.Key(key)
+	}
+	return p
+}
+
+// ParsePath parses a JSON Pointer. Numeric tokens remain unresolved until schema
+// or document traversal determines whether the containing value is a mapping or
+// sequence.
+func ParsePath(pointer string) (Path, error) {
+	if pointer == "" {
+		return Path{}, nil
+	}
+	if pointer[0] != '/' {
+		return Path{}, errors.New("configkit: path must be empty or begin with slash")
+	}
+
+	parts := strings.Split(pointer[1:], "/")
+	segments := make([]pathSegment, 0, len(parts))
+	for _, part := range parts {
+		key, ok := unescapePathToken(part)
+		if !ok {
+			return Path{}, errors.New("configkit: path contains an invalid tilde escape")
+		}
+		segments = append(segments, pathSegment{kind: pathSegmentParsed, key: key})
+	}
+	return Path{segments: segments}, nil
+}
+
+// Key returns a new path with key appended as a mapping-key segment.
+func (p Path) Key(key string) Path {
+	segments := clonePathSegments(p.segments, 1)
+	segments = append(segments, pathSegment{kind: pathSegmentKey, key: key})
+	return Path{segments: segments, invalid: p.invalid}
+}
+
+// Index returns a new path with index appended as a sequence-index segment.
+// A negative index produces an invalid path that configuration operations reject.
+func (p Path) Index(index int) Path {
+	segments := clonePathSegments(p.segments, 1)
+	segments = append(segments, pathSegment{kind: pathSegmentIndex, index: index})
+	return Path{segments: segments, invalid: p.invalid || index < 0}
+}
+
+// String returns the path's canonical JSON Pointer spelling.
+func (p Path) String() string {
+	if len(p.segments) == 0 {
+		return ""
+	}
+
+	var b strings.Builder
+	for _, segment := range p.segments {
+		b.WriteByte('/')
+		if segment.kind == pathSegmentIndex {
+			b.WriteString(strconv.Itoa(segment.index))
+			continue
+		}
+		b.WriteString(escapePathToken(segment.key))
+	}
+	return b.String()
+}
+
+func (p Path) clone() Path {
+	return Path{segments: clonePathSegments(p.segments, 0), invalid: p.invalid}
+}
+
+func (p Path) valid() bool {
+	return !p.invalid
+}
+
+func clonePathSegments(segments []pathSegment, extra int) []pathSegment {
+	if len(segments) == 0 && extra == 0 {
+		return nil
+	}
+	cloned := make([]pathSegment, len(segments), len(segments)+extra)
+	copy(cloned, segments)
+	return cloned
+}
+
+func escapePathToken(token string) string {
+	token = strings.ReplaceAll(token, "~", "~0")
+	return strings.ReplaceAll(token, "/", "~1")
+}
+
+func unescapePathToken(token string) (string, bool) {
+	if !strings.Contains(token, "~") {
+		return token, true
+	}
+
+	var b strings.Builder
+	for i := 0; i < len(token); i++ {
+		if token[i] != '~' {
+			b.WriteByte(token[i])
+			continue
+		}
+		if i+1 >= len(token) {
+			return "", false
+		}
+		i++
+		switch token[i] {
+		case '0':
+			b.WriteByte('~')
+		case '1':
+			b.WriteByte('/')
+		default:
+			return "", false
+		}
+	}
+	return b.String(), true
+}