package engine

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 ""
	}

	// Include each separator and the extra byte introduced by escaping a token.
	// Use stack storage for sizing indexes without allocating another string.
	size := len(p.segments)
	var indexText [20]byte
	for _, segment := range p.segments {
		if segment.kind == pathSegmentIndex {
			size += len(strconv.AppendInt(indexText[:0], int64(segment.index), 10))
		} else {
			size += len(segment.key) + strings.Count(segment.key, "~") + strings.Count(segment.key, "/")
		}
	}

	var b strings.Builder
	b.Grow(size)
	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
}