package identity

import (
	"errors"
	"strings"
	"unicode"
	"unicode/utf8"
)

// Aliases (INV-7).
//
// An alias is a human-readable label that an identity asserts about itself. It
// is decoration for user interfaces and nothing else:
//
//   - It is not unique. Two identities may claim the same alias, and the
//     protocol does nothing to prevent that.
//   - It is not verified. "Self-asserted" means exactly that; no one checks it.
//   - It never participates in signature verification or in any authorization
//     decision.
//
// Alias spoofing is therefore not a protocol vulnerability but a user
// interface concern. The mitigation is presentational: never show an alias
// without the address it belongs to. Display code should render
//
//	NikoCraft (trust1q...w4np)
//
// and never "NikoCraft" alone.
//
// The validation here is not a security control. It exists to bound size and
// to strip characters that let one alias impersonate another visually or
// corrupt terminal output.

const (
	// MaxAliasLen is the maximum length of an alias in bytes.
	MaxAliasLen = 64

	// MaxAliasRunes is the maximum length of an alias in runes, bounding the
	// visual width independently of UTF-8 encoding length.
	MaxAliasRunes = 32
)

var (
	// ErrAliasTooLong is returned for an alias exceeding the size limits.
	ErrAliasTooLong = errors.New("identity: alias too long")

	// ErrAliasInvalidUTF8 is returned for an alias that is not valid UTF-8.
	ErrAliasInvalidUTF8 = errors.New("identity: alias is not valid UTF-8")

	// ErrAliasControlChar is returned for an alias containing control,
	// bidirectional-override or other non-printing characters.
	ErrAliasControlChar = errors.New("identity: alias contains a disallowed character")

	// ErrAliasWhitespace is returned for an alias with leading or trailing
	// whitespace, which would otherwise create look-alike aliases.
	ErrAliasWhitespace = errors.New("identity: alias has leading or trailing whitespace")
)

// Alias is a validated display label. Its zero value is the empty alias, which
// is always acceptable: an identity is under no obligation to name itself.
type Alias struct {
	s string
}

// ParseAlias validates a self-asserted display label.
//
// An empty alias is valid and yields the zero Alias.
func ParseAlias(s string) (Alias, error) {
	if s == "" {
		return Alias{}, nil
	}
	if len(s) > MaxAliasLen {
		return Alias{}, ErrAliasTooLong
	}
	if !utf8.ValidString(s) {
		return Alias{}, ErrAliasInvalidUTF8
	}
	if utf8.RuneCountInString(s) > MaxAliasRunes {
		return Alias{}, ErrAliasTooLong
	}
	if strings.TrimSpace(s) != s {
		return Alias{}, ErrAliasWhitespace
	}
	for _, r := range s {
		if !allowedAliasRune(r) {
			return Alias{}, ErrAliasControlChar
		}
	}
	return Alias{s: s}, nil
}

// allowedAliasRune reports whether r may appear in an alias.
//
// Rejected: control characters, format characters (which include the
// bidirectional overrides U+202A..U+202E and U+2066..U+2069 used to make text
// render in a misleading order), unassigned code points, surrogates, private
// use characters, and every space character other than a plain ASCII space.
func allowedAliasRune(r rune) bool {
	if r == ' ' {
		return true
	}
	if unicode.IsControl(r) || unicode.IsSpace(r) {
		return false
	}
	if unicode.In(r, unicode.Cf, unicode.Cs, unicode.Co, unicode.Cn) {
		return false
	}
	return unicode.IsPrint(r)
}

// String returns the alias text.
func (a Alias) String() string { return a.s }

// IsEmpty reports whether the alias is unset.
func (a Alias) IsEmpty() bool { return a.s == "" }

// Display renders an identity for a user interface.
//
// The address is always included, because an alias on its own is not evidence
// of anything. Callers must not build their own alias-only display strings.
func Display(alias Alias, id Identity) string {
	addr := id.String()
	if alias.IsEmpty() {
		return addr
	}
	return alias.String() + " (" + shortAddress(addr) + ")"
}

// shortAddress abbreviates an address for display while keeping enough of both
// ends to make substitution visible.
func shortAddress(addr string) string {
	const head, tail = 10, 6
	if len(addr) <= head+tail+1 {
		return addr
	}
	return addr[:head] + "\u2026" + addr[len(addr)-tail:]
}