new message on login

internal/domain/emailphone.go

@@ -1,9 +1,6 @@
 package domain
 
-import (
-	"math/big"
-	"strings"
-)
+import "strings"
 
 // EmailPhonePrefix marks a "phone number" as a synthetic identity encoding an
 // email address, not a real phone. It reuses Telegram's own +888 "Anonymous
@@ -17,24 +14,69 @@ const EmailPhonePrefix = "888"
 // 0087) and ValidPhone's upper bound.
 const MaxEmailSignupPhoneLen = 200
 
+// emailPhoneEscape marks a 2-character escape sequence standing in for one
+// punctuation byte email addresses may contain but a "phone number" string
+// otherwise can't (see NormalizePhone, which only preserves letters/digits
+// for values recognized as email-signup phones). Every encoded value
+// contains at least one 'q' (from the mandatory '@' escape), which is what
+// lets IsEmailSignupPhone tell an encoded phone apart from a real,
+// all-digit, "888"-area-code phone number without any extra bookkeeping.
+const emailPhoneEscape = 'q'
+
+var emailPhoneEscapeEncode = map[rune]byte{
+	'@': '0',
+	'.': '1',
+	'-': '2',
+	'_': '3',
+	'+': '4',
+	emailPhoneEscape: '5',
+}
+
+var emailPhoneEscapeDecode = map[byte]rune{
+	'0': '@',
+	'1': '.',
+	'2': '-',
+	'3': '_',
+	'4': '+',
+	'5': emailPhoneEscape,
+}
+
 // EncodeEmailPhone deterministically and reversibly encodes an email address
-// into a synthetic "888"-prefixed all-digit phone number: the email's
-// lowercased/trimmed UTF-8 bytes, read as a big-endian unsigned integer, then
-// printed in decimal. This lets the existing phone-based sendCode/signUp/
-// signIn/changePhone flow carry an email address end to end unchanged — no
-// new TL constructors, no server-side reverse-lookup table required.
+// into a synthetic "888"-prefixed phone-number-shaped string: letters and
+// digits pass through unchanged, and the handful of punctuation characters
+// real email addresses use are each replaced by a 2-character escape
+// ('q' + a digit). This keeps the encoded length close to the email's own
+// length (unlike a byte-for-byte big-integer encoding, which runs ~2.4x
+// longer), while staying fully reversible with no server-side lookup table
+// and no new TL constructors — the existing phone-based sendCode/signUp/
+// signIn/changePhone flow carries it end to end unchanged.
 //
-// ok is false if email is empty/invalid or the encoded result would not fit
-// the users.phone column (MaxEmailSignupPhoneLen) — this comfortably covers
-// realistic email addresses (roughly up to 80 bytes).
+// ok is false if email is empty/invalid, contains a character outside
+// [a-z0-9@._+-], or the encoded result would not fit the users.phone column
+// (MaxEmailSignupPhoneLen).
 func EncodeEmailPhone(email string) (phone string, ok bool) {
 	normalized := NormalizeEmailForPhone(email)
 	if normalized == "" || !strings.Contains(normalized, "@") {
 		return "", false
 	}
-	n := new(big.Int).SetBytes([]byte(normalized))
-	digits := n.String()
-	phone = EmailPhonePrefix + digits
+	var b strings.Builder
+	b.Grow(len(normalized) * 2)
+	for _, r := range normalized {
+		switch {
+		case r >= 'a' && r <= 'z' && r != rune(emailPhoneEscape):
+			b.WriteRune(r)
+		case r >= '0' && r <= '9':
+			b.WriteRune(r)
+		default:
+			digit, escapable := emailPhoneEscapeEncode[r]
+			if !escapable {
+				return "", false
+			}
+			b.WriteByte(emailPhoneEscape)
+			b.WriteByte(digit)
+		}
+	}
+	phone = EmailPhonePrefix + b.String()
 	if len(phone) > MaxEmailSignupPhoneLen {
 		return "", false
 	}
@@ -44,20 +86,46 @@ func EncodeEmailPhone(email string) (phone string, ok bool) {
 // DecodeEmailPhone reverses EncodeEmailPhone. ok is false if phone does not
 // carry the "888" prefix or does not decode to a plausible email address.
 func DecodeEmailPhone(phone string) (email string, ok bool) {
-	phone = NormalizePhone(strings.TrimSpace(phone))
-	digits, found := strings.CutPrefix(phone, EmailPhonePrefix)
-	if !found || digits == "" {
+	lower := strings.ToLower(strings.TrimSpace(phone))
+	body, found := strings.CutPrefix(lower, EmailPhonePrefix)
+	if !found || body == "" {
 		return "", false
 	}
-	n, valid := new(big.Int).SetString(digits, 10)
-	if !valid {
+	var b strings.Builder
+	b.Grow(len(body))
+	runes := []rune(body)
+	for i := 0; i < len(runes); i++ {
+		r := runes[i]
+		if r != rune(emailPhoneEscape) {
+			b.WriteRune(r)
+			continue
+		}
+		i++
+		if i >= len(runes) {
+			return "", false
+		}
+		digitByte, ok := asciiDigitByte(runes[i])
+		if !ok {
+			return "", false
+		}
+		decoded, known := emailPhoneEscapeDecode[digitByte]
+		if !known {
+			return "", false
+		}
+		b.WriteRune(decoded)
+	}
+	email = b.String()
+	if email == "" || !strings.Contains(email, "@") {
 		return "", false
 	}
-	decoded := string(n.Bytes())
-	if decoded == "" || !strings.Contains(decoded, "@") {
-		return "", false
+	return email, true
+}
+
+func asciiDigitByte(r rune) (byte, bool) {
+	if r < '0' || r > '9' {
+		return 0, false
 	}
-	return decoded, true
+	return byte(r), true
 }
 
 // NormalizeEmailForPhone lowercases and trims an email so the same address
@@ -67,8 +135,20 @@ func NormalizeEmailForPhone(email string) string {
 	return strings.ToLower(strings.TrimSpace(email))
 }
 
-// IsEmailSignupPhone reports whether phone was produced by EncodeEmailPhone
-// (i.e. carries the synthetic "888" prefix), without decoding it.
+// IsEmailSignupPhone reports whether phone was produced by EncodeEmailPhone.
+// Every encoded value contains at least one letter (the mandatory '@'
+// escape's 'q' marker byte), which real, all-digit phone numbers — even
+// ones that happen to start with the 888 area code — never do; this keeps
+// the check unambiguous without any extra prefix bookkeeping.
 func IsEmailSignupPhone(phone string) bool {
-	return strings.HasPrefix(NormalizePhone(strings.TrimSpace(phone)), EmailPhonePrefix)
+	lower := strings.ToLower(strings.TrimSpace(phone))
+	if !strings.HasPrefix(lower, EmailPhonePrefix) {
+		return false
+	}
+	for _, r := range lower {
+		if r >= 'a' && r <= 'z' {
+			return true
+		}
+	}
+	return false
 }