"""Codec para UTF-7 modificado (RFC 3501), usado por IMAP para nombres de carpetas."""

import base64

_B64_CHARS = "-,ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+"


def encode(name: str) -> bytes:
    """Codifica un nombre de carpeta Unicode a UTF-7 modificado (IMAP)."""
    res = []
    buffer = []

    def flush():
        if not buffer:
            return
        raw = "".join(buffer).encode("utf-16-be")
        b64 = base64.b64encode(raw).decode("ascii").rstrip("=")
        b64 = b64.replace("/", ",")
        res.append("&" + b64 + "-")
        buffer.clear()

    for ch in name:
        code = ord(ch)
        if ch == "&":
            flush()
            res.append("&-")
        elif 0x20 <= code <= 0x7E:
            flush()
            res.append(ch)
        else:
            buffer.append(ch)
    flush()
    return "".join(res).encode("ascii")


def decode(data: bytes) -> str:
    """Decodifica un nombre de carpeta IMAP en UTF-7 modificado a texto Unicode."""
    s = data.decode("ascii")
    res = []
    i = 0
    n = len(s)
    while i < n:
        ch = s[i]
        if ch == "&":
            end = s.find("-", i + 1)
            if end == -1:
                end = n
            segment = s[i + 1:end]
            if segment == "":
                res.append("&")
            else:
                segment = segment.replace(",", "/")
                padding = "=" * (-len(segment) % 4)
                raw = base64.b64decode(segment + padding)
                res.append(raw.decode("utf-16-be"))
            i = end + 1
        else:
            res.append(ch)
            i += 1
    return "".join(res)