r"""UTF-8 sanitizer.

Python's UTF-8 parser is quite relaxed, this creates problems when
talking with other software that uses stricter parsers.

>>> safe_utf8_decode("foobar")
(True, u'foobar')
>>> safe_utf8_decode('X\xed\xa0\x80Y\xed\xb0\x89Z')
(False, u'X\ufffdY\ufffdZ')
>>> safe_utf8_decode('X\xed\xa0\x80\xed\xb0\x89Z')
(False, u'X\U00010009Z')
>>> safe_utf8_decode('X\0Z')
(False, u'X\ufffdZ')
>>> safe_utf8_decode('OK')
(True, u'OK')
>>> safe_utf8_decode('X\xF1Y')
(False, u'X\ufffdY')
"""

import re, codecs

__all__ = ['safe_utf8_decode']

# by default, use same symbol as 'replace'
REPLACEMENT_SYMBOL = unichr(0xFFFD)

def _fix_utf8(m):
    """Merge UTF16 surrogates, replace others"""
    u = m.group()
    if len(u) == 2:
        # merge into single symbol
        c1 = ord(u[0])
        c2 = ord(u[1])
        c = 0x10000 + ((c1 & 0x3FF) << 10) + (c2 & 0x3FF)
        return unichr(c)
    else:
        # use replacement symbol
        return REPLACEMENT_SYMBOL

_urc = None

def sanitize_unicode(u):
    """Fix invalid symbols in unicode string."""
    global _urc

    assert isinstance(u, unicode)

    # regex for finding invalid chars, works on unicode string
    if not _urc:
        rx = u"[\uD800-\uDBFF] [\uDC00-\uDFFF]? | [\0\uDC00-\uDFFF]"
        _urc = re.compile(rx, re.X)

    # now find and fix UTF16 surrogates
    m = _urc.search(u)
    if m:
        u = _urc.sub(_fix_utf8, u)
    return u

def safe_replace(exc):
    """Replace only one symbol at a time.

    Builtin .decode('xxx', 'replace') replaces several symbols
    together, which is unsafe.
    """
    if not isinstance(exc, UnicodeDecodeError):
        raise exc
    c2 = REPLACEMENT_SYMBOL

    # we could assume latin1
    if 0:
        c1 = exc.object[exc.start]
        c2 = unichr(ord(c1))

    return c2, exc.start + 1

# register, it will be globally available
codecs.register_error("safe_replace", safe_replace)

def safe_utf8_decode(s):
    """Decode UTF-8 safely.

    Acts like str.decode('utf8', 'replace') but also fixes
    UTF16 surrogates and NUL bytes, which Python's default
    decoder does not do.
    
    @param s: utf8-encoded byte string
    @return: tuple of (was_valid_utf8, unicode_string) 
    """

    # decode with error detection
    ok = True
    try:
        # expect no errors by default
        u = s.decode('utf8')
    except UnicodeDecodeError:
        u = s.decode('utf8', 'safe_replace')
        ok = False
    
    u2 = sanitize_unicode(u)
    if u is not u2:
        ok = False
    return (ok, u2)

if __name__ == '__main__':
    import doctest
    doctest.testmod()