Originally, `mb_detect_encoding` essentially just checked all candidate
encodings to see which ones the input string was valid in. However, it
was only able to do this for a limited few of all the text encodings
which are officially supported by mbstring.
In 3e7acf901d, I modified it so it could 'detect' any text encoding
supported by mbstring. While this is arguably an improvement, if the
only text encodings one is interested in are those which
`mb_detect_encoding` could originally handle, the old
`mb_detect_encoding` may have been preferable. Because the new one has
more possible encodings which it can guess, it also has more chances to
get the answer wrong.
This commit adjusts the detection heuristics to provide accurate
detection in a wider variety of scenarios. While the previous detection
code would frequently confuse UTF-32BE with UTF-32LE or UTF-16BE with
UTF-16LE, the adjusted code is extremely accurate in those cases.
Detection for Chinese text in Chinese encodings like GB18030 or BIG5
and for Japanese text in Japanese encodings like EUC-JP or SJIS is
greatly improved. Detection of UTF-7 is also greatly improved. An 8KB
table, with one bit for each codepoint from U+0000 up to U+FFFF, is
used to achieve this.
One significant constraint is that the heuristics are completely based
on looking at each codepoint in a string in isolation, treating some
codepoints as 'likely' and others as 'unlikely'. It might still be
possible to achieve great gains in detection accuracy by looking at
sequences of codepoints rather than individual codepoints. However,
this might require huge tables. Further, we might need a huge corpus
of text in various languages to derive those tables.
Accuracy is still dismal when trying to distinguish single-byte
encodings like ISO-8859-1, ISO-8859-2, KOI8-R, and so on. This is
because the valid bytes in these encodings are basically all the same,
and all valid bytes decode to 'likely' codepoints, so our method of
detection (which is based on rating codepoints as likely or unlikely)
cannot tell any difference between the candidates at all. It just
selects the first encoding in the provided list of candidates.
Speaking of which, if one wants to get good results from
`mb_detect_encoding`, it is important to order the list of candidate
encodings according to your prior belief of which are more likely to
be correct. When the function cannot tell any difference between two
candidates, it returns whichever appeared earlier in the array.
mb_convert_kana is controlled by user-provided flags, which specify what it should convert
and to what. These flags come in inverse pairs, for example "fullwidth numerals to halfwidth
numerals" and "halfwidth numerals to fullwidth numerals". It does not make sense to combine
inverse flags.
But, clever reader of commit logs, you will surely say: What if I want all my halfwidth
numerals to become fullwidth, and all my fullwidth numerals to become halfwidth? Much too
clever, you are! Let's put aside the fact that this bizarre switch-up is ridiculous and
will never be used, and face up to another stark reality: mb_convert_kana does not work
for that case, and never has. This was probably never noticed because nobody ever tried.
Disallowing useless combinations of flags gives freedom to rearrange the kana conversion
code without changing behavior.
We can also reject unrecognized flags. This may help users to catch bugs.
Interestingly, the existing tests used a 'Z' flag, which is useless (it's not recognized
at all).
Rather than doing a linear search of a table of fullwidth codepoint
ranges for every input character,
1) Short-cut the search if the codepoint is below the first such range
2) Otherwise, do a binary (rather than linear) search
Rather than using pointers to pointers to pointers (3 levels of indirection), what
makes sense is two levels. This reduces unnecessary pointer dereference operations.
* PHP-8.1:
Bug #81390: mb_detect_encoding should not prematurely stop processing input
mb_detect_encoding with only one candidate encoding uses mb_check_encoding
Optimize text encoding detection for speed (eliminate Unicode property lookups)
As a performance optimization, mb_detect_encoding tries to stop
processing the input string early when there is only one 'candidate'
encoding which the input string is valid in. However, the code which
keeps count of how many candidate encodings have already been rejected
was buggy. This caused mb_detect_encoding to prematurely stop
processing the input when it should have continued.
As a result, it did not notice that in the test case provided by Alec,
the input string was not valid in UTF-16.
...By just testing the input codepoints if they are within a few fixed
ranges instead. This avoids hash lookups in property tables.
From (micro-)benchmarking on my PC, this looks to be a bit less than 4x
faster than the existing code.
mb_convert_kana is able to convert fullwidth katakana to fullwidth
hiragana (and vice versa). The constants referring to these modes had
names like MBFL_FILT_TL_ZEN2HAN_KANA2HIRA.
The "ZEN2HAN" part of the name is misleading, since these modes do not
convert fullwidth (zenkaku) kana to halfwidth (hankaku). The converted
characters are fullwidth both before and after the conversion. So...
let's name the constants accordingly.
mb_convert_kana has conversion modes selected using 'M'/'m', which
convert a few various punctuation and symbol characters between
'ordinary' and full-width forms. The constants which refer to these
modes have names ending with COMPAT1.
Internally, there are similar conversion modes with names ending in
COMPAT2. They are like COMPAT1 modes, but they operate on a smaller
set of characters. But... that is all just dead code, because there is
no way for user code to select the COMPAT2 modes.
I have no idea what the original author intended those COMPAT2 modes to
actually be used for. Guess it doesn't really matter, anyways. At this
point, it's just more food for the flames.
Whoever originally wrote mbstring seems to have a deathly fear of NULL
pointers lurking behind every corner. A common pattern is that one
function will check if a pointer is NULL, then pass it to another
function, which will again check if it is NULL, then pass to yet another
function, which will yet again check if it is NULL... it's NULL checks
all the way down.
Remove all the NULL checks in places where pointers could not possibly
be NULL.
mbstring has a great deal of dead code. Some common types are:
- Default switch clauses which will never be taken
- If clauses intended to convert codepoints which were not present in
a conversion table... but the codepoint in question *is* in the table,
so the if clause is not needed.
- Bounds checks in places where it is not possible for a value to ever
be out of bounds.
- Checks to see if an unmatched Unicode codepoint is in CP932 extension
range 3... but every codepoint in range 3 is also in range 2, so no
codepoint will ever be matched and converted by that code.
mbstring has always had the conversion tables to support CP932 codes
in ku 115-119, and the conversion code for CP5022x has an 'if' clause
specifically to handle such characters... but that 'if' clause was dead
code, since a guard clause earlier in the same function prevented it
from accepting 2-byte characters with a starting byte of 0x93-0x97.
Adjust the guard clause so that these characters can be converted as
the original author apparently intended.
The code which handles ku 115-119 is the part which reads:
} else if (s >= cp932ext3_ucs_table_min && s < cp932ext3_ucs_table_max) {
w = cp932ext3_ucs_table[s - cp932ext3_ucs_table_min];
Previously, mbstring had a special mode whereby it would convert
erroneous input byte sequences to output like "BAD+XXXX", where "XXXX"
would be the erroneous bytes expressed in hexadecimal. This mode could
be enabled by calling `mb_substitute_character("long")`.
However, accurately reproducing input byte sequences from the cached
state of a conversion filter is often tricky, and this significantly
complicates the implementation. Further, the means used for passing
the erroneous bytes through to where the "BAD+XXXX" text is generated
only allows for up to 3 bytes to be passed, meaning that some erroneous
byte sequences are truncated anyways.
More to the point, a search of publically available PHP code indicates
that nobody is really using this feature anyways.
Incidentally, this feature also provided error output like "JIS+XXXX"
if the input 'should have' represented a JISX 0208 codepoint, but it
decodes to a codepoint which does not exist in the JISX 0208 charset.
Similarly, specific error output was provided for non-existent
JISX 0212 codepoints, and likewise for JISX 0213, CP932, and a few
other charsets. All of that is now consigned to the flames.
However, "long" error markers also include a somewhat more useful
"U+XXXX" marker for Unicode codepoints which were successfully
decoded from the input text, but cannot be represented in the output
encoding. Those are still supported.
With this change, there is no need to use a variety of special values
in the high bits of a wchar to represent different types of error
values. We can (and will) just use a single error value. This will be
equal to -1.
One complicating factor: Text conversion functions return an integer to
indicate whether the conversion operation should be immediately
aborted, and the magic 'abort' marker is -1. Also, almost all of these
functions would return the received byte/codepoint to indicate success.
That doesn't work with the new error value; if an input filter detects
an error and passes -1 to the output filter, and the output filter
returns it back, that would be taken to mean 'abort'.
Therefore, amend all these functions to return 0 for success.
This is to match the way that we handle UCS-2. When a BOM is found at
the beginning of a 'UCS-2' string (NOT 'UCS-2BE' or 'UCS-2LE'), we take
note of the intended byte order and handle the string accordingly, but
do NOT emit a BOM to the output. Rather, we just use the default byte
order for the requested output encoding.
Some might argue that if the input string used a BOM, and we are
emitting output in a text encoding where both big-endian and
little-endian byte orders are possible, we should include a BOM in the
output string. To such hypothetical debaters of minutiae, I can only
offer you a shoulder shrug. No reasonable program which handles UCS-2
and UCS-4 text should require a BOM.
Really, the concept of the BOM is a poor idea and should not have been
included in Unicode. Standardizing on a single byte order would have
been much better, similar to 'network byte order' for the Internet
Protocol. But this is not the place to speak at length of such things.
Sigh. I included tests which were intended to check this case in the
test suite for ISO-2022-JP-MS, but those tests were faulty and didn't
actually test what they were supposed to.
Fixing the tests revealed that there were still bugs in this area.
There was a bit of legacy code here which looks like the original author
of mbstring intended to allow conversion of Unicode Private Use Area
codepoints to ISO-2022-JP-KDDI. However, that code never worked.
It set the output variable to values which were not matched by any
of the 'if' clauses below, which meant that nothing was actually
emitted to the output. In other words, if one tried to convert Unicode
to ISO-2022-JP-KDDI, and the Unicode string contained PUA codepoints,
they would be quietly 'swallowed' and disappear.
I don't know what ISO-2022-JP-KDDI byte sequences the author wanted
to map those PUA codepoints to, and anyways, this use case is so obscure
that there is little point in worrying about it. However, it is better
to remove the non-functioning code than to leave it in.
This means that if now one tries to convert PUA codepoints to
ISO-2022-JP-KDDI, those codepoints will be treated as erroneous rather
than silently ignored.
After mb_substitute_character("long"), mbstring will respond to
erroneous input by inserting 'long' error markers into the output.
Depending on the situation, these error markers will either look like
BAD+XXXX (for general bad input), U+XXXX (when the input is OK, but it
converts to Unicode codepoints which cannot be represented in the
output encoding), or an encoding-specific marker like JISX+XXXX or
W932+XXXX.
We have almost no tests for this feature. Add a bunch of tests to
ensure that all our legacy encoding handlers work in a reasonable
way when 'long' error markers are enabled.
Some text encodings supported by mbstring (such as UCS-4) accept 4-byte
characters. When mbstring encounters an illegal byte sequence for the
encoding it is using, it should emit an 'illegal character' marker,
which can either be a single character like '?', an HTML hexadecimal
entity, or a marker string like 'BAD+XXXX'.
Because of the use of signed integers to hold 4-byte characters,
illegal 4-byte sequences with a 'negative' value (one with the high
bit set) were not handled correctly when emitting the illegal char
marker. The result is that such illegal sequences were just skipped
over (and the marker was not emitted to the output). Fix that.
The "wchar" encoding isn't really an encoding -- it's what we
internally use as the representation of decoded characters.
In practice, it tends to behave a lot like the 8bit encoding when
used from userland, because input code units end up being treated
as code points.
This patch removes the wchar encoding from the public encoding
list and reserves it for internal use only.
The ascii to wchar was reporting errors using conv_illegal_output,
while it should have been using WCSGROUP_THROUGH. Effectively that
replaced illegal characters with '?' for the purpose of
identification.
opaque is used by the htmlentities filter, which means that we
end up trying to free the score value as a pointer. Don't try to
be overly tricky here and simply allocate a separate structure
to hold the number of illegal characters and the score.
- Truncated multi-byte characters are treated as an error
- Reject GB18030 4-byte codes which translate to (non-existent)
Unicode codepoints above 0x10FFFF
- Add a number of missing mappings from the GB18030 standards
(These mappings are supported by iconv. I don't know why they were
missing from mbstring.)
- Truncated multi-byte characters are treated as an error
- Truncated or unrecognized escape sequences are treated as an error
- ASCII control characters are not allowed to appear in the middle
of a multi-byte character
