Utf8.h
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
* UTF-8-related functionality, including a type-safe structure representing a
* UTF-8 code unit.
*/
#ifndef mozilla_Utf8_h
#define mozilla_Utf8_h
#include "mozilla/Casting.h" // for mozilla::AssertedCast
#include "mozilla/Likely.h" // for MOZ_UNLIKELY
#include "mozilla/Maybe.h" // for mozilla::Maybe
#include "mozilla/Span.h" // for mozilla::Span
#include "mozilla/TextUtils.h" // for mozilla::IsAscii and via Latin1.h for
// encoding_rs_mem.h and MOZ_HAS_JSRUST.
#include "mozilla/Types.h" // for MFBT_API
#include <limits> // for std::numeric_limits
#include <limits.h> // for CHAR_BIT
#include <stddef.h> // for size_t
#include <stdint.h> // for uint8_t
#if MOZ_HAS_JSRUST()
// Can't include mozilla/Encoding.h here.
extern "C" {
// Declared as uint8_t instead of char to match declaration in another header.
size_t encoding_utf8_valid_up_to(uint8_t const* buffer, size_t buffer_len);
}
#else
namespace mozilla {
namespace detail {
extern MFBT_API bool IsValidUtf8(const void* aCodeUnits, size_t aCount);
}; // namespace detail
}; // namespace mozilla
#endif // MOZ_HAS_JSRUST
namespace mozilla {
union Utf8Unit;
static_assert(CHAR_BIT == 8,
"Utf8Unit won't work so well with non-octet chars");
/**
* A code unit within a UTF-8 encoded string. (A code unit is the smallest
* unit within the Unicode encoding of a string. For UTF-8 this is an 8-bit
* number; for UTF-16 it would be a 16-bit number.)
*
* This is *not* the same as a single code point: in UTF-8, non-ASCII code
* points are constituted by multiple code units.
*/
union Utf8Unit {
private:
// Utf8Unit is a union wrapping a raw |char|. The C++ object model and C++
// requirements as to how objects may be accessed with respect to their actual
// types (almost?) uniquely compel this choice.
//
// Our requirements for a UTF-8 code unit representation are:
//
// 1. It must be "compatible" with C++ character/string literals that use
// the UTF-8 encoding. Given a properly encoded C++ literal, you should
// be able to use |Utf8Unit| and friends to access it; given |Utf8Unit|
// and friends (particularly UnicodeData), you should be able to access
// C++ character types for their contents.
// 2. |Utf8Unit| and friends must convert to/from |char| and |char*| only by
// explicit operation.
// 3. |Utf8Unit| must participate in overload resolution and template type
// equivalence (that is, given |template<class> class X|, when |X<T>| and
// |X<U>| are the same type) distinctly from the C++ character types.
//
// And a few nice-to-haves (at least for the moment):
//
// 4. The representation should use unsigned numbers, to avoid undefined
// behavior that can arise with signed types, and because Unicode code
// points and code units are unsigned.
// 5. |Utf8Unit| and friends should be convertible to/from |unsigned char|
// and |unsigned char*|, for APIs that (because of #4 above) use those
// types as the "natural" choice for UTF-8 data.
//
// #1 requires that |Utf8Unit| "incorporate" a C++ character type: one of
// |{,{un,}signed} char|.[0] |uint8_t| won't work because it might not be a
// C++ character type.
//
// #2 and #3 mean that |Utf8Unit| can't *be* such a type (or a typedef to one:
// typedefs don't generate *new* types, just type aliases). This requires a
// compound type.
//
// The ultimate representation (and character type in it) is constrained by
// C++14 [basic.lval]p10 that defines how objects may be accessed, with
// respect to the dynamic type in memory and the actual type used to access
// them. It reads:
//
// If a program attempts to access the stored value of an object
// through a glvalue of other than one of the following types the
// behavior is undefined:
//
// 1. the dynamic type of the object,
// 2. a cv-qualified version of the dynamic type of the object,
// ...other types irrelevant here...
// 3. an aggregate or union type that includes one of the
// aforementioned types among its elements or non-static data
// members (including, recursively, an element or non-static
// data member of a subaggregate or contained union),
// ...more irrelevant types...
// 4. a char or unsigned char type.
//
// Accessing (wrapped) UTF-8 data as |char|/|unsigned char| is allowed no
// matter the representation by #4. (Briefly set aside what values are seen.)
// (And #2 allows |const| on either the dynamic type or the accessing type.)
// (|signed char| is really only useful for small signed numbers, not
// characters, so we ignore it.)
//
// If we interpret contents as |char|/|unsigned char| contrary to the actual
// type stored there, what happens? C++14 [basic.fundamental]p1 requires
// character types be identically aligned/sized; C++14 [basic.fundamental]p3
// requires |signed char| and |unsigned char| have the same value
// representation. C++ doesn't require identical bitwise representation, tho.
// Practically we could assume it, but this verges on C++ spec bits best not
// *relied* on for correctness, if possible.
//
// So we don't expose |Utf8Unit|'s contents as |unsigned char*|: only |char|
// and |char*|. Instead we safely expose |unsigned char| by fully-defined
// *integral conversion* (C++14 [conv.integral]p2). Integral conversion from
// |unsigned char| → |char| has only implementation-defined behavior. It'd be
// better not to depend on that, but given twos-complement won, it should be
// okay. (Also |unsigned char*| is awkward enough to work with for strings
// that it probably doesn't appear in string manipulation much anyway, only in
// places that should really use |Utf8Unit| directly.)
//
// The opposite direction -- interpreting |char| or |char*| data through
// |Utf8Unit| -- isn't tricky as long as |Utf8Unit| contains a |char| as
// decided above, using #3. An "aggregate or union" will work that contains a
// |char|. Oddly, an aggregate won't work: C++14 [dcl.init.aggr]p1 says
// aggregates must have "no private or protected non-static data members", and
// we want to keep the inner |char| hidden. So a |struct| is out, and only
// |union| remains.
//
// (Enums are not "an aggregate or union type", so [maybe surprisingly] we
// can't make |Utf8Unit| an enum class with |char| underlying type, because we
// are given no license to treat |char| memory as such an |enum|'s memory.)
//
// Therefore |Utf8Unit| is a union type with a |char| non-static data member.
// This satisfies all our requirements. It also supports the nice-to-haves of
// creating a |Utf8Unit| from an |unsigned char|, and being convertible to
// |unsigned char|. It doesn't satisfy the nice-to-haves of using an
// |unsigned char| internally, nor of letting us wrap an existing
// |unsigned char| or pointer to one. We probably *could* do these, if we
// were willing to rely harder on implementation-defined behaviors, but for
// now we privilege C++'s main character type over some conceptual purity.
//
// 0. There's a proposal for a UTF-8 character type distinct from the existing
// C++ narrow character types:
//
// http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0482r0.html
//
// but it hasn't been standardized (and might never be), and none of the
// compilers we really care about have implemented it. Maybe someday we
// can change our implementation to it without too much trouble, if we're
// lucky...
char mValue = '\0';
public:
Utf8Unit() = default;
explicit constexpr Utf8Unit(char aUnit) : mValue(aUnit) {}
explicit constexpr Utf8Unit(unsigned char aUnit)
: mValue(static_cast<char>(aUnit)) {
// Per the above comment, the prior cast is integral conversion with
// implementation-defined semantics, and we regretfully but unavoidably
// assume the conversion does what we want it to.
}
#ifdef __cpp_char8_t
explicit constexpr Utf8Unit(char8_t aUnit)
: mValue(static_cast<char>(aUnit)) {}
#endif
constexpr bool operator==(const Utf8Unit& aOther) const {
return mValue == aOther.mValue;
}
constexpr bool operator!=(const Utf8Unit& aOther) const {
return !(*this == aOther);
}
/** Convert a UTF-8 code unit to a raw char. */
constexpr char toChar() const {
// Only a |char| is ever permitted to be written into this location, so this
// is both permissible and returns the desired value.
return mValue;
}
/** Convert a UTF-8 code unit to a raw unsigned char. */
constexpr unsigned char toUnsignedChar() const {
// Per the above comment, this is well-defined integral conversion.
return static_cast<unsigned char>(mValue);
}
/** Convert a UTF-8 code unit to a uint8_t. */
constexpr uint8_t toUint8() const {
// Per the above comment, this is well-defined integral conversion.
return static_cast<uint8_t>(mValue);
}
// We currently don't expose |&mValue|. |UnicodeData| sort of does, but
// that's a somewhat separate concern, justified in different comments in
// that other code.
};
/**
* Reinterpret the address of a UTF-8 code unit as |const unsigned char*|.
*
* Assuming proper backing has been set up, the resulting |const unsigned char*|
* may validly be dereferenced.
*
* No access is provided to mutate this underlying memory as |unsigned char|.
* Presently memory inside |Utf8Unit| is *only* stored as |char|, and we are
* loath to offer a way to write non-|char| data until absolutely necessary.
*/
inline const unsigned char* Utf8AsUnsignedChars(const Utf8Unit* aUnits) {
static_assert(sizeof(Utf8Unit) == sizeof(unsigned char),
"sizes must match to permissibly reinterpret_cast<>");
static_assert(alignof(Utf8Unit) == alignof(unsigned char),
"alignment must match to permissibly reinterpret_cast<>");
// The static_asserts above only enable the reinterpret_cast<> to occur.
//
// Dereferencing the resulting pointer is a separate question. Any object's
// memory may be interpreted as |unsigned char| per C++11 [basic.lval]p10, but
// this doesn't guarantee what values will be observed. If |char| is
// implemented to act like |unsigned char|, we're good to go: memory for the
// |char| in |Utf8Unit| acts as we need. But if |char| is implemented to act
// like |signed char|, dereferencing produces the right value only if the
// |char| types all use two's-complement representation. Every modern
// compiler does this, and there's a C++ proposal to standardize it.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0907r0.html So
// *technically* this is implementation-defined -- but everyone does it and
// this behavior is being standardized.
return reinterpret_cast<const unsigned char*>(aUnits);
}
/** Returns true iff |aUnit| is an ASCII value. */
constexpr bool IsAscii(Utf8Unit aUnit) {
return IsAscii(aUnit.toUnsignedChar());
}
/**
* Return true if the given span of memory consists of a valid UTF-8
* string and false otherwise.
*
* The string *may* contain U+0000 NULL code points.
*/
inline bool IsUtf8(mozilla::Span<const char> aString) {
#if MOZ_HAS_JSRUST()
size_t length = aString.Length();
const uint8_t* ptr = reinterpret_cast<const uint8_t*>(aString.Elements());
// For short strings, the function call is a pessimization, and the SIMD
// code won't have a chance to kick in anyway.
if (length < 16) {
for (size_t i = 0; i < length; i++) {
if (ptr[i] >= 0x80U) {
ptr += i;
length -= i;
goto end;
}
}
return true;
}
end:
return length == encoding_utf8_valid_up_to(ptr, length);
#else
return detail::IsValidUtf8(aString.Elements(), aString.Length());
#endif
}
#if MOZ_HAS_JSRUST()
// See Latin1.h for conversions between Latin1 and UTF-8.
/**
* Returns the index of the start of the first malformed byte
* sequence or the length of the string if there are none.
*/
inline size_t Utf8ValidUpTo(mozilla::Span<const char> aString) {
return encoding_utf8_valid_up_to(
reinterpret_cast<const uint8_t*>(aString.Elements()), aString.Length());
}
/**
* Converts potentially-invalid UTF-16 to UTF-8 replacing lone surrogates
* with the REPLACEMENT CHARACTER.
*
* The length of aDest must be at least the length of aSource times three.
*
* Returns the number of code units written.
*/
inline size_t ConvertUtf16toUtf8(mozilla::Span<const char16_t> aSource,
mozilla::Span<char> aDest) {
return encoding_mem_convert_utf16_to_utf8(
aSource.Elements(), aSource.Length(), aDest.Elements(), aDest.Length());
}
/**
* Converts potentially-invalid UTF-8 to UTF-16 replacing malformed byte
* sequences with the REPLACEMENT CHARACTER with potentially insufficient
* output space.
*
* Returns the number of code units read and the number of bytes written.
*
* If the output isn't large enough, not all input is consumed.
*
* The conversion is guaranteed to be complete if the length of aDest is
* at least the length of aSource times three.
*
* The output is always valid UTF-8 ending on scalar value boundary
* even in the case of partial conversion.
*
* The semantics of this function match the semantics of
* TextEncoder.encodeInto.
* https://encoding.spec.whatwg.org/#dom-textencoder-encodeinto
*/
inline std::tuple<size_t, size_t> ConvertUtf16toUtf8Partial(
mozilla::Span<const char16_t> aSource, mozilla::Span<char> aDest) {
size_t srcLen = aSource.Length();
size_t dstLen = aDest.Length();
encoding_mem_convert_utf16_to_utf8_partial(aSource.Elements(), &srcLen,
aDest.Elements(), &dstLen);
return std::make_tuple(srcLen, dstLen);
}
/**
* Converts potentially-invalid UTF-8 to UTF-16 replacing malformed byte
* sequences with the REPLACEMENT CHARACTER.
*
* Returns the number of code units written.
*
* The length of aDest must be at least one greater than the length of aSource
* even though the last slot isn't written to.
*
* If you know that the input is valid for sure, use
* UnsafeConvertValidUtf8toUtf16() instead.
*/
inline size_t ConvertUtf8toUtf16(mozilla::Span<const char> aSource,
mozilla::Span<char16_t> aDest) {
return encoding_mem_convert_utf8_to_utf16(
aSource.Elements(), aSource.Length(), aDest.Elements(), aDest.Length());
}
/**
* Converts known-valid UTF-8 to UTF-16. If the input might be invalid,
* use ConvertUtf8toUtf16() or ConvertUtf8toUtf16WithoutReplacement() instead.
*
* Returns the number of code units written.
*
* The length of aDest must be at least the length of aSource.
*/
inline size_t UnsafeConvertValidUtf8toUtf16(mozilla::Span<const char> aSource,
mozilla::Span<char16_t> aDest) {
return encoding_mem_convert_str_to_utf16(aSource.Elements(), aSource.Length(),
aDest.Elements(), aDest.Length());
}
/**
* Converts potentially-invalid UTF-8 to valid UTF-16 signaling on error.
*
* Returns the number of code units written or `mozilla::Nothing` if the
* input was invalid.
*
* The length of the destination buffer must be at least the length of the
* source buffer.
*
* When the input was invalid, some output may have been written.
*
* If you know that the input is valid for sure, use
* UnsafeConvertValidUtf8toUtf16() instead.
*/
inline mozilla::Maybe<size_t> ConvertUtf8toUtf16WithoutReplacement(
mozilla::Span<const char> aSource, mozilla::Span<char16_t> aDest) {
size_t written = encoding_mem_convert_utf8_to_utf16_without_replacement(
aSource.Elements(), aSource.Length(), aDest.Elements(), aDest.Length());
if (MOZ_UNLIKELY(written == std::numeric_limits<size_t>::max())) {
return mozilla::Nothing();
}
return mozilla::Some(written);
}
#endif // MOZ_HAS_JSRUST
/**
* Returns true iff |aUnit| is a UTF-8 trailing code unit matching the pattern
* 0b10xx'xxxx.
*/
inline bool IsTrailingUnit(Utf8Unit aUnit) {
return (aUnit.toUint8() & 0b1100'0000) == 0b1000'0000;
}
/**
* Given |aLeadUnit| that is a non-ASCII code unit, a pointer to an |Iter aIter|
* that (initially) itself points one unit past |aLeadUnit|, and
* |const EndIter& aEnd| that denotes the end of the UTF-8 data when compared
* against |*aIter| using |aEnd - *aIter|:
*
* If |aLeadUnit| and subsequent code units computed using |*aIter| (up to
* |aEnd|) encode a valid code point -- not exceeding Unicode's range, not a
* surrogate, in shortest form -- then return Some(that code point) and advance
* |*aIter| past those code units.
*
* Otherwise decrement |*aIter| (so that it points at |aLeadUnit|) and return
* Nothing().
*
* |Iter| and |EndIter| are generalized concepts most easily understood as if
* they were |const char*|, |const unsigned char*|, or |const Utf8Unit*|:
* iterators that when dereferenced can be used to construct a |Utf8Unit| and
* that can be compared and modified in certain limited ways. (Carefully note
* that this function mutates |*aIter|.) |Iter| and |EndIter| are template
* parameters to support more-complicated adaptor iterators.
*
* The template parameters after |Iter| allow users to implement custom handling
* for various forms of invalid UTF-8. A version of this function that defaults
* all such handling to no-ops is defined below this function. To learn how to
* define your own custom handling, consult the implementation of that function,
* which documents exactly how custom handler functors are invoked.
*
* This function is MOZ_ALWAYS_INLINE: if you don't need that, use the version
* of this function without the "Inline" suffix on the name.
*/
template <typename Iter, typename EndIter, class OnBadLeadUnit,
class OnNotEnoughUnits, class OnBadTrailingUnit, class OnBadCodePoint,
class OnNotShortestForm>
MOZ_ALWAYS_INLINE Maybe<char32_t> DecodeOneUtf8CodePointInline(
const Utf8Unit aLeadUnit, Iter* aIter, const EndIter& aEnd,
OnBadLeadUnit aOnBadLeadUnit, OnNotEnoughUnits aOnNotEnoughUnits,
OnBadTrailingUnit aOnBadTrailingUnit, OnBadCodePoint aOnBadCodePoint,
OnNotShortestForm aOnNotShortestForm) {
MOZ_ASSERT(Utf8Unit((*aIter)[-1]) == aLeadUnit);
char32_t n = aLeadUnit.toUint8();
MOZ_ASSERT(!IsAscii(n));
// |aLeadUnit| determines the number of trailing code units in the code point
// and the bits of |aLeadUnit| that contribute to the code point's value.
uint8_t remaining;
uint32_t min;
if ((n & 0b1110'0000) == 0b1100'0000) {
remaining = 1;
min = 0x80;
n &= 0b0001'1111;
} else if ((n & 0b1111'0000) == 0b1110'0000) {
remaining = 2;
min = 0x800;
n &= 0b0000'1111;
} else if ((n & 0b1111'1000) == 0b1111'0000) {
remaining = 3;
min = 0x10000;
n &= 0b0000'0111;
} else {
*aIter -= 1;
aOnBadLeadUnit();
return Nothing();
}
// If the code point would require more code units than remain, the encoding
// is invalid.
auto actual = aEnd - *aIter;
if (MOZ_UNLIKELY(actual < remaining)) {
*aIter -= 1;
aOnNotEnoughUnits(AssertedCast<uint8_t>(actual + 1), remaining + 1);
return Nothing();
}
for (uint8_t i = 0; i < remaining; i++) {
const Utf8Unit unit(*(*aIter)++);
// Every non-leading code unit in properly encoded UTF-8 has its high
// bit set and the next-highest bit unset.
if (MOZ_UNLIKELY(!IsTrailingUnit(unit))) {
uint8_t unitsObserved = i + 1 + 1;
*aIter -= unitsObserved;
aOnBadTrailingUnit(unitsObserved);
return Nothing();
}
// The code point being encoded is the concatenation of all the
// unconstrained bits.
n = (n << 6) | (unit.toUint8() & 0b0011'1111);
}
// UTF-16 surrogates and values outside the Unicode range are invalid.
if (MOZ_UNLIKELY(n > 0x10FFFF || (0xD800 <= n && n <= 0xDFFF))) {
uint8_t unitsObserved = remaining + 1;
*aIter -= unitsObserved;
aOnBadCodePoint(n, unitsObserved);
return Nothing();
}
// Overlong code points are also invalid.
if (MOZ_UNLIKELY(n < min)) {
uint8_t unitsObserved = remaining + 1;
*aIter -= unitsObserved;
aOnNotShortestForm(n, unitsObserved);
return Nothing();
}
return Some(n);
}
/**
* Identical to the above function, but not forced to be instantiated inline --
* the compiler is permitted to common up separate invocations if it chooses.
*/
template <typename Iter, typename EndIter, class OnBadLeadUnit,
class OnNotEnoughUnits, class OnBadTrailingUnit, class OnBadCodePoint,
class OnNotShortestForm>
inline Maybe<char32_t> DecodeOneUtf8CodePoint(
const Utf8Unit aLeadUnit, Iter* aIter, const EndIter& aEnd,
OnBadLeadUnit aOnBadLeadUnit, OnNotEnoughUnits aOnNotEnoughUnits,
OnBadTrailingUnit aOnBadTrailingUnit, OnBadCodePoint aOnBadCodePoint,
OnNotShortestForm aOnNotShortestForm) {
return DecodeOneUtf8CodePointInline(aLeadUnit, aIter, aEnd, aOnBadLeadUnit,
aOnNotEnoughUnits, aOnBadTrailingUnit,
aOnBadCodePoint, aOnNotShortestForm);
}
/**
* Like the always-inlined function above, but with no-op behavior from all
* trailing if-invalid notifier functors.
*
* This function is MOZ_ALWAYS_INLINE: if you don't need that, use the version
* of this function without the "Inline" suffix on the name.
*/
template <typename Iter, typename EndIter>
MOZ_ALWAYS_INLINE Maybe<char32_t> DecodeOneUtf8CodePointInline(
const Utf8Unit aLeadUnit, Iter* aIter, const EndIter& aEnd) {
// aOnBadLeadUnit is called when |aLeadUnit| itself is an invalid lead unit in
// a multi-unit code point. It is passed no arguments: the caller already has
// |aLeadUnit| on hand, so no need to provide it again.
auto onBadLeadUnit = []() {};
// aOnNotEnoughUnits is called when |aLeadUnit| properly indicates a code
// point length, but there aren't enough units from |*aIter| to |aEnd| to
// satisfy that length. It is passed the number of code units actually
// available (according to |aEnd - *aIter|) and the number of code units that
// |aLeadUnit| indicates are needed. Both numbers include the contribution
// of |aLeadUnit| itself: so |aUnitsAvailable <= 3|, |aUnitsNeeded <= 4|, and
// |aUnitsAvailable < aUnitsNeeded|. As above, it also is not passed the lead
// code unit.
auto onNotEnoughUnits = [](uint8_t aUnitsAvailable, uint8_t aUnitsNeeded) {};
// aOnBadTrailingUnit is called when one of the trailing code units implied by
// |aLeadUnit| doesn't match the 0b10xx'xxxx bit pattern that all UTF-8
// trailing code units must satisfy. It is passed the total count of units
// observed (including |aLeadUnit|). The bad trailing code unit will
// conceptually be at |(*aIter)[aUnitsObserved - 1]| if this functor is
// called, and so |aUnitsObserved <= 4|.
auto onBadTrailingUnit = [](uint8_t aUnitsObserved) {};
// aOnBadCodePoint is called when a structurally-correct code point encoding
// is found, but the *value* that is encoded is not a valid code point: either
// because it exceeded the U+10FFFF Unicode maximum code point, or because it
// was a UTF-16 surrogate. It is passed the non-code point value and the
// number of code units used to encode it.
auto onBadCodePoint = [](char32_t aBadCodePoint, uint8_t aUnitsObserved) {};
// aOnNotShortestForm is called when structurally-correct encoding is found,
// but the encoded value should have been encoded in fewer code units (e.g.
// mis-encoding U+0000 as 0b1100'0000 0b1000'0000 in two code units instead of
// as 0b0000'0000). It is passed the mis-encoded code point (which will be
// valid and not a surrogate) and the count of code units that mis-encoded it.
auto onNotShortestForm = [](char32_t aBadCodePoint, uint8_t aUnitsObserved) {
};
return DecodeOneUtf8CodePointInline(aLeadUnit, aIter, aEnd, onBadLeadUnit,
onNotEnoughUnits, onBadTrailingUnit,
onBadCodePoint, onNotShortestForm);
}
/**
* Identical to the above function, but not forced to be instantiated inline --
* the compiler/linker are allowed to common up separate invocations.
*/
template <typename Iter, typename EndIter>
inline Maybe<char32_t> DecodeOneUtf8CodePoint(const Utf8Unit aLeadUnit,
Iter* aIter,
const EndIter& aEnd) {
return DecodeOneUtf8CodePointInline(aLeadUnit, aIter, aEnd);
}
} // namespace mozilla
#endif /* mozilla_Utf8_h */
