is_utf8 0.1.0-alpha

Provides functions to determine if a sequence of bytes is valid utf-8.
Documentation 
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#[cfg(target_arch = "x86")]
use core::arch::x86::{
    __m128i, _mm_add_epi8, _mm_alignr_epi8, _mm_and_si128, _mm_cmpeq_epi8, _mm_cmpgt_epi8,
    _mm_loadu_si128, _mm_or_si128, _mm_set1_epi8, _mm_set_epi8, _mm_setr_epi8, _mm_setzero_si128,
    _mm_shuffle_epi8, _mm_srli_epi16, _mm_subs_epu8, _mm_testz_si128,
};
#[cfg(target_arch = "x86_64")]
use core::arch::x86_64::{
    __m128i, _mm_add_epi8, _mm_alignr_epi8, _mm_and_si128, _mm_cmpeq_epi8, _mm_cmpgt_epi8,
    _mm_loadu_si128, _mm_or_si128, _mm_set1_epi8, _mm_setr_epi8, _mm_setzero_si128,
    _mm_shuffle_epi8, _mm_srli_epi16, _mm_subs_epu8, _mm_testz_si128,
};
use core::default::Default;
use core::ptr;

#[derive(Copy, Clone)]
struct ProcessedUtfBytes {
    rawbytes: __m128i,
    high_nibbles: __m128i,
    carried_continuations: __m128i,
}

impl Default for ProcessedUtfBytes {
    fn default() -> Self {
        unsafe {
            ProcessedUtfBytes {
                rawbytes: _mm_setzero_si128(),
                high_nibbles: _mm_setzero_si128(),
                carried_continuations: _mm_setzero_si128(),
            }
        }
    }
}

/*
 * legal utf-8 byte sequence
 * http://www.unicode.org/versions/Unicode6.0.0/ch03.pdf - page 94
 *
 *  Code Points        1st       2s       3s       4s
 * U+0000..U+007F     00..7F
 * U+0080..U+07FF     C2..DF   80..BF
 * U+0800..U+0FFF     E0       A0..BF   80..BF
 * U+1000..U+CFFF     E1..EC   80..BF   80..BF
 * U+D000..U+D7FF     ED       80..9F   80..BF
 * U+E000..U+FFFF     EE..EF   80..BF   80..BF
 * U+10000..U+3FFFF   F0       90..BF   80..BF   80..BF
 * U+40000..U+FFFFF   F1..F3   80..BF   80..BF   80..BF
 * U+100000..U+10FFFF F4       80..8F   80..BF   80..BF
 *
 */

// all byte values must be no larger than 0xF4
#[inline]
unsafe fn check_smaller_than_0xf4(current_bytes: __m128i, has_error: *mut __m128i) {
    // unsigned, saturates to 0 below max
    *has_error = _mm_or_si128(
        *has_error,
        _mm_subs_epu8(current_bytes, _mm_set1_epi8(0xf4i32 as libc::c_char)),
    );
}

#[inline]
fn continuation_lengths(high_nibbles: __m128i) -> __m128i {
    unsafe {
        return _mm_shuffle_epi8(
            _mm_setr_epi8(1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 2, 2, 3, 4),
            high_nibbles,
        );
    }
}

#[inline]
fn carry_continuations(initial_lengths: __m128i, previous_carries: __m128i) -> __m128i {
    unsafe {
        let right1 = _mm_subs_epu8(
            _mm_alignr_epi8(initial_lengths, previous_carries, 16i32 - 1i32),
            _mm_set1_epi8(1i8),
        );
        let sum = _mm_add_epi8(initial_lengths, right1);
        let right2 = _mm_subs_epu8(
            _mm_alignr_epi8(sum, previous_carries, 16i32 - 2i32),
            _mm_set1_epi8(2i8),
        );
        return _mm_add_epi8(sum, right2);
    }
}

#[inline]
unsafe fn check_continuations(initial_lengths: __m128i, carries: __m128i, has_error: *mut __m128i) {
    let overunder = _mm_cmpeq_epi8(
        _mm_cmpgt_epi8(carries, initial_lengths),
        _mm_cmpgt_epi8(initial_lengths, _mm_setzero_si128()),
    );
    *has_error = _mm_or_si128(*has_error, overunder);
}

// when 0xED is found, next byte must be no larger than 0x9F
// when 0xF4 is found, next byte must be no larger than 0x8F
// next byte must be continuation, ie sign bit is set, so signed < is ok
#[inline]
unsafe fn check_first_continuation_max(
    current_bytes: __m128i,
    off1_current_bytes: __m128i,
    has_error: *mut __m128i,
) {
    let mask_ed = _mm_cmpeq_epi8(off1_current_bytes, _mm_set1_epi8(0xedi32 as libc::c_char));
    let mask_f4 = _mm_cmpeq_epi8(off1_current_bytes, _mm_set1_epi8(0xf4i32 as libc::c_char));
    let bad_follow_ed = _mm_and_si128(
        _mm_cmpgt_epi8(current_bytes, _mm_set1_epi8(0x9fi32 as libc::c_char)),
        mask_ed,
    );
    let bad_follow_f4 = _mm_and_si128(
        _mm_cmpgt_epi8(current_bytes, _mm_set1_epi8(0x8fi32 as libc::c_char)),
        mask_f4,
    );
    *has_error = _mm_or_si128(*has_error, _mm_or_si128(bad_follow_ed, bad_follow_f4));
}

// map off1_hibits => error condition
// hibits     off1    cur
// C       => < C2 && true
// E       => < E1 && < A0
// F       => < F1 && < 90
// else      false && false
#[inline]
unsafe fn check_overlong(
    current_bytes: __m128i,
    off1_current_bytes: __m128i,
    hibits: __m128i,
    previous_hibits: __m128i,
    has_error: *mut __m128i,
) {
    let off1_hibits = _mm_alignr_epi8(hibits, previous_hibits, 16i32 - 1i32);
    let initial_mins = _mm_shuffle_epi8(
        _mm_setr_epi8(
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            0xc2i32 as libc::c_char,
            -(128i32) as libc::c_char,
            0xe1i32 as libc::c_char,
            0xf1i32 as libc::c_char,
        ),
        off1_hibits,
    );
    let initial_under = _mm_cmpgt_epi8(initial_mins, off1_current_bytes);
    let second_mins = _mm_shuffle_epi8(
        _mm_setr_epi8(
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            -(128i32) as libc::c_char,
            127i8,
            127i8,
            0xa0i32 as libc::c_char,
            0x90i32 as libc::c_char,
        ),
        off1_hibits,
    );
    let second_under = _mm_cmpgt_epi8(second_mins, current_bytes);
    *has_error = _mm_or_si128(*has_error, _mm_and_si128(initial_under, second_under));
}

#[inline]
fn count_nibbles(bytes: __m128i, mut answer: &mut ProcessedUtfBytes) {
    answer.rawbytes = bytes;
    answer.high_nibbles =
        unsafe { _mm_and_si128(_mm_srli_epi16(bytes, 4i32), _mm_set1_epi8(0xfi8)) };
}

// check whether the current bytes are valid UTF-8
// at the end of the function, previous gets updated
unsafe fn check_utf8_bytes(
    current_bytes: __m128i,
    previous: &mut ProcessedUtfBytes,
    has_error: *mut __m128i,
) -> ProcessedUtfBytes {
    let mut pb = ProcessedUtfBytes::default();
    count_nibbles(current_bytes, &mut pb);
    check_smaller_than_0xf4(current_bytes, has_error);
    let initial_lengths = continuation_lengths(pb.high_nibbles);
    pb.carried_continuations = carry_continuations(initial_lengths, previous.carried_continuations);
    check_continuations(initial_lengths, pb.carried_continuations, has_error);
    let off1_current_bytes = _mm_alignr_epi8(pb.rawbytes, previous.rawbytes, 16i32 - 1i32);
    check_first_continuation_max(current_bytes, off1_current_bytes, has_error);
    check_overlong(
        current_bytes,
        off1_current_bytes,
        pb.high_nibbles,
        previous.high_nibbles,
        has_error,
    );
    return pb;
}

pub fn is_utf8(bytes: &[u8]) -> bool {
    unsafe {
        let len = bytes.len();
        let mut i = 0;
        let mut has_error = _mm_setzero_si128();
        let mut previous = ProcessedUtfBytes::default();
        if len >= 16 {
            while i <= len - 16 {
                let current_bytes =
                    _mm_loadu_si128(bytes.as_ptr().offset(i as isize) as *const __m128i);
                previous = check_utf8_bytes(current_bytes, &mut previous, &mut has_error);
                i += 16
            }
        }
        // last part
        if i < len {
            let mut buffer = [0; 16];
            ptr::write_bytes(buffer.as_mut_ptr(), 0, 16);
            ptr::copy(
                bytes.as_ptr().offset(i as isize),
                buffer.as_mut_ptr(),
                len - i,
            );
            let current_bytes_0 = _mm_loadu_si128(buffer.as_mut_ptr() as *const __m128i);
            check_utf8_bytes(current_bytes_0, &mut previous, &mut has_error);
        } else {
            has_error = _mm_or_si128(
                _mm_cmpgt_epi8(
                    previous.carried_continuations,
                    _mm_setr_epi8(9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 1),
                ),
                has_error,
            )
        }
        return _mm_testz_si128(has_error, has_error) != 0;
    }
}