Sheng gist

sheng-gist

struct Sheng {
    typedef m128 State;
    m128 transitions[256];
    State start_state;

    Sheng(std::vector<std::tuple<u32, u32, u8>> & trans_vec, u8 start_state_, u8 default_state) {
        // fill all transitions with default state
        for (u32 i = 0; i < 256; ++i) {
            transitions[i] = _mm_set1_epi8(default_state);
        }
        // fill in state transition for slot 'from' to point to 'to' for our character transition c
        for (auto p : trans_vec) {
            u32 from, to;
            u8 c;
            std::tie(from, to, c) = p;
            set_byte_at_offset(transitions[c], from, to);
        }
        start_state =  _mm_set1_epi8(start_state_); // put everyone into start state - why not?
    }

    State apply(const u8 * data, size_t len, State s) {
        size_t i = 0;
        for (; i+7 < len; i+=8) {
            u8 c1 = data[i+0];
            u8 c2 = data[i+1];
            u8 c3 = data[i+2];
            u8 c4 = data[i+3];
            u8 c5 = data[i+4];
            u8 c6 = data[i+5];
            u8 c7 = data[i+6];
            u8 c8 = data[i+7];
            s = _mm_shuffle_epi8(transitions[c1], s);
            s = _mm_shuffle_epi8(transitions[c2], s);
            s = _mm_shuffle_epi8(transitions[c3], s);
            s = _mm_shuffle_epi8(transitions[c4], s);
            s = _mm_shuffle_epi8(transitions[c5], s);
            s = _mm_shuffle_epi8(transitions[c6], s);
            s = _mm_shuffle_epi8(transitions[c7], s);
            s = _mm_shuffle_epi8(transitions[c8], s);
        }
        for (; i < len; ++i) {
            s = _mm_shuffle_epi8(transitions[data[i]], s);
        }
        return s;
    }