Ponderous older version of our "are we inside quotes" code

old-quote-detection

        ////////////////////////////////////////////////////////////////////////////////////////////
        //     Step 2: detect insides of quote pairs 
        ////////////////////////////////////////////////////////////////////////////////////////////

        u64 quote_bits = cmp_mask_against_input(input_lo, input_hi, _mm256_set1_epi8('"'));
        quote_bits = quote_bits & ~odd_ends;
        dumpbits(quote_bits, "quote_bits");
        // pdep pattern is alternating 0 and 1 bits, starting with 0 or 1 depending on whether
        // we're in a quote-pair from the previous iteration
        u64 pdep_pattern = even_bits ^ prev_iter_inside_quote;
        u64 starting_quotes = _pdep_u64(pdep_pattern, quote_bits);
        // now starting quotes is the opening quote in each pair
        dumpbits(pdep_pattern, "pdep_pattern");
        dumpbits(starting_quotes, "starting quotes");

        // record whether we borrow from out of bit 63 (whether we're in a quote at iteration end)

        // we also need to subtract 1 if we need to; this reflects
        // previous iteration status. This needs to be effectively subtracted from our starting
        // quotes as we need to get a carry even if there aren't any quote bits in order to preserve
        // our 'in a quote' status inside a long string

        // aside from that it is simple: just clear starting quotes and subtract them, leaving us
        // with one bits where we are inside quotes (a half-open range including the start quote
        // but not the end quote).
        
        u64 subtracted_starts;
        bool iter_ends_quote = __builtin_usubll_overflow(
            (quote_bits ^ starting_quotes),
            starting_quotes - prev_iter_inside_quote,
            &subtracted_starts);
        prev_iter_inside_quote = iter_ends_quote ? 0xffffffffffffffffULL : 0x0ULL;

        dumpbits(subtracted_starts, "subtracted_starts");