But let’s start at the beginning. We’re also going to switch from a monolithic getbits function to something a bit more fine-grained. Pegasus spywareThis time, we’re going to switch to the stateful style employed by most bit readers you’re likely to encounter (because it ends up cheaper). If you want specifics on what exactly is affected, Apple has said the following: All iPhones with iOS versions prior to 14.8, All Mac computers with operating system versions prior to OSX Big Sur 11.6, Security Update 2021-005 Catalina, and all Apple Watches prior to watchOS 7.6.2. If you’re running a business with multiple PCs, you can take advantage of the. However, Outbyte is running a promo right now and has slashed the price by half, reducing the prices to 29.95 for the single license and 39.95 for the 5-PC license.We need to store that data in a variable that I will call the “bit buffer”:// Again, this is understood to be per-bit-reader state or localUint64_t bitbuf = 0 // value of bits in bufferInt bitcount = 0 // number of bits in bufferWhile processing input, we will always be seesawing between putting more bits into that buffer when we’re running low, and then consuming bits from that buffer while we can.Without further ado, let’s do our first stateful getbits implementation, reading one byte at a time, and starting with MSB-first this time:// Invariant: there are "bitcount" bits in "bitbuf", stored from the// MSB downwards the remaining bits in "bitbuf" are 0.Assert(count >= 1 & count > (64 - count) Writing getbits as the composition of these three smaller primitives is not always optimal. This is not always possible or desirable so let’s investigate the other extreme, a bit reader that requests additional bytes one at a time, and only when they’re needed.In general, our stateful variants will dribble in input a few bytes at a time, and have partially processed bytes lying around. Start a New Service Ticket We just need a couple things to get started: First name Last Continue reading Our “extract”-style reader assumed the entire bitstream was available in memory ahead of time. All prices are in Canadian dollars (CA) and are subject to tax.Welcome You may begin a check-in to get started with a new request, or check the status of an existing request. These fees apply in Canada. Screen or external enclosure only (with AppleCare+) Other damage (with AppleCare+) All Mac models.
![]() But this is silly we know in advance that we’re going to be reading 4+3+5=12 bits in one go, so we might as well grab them all at once:Where getbits_no_refill is yet another getbits variant that does peekbits and consume, but, as the name suggests, no refilling. Let’s start with a simple toy example: say we want to read our three example bit fields from the last part:With getbits implemented as above, this will do the refill check (and potentially some actual refilling) up to 3 times. LookaheadThe most important such transform is amortizing refills over multiple decode operations. However, breaking it down into these individual steps is still a useful thing to do, because once you conceptualize these three phases as distinct things, you can start putting them together differently. ![]() How Much Next Byte Charge Repair Full Bytes TheseThis exact variant is, to the best of my knowledge, another RAD special – discovered by my colleague Charles Bloom and me while working on Kraken ( UPDATE: as Yann points out in the comments, this basic idea was apparently used in Eric Biggers’ “Xpack” long before Kraken was launched I wasn’t aware of this and I don’t think Charles was either, but that means we’re definitely not the first ones to come up with the idea. Variant 4: a different kind of lookaheadAnd now that we’re here, let’s do another branchless lookahead variant. It’s also completely branchless! It does expect unaligned 64-bit big-endian reads to exist and be reasonably cheap (not a problem on mainstream x86s or ARMs), and of course a realistic implementation needs to include handling of the end-of-buffer cases see the discussion in the “lookahead” section. Once we break the bit extraction logic into separate “refill” and “peek”/”consume” pieces, it becomes clear how all of the individual pieces are fairly small and clean. For example, note how peekbits tests whether count > 3 // Number of bits in the current byte we've already consumed// (we took care of the full bytes these are the leftoverAssert(bitpos + count = 1 & count > (64 - count) This time, I’ve also left out the getbits built from refill / peek / consume calls, because that’s yet another pattern that should be pretty clear by now.It’s a pretty sweet variant. If not, we switch over to the bits from the lookahead value (shifting out however many of them we consumed) and clear the have_lookahead flag to indicate that what used to be our lookahead value is now just the contents of our bit buffer.There are some contortions in this code to ensure we don’t do out-of-range (undefined-behavior-inducing) shifts. Finding product key for microsoft office 2011 macBut the bitptr / bitcount manipulation needs some explanation.It’s actually easier to start with the bitcount part. Reading 64 little-endian bits and shifting them up to align with the top of our current bit buffer should be straightforward at this point. Now all branchless (well, branchless if you ignore end-of-buffer checking in the refill etc.) bit readers look very much alike, but this particular variant has a few interesting properties (some of which I’ll only discuss later because we’re lacking a bit of necessary background right now), and that I haven’t seen anywhere else in this combination if someone else did it first, feel free to inform me in the comments, and I’ll add the proper attribution! Here goes back to LSB-first again, because I’m committed to hammering home just how similar and interchangeable LSB-first/MSB-first are at this level, holy wars notwithstanding.Const uint8_t *bitptr // Pointer to next byte to insert into buf// Grab the next few bytes and insert them right aboveBitcount |= 56 // now bitcount is in The peek and consume phases are nothing we haven’t seen before, although this time the maximum permissible bit width seems to have shrunk by one more bit down to 56 bits for some reason.That reason is in the refill phase, which works slightly differently from the ones we’ve seen so far. Well, let’s just look at the values of the original bitcount: And if we refill to a target of bits in the buffer, we can compute the updated bit count with a single binary OR operation.Which brings me to the question of how many bytes we should advance the pointer by. But why? Well, inserting some integer number of bytes means bitcount is going to be incremented by some multiple of 8 during the refill that means that bitcount & 7 (the low 3 bits) won’t change. This version instead targets having between 56 and 63 bits in the buffer (which is also why the limit on count went down by one). If 48 ≤ bitcount ≤ 55, we’re adding exactly 1 byte (and so want to advance bit_ptr by that much).
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