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Language: en-GB

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I've had a lot of requests to talk about the
Effective Power bug on iPhones - the one that

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means that if you send this string via iMessage,
it crashes the recipient's phone.

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Now there are all sorts of theories out there;
Apple hasn't confirmed any of them, and I

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haven't seen any in-depth analysis of this:
so this is a combination of the most plausible

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explanations I can find -- Snopes, of course,
has a pretty good rundown of the history -- and

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my own testing on this one phone.

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Clue number 1: It doesn't always work. This
isn't like the old iPhone bug that'd fail

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if it tried to render one particular bit of
text anywhere in the operating system. This

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is specific -- it has to appear in a banner
notification that drops down, and it only

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works sometimes. So that's a clue right there.
What happens to text in banner notifications

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that doesn't happen elsewhere in the messaging
app? It's shortened. You see the first few

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characters, the first few words, and then
there's an ellipsis, a dot-dot-dot at the

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end.

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Immediately, that sets off some warning signs.
'Cos shortening text isn't quite as easy as

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you might think.

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Now Apple are smart. They're not cutting based
on the number of bytes, the naive way of doing

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it. If they did that, phones would be crashing
regularly for anyone who used emoji. And they're

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not cutting just on a fixed number of characters,
either.

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The iPhone works out how much text it can
display, given the size of your screen, the

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font size your notifications are displayed
in, and the content of the message itself.

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It cuts always on a character, not a bytes.
Can it fit the full message in? Great, show

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it all. Is it too big? Then how many letters
can we fit?

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So what else is strange about the message?
Well, for folks in England, the US, and most

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of the rest of the world, it includes Arabic
text. And not just that: weird Arabic text

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mixed with Devangari diacritics, those little
marks that go above and below characters.

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That's where the bug is. The words "effective
power" and that letter on the end isn't special,

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you can put any text there, as long as it
pads out the message enough so that dot-dot-dot

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lands somewhere in the middle of that jumble
of Arabic characters.

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So we can be fairly sure that the bug is in
Arabic text handling. That is almost definite.

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The iPhone tries to cut the weird Arabic word
in half, fails for some reason, and somewhere

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in there is a bug that causes it to crash.

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This part we're fairly certain about. Here's
my speculation: it seems logical that when

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you remove a character by pressing backspace,
the string of text will become shorter. But

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in Arabic, that isn't always the case.

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Arabic is a really interesting script: letters
have multiple forms, and the one you use changes

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depending on whether it's at the start, middle,
or end of a word I've found some sequences

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-- I don't know if they come up in everyday
Arabic -- where adding a letter can turn one

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wide character into two narrow ones that are
shorter together. So if you cut half way through

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that sequence, the resulting text would be
wider than the one you started with.

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So I reckon, that as the phone tries to work
out how many letters it can fit on screen,

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it finds itself with a combination of weird
text, strange characters, and text that somehow

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gets bigger when it removes some of it. That
sounds to me like the kind of unexpected weirdness

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that would cause a crash bug in code that
is written mostly by people who write in English.

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But what I do know? We'll have to wait and
see if Apple actually release some sort of

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post-mortem, or if they just say that it's
fixed, as if by magic, in the next release.

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