<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Sep 22, 2014 at 1:47 PM, Nicolas Cellier <span dir="ltr"><<a href="mailto:nicolas.cellier.aka.nice@gmail.com" target="_blank">nicolas.cellier.aka.nice@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><span class="">2014-09-22 22:34 GMT+02:00 Bert Freudenberg <span dir="ltr"><<a href="mailto:bert@freudenbergs.de" target="_blank">bert@freudenbergs.de</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span><br>
On 22.09.2014, at 22:21, Ron Teitelbaum <<a href="mailto:ron@usmedrec.com" target="_blank">ron@usmedrec.com</a>> wrote:<br>
<br>
><br>
>> From: Bert Freudenberg<br>
>> Sent: Monday, September 22, 2014 4:00 PM<br>
>><br>
>> I just found out why PNG decoding was broken in SqueakJS [*]. Wouldn't<br>
>> have believed that any code actually relied on the following behavior:<br>
>><br>
>> | a |<br>
>> a := (1 to: 20) asArray.<br>
>> a replaceFrom: 11 to: 20 with: a startingAt: 8.<br>
>><br>
>> ==> #(1 2 3 4 5 6 7 8 9 10 8 9 10 8 9 10 8 9 10 8)<br>
>><br>
> That's funky! Hope it wasn't my code! Seems wrong to not copy the array<br>
> before modifying it (Like your buggy VM did).<br>
<br>
</span>It's not a copy. It moved the numbers 8-17 to index 11. Which is what I erroneously had assumed to be the desired behavior.<br>
<span><font color="#888888"><br>
- Bert -<br>
</font></span><div><div><br></div></div></blockquote><div><br></div></span><div>It's a known problem.<br></div><div>The primitive should use memmove rather than memcpy.<br></div><div>So the solution is quite simple, it's just that it's waiting for a good soul to integrate it.<br></div></div></div></div></blockquote><div><br></div><div>Um, no. It has ever been so and must therefore remain. This is the method (as in the Smalltalk-80 V2 sources) that the primitive optimizes:</div><div><br></div><div> replaceFrom: start to: stop with: replacement startingAt: repStart </div><div><span class="Apple-tab-span" style="white-space:pre">        </span>"This destructively replaces elements from start to stop in the receiver</div><div><span class="Apple-tab-span" style="white-space:pre">        </span>starting at index, repStart, in the collection, replacement. Answer the</div><div><span class="Apple-tab-span" style="white-space:pre">        </span>receiver. No range checks are performed - this may be primitively implemented."</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre">        </span>| index repOff |</div><div><span class="Apple-tab-span" style="white-space:pre">        </span>repOff _ repStart - start.</div><div><span class="Apple-tab-span" style="white-space:pre">        </span>index _ start - 1.</div><div><span class="Apple-tab-span" style="white-space:pre">        </span>[(index _ index + 1) <= stop]</div><div><span class="Apple-tab-span" style="white-space:pre">                </span>whileTrue: [self at: index put: (replacement at: repOff + index)]</div><div><br></div><div>So the primitive is correct. This is memcpy behaviour. IIRC BitBlt is the same. There were some BitBlt ticks played with image fill that depended on this.</div><div><br></div><div><div>| oc |</div><div>oc := OrderedCollection withAll: (1 to: 20).</div><div>oc replaceFrom: 11 to: 20 with: oc startingAt: 8.</div><div>oc => an OrderedCollection(1 2 3 4 5 6 7 8 9 10 8 9 10 8 9 10 8 9 10 8)</div></div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><span class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div>
> Ron<br>
><br>
>> In my (buggy) VM it answered<br>
>><br>
>> ==> #(1 2 3 4 5 6 7 8 9 10 8 9 10 11 12 13 14 15 16 17)<br>
>><br>
>> which apparently did not at all match what The Creators intended ;)<br>
>><br>
>> - Bert -<br>
>><br>
>> [*] specifically, #decompressBlock:with:<br>
><br>
><br>
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<br></blockquote></div><br><br clear="all"><div><br></div>-- <br>best,<div>Eliot</div>
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