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From: fir <fir@grunge.pl>
Newsgroups: comp.lang.c
Subject: Re: filling area by color atack safety
Date: Wed, 20 Mar 2024 01:48:54 +0100
Organization: i2pn2 (i2pn.org)
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bart wrote:
> On 16/03/2024 04:11, fir wrote:
>> i was writing simple editor (something like paint but more custom for
>> my eventual needs) for big pixel (low resolution) drawing
>>
>> it showed in a minute i need a click for changing given drawed area of
>> of one color into another color (becouse if no someone would need to
>> do it  by hand pixel by pixel and the need to change color of given
>> element is very common)
>>
>> there is very simple method of doing it - i men i click in given color
>> pixel then replace it by my color and call the same function on
>> adjacent 4 pixels (only need check if it is in screen at all and if
>> the color to change is that initial color
>>
>> int RecolorizePixelAndAdjacentOnes(int x, int y, unsigned old_color,
>> unsigned new_color)
>> {
>>    if(old_color == new_color) return 0;
>>
>>    if(XYIsInScreen( x,  y))
>>    if(GetPixelUnsafe(x,y)==old_color)
>>    {
>>      SetPixelSafe(x,y,new_color);
>>      RecolorizePixelAndAdjacentOnes(x+1, y,  old_color, new_color);
>>      RecolorizePixelAndAdjacentOnes(x-1, y,  old_color, new_color);
>>      RecolorizePixelAndAdjacentOnes(x, y-1,  old_color, new_color);
>>      RecolorizePixelAndAdjacentOnes(x, y+1,  old_color, new_color);
>>      return 1;
>>    }
>>
>>    return 0;
>> }
>>
>> it work but im not quite sure how to estimate the safety of this  -
>
> On my machine, it's OK up to a 400x400 image (starting with all one
> colour and filling from the centre with another colour).
>
> At 500x500, I get stack overflow. The 400x400 the maximum recursion
> depth is 80,000 calls.
>

i was slightly thinking a bit of this recursion more generally and
i observed that those very long depth chains are kinda problem of this 
recursion becouse maybe it is more fitted to be run parrallel

if yu would just 'fork' that one call on 4 parallel calls you dont get 
that problem - as it then works like 'horisontal' (shallow, like in 
shallow searh) not 'vertical' (in-depth, deep search)

and if someone would rewrite in on non recursion way then it would be 
natural to rewrite it to work horisontal -w hich is better


if someone would fork it in really parallel then the program of 
sybchronistation of ram accesses appears

this observation hovewer may be seen as a strength of resursion -
as it naturally shows it works good with micro-paralelisation
(crowd of execution channels)