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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: filling area by color atack safety
Date: Wed, 20 Mar 2024 09:29:54 +0100
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On 19/03/2024 18:16, bart wrote:
> On 19/03/2024 15:05, fir wrote:

>> if this is 100x 100 square and i put the initioation
>> in middle it would go 50x right then at depth 50
>> it would go one up than i guess 100 times left
>>
>> then just about this line up until up edge of picture
>> - then it probably revert back (with a lot
>> of false is) to first line and then go down
> 
> That's what I thought until I tried it.
> 
> If I start with an 18x18 image of all zeros, then fill starting from the 
> centre with a 'colour' that is an incrementing value, then the final 
> image displayed as a table of integers looks like this:
> 
> 
> 171 170 169 168 167 166 165 164 163 162 161 160 159 158 157 156 155 154
> 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153
> 135 134 133 132 131 130 129 128 127 126 125 124 123 122 121 120 119 118
> 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117
>   99  98  97  96  95  94  93  92  91  90  89  88  87  86  85  84  83  82
>   64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81
>   63  62  61  60  59  58  57  56  55  54  53  52  51  50  49  48  47  46
>   28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45
>   27  26  25  24  23  22  21  20  19  18  17  16  15  14  13  12  11  10
> 172 173 174 175 176 177 178 179 180   1   2   3   4   5   6   7   8   9
> 209 210 211 212 213 214 215 216 181 182 183 184 185 186 187 188 189 190
> 208 207 206 205 204 203 202 201 200 199 198 197 196 195 194 193 192 191
> 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234
> 252 251 250 249 248 247 246 245 244 243 242 241 240 239 238 237 236 235
> 253 254 255 325 257 258 259 260 261 262 263 264 265 266 267 268 269 270
> 288 287 286 285 284 283 282 281 280 279 278 277 276 275 274 273 272 271
> 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306
> 324 323 322 321 320 319 318 317 316 315 314 313 312 311 310 309 308 307
> 
> By following the sequence starting from 1, you can see the fill-pattern.
> 
> It's not clear how it gets from 171 at top left to 172 half-way down the 
> left edge.
> 

After the sequence hits the end at 171, it backtracks down the numbers. 
27 is the first it reaches where there is a zero square neighbour, so it 
goes down from there - and the next number in the sequence is 172.  Then 
it is free to move to the right again (then down after moving right is 
blocked at 180).

I think your posts here gives a very nice and clear way to view the 
working of the algorithm.  Thanks for doing that.