Separate the bytes that form an integer?

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3

Assuming I have this positive number: 

local uint = 2000;

How can I get the bytes that compose it separately? For example: 

print(sepbytes(uint));
-- 7, 208

My attempt: 

local function sepbytes(cur)
  local t = {};
  repeat
    cur = cur / 16;
    table.insert(t, 1, cur);
  until cur <= 0
  return table.unpack(t);
end

print(sepbytes(2e3));

Result: 

0   9.8813129168249e-324    1.5316035021079e-322    2.4703282292062e-321    3.95252516673e-320  6.324040266768e-319 1.0118464426829e-317    1.6189543082926e-316    2.5903268932682e-315    4.144523029229e-314 6.6312368467665e-313    1.0609978954826e-311    1.6975966327722e-310    2.7161546124355e-309    4.3458473798969e-308    6.953355807835e-307 1.1125369292536e-305    1.7800590868058e-304    2.8480945388892e-303    4.5569512622227e-302    7.2911220195564e-301    1.166579523129e-299 1.8665272370064e-298    2.9864435792103e-297    4.7783097267365e-296    7.6452955627784e-295    1.2232472900445e-293    1.9571956640713e-292    3.131513062514e-291 5.0104209000224e-290    8.0166734400359e-289    1.2826677504057e-287    2.0522684006492e-286    3.2836294410387e-285    5.2538071056619e-284    8.4060913690591e-283    1.3449746190495e-281    2.1519593904791e-280    3.4431350247666e-279    5.5090160396266e-278    8.8144256634025e-277    1.4103081061444e-275    2.256492969831e-274 3.6103887517297e-273    5.7766220027675e-272    9.2425952044279e-271    1.4788152327085e-269    2.3661043723335e-268    3.7857669957337e-267    6.0572271931739e-266    9.6915635090782e-265    1.5506501614525e-263    2.481040258324e-262 3.9696644133184e-261    6.3514630613095e-260    1.0162340898095e-258    1.6259745436952e-257    2.6015592699124e-256    4.1624948318598e-255    6.6599917309757e-254    1.0655986769561e-252    1.7049578831298e-251    2.7279326130076e-250    4.3646921808122e-249    6.9835074892995e-248    1.1173611982879e-246    1.7877779172607e-245    2.8604446676171e-244    4.5767114681874e-243    7.3227383490998e-242    1.171638135856e-240 1.8746210173695e-239    2.9993936277913e-238    4.799029804466e-237 7.6784476871456e-236    1.2285516299433e-234    1.9656826079093e-233    3.1450921726549e-232    5.0321474762478e-231    8.0514359619964e-230    1.2882297539194e-228    2.0611676062711e-227    3.2978681700337e-226    5.276589072054e-225 8.4425425152864e-224    1.3508068024458e-222    2.1612908839133e-221    3.4580654142613e-220    5.5329046628181e-219    8.8526474605089e-218    1.4164235936814e-216    2.2662777498903e-215    3.6260443998244e-214    5.8016710397191e-213    9.2826736635506e-212    1.4852277861681e-210    2.3763644578689e-209    3.8021831325903e-208    6.0834930121445e-207    9.7335888194312e-206    1.557374211109e-204 2.4917987377744e-203    3.986877980439e-202 6.3790047687024e-201    1.0206407629924e-199    1.6330252207878e-198    2.6128403532605e-197    4.1805445652168e-196    6.6888713043469e-195    1.0702194086955e-193    1.7123510539128e-192    2.7397616862605e-191    4.3836186980168e-190    7.0137899168269e-189    1.1222063866923e-187    1.7955302187077e-186    2.8728483499323e-185    4.5965573598917e-184    7.3544917758267e-183    1.1767186841323e-181    1.8827498946116e-180    3.0123998313786e-179    4.8198397302058e-178    7.7117435683292e-177    1.2338789709327e-175    1.9742063534923e-174    3.1587301655877e-173    5.0539682649402e-172    8.0863492239044e-171    1.2938158758247e-169    2.0701054013195e-168    3.3121686421112e-167    5.299469827378e-166 8.4791517238048e-165    1.3566642758088e-163    2.170662841294e-162 3.4730605460704e-161    5.5568968737127e-160    8.8910349979403e-159    1.4225655996704e-157    2.2761049594727e-156    3.6417679351564e-155    5.8268286962502e-154    9.3229259140003e-153    1.49166814624e-151  2.3866690339841e-150    3.8186704543745e-149    6.1098727269992e-148    9.7757963631987e-147    1.5641274181118e-145    2.5026038689789e-144    4.0041661903662e-143    6.4066659045859e-142    1.0250665447337e-140    1.640106471574e-139 2.6241703545184e-138    4.1986725672294e-137    6.7178761075671e-136    1.0748601772107e-134    1.7197762835372e-133    2.7516420536595e-132    4.4026272858552e-131    7.0442036573683e-130    1.1270725851789e-128    1.8033161362863e-127    2.885305818058e-126 4.6164893088929e-125    7.3863828942286e-124    1.1818212630766e-122    1.8909140209225e-121    3.025462433476e-120 4.8407398935616e-119    7.7451838296986e-118    1.2392294127518e-116
1.9827670604029e-115    3.1724272966446e-114    5.0758836746313e-113    8.1214138794101e-112    1.2994262207056e-110    2.079081953129e-109 3.3265311250064e-108    5.3224498000102e-107    8.5159196800163e-106    1.3625471488026e-104    2.1800754380842e-103    3.4881207009347e-102    5.5809931214955e-101    8.9295889943928e-100    1.4287342391028e-98 2.2859747825645e-97 3.6575596521033e-96 5.8520954433652e-95 9.3633527093844e-94 1.4981364335015e-92 2.3970182936024e-91 3.8352292697638e-90 6.1363668316222e-89 9.8181869305955e-88 1.5709099088953e-86 2.5134558542324e-85 4.0215293667719e-84 6.434446986835e-83  1.0295115178936e-81 1.6472184286298e-80 2.6355494858076e-79 4.2168791772922e-78 6.7470066836675e-77 1.0795210693868e-75 1.7272337110189e-74 2.7635739376302e-73 4.4217183002084e-72 7.0747492803334e-71 1.1319598848533e-69 1.8111358157653e-68 2.8978173052245e-67 4.6365076883593e-66 7.4184123013748e-65 1.18694596822e-63   1.899113549152e-62  3.0385816786431e-61 4.861730685829e-60  7.7787690973264e-59 1.2446030555722e-57 1.9913648889156e-56 3.1861838222649e-55 5.0978941156238e-54 8.1566305849982e-53 1.3050608935997e-51 2.0880974297595e-50 3.3409558876152e-49 5.3455294201844e-48 8.552847072295e-47  1.3684555315672e-45 2.1895288505075e-44 3.503246160812e-43  5.6051938572993e-42 8.9683101716788e-41 1.4349296274686e-39 2.2958874039498e-38 3.6734198463196e-37 5.8774717541114e-36 9.4039548065783e-35 1.5046327690525e-33 2.407412430484e-32  3.8518598887745e-31 6.1629758220392e-30 9.8607613152626e-29 1.577721810442e-27  2.5243548967072e-26 4.0389678347316e-25 6.4623485355705e-24 1.0339757656913e-22 1.6543612251061e-21 2.6469779601697e-20 4.2351647362715e-19 6.7762635780344e-18 1.0842021724855e-16 1.7347234759768e-15 2.7755575615629e-14 4.4408920985006e-13 7.105427357601e-12  1.1368683772162e-10 1.8189894035459e-09 2.9103830456734e-08 4.6566128730774e-07 7.4505805969238e-06 0.00011920928955078 0.0019073486328125  0.030517578125  0.48828125  7.8125  125

Expected result: 

7, 208
    
asked by anonymous 05.01.2017 / 19:10

2 answers

3

You can use bit operations for this, without messing around with what you were doing: 

local function sepbytes(num)
    local t = {};
    local byte = 0;
    repeat
        table.insert(t, 1, bit32.band(bit32.rshift(num, byte), 0xff));
        byte = byte + 8
    until byte > 24
    return table.unpack(t);
end

print(sepbytes(2000));

See running on ideone .

This depends on endianess . If the architecture is different then it needs to go from 24 to 0 and not more than 0 to 24. If you can not guarantee that it will run in a specific architecture, you need to find out what it is and use one or the other, that is another problem.

You can check for more or fewer bytes, which checks 0, 8, 16, and 24, that is, 4 bytes (32 bits). Just change as far as it goes.

If you need to discard those that have non-significant 0 values, you could filter this over with one condition. I think it would be wrong to do that.

    
05.01.2017 / 19:42
1

With the help of some comments in my other question I was able to simplify my function to return 2 fixed bytes: 

local function InParts(num)
    return (num & (0xff << 8)) >> 8, num & 0xff;
end
    
05.01.2017 / 23:12
Check ip, register in array, convert to json and write to file Error creating foreign key in MariaDB