AOC 2020 Day 1 in CBM Basic

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- I implemented the - Advent of Code 2020 Day 1 - challenge in CBM BASIC on a real Commodore 64. I haven't done anything - in Basic in a long time, and probably never did anything actually - meaningful with it. Part 1 of the challenge was to take a list of - numbers, find the two that summed to 2020, and then multiply those two - numbers together. Part two was to perform part 1 but with three numbers - instead of two. -

- Now I wanted to actually write the code on the Commodore 64 itself, but - I gave myself some leniency. Instead of manually typing in all 200 - entries of input data (and inevitably making a breaking mistake) I used - Vim on my PC to format the DATA entries at the start of the - code. I then dropped that onto a 1541 disk image, plopped it on an SD - card, and used my SD2IEC to mount the SD card's image on the Commodore. - The rest of the programming was done on the Commodore itself. -

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Here is my solution for Day 1 Part 1:

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-10 DATA 1686,  1983,  1801, 1890, 1910, 1722, 1571, 1952, 1602, 1551, 1144
-11 DATA 1208,  1335,  1914, 1656, 1515, 1600, 1520, 1683, 1679, 1800, 1889
-12 DATA 1717,  1592,  1617, 1756, 1646, 1596, 1874, 1595, 1660, 1748, 1946
-13 DATA 1734,  1852,  2006, 1685, 1668, 1607, 1677, 403 , 1312, 1828, 1627
-14 DATA 1925,  1657,  1536, 1522, 1557, 1636, 1586, 1654, 1541, 1363, 1844
-15 DATA 1951,  1765,  1872, 696,  1764, 1718, 1540, 1493, 1947, 1786, 1548
-16 DATA 1981,  1861,  1589, 1707, 1915, 1755, 1906, 1911, 1628, 1980, 1986
-17 DATA 1780,  1645,  741 , 1727, 524 , 1690, 1732, 1956, 1523, 1534, 1498
-18 DATA 1510,  372 ,  1777, 1585, 1614, 1712, 1650, 702 , 1773, 1713, 1797
-19 DATA 1691,  1758,  1973, 1560, 1615, 1933, 1281, 1899, 1845, 1752, 1542
-20 DATA 1694,  1950,  1879, 1684, 1809, 1988, 1978, 1843, 1730, 1377, 1507
-21 DATA 1506,  1566,  935 , 1851, 1995, 1796, 1900, 896 , 171,  1728, 1635
-22 DATA 1810,  2003,  1580, 1789, 1709, 2007, 1639, 1726, 1537, 1976, 1538
-23 DATA 1544,  1626,  1876, 1840, 1953, 1710, 1661, 1563, 1836, 1358, 1550
-24 DATA 1112,  1832,  1555, 1394, 1912, 1884, 1524, 1689, 1775, 1724, 1366
-25 DATA 1966,  1549,  1931, 1975, 1500, 1667, 1674, 1771, 1631, 1662, 1902
-26 DATA 1970,  1864,  2004, 2010, 504 , 1714, 1917, 1907, 1704, 1501, 1812
-27 DATA 1349,  1577,  1638, 1886, 1157, 1761, 1676, 1731, 2001, 1261, 1154
-28 DATA 1769,  1529
-100 DIM A(200)
-110 FOR I=0TO199
-120 READ A(I)
-140 NEXT
-150 FOR I=0TO199
-160 B=A(I)
-170 FOR J=0TO199
-180 IF I=J THEN 210
-190 C=A(J)
-200 IF B+C=2020 THEN PRINT "!",B,C,B*C:STOP
-210 NEXT J
-220 NEXT I 
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- I basically put all 200 numbers into data fields, and then defined an - array large enough to read them into with DIM. Then I - iterated over the array twice, checking each element against each other - element to see if they summed to 2020. If they did, I printed them both - and the product of the two found numbers and stopped further execution. -

- There weren't really any special tricks to this implementation except - remembering that I shouldn't be checking whether a number could sum to - 2020 with itself. -

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- Then I got to move onto Part 2, and this is where things got - interesting. Comparing any three numbers from the data meant the - cognitively easiest way to solve the problem was a triple loop. This of - course meant O(n^3) time, which the Commodore struggled - with. I waited about an hour before I decided I could optimize just a - little bit to speed up the search. -

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- I figured that for three numbers to sum to 2020, they all had to be - pretty small. Most likely they were most (if not all) three digits - instead of four. So I figured I could sort the entry data to make the - search finish probably near the start of the first layer of iteration. - Keep in mind I didn't want to pre-sort the data, I wanted the Commodore - to work with the same exact input set it had for Part 1. So I turned to - the simplest sorting algorithm I could remember: - bubble sort. -

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Here is my solution for Day 1 Part 2:

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-10 DATA 1686,  1983,  1801, 1890, 1910, 1722, 1571, 1952, 1602, 1551, 1144
-11 DATA 1208,  1335,  1914, 1656, 1515, 1600, 1520, 1683, 1679, 1800, 1889
-12 DATA 1717,  1592,  1617, 1756, 1646, 1596, 1874, 1595, 1660, 1748, 1946
-13 DATA 1734,  1852,  2006, 1685, 1668, 1607, 1677, 403 , 1312, 1828, 1627
-14 DATA 1925,  1657,  1536, 1522, 1557, 1636, 1586, 1654, 1541, 1363, 1844
-15 DATA 1951,  1765,  1872, 696,  1764, 1718, 1540, 1493, 1947, 1786, 1548
-16 DATA 1981,  1861,  1589, 1707, 1915, 1755, 1906, 1911, 1628, 1980, 1986
-17 DATA 1780,  1645,  741 , 1727, 524 , 1690, 1732, 1956, 1523, 1534, 1498
-18 DATA 1510,  372 ,  1777, 1585, 1614, 1712, 1650, 702 , 1773, 1713, 1797
-19 DATA 1691,  1758,  1973, 1560, 1615, 1933, 1281, 1899, 1845, 1752, 1542
-20 DATA 1694,  1950,  1879, 1684, 1809, 1988, 1978, 1843, 1730, 1377, 1507
-21 DATA 1506,  1566,  935 , 1851, 1995, 1796, 1900, 896 , 171,  1728, 1635
-22 DATA 1810,  2003,  1580, 1789, 1709, 2007, 1639, 1726, 1537, 1976, 1538
-23 DATA 1544,  1626,  1876, 1840, 1953, 1710, 1661, 1563, 1836, 1358, 1550
-24 DATA 1112,  1832,  1555, 1394, 1912, 1884, 1524, 1689, 1775, 1724, 1366
-25 DATA 1966,  1549,  1931, 1975, 1500, 1667, 1674, 1771, 1631, 1662, 1902
-26 DATA 1970,  1864,  2004, 2010, 504 , 1714, 1917, 1907, 1704, 1501, 1812
-27 DATA 1349,  1577,  1638, 1886, 1157, 1761, 1676, 1731, 2001, 1261, 1154
-28 DATA 1769,  1529
-100 DIM A(200)
-110 FOR I=0TO199
-120 READ A(I)
-140 NEXT
-141 GOSUB 300
-150 FOR I=0TO199
-160 B=A(I)
-170 FOR J=0TO199
-180 IF J=I THEN 250
-190 C=A(J)
-200 FOR K=0TO199
-210 IF K=I OR K=J THEN 240
-220 D=A(K)
-230 IF B+C+D=2020 THEN PRINT "!",B,C,D,B*C*D:STOP
-240 NEXT K
-250 NEXT J
-260 NEXT I
-300 REM BUBBLE SORT
-301 X=200
-310 N=200
-320 FOR I=0TON-2
-330 FOR J=0TON-I-2
-340 X=A(J):Y=A(J+1)
-350 IF X>Y THEN A(J)=Y:A(J+1)=X
-360 NEXT : NEXT
-370 RETURN 
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- I added a subroutine starting on line 300 to perform a basic bubble sort - on top of the original array of data. Now bubble sort isn't fast by any - means, but the Commodore was able to finish it in a couple of minutes. - And the results were worth it because the subsequent triple - FOR-loop completed in another few minutes. My instinct was - right and two of the solution numbers were triple-digit. -

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- So there you have it, Advent of Code 2020 Day 1 in Commodore 64 Basic - V2. You can run these samples on real hardware of course, or in an - emulator. You can also run them with the - cbmbasic - interpreter, which is a neat native C64 Basic interpreter for modern - architectures. (Oh and I tested my samples on cbmbasic and - they finished instantaneously. It helps to have a - thousands-of-times-faster processor.) -

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- I was going to keep going with the challenge and finish them all in CBM - basic for fun, but the Day 2 input data set was 1000 entries. No - problem, I can just read them from a SEQ file. The only - blocker I realized was the challenge requires string character counting, - which I don't think there's a function for in CBM basic. Maybe I have to - do a few PEEKs and POKEs to check memory - locations for ASCII/PETSCII character codes. Or I could just put it off - til next year :) -