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+ <article>
+ <h1>AOC 2020 Day 1 in CBM Basic</h1>
+
+ <p class="description">
+ I implemented the
+ <a href="https://adventofcode.com/2020">Advent of Code 2020</a> 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.
+ </p>
+ <p>
+ 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 <code>DATA</code> 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.
+ </p>
+
+ <p>Here is my solution for Day 1 Part 1:</p>
+ <pre>
+ <code>
+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
+ </code></pre>
+
+ <p>
+ I basically put all 200 numbers into data fields, and then defined an
+ array large enough to read them into with <code>DIM</code>. 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.
+ </p>
+ <p>
+ 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.
+ </p>
+
+ <p>
+ 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 <code>O(n^3)</code> 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.
+ </p>
+
+ <p>
+ 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:
+ <a href="https://en.wikipedia.org/wiki/Bubble_sort">bubble sort</a>.
+ </p>
+
+ <p>Here is my solution for Day 1 Part 2:</p>
+
+ <pre>
+ <code>
+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
+ </code>
+ </pre>
+
+ <p>
+ 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
+ <code>FOR</code>-loop completed in another few minutes. My instinct was
+ right and two of the solution numbers were triple-digit.
+ </p>
+
+ <p>
+ 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
+ <a href="https://github.com/mist64/cbmbasic"><code>cbmbasic</code></a>
+ interpreter, which is a neat native C64 Basic interpreter for modern
+ architectures. (Oh and I tested my samples on <code>cbmbasic</code> and
+ they finished instantaneously. It helps to have a
+ thousands-of-times-faster processor.)
+ </p>
+
+ <p>
+ 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 <code>SEQ</code> 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 <code>PEEK</code>s and <code>POKE</code>s to check memory
+ locations for ASCII/PETSCII character codes. Or I could just put it off
+ til next year :)
+ </p>
+ </article>
+ </body>
+</html>
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