Solve day 24.

README.md

@@ -4,6 +4,8 @@ Thanks to Eric Wastl for creating this enjoyable event.
 
 Requires `lib.py` from [aocpy](https://git.felixm.de/felixm/aocpy) repository.
 
+Requires `sympy` for day 24.
+
 
 # Times
 
@@ -40,6 +42,9 @@ Requires `lib.py` from [aocpy](https://git.felixm.de/felixm/aocpy) repository.
   something else before returning to the idea and it worked flawlessly.
 - Day 21:
 - Day 22: Not too hard, but definitely way too slow for leaderboard.
-- Day 23:
-- Day 24:
+- Day 23: I found this fun because it required some creativity for part 2. Slow
+  af, of course.
+- Day 24: Solve problem with sympy. I first used numpy to solve part 1 and it was
+  much faster than using sympy, but I lost that solution when switching to sympy.
+  Takes about three minutes to run for part 1 and then part 2 is under a second.
 - Day 25:

d24.py

@@ -0,0 +1,74 @@
+from lib import *
+import sympy as sp
+
+EXAMPLE = """19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3
+"""
+
+
+def solve1(input: Input):
+    if len(input.lines()) == 5:
+        lb = 7
+        ub = 27
+    else:
+        lb = 200000000000000
+        ub = 400000000000000
+
+    # On paper:
+    # (px - sx1) / vx1 = (py - sy1) / vy1
+    # (px - sx1) * vy1 = (py - sy1) * vx1
+    # (px - sx1) * vy1 - (py - sy1) * vx1 = 0
+    res = 0
+    eqs = [str_to_ints(l) for l in input.lines()]
+    for i, eq1 in enumerate(eqs):
+        for eq2 in eqs[:i]:
+            sx1, sy1, _, vx1, vy1, _ = eq1
+            sx2, sy2, _, vx2, vy2, _ = eq2
+
+            px, py = sp.symbols("px py")
+            es = [
+                vy1 * (px - sx1) - vx1 * (py - sy1),
+                vy2 * (px - sx2) - vx2 * (py - sy2),
+            ]
+            r = sp.solve(es)
+            if not r:
+                continue
+
+            x, y = r[px], r[py]
+            if lb <= x <= ub and lb <= y < ub:
+                t1 = (x - sx1) / vx1
+                t2 = (x - sx2) / vx2
+                if (t1 > 0 and t2 > 0):
+                    res += 1
+
+    return res
+
+
+def solve2(input: Input):
+    eqs = [str_to_ints(l) for l in input.lines()]
+    px, py, pz, vxo, vyo, vzo = sp.symbols("px py pz vxo vyo vzo")
+    es = []
+
+    # The first six equations are enough to find a solution for my problem set.
+    # Might have to be increased depending on input.
+    for i, (x, y, z, vx, vy, vz) in enumerate(eqs[:6]):
+        t = sp.symbols(f"t{i}")
+        es.append(px + vxo * t - x - vx * t)
+        es.append(py + vyo * t - y - vy * t)
+        es.append(pz + vzo * t - z - vz * t)
+    r = sp.solve(es)[0]
+    return r[px] + r[py] + r[pz]
+
+def main():
+    DAY_INPUT = "i24.txt"
+    print("Solution 1:", solve1(Input(EXAMPLE)))
+    print("Solution 1:", solve1(Input(DAY_INPUT)))
+    print("Example  2:", solve2(Input(EXAMPLE)))
+    print("Solution 2:", solve2(Input(DAY_INPUT)))
+    return
+
+if __name__ == "__main__":
+    main()