Day 24: Never Tell Me The Odds (part 2)

ephemient · Dec 26, 2023 · 39060b5 · 39060b5
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diff --git a/README.md b/README.md
@@ -28,5 +28,5 @@ Development occurs in language-specific directories:
 |[Day21.hs](hs/src/Day21.hs)|[Day21.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day21.kt)|[day21.py](py/aoc2023/day21.py)|[day21.rs](rs/src/day21.rs)|
 |[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day22.kt)|[day22.py](py/aoc2023/day22.py)|[day22.rs](rs/src/day22.rs)|
 |[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day23.kt)||[day23.rs](rs/src/day23.rs)|
-|[Day24.hs](hs/src/Day24.hs)|[Day24.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day24.kt) ½|[day24.py](py/aoc2023/day24.py) ½|[day24.rs](rs/src/day24.rs) ½|
+|[Day24.hs](hs/src/Day24.hs)|[Day24.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day24.kt) ½|[day24.py](py/aoc2023/day24.py)|[day24.rs](rs/src/day24.rs) ½|
 |[Day25.hs](hs/src/Day25.hs)|[Day25.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day25.kt)|[day25.py](py/aoc2023/day25.py)|[day25.rs](rs/src/day25.rs)|
diff --git a/py/aoc2023/day24.py b/py/aoc2023/day24.py
@@ -2,6 +2,8 @@
 Day 24: Never Tell Me The Odds
 """
 
+from fractions import Fraction
+
 SAMPLE_INPUT = """19, 13, 30 @ -2,  1, -2
 18, 19, 22 @ -1, -1, -2
 20, 25, 34 @ -2, -2, -4
@@ -50,11 +52,52 @@ def part1(data, lo=200000000000000, hi=400000000000000):
 
 def part2(data):
     """
-    >>> part2(SAMPLE_INPUT) # doctest: +SKIP
+    >>> part2(SAMPLE_INPUT)
     47
     """
-    _parse(data)
-    raise NotImplementedError()
+    points = list(_parse(data))
+    for i, ((x0, y0, z0), (vx0, vy0, vz0)) in enumerate(points):
+
+        # pylint: disable=cell-var-from-loop
+        def offset(point):
+            (x1, y1, z1), (vx1, vy1, vz1) = point
+            return (x1 - x0, y1 - y0, z1 - z0), (vx1 - vx0, vy1 - vy0, vz1 - vz0)
+
+        for j, ((x1, y1, z1), (vx1, vy1, vz1)) in enumerate(map(offset, points[:i])):
+            px1 = y1 * vz1 - z1 * vy1
+            py1 = z1 * vx1 - x1 * vz1
+            pz1 = x1 * vy1 - y1 * vx1
+
+            for (x2, y2, z2), (vx2, vy2, vz2) in map(offset, points[:j]):
+                px2 = y2 * vz2 - z2 * vy2
+                py2 = z2 * vx2 - x2 * vz2
+                pz2 = x2 * vy2 - y2 * vx2
+
+                mx = py1 * pz2 - pz1 * py2
+                my = pz1 * px2 - px1 * pz2
+                mz = px1 * py2 - py1 * px2
+
+                if my * vx1 == mx * vy1 or my * vx2 == mx * vy2:
+                    continue
+
+                u1 = Fraction(y1 * vx1 - x1 * vy1, my * vx1 - mx * vy1)
+                u2 = Fraction(y2 * vx2 - x2 * vy2, my * vx2 - mx * vy2)
+                t1, t2 = (
+                    next(
+                        (m * u - p) / v
+                        for m, p, v in zip((mx, my, mz), (x, y, z), (vx, vy, vz))
+                        if v
+                    )
+                    for u, x, y, z, vx, vy, vz in (
+                        (u1, x1, y1, z1, vx1, vy1, vz1),
+                        (u2, x2, y2, z2, vx2, vy2, vz2),
+                    )
+                )
+                return int(
+                    x0 + y0 + z0 + (mx + my + mz) * (u1 * t2 - u2 * t1) / (t2 - t1)
+                )
+
+    return None
 
 
-parts = (part1,)
+parts = (part1, part2)