catch up on solutions up to day 20

1 files changed, 39 insertions, 0 deletions

diff --git a/day17.py b/day17.py
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+#!/usr/bin/env python
+
+# target area: 236 <= x <= 262, -78 <= y <= -58
+# i was originally going to do a brute force solution
+# since the problem implies that the displacement on the x axis is the Vx'th triangular number
+# which gives a lower bound on the number of steps to calculate the displacement on the y axis
+# but there's actually a really brilliant solution located at
+# https://github.com/prendradjaja/advent-of-code-2021/blob/main/17--trick-shot/a.py
+# which I am paraphrasing here
+
+yvel = 77
+print(sum(n for n in range(1, yvel+1)))
+
+
+# part 2
+xMin, xMax, yMin, yMax = 236, 262, -78, -58
+
+def is_hit(vel):
+    for pos in trajectory(vel):
+        if xMin <= pos[0] <= xMax and yMin <= pos[1] <= yMax:
+            return True
+    return False
+
+def trajectory(vel):
+    pos = (0,0)
+    while pos[0] <= xMax and pos[1] >= yMin:
+        yield pos
+        pos = (pos[0] + vel[0], pos[1] + vel[1])
+        vel = (
+            max(0, vel[0] - 1),
+            vel[1] - 1
+        )
+
+result = 0
+for xvel in range(0, xMax+1):
+    for yvel in range(yMin, -yMin):
+        if is_hit((xvel, yvel)):
+            result += 1
+print(result)