Add day 9.

2022/day9.py

@@ -0,0 +1,73 @@
+# -*- encoding: utf-8 -*-
+
+import sys
+
+
+def move(head: tuple[int, int], command: str) -> tuple[int, int]:
+
+    h_col, h_row = head
+
+    if command == "L":
+        head = (h_col - 1, h_row)
+    elif command == "R":
+        head = (h_col + 1, h_row)
+    elif command == "U":
+        head = (h_col, h_row + 1)
+    elif command == "D":
+        head = (h_col, h_row - 1)
+
+    return head
+
+
+def follow(head: tuple[int, int], tail: tuple[int, int]) -> tuple[int, int]:
+
+    h_col, h_row = head
+    t_col, t_row = tail
+
+    # cover, nothing to do
+    if head == tail:
+        pass
+    elif h_col == t_col and abs(h_row - t_row) > 1:
+        diff = (h_row - t_row) // abs(h_row - t_row)
+        tail = (t_col, t_row + diff)
+    elif h_row == t_row and abs(h_col - t_col) > 1:
+        diff = (h_col - t_col) // abs(h_col - t_col)
+        tail = (t_col + diff, t_row)
+    elif abs(h_col - t_col) > 1 or abs(h_row - t_row) > 1:
+        c_diff = (h_col - t_col) // abs(h_col - t_col)
+        r_diff = (h_row - t_row) // abs(h_row - t_row)
+        tail = (t_col + c_diff, t_row + r_diff)
+
+    return tail
+
+
+def run(commands: list[str], n_blocks: int) -> list[tuple[int, int]]:
+
+    blocks = [(0, 0) for _ in range(n_blocks)]
+    visited = [blocks[-1]]
+
+    for command in commands:
+        blocks[0] = move(blocks[0], command)
+
+        for i in range(0, n_blocks - 1):
+            blocks[i + 1] = follow(blocks[i], blocks[i + 1])
+
+        visited.append(blocks[-1])
+
+    return visited
+
+
+lines = sys.stdin.read().splitlines()
+
+# flatten the commands
+commands: list[str] = []
+for line in lines:
+    d, c = line.split()
+    commands.extend(d * int(c))
+
+
+visited_1 = run(commands, n_blocks=2)
+print(f"answer 1 is {len(set(visited_1))}")
+
+visited_2 = run(commands, n_blocks=10)
+print(f"answer 2 is {len(set(visited_2))}")