From 62c52065aaacaca3b466973c83bf62d53f1d0cad Mon Sep 17 00:00:00 2001
From: wieerwill <robert.jeutter@gmx.de>
Date: Sun, 10 Dec 2023 13:22:58 +0100
Subject: [PATCH] solve Day09 in python

---
 Day09/python/solution2.py | 68 +++++++++++++++++++++++++++++++++++++++
 README.md                 |  2 +-
 2 files changed, 69 insertions(+), 1 deletion(-)
 create mode 100644 Day09/python/solution2.py

diff --git a/Day09/python/solution2.py b/Day09/python/solution2.py
new file mode 100644
index 0000000..ac873bf
--- /dev/null
+++ b/Day09/python/solution2.py
@@ -0,0 +1,68 @@
+def generate_difference_table(history):
+    """Generates a difference table for a given history."""
+    table = [history]
+    while not all(value == 0 for value in table[-1]):
+        try:
+            next_row = [table[-1][i+1] - table[-1][i] for i in range(len(table[-1]) - 1)]
+        except IndexError as e:
+            print(f"IndexError in generate_difference_table: {e}")
+            print(f"Current table: {table}")
+            raise
+        table.append(next_row)
+    return table
+
+def extrapolate_previous_value(table):
+    """Extrapolates the previous value from the difference table."""
+    try:
+        for i in range(len(table) - 2, -1, -1):
+            table[i].insert(0, table[i][0] - table[i+1][0])
+    except IndexError as e:
+        print(f"IndexError in extrapolate_previous_value: {e}")
+        print(f"Current table: {table}")
+        raise
+    return table[0][0]
+
+def solve_puzzle(filename):
+    """Solves the puzzle by reading histories from the file and summing their extrapolated previous values."""
+    total = 0
+    try:
+        with open(filename, 'r') as file:
+            for line in file:
+                try:
+                    history = [int(x) for x in line.split()]
+                    diff_table = generate_difference_table(history)
+                    prev_value = extrapolate_previous_value(diff_table)
+                    total += prev_value
+                except ValueError as e:
+                    print(f"ValueError processing line '{line}': {e}")
+                except IndexError as e:
+                    print(f"IndexError processing line '{line}': {e}")
+    except FileNotFoundError as e:
+        print(f"FileNotFoundError: The file {filename} was not found.")
+        raise
+    except Exception as e:
+        print(f"Unexpected error processing file {filename}: {e}")
+        raise
+    return total
+
+def test():
+    """Runs the test using the test.txt file and asserts the expected outcome for the second part of the puzzle."""
+    expected = 2  # Expected result from the test data for the second part
+    try:
+        result = solve_puzzle("../test.txt")
+        assert result == expected, f"Test failed: Expected {expected}, got {result}"
+        print("Test passed successfully.")
+    except AssertionError as e:
+        print(e)
+        raise
+
+def main():
+    """Main function to run the test and then solve the puzzle."""
+    print("Running test for the second part...")
+    test()
+    print("Test completed. Solving the puzzle for the second part...")
+    result = solve_puzzle("../input.txt")
+    print(f"Puzzle result for the second part: {result}")
+
+if __name__ == "__main__":
+    main()
diff --git a/README.md b/README.md
index 62d9f73..dd22311 100644
--- a/README.md
+++ b/README.md
@@ -40,7 +40,7 @@ I aim to complete each day's puzzle on the same day, but as with any challenge,
 | 06  | ✅       | ✅       | [Day06 README](/Day06/README.md) |
 | 07  | ✅       | ✅       | [Day07 README](/Day07/README.md) |
 | 08  | ✅       | ✅       | [Day08 README](/Day08/README.md) |
-| 09  | ✅       | ❓       | [Day09 README](/Day09/README.md) |
+| 09  | ✅       | ✅       | [Day09 README](/Day09/README.md) |
 | 10  | ❓       | ❓       | [Day10 README](/Day10/README.md) |
 | 11  | ❓       | ❓       | [Day11 README](/Day11/README.md) |
 | 12  | ❓       | ❓       | [Day12 README](/Day12/README.md) |