add-day2 (#1)

Reviewed-on: #1
Co-authored-by: josh <josh@joshuaschuett.com>
Co-committed-by: josh <josh@joshuaschuett.com>

day2/day2_sample.py

@@ -0,0 +1,96 @@
+#!/usr/bin/env python3
+"""
+Counts how many reports are "safe".
+
+A report (one line of space-separated integers) is safe if:
+1) Levels are strictly monotonic (all increasing OR all decreasing).
+2) Adjacent levels differ by at least 1 and at most 3 (inclusive).
+
+Usage:
+  python day2/aoc_2024_day2.py input.txt
+"""
+
+import argparse
+
+
+class Report:
+    def __init__(self, levels):
+        self.levels = levels
+
+    @classmethod
+    def from_line(cls, line):
+        parts = line.split()
+        levels = []
+        for part in parts:
+            levels.append(int(part))
+        return cls(levels=levels)
+
+    def is_safe(self):
+        if len(self.levels) < 2:
+            return True
+
+        # Determine direction (increasing/decreasing) from the first valid step.
+        #  0  => not decided yet
+        # +1  => increasing
+        # -1  => decreasing
+        direction = 0
+
+        for i in range(1, len(self.levels)):
+            previous_level = self.levels[i - 1]
+            current_level = self.levels[i]
+            delta = current_level - previous_level
+
+            # Adjacent levels must change (no equals).
+            if delta == 0:
+                return False
+
+            # Differ by at least one and at most three.
+            if abs(delta) > 3:
+                return False
+
+            step_direction = 1 if delta > 0 else -1
+            if direction == 0:
+                direction = step_direction
+            # The levels are either all increasing or all decreasing.
+            elif step_direction != direction:
+                return False
+
+        return True
+
+
+def parse_reports(path):
+    reports = []
+    with open(path, "r", encoding="utf-8") as f:
+        for raw in f:
+            line = raw.strip()
+            if not line:
+                continue
+            reports.append(Report.from_line(line))
+    return reports
+
+
+def count_safe(reports):
+    safe_count = 0
+    for report in reports:
+        if report.is_safe():
+            safe_count += 1
+    return safe_count
+
+
+def count_safe_in_file(path):
+    return count_safe(parse_reports(path))
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser("CLI to read sample files.")
+    parser.add_argument(
+        "input",
+        help="Input file path.",
+    )
+
+    args = parser.parse_args(argv)
+    return count_safe_in_file(args.input)
+
+
+if __name__ == "__main__":
+    print(main())