#!/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
  cat input.txt | python day2/aoc_2024_day2.py
"""

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(lines):
    reports = []
    for raw in lines:
        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 main():
    parser = argparse.ArgumentParser("CLI to read sample files.")
    parser.add_argument("input", help="Input file path (defaults to stdin). Use '-' for stdin.")
    
    args = parser.parse_args()
    reports = parse_reports(args.input)

    print(count_safe(reports))
    return 0


if __name__ == "__main__":
    main()