import * as fs from "node:fs/promises";

export function readInput(filename: string): Promise<number[][]> {
    return fs
        .readFile(filename, { encoding: "utf8" })
        .then((lines: string) => lines.split(/\n/))
        .then((lines: string[]) =>
            lines.filter((x) => x).map((x) =>
                x.split(/ +/).map((x) => Number(x))
            )
        );
}

export function isSafe(report: readonly number[]): boolean {
    const r = [ ...report ];

    const asc = r[1] - r[0];

    let level = r.shift();

    while (r.length) {
        const next = r.shift();
        const delta = next! - level!;
        if (delta * asc < 0) return false;
        if (Math.abs(delta) < 1 || Math.abs(delta) > 3) return false;
        level = next;
    }

    return true;
}

export function solve(reports: number[][]): number {
    return reports.filter(isSafe).length;
}