aoc2021day7.zig

const std = @import("std");
const mem = std.mem;
const fs = std.fs;
const sort = std.sort;
const print = std.debug.print;

fn getLines(path: []const u8, allocator: mem.Allocator) !std.ArrayList(u32) {
    const file = try fs.cwd().openFile(path, .{});
    defer file.close();

    var buf_reader = std.io.bufferedReader(file.reader());
    const reader = buf_reader.reader();

    var nums = std.ArrayList(u32).init(allocator);

    var line = std.ArrayList(u8).init(allocator);
    defer line.deinit();

    const writer = line.writer();
    try reader.streamUntilDelimiter(writer, '\n', null);

    var it = mem.split(u8, line.items, ",");
    while (it.next()) |str| {
        try nums.append(try std.fmt.parseInt(u32, str, 10));
    }

    return nums;
}

fn part1(nums: std.ArrayList(u32)) !void {
    var res: u32 = 0;

    mem.sort(u32, nums.items, {}, sort.asc(u32));

    const median = nums.items[nums.items.len / 2];

    for (nums.items) |num| {
        res += if (num > median) num - median else median - num;
    }

    print("Part1: {d}\n", .{res});
}

fn part2(nums: std.ArrayList(u32)) !void {
    var res: usize = 0;

    var sum: u32 = 0;
    for (nums.items) |num| {
        sum += num;
    }
    const average = sum / nums.items.len;

    for (nums.items) |num| {
        var cost: usize = 0;
        const to_move = if (num > average) num - average else average - num;
        for (0..to_move + 1) |i| {
            cost += i;
        }
        res += cost;
    }

    print("part2: {d}\n", .{res});
}

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer _ = gpa.deinit();
    const alloc = gpa.allocator();

    const filename = "day07.txt";
    const input = try getLines(filename, alloc);
    defer input.deinit();

    try part1(input);
    try part2(input);
}