advent-of-code-2023/day9/main.zig

const std = @import("std");

pub fn main() !void {
    const content = @embedFile("data.txt");
    var lines = std.mem.split(u8, content, "\n");

    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();

    var list = std.ArrayList(std.ArrayList(i32)).init(allocator);
    defer list.deinit();

    while (lines.next()) |line| {
        if (line.len == 0) {
            break;
        }
        var history = std.ArrayList(i32).init(allocator);
        var nums = std.mem.split(u8, line, " ");
        while (nums.next()) |num| {
            const parsed = try std.fmt.parseInt(i32, num, 10);
            try history.append(parsed);
        }

        try list.append(history);
    }

    var part1: i32 = 0;
    var part2: i32 = 0;

    for (list.items) |history| {
        var derived = std.ArrayList(std.ArrayList(i32)).init(allocator);
        try derived.append(history);

        var cur = history;
        while (true) {
            var d = try derive(cur, allocator);
            try derived.append(d);
            cur = d;

            var allZero = true;
            for (d.items) |item| {
                if (item != 0) {
                    allZero = false;
                }
            }
            if (allZero) {
                break;
            }
        }

        var newItem: i32 = 0;
        var newItem2: i32 = 0;
        for (derived.items, 0..) |_, j| {
            // part 1
            const idx = derived.items.len - j - 1;
            const lastItem = derived.items[idx].getLast();
            newItem = lastItem + newItem;

            // part 2
            const firstItem = derived.items[idx].items[0];
            newItem2 = firstItem - newItem2;
        }

        part1 += newItem;
        part2 += newItem2;
    }

    std.debug.print("Part 1: {d}\n", .{part1});
    std.debug.print("Part 2: {d}\n", .{part2});
}

fn derive(history: std.ArrayList(i32), allocator: std.mem.Allocator) !std.ArrayList(i32) {
    var list = std.ArrayList(i32).init(allocator);
    for (0..history.items.len - 1) |idx| {
        try list.append(history.items[idx + 1] - history.items[idx]);
    }
    return list;
}