adventofcode2025/day4/zig/main.zig

const std = @import("std");

const Tile = enum { empty, paper };

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

    var stdin_buffer: [1024]u8 = undefined;
    var stdin_reader = std.fs.File.stdin().reader(&stdin_buffer);

    const stdin = &stdin_reader.interface;

    const allocator = gpa.allocator();
    var rows = try std.ArrayList(std.ArrayList(Tile)).initCapacity(allocator, 1);
    defer rows.deinit(allocator);

    while (try stdin.takeDelimiter('\n')) |line| {
        var row = try std.ArrayList(Tile).initCapacity(allocator, 1);
        for (line) |c| {
            switch (c) {
                '.' => {
                    try row.append(allocator, Tile.empty);
                },
                '@' => {
                    try row.append(allocator, Tile.paper);
                },
                else => unreachable,
            }
        }
        try rows.append(allocator, row);
    }

    var task1: usize = 0;

    var i: isize = 0;
    while (i < rows.items.len) : (i += 1) {
        var j: isize = 0;
        while (j < rows.items[0].items.len) : (j += 1) {
            const i_u: usize = @intCast(i);
            const j_u: usize = @intCast(j);
            if (rows.items[i_u].items[j_u] != Tile.paper) {
                continue;
            }

            const paper_count = countNeighbors(j, i, rows);

            if (paper_count < 4) {
                task1 += 1;
            }
        }
    }

    var removed_count: usize = 1;
    var task2: usize = 0;

    while (removed_count > 0) {
        removed_count = 0;
        var ii: isize = 0;
        while (ii < rows.items.len) : (ii += 1) {
            var j: isize = 0;
            while (j < rows.items[0].items.len) : (j += 1) {
                const i_u: usize = @intCast(ii);
                const j_u: usize = @intCast(j);
                if (rows.items[i_u].items[j_u] != Tile.paper) {
                    continue;
                }

                const paper_count = countNeighbors(j, ii, rows);

                if (paper_count < 4) {
                    task2 += 1;
                    rows.items[i_u].items[j_u] = Tile.empty;
                    removed_count += 1;
                }
            }
        }
    }

    for (rows.items) |*row| {
        row.deinit(allocator);
    }

    std.debug.print("{d}\n", .{task1});
    std.debug.print("{d}\n", .{task2});
}

fn countNeighbors(x: isize, y: isize, rows: std.ArrayList(std.ArrayList(Tile))) usize {
    var paper_count: usize = 0;
    var i: isize = -1;
    while (i < 2) : (i += 1) {
        var j: isize = -1;
        while (j < 2) : (j += 1) {
            if (i == 0 and j == 0) {
                continue;
            }
            const x_new = x + i;
            const y_new = y + j;

            if (x_new < 0 or x_new >= rows.items[0].items.len or y_new < 0 or y_new >= rows.items.len) {
                continue;
            }

            const y_new_u: usize = @intCast(y_new);
            const z_new_u: usize = @intCast(x_new);
            if (rows.items[y_new_u].items[z_new_u] == Tile.paper) {
                paper_count += 1;
            }
        }
    }

    return paper_count;
}