aoc2025/day2/day2b.zig

const std = @import("std");

const allocator = std.heap.page_allocator;

fn isRepeatedSequence(sequence: []const u8, prefix: []const u8) !bool {
    if (sequence.len == 0) {
        return true;
    }

    if (!std.mem.startsWith(u8, sequence, prefix)) {
        return false;
    }

    return isRepeatedSequence(sequence[prefix.len..], prefix);
}

fn isSillyId(idText: []const u8) !bool {
    for (1..idText.len/2+1) |prefixLen| {
        if (try isRepeatedSequence(idText[prefixLen..], idText[0..prefixLen])) {
            return true;
        }
    }

    return false;
}

pub fn main() !void {
    var printBuf: [1024]u8 = undefined;

    const inputFile = try std.fs.cwd().openFile(
        "input.txt",
        .{
            .mode = .read_only,
        },
    );
    defer inputFile.close();

    var inputFileBuffer: [1024]u8 = undefined;
    var inputFileReader = inputFile.reader(&inputFileBuffer);
    const inputFileSize = try inputFileReader.getSize();

    const inputBuffer = try allocator.alloc(u8, inputFileSize);
    try inputFileReader.interface.readSliceAll(inputBuffer);

    var counter:u64 = 0;

    var it = std.mem.tokenizeAny(u8, inputBuffer, ",\n");
    while (it.next()) |token| {
        const dashPos = std.mem.indexOf(u8, token, "-").?;
        const startRangeText = token[0..dashPos];
        const endRangeText = token[dashPos+1..];

        const startRange = try std.fmt.parseInt(u64, startRangeText, 10);
        const endRange = try std.fmt.parseInt(u64, endRangeText, 10);

        for (startRange..endRange+1) |id| {
            const idText = try std.fmt.bufPrint(&printBuf, "{d}", .{id});
            if (try isSillyId(idText)) {
                counter += id;
            }
        }
    }

    try std.fs.File.stdout().writeAll(
      try std.fmt.bufPrint(&printBuf, "{}\n", .{counter}));
}