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const std = @import("std");
const aoc = @import("aoc");
fn transform(subject_num: u64, loops: u64) u64 {
var num: u64 = 1;
var i: u64 = 0;
while (i < loops) : (i += 1) {
num *= subject_num;
num %= 20201227;
}
return num;
}
fn bfLoops(subject_num: u64, pubkey: u64) ?u64 {
var i: u64 = 0;
var tmp: u64 = 1;
while (i < ~@as(u64, 0)) : (i += 1) {
if (tmp == pubkey) return i;
tmp *= subject_num;
tmp %= 20201227;
}
return null;
}
fn parseInput(door_pubkey: *u64, card_pubkey: *u64, input: []const u8) !void {
var lineit = std.mem.tokenize(u8, input, "\n");
door_pubkey.* = try std.fmt.parseInt(u64, try aoc.unwrap(lineit.next()), 10);
card_pubkey.* = try std.fmt.parseInt(u64, try aoc.unwrap(lineit.next()), 10);
}
fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
var door_pubkey: u64 = undefined;
var card_pubkey: u64 = undefined;
try parseInput(&door_pubkey, &card_pubkey, input);
if (args.len == 0 or std.mem.eql(u8, args[0], "bf_door")) {
if (bfLoops(7, door_pubkey)) |door_loops| {
std.debug.print("{}\n", .{transform(card_pubkey, door_loops)});
return null;
}
} else if (args.len > 0 and std.mem.eql(u8, args[0], "bf_card")) {
if (bfLoops(7, card_pubkey)) |card_loops| {
const answer = transform(door_pubkey, card_loops);
return try std.fmt.allocPrint(allocator, "{}", .{answer});
}
}
return null;
}
fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
_ = input;
_ = args;
return try std.fmt.allocPrint(allocator, "", .{});
}
pub const main = aoc.main(part1, part2, .{ "15217943", "" });
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