1 files changed, 58 insertions, 0 deletions
diff --git a/src/25/main.zig b/src/25/main.zig
new file mode 100644
index 0000000..0008550
--- /dev/null
+++ b/src/25/main.zig
@@ -0,0 +1,58 @@
+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", "" });