aoc2025: day 1

1 files changed, 95 insertions, 0 deletions

diff --git a/aoc25/src/day1.zig b/aoc25/src/day1.zig
new file mode 100644
index 0000000..74cb2a4
--- /dev/null
+++ b/aoc25/src/day1.zig
@@ -0,0 +1,95 @@
+const std = @import("std");
+
+const test_input =
+    \\L68
+    \\L30
+    \\R48
+    \\L5
+    \\R60
+    \\L55
+    \\L1
+    \\L99
+    \\R14
+    \\L82
+;
+
+test part1 {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    try std.testing.expectEqual(3, try part1(allocator, try parse(allocator, test_input)));
+}
+
+test part2 {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    try std.testing.expectEqual(6, try part2(allocator, try parse(allocator, test_input)));
+}
+
+test "part2 a" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    try std.testing.expectEqual(1, try part2(allocator, try parse(allocator,
+        \\L50
+    )));
+}
+
+
+const Direction = enum { left, right };
+
+const Input = []struct {
+    direction: Direction,
+    amount: u32,
+};
+
+pub fn parse(allocator: std.mem.Allocator, input: []const u8) !Input {
+    var lines = std.mem.splitScalar(u8, input, '\n');
+
+    var rotations = try std.ArrayList(@typeInfo(Input).pointer.child).initCapacity(allocator, 0);
+
+    while (lines.next()) |line| {
+        if (line.len == 0) continue;
+        const direction: Direction = switch (line[0]) {
+            'L' => .left,
+            'R' => .right,
+            else => return error.InvalidDirection,
+        };
+        const amount = try std.fmt.parseInt(u32, line[1..], 10);
+        try rotations.append(allocator, .{ .direction = direction, .amount = amount });
+    }
+
+    return rotations.toOwnedSlice(allocator);
+}
+
+pub fn part1(_: std.mem.Allocator, input: Input) !u32 {
+    var dial: i32 = 50;
+    var password: u32 = 0;
+    for (input) |rotation| {
+        dial += switch (rotation.direction) {
+            .left => -@as(i32, @intCast(rotation.amount)),
+            .right => @intCast(rotation.amount),
+        };
+        dial = @mod(dial, 100);
+        if (dial == 0) password += 1;
+    }
+    return password;
+}
+
+pub fn part2(_: std.mem.Allocator, input: Input) !u32 {
+    var dial: i32 = 50;
+    var password: u32 = 0;
+    for (input) |rotation| {
+        // Smoothbrain solution right here
+        for (0..rotation.amount) |_| {
+            dial += switch (rotation.direction) { .left => -1, .right => 1 };
+            dial = @mod(dial, 100);
+            if (dial == 0) password += 1;
+        }
+    }
+    return password;
+}