1const std = @import("std");
 2
 3pub fn pt1(input: []const u8) u64 {
 4    var pointer: i64 = 50;
 5    var counter: u64 = 0;
 6
 7    var lines = std.mem.splitScalar(u8, input, '\n');
 8
 9    while (lines.next()) |l| {
10        if (l.len == 0) {
11            // this happens at the end so we are safe to return here.
12            return counter;
13        }
14
15        const op = l[0];
16        const rotation = std.fmt.parseInt(i64, l[1..], 10) catch undefined;
17
18        if (op == 'R') {
19            pointer = @mod(pointer + rotation, 100);
20        } else {
21            pointer = @mod(pointer - rotation, 100);
22        }
23
24        if (pointer == 0) {
25            counter += 1;
26        }
27    }
28
29    return counter;
30}
31
32
33const operator = enum { left, right };
34pub fn pt2(input: []const u8) u64 {
35    var pointer: i64 = 50;
36    var counter: u64 = 0;
37
38    var lines = std.mem.splitScalar(u8, input, '\n');
39
40    while (lines.next()) |l| {
41        if (l.len == 0) {
42            // this happens at the end so we are safe to return here.
43            return counter;
44        }
45
46        const op = l[0];
47        const rotation = std.fmt.parseInt(i64, l[1..], 10) catch undefined;
48
49        if (op == 'R') {
50            for (0..@intCast(rotation)) |_| {
51                pointer = step(pointer, .right);
52                if (pointer == 0) counter += 1;
53            }
54        } else {
55            for (0..@intCast(rotation)) |_| {
56                pointer = step(pointer, .left);
57                if (pointer == 0) counter += 1;
58            }
59        }
60    }
61
62    return counter;
63}
64
65fn step(p: i64, comptime o: operator) i64 {
66    switch (o) {
67        operator.left => {
68            if (p == 99) return 0;
69            return p + 1;
70        },
71        operator.right => {
72            if (p == 0) return 99;
73            return p - 1;
74        },
75    }
76
77    return p;
78}
79
80test "Day 1 part 1" {
81    const res = pt1(@embedFile("./input/day1"));
82    try std.testing.expect(res == 1165);
83}
84
85
86test "Day 1 part 2" {
87    const res = pt2(@embedFile("./input/day1"));
88    try std.testing.expect(res == 6496);
89}