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