day08 part2

src/day08.zig

@@ -0,0 +1,104 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day08.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.a});
+    std.debug.print("{d}\n", .{sln.b});
+}
+
+const Solution = struct {
+    a: usize,
+    b: usize,
+};
+
+const Step = struct {
+    left: [3]u8,
+    right: [3]u8,
+};
+
+fn gcd(a: usize, b: usize) usize {
+    return if (b == 0) a else gcd(b, a % b);
+}
+
+fn lcm(ns: []u32) usize {
+    var acc: usize = ns[0];
+    for (ns[1..]) |n| {
+        const nsize = @as(usize, n);
+        acc = acc / gcd(acc, nsize) * nsize;
+    }
+    return acc;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var states = std.AutoHashMap([3]u8, Step).init(alloc);
+    defer states.deinit();
+
+    var it = util.splitLines(input);
+    const steps = it.next().?;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        try states.put(line[0..3].*, .{ .left = line[7..10].*, .right = line[12..15].* });
+    }
+
+    var a_nodes = std.AutoHashMap([3]u8, Step).init(alloc);
+    defer a_nodes.deinit();
+    var state_it = states.iterator();
+    while (state_it.next()) |entry| {
+        if (entry.key_ptr[2] == 'A') {
+            try a_nodes.put(entry.key_ptr.*, entry.value_ptr.*);
+        }
+    }
+
+    var all_steps = std.ArrayList(u32).init(alloc);
+    defer all_steps.deinit();
+    var a_it = a_nodes.iterator();
+    while (a_it.next()) |start| {
+        std.debug.print("{s}\n", .{start.key_ptr});
+        var position: [3]u8 = start.key_ptr.*;
+        var count: u32 = 0;
+        find: while (position[2] != 'Z') : (count += 1) {
+            const curr_state = states.get(position) orelse break :find; // ???????
+            position = if (steps[count % steps.len] == 'L') curr_state.left else curr_state.right;
+        }
+        try all_steps.append(count);
+    }
+
+    return .{ .a = 2, .b = lcm(all_steps.items) };
+}
+
+test "silver" {
+    const input =
+        \\RL
+        \\
+        \\AAA = (BBB, CCC)
+        \\BBB = (DDD, EEE)
+        \\CCC = (ZZZ, GGG)
+        \\DDD = (DDD, DDD)
+        \\EEE = (EEE, EEE)
+        \\GGG = (GGG, GGG)
+        \\ZZZ = (ZZZ, ZZZ)
+    ;
+    _ = input;
+    // const sln = try solve(std.testing.allocator, input);
+    // try std.testing.expectEqual(@as(usize, 2), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\LR
+        \\
+        \\11A = (11B, XXX)
+        \\11B = (XXX, 11Z)
+        \\11Z = (11B, XXX)
+        \\22A = (22B, XXX)
+        \\22B = (22C, 22C)
+        \\22C = (22Z, 22Z)
+        \\22Z = (22B, 22B)
+        \\XXX = (XXX, XXX)
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 6), sln.b);
+}