Day 8. After doing AoC for a number of years, I pretty much knew what to do here for Part 2

day8/main.zig

@@ -0,0 +1,91 @@
+const std = @import("std");
+
+pub fn main() !void {
+    const content = @embedFile("data.txt");
+    var lines = std.mem.split(u8, content, "\n");
+
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    const allocator = gpa.allocator();
+
+    const instructions = lines.next().?;
+    _ = lines.next();
+
+    var map = std.StringArrayHashMap([]const u8).init(allocator);
+    defer map.deinit();
+
+    while (lines.next()) |line| {
+        if (line.len == 0) {
+            break;
+        }
+
+        try map.put(line[0..3], line[7..15]);
+    }
+
+    var i: u16 = 0;
+    var cur = try allocator.dupe(u8, "AAA");
+    while (true) : (i += 1) {
+        if (std.mem.eql(u8, cur, "ZZZ")) {
+            break;
+        }
+        const ins = instructions[i % instructions.len];
+        const nodes = map.get(cur).?;
+        if (ins == 'L') {
+            std.mem.copy(u8, cur, nodes[0..3]);
+        } else {
+            std.mem.copy(u8, cur, nodes[5..8]);
+        }
+    }
+
+    std.debug.print("Part 1: {d}\n", .{i});
+
+    // Part 2
+    var nodes = std.ArrayList([]const u8).init(allocator);
+    defer nodes.deinit();
+
+    var steps = std.ArrayList(u32).init(allocator);
+    defer steps.deinit();
+
+    // Find starting nodes
+    for (map.keys()) |k| {
+        if (k[2] == 'A') {
+            try nodes.append(k);
+        }
+    }
+
+    // for each starting node, how long until we get to a Z-ending node
+    for (nodes.items) |node| {
+        var j: u32 = 0;
+        cur = try allocator.dupe(u8, node);
+        while (true) : (j += 1) {
+            // std.debug.print("checking node: {s}\n", .{cur});
+            if (cur[2] == 'Z') {
+                break;
+            }
+            const ins = instructions[j % instructions.len];
+
+            const next = map.get(cur).?;
+            if (ins == 'L') {
+                std.mem.copy(u8, cur, next[0..3]);
+            } else {
+                std.mem.copy(u8, cur, next[5..8]);
+            }
+        }
+        try steps.append(j);
+    }
+
+    var lowest: u64 = steps.items[0];
+    for (0..steps.items.len - 1) |j| {
+        lowest = lcm(lowest, steps.items[j + 1]).?;
+    }
+    std.debug.print("Part 2: {d}\n", .{lowest});
+}
+
+fn lcm(left: u64, right: u64) ?u64 {
+    var i: u64 = @max(left, right);
+    while (true) : (i += @max(left, right)) {
+        if (i % left == 0 and i % right == 0) {
+            return @as(u64, @intCast(i));
+        }
+    }
+    return null;
+}