2024 Day 22 Part 2

2024/src/day22.zig

@@ -27,24 +27,96 @@ pub fn main() !void {
     var bw = std.io.bufferedWriter(std.io.getStdOut().writer());
     const stdout = bw.writer();
 
-    try stdout.print("{}\n", .{part1(seeds.items)});
+    try stdout.print("{}\n{}\n", try solve(allocator, seeds.items));
     try bw.flush();
 }
 
-fn part1(seeds: []u64) u64 {
+fn solve(allocator: Allocator, seeds: []u64) !struct { u64, u64 } {
+    var totals = Map([4]i64, u64).init(allocator);
+    defer totals.deinit();
+
     var sum: u64 = 0;
     for (seeds) |seed| {
-        sum += simulate(seed, 2000);
+        var profit = Map([4]i64, u64).init(allocator);
+        defer profit.deinit();
+
+        sum += try populate(seed, 2000, &profit);
+
+        var it = profit.iterator();
+        while (it.next()) |entry| {
+            const changes, const bananas = .{ entry.key_ptr.*, entry.value_ptr.* };
+
+            const total = try totals.getOrPutValue(changes, 0);
+            total.value_ptr.* += bananas;
+        }
     }
-    return sum;
+
+    var best: u64 = 0;
+    var it = totals.valueIterator();
+    while (it.next()) |p| {
+        best = @max(best, p.*);
+    }
+    return .{ sum, best };
 }
 
-fn simulate(seed: u64, steps: usize) u64 {
+fn populate(seed: u64, steps: usize, profit: *Map([4]i64, u64)) !u64 {
     var n: u64 = seed;
-    for (0..steps) |_| {
+    var prices = [_]u64{0} ** 4;
+
+    // Pre-fill the first few updates, since those don't have enough changes yet.
+    for (0..3) |i| {
+        const price = n % 10;
+        prices[i] = price;
+
         n = ((n * 64) ^ n) % 16777216;
         n = ((n / 32) ^ n) % 16777216;
         n = ((n * 2048) ^ n) % 16777216;
     }
+
+    var changes = [_]i64{0} ** 4;
+    for (1..4) |i| {
+        const next: i64 = @intCast(prices[i]);
+        const prev: i64 = @intCast(prices[i - 1]);
+        changes[i] = next - prev;
+    }
+
+    // This is the first time we have a usable change sequence.
+    _ = try profit.getOrPutValue(changes, prices[3]);
+
+    for (3..steps) |_| {
+        n = ((n * 64) ^ n) % 16777216;
+        n = ((n / 32) ^ n) % 16777216;
+        n = ((n * 2048) ^ n) % 16777216;
+
+        const price = n % 10;
+
+        std.mem.copyForwards(u64, &prices, prices[1..]);
+        prices[3] = price;
+
+        std.mem.copyForwards(i64, &changes, changes[1..]);
+        {
+            const a: i64 = @intCast(prices[3]);
+            const b: i64 = @intCast(prices[2]);
+            changes[3] = a - b;
+        }
+        _ = try profit.getOrPutValue(changes, prices[3]);
+    }
+
+    // Everything but the update, because we *can* still sell here.
+    {
+        const price = n % 10;
+
+        std.mem.copyForwards(u64, &prices, prices[1..]);
+        prices[3] = price;
+
+        std.mem.copyForwards(i64, &changes, changes[1..]);
+        {
+            const a: i64 = @intCast(prices[3]);
+            const b: i64 = @intCast(prices[2]);
+            changes[3] = a - b;
+        }
+        _ = try profit.getOrPutValue(changes, prices[3]);
+    }
+
     return n;
 }