Dart

Well, that was certainly a bit easier than yesterday...

I know running a window over each full list of 2000 prices rather than looking for cycles etc means I'm doing a lot of unnecessary work, but it only takes a couple of seconds, so that'll do.

import 'package:collection/collection.dart';
import 'package:more/more.dart';

rng(int i) {
  i = ((i << 6) ^ i) % 16777216;
  i = ((i >> 5) ^ i) % 16777216;
  i = ((i << 11) ^ i) % 16777216;
  return i;
}

Iterable<int> getPrices(int val, int rounds) {
  var ret = [val];
  for (var _ in 1.to(rounds)) {
    ret.add(val = rng(val));
  }
  return ret.map((e) => e % 10);
}

int run(int val, int rounds) => 0.to(rounds).fold(val, (s, t) => s = rng(s));
part1(lines) => [for (var i in lines.map(int.parse)) run(i, 2000)].sum;

part2(List<String> lines) {
  var market = <int, int>{}.withDefault(0);
  for (var seed in lines.map(int.parse)) {
    var seen = <int>{};
    for (var w in getPrices(seed, 2000).window(5)) {
      var key = // Can't use lists as keys, so make cheap hash.
          w.window(2).map((e) => e[1] - e[0]).reduce((s, t) => (s << 4) + t);
      if (seen.contains(key)) continue;
      seen.add(key);
      market[key] += w.last;
    }
  }
  return market.values.max;
}