private static int NumberOfShortestCombinations(int[] list, int initialGoal)
{
var memo = new SimpleMemo <ulong>();
var lengths = new SafeDictionary <int, int>();
Explore(list.Length, 0, initialGoal);
var shortestCombinations = lengths.OrderBy(x => x.Key).First().Value;
return(shortestCombinations);
void Explore(int size, uint pickmask, int goal)
{
if (goal == 0)
{
var len = pickmask.NumberOfSetBits();
lengths[len]++;
return;
}
if (goal < 0 || size == 0)
{
return;
}
if (memo.IsSeenBefore((ulong)size << 32 | pickmask))
{
return;
}
// For skipped item, pass 0 into the bit-mask (ie do nothing); for used item pass 1.
Explore(size - 1, pickmask << 1, goal);
Explore(size - 1, (pickmask << 1) + 1, goal - list[size - 1]);
}
}
protected override string Part2(string[] input)
{
var ids = input;
// Instead of doing the trivial N^2 set of comparisons (where each comparison
// would even need to step through all chars to find just one mismatch) do this:
// For every string, remember n versions of it with each letter, one at a time,
// swapped out with a dot. If we ever come across a similar string then we've
// found the single-letter-mismatch. The result is the string except that dot
// at the mismatched position - very nice.
var seen = new SimpleMemo <string>();
foreach (var id in ids)
{
var letters = id.ToCharArray();
for (var i = 0; i < letters.Length; i++)
{
var tmp = letters[i];
letters[i] = '.';
var s = new string(letters);
if (seen.IsSeenBefore(s))
{
// We found the mismatch
return(s.Replace(".", ""));
}
letters[i] = tmp;
}
}
throw new Exception("Not found");
}
private static int FindFirstCycle(int[] banks)
{
var memo = new SimpleMemo <ulong>();
var cycle = 0;
do
{
Redistribute(banks);
cycle++;
} while (!memo.IsSeenBefore(Hashing.KnuthHash(banks)));
return(cycle);
}
protected override int Part2(string[] input)
{
var deltas = input.Select(int.Parse).ToArray();
// Loop deltas to build frequency, until one is reached we've
// seen before. Using while(true) + for() is faster than %.
var seen = new SimpleMemo <int>();
var freq = 0;
while (true)
{
foreach (var d in deltas)
{
freq += d;
if (seen.IsSeenBefore(freq))
{
return(freq);
}
}
}
}
protected override int Part1(string[] input)
{
var claims = ReadClaims(input);
// Fastest way seem to be to simple hash memo-tables that register
// every square and therefore can count the first time a square is
// seen. It's faster than counting up all the overlappings and then
// searching for those with >1 overlap.
var once = new SimpleMemo <int>();
var twice = new SimpleMemo <int>();
var overlaps = 0;
foreach (var pos in claims.SelectMany(c => c.Squares()))
{
if (once.IsSeenBefore(pos) && !twice.IsSeenBefore(pos))
{
overlaps++;
}
}
return(overlaps);
}