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Day18.cs
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Day18.cs
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using AdventOfCode.CSharp.Common;
using System;
using System.Numerics;
using System.Runtime.CompilerServices;
namespace AdventOfCode.CSharp.Y2021.Solvers;
public class Day18 : ISolver
{
public static void Solve(ReadOnlySpan<byte> input, Solution solution)
{
const int MaxSnailfishCount = 128;
Span<byte> snailFishes = stackalloc byte[16 * MaxSnailfishCount];
snailFishes.Fill(255); // 255 indicates that number is empty
int snailFishCount = ParseAllSnailfish(input, snailFishes);
Span<byte> snailfishSum = stackalloc byte[16];
snailFishes.Slice(0, 16).CopyTo(snailfishSum);
int part2 = 0;
for (int i = 1; i < snailFishCount; i++)
{
// Part 1
Span<byte> snailFish = snailFishes.Slice(i * 16, 16);
AddSnailfish(snailfishSum, snailFish);
// Part 2
for (int j = 0; j < i; j++)
{
Span<byte> otherSnailfish = snailFishes.Slice(j * 16, 16);
int magnitude1 = AddAndGetMagnitude(snailFish, otherSnailfish);
if (magnitude1 > part2)
part2 = magnitude1;
int magnitude2 = AddAndGetMagnitude(otherSnailfish, snailFish);
if (magnitude2 > part2)
part2 = magnitude2;
}
}
int part1 = GetMagnitude(snailfishSum);
solution.SubmitPart1(part1);
solution.SubmitPart2(part2);
}
private static int AddAndGetMagnitude(Span<byte> fish1, Span<byte> fish2)
{
Span<byte> part2SnailfishSum = stackalloc byte[16];
fish1.CopyTo(part2SnailfishSum);
AddSnailfish(part2SnailfishSum, fish2);
return GetMagnitude(part2SnailfishSum);
}
private static int ParseAllSnailfish(ReadOnlySpan<byte> input, Span<byte> snailFishes)
{
int snailFishCount = 0;
int inputIndex = 0;
while (inputIndex < input.Length)
{
ParseSnailfishLine(input, ref inputIndex, snailFishes.Slice(16 * snailFishCount++, 16));
inputIndex++; // skip newline
}
return snailFishCount;
}
private static void ParseSnailfishLine(ReadOnlySpan<byte> input, ref int inputIndex, Span<byte> snailfish)
{
byte c = input[inputIndex++];
if (c == '[')
{
int halfLen = snailfish.Length / 2;
ParseSnailfishLine(input, ref inputIndex, snailfish.Slice(0, halfLen));
inputIndex++; // skip comma
ParseSnailfishLine(input, ref inputIndex, snailfish.Slice(halfLen));
inputIndex++; // skip end bracket
}
else
{
snailfish[0] = (byte)(c - '0');
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static void AddSnailfish(Span<byte> fish, ReadOnlySpan<byte> fishToAdd)
{
AddSnailfishWithExplosions(fish, fishToAdd, out int fishBitset, out int needsSplittingBitset);
while (needsSplittingBitset != 0)
{
// Get index of next number that needs splitting
// See https://lemire.me/blog/2018/02/21/iterating-over-set-bits-quickly/ for explanation
int t = needsSplittingBitset & -needsSplittingBitset;
int indexToSplit = BitOperations.TrailingZeroCount(t);
needsSplittingBitset ^= t;
SplitAtIndex(fish, indexToSplit, ref fishBitset, ref needsSplittingBitset);
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static void AddSnailfishWithExplosions(Span<byte> fish, ReadOnlySpan<byte> fishToAdd, out int fishBitset, out int needsSplittingBitset)
{
byte carryRight = 0;
int leftIndex = 255;
fishBitset = 0;
needsSplittingBitset = 0;
// Add first fish
for (int i = 0; i < 8; i++)
{
byte left = fish[i * 2];
if (left != 255)
{
left += carryRight;
carryRight = 0;
byte right = fish[i * 2 + 1];
if (right != 255)
{
if (leftIndex != 255)
{
int newAmount = fish[leftIndex] += left;
if (newAmount >= 10)
needsSplittingBitset |= 1 << leftIndex;
}
left = 0;
carryRight = right;
}
fishBitset |= 1 << i;
leftIndex = i;
if (left >= 10)
needsSplittingBitset |= 1 << i;
}
fish[i] = left;
}
// Add second fish
for (int i = 0; i < 8; i++)
{
int fishIndex = i + 8;
byte left = fishToAdd[i * 2];
if (left != 255)
{
left += carryRight;
carryRight = 0;
byte right = fishToAdd[i * 2 + 1];
if (right != 255)
{
if (leftIndex != 255)
{
int newAmount = fish[leftIndex] += left;
if (newAmount >= 10)
needsSplittingBitset |= 1 << leftIndex;
}
left = 0;
carryRight = right;
}
fishBitset |= 1 << fishIndex;
leftIndex = fishIndex;
if (left >= 10)
needsSplittingBitset |= 1 << fishIndex;
}
fish[fishIndex] = left;
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static void SplitAtIndex(Span<byte> fish, int i, ref int fishBitset, ref int needsSplittingBitset)
{
byte number = fish[i];
byte splitLeft = (byte)(number / 2);
byte splitRight = (byte)(number - splitLeft);
if ((i & 1) == 1 || (fishBitset & (1 << (i + 1))) != 0)
{
// Handle where split causes an explosion
fish[i] = 0;
// If the explosion causes a number to the left to go past 10 then we need to set i back to that point
for (int j = i - 1; j >= 0; j--)
{
if ((fishBitset & (1U << j)) != 0)
{
int newAmount = fish[j] += splitLeft;
if (newAmount >= 10)
needsSplittingBitset |= 1 << j;
break;
}
}
for (int j = i + 1; j < fish.Length; j++)
{
if ((fishBitset & (1U << j)) != 0)
{
int newAmount = fish[j] += splitRight;
if (newAmount >= 10)
needsSplittingBitset |= 1 << j;
break;
}
}
}
else
{
int distanceToNextFish = 2;
while ((i & distanceToNextFish) == 0 && (fishBitset & (1 << (i + distanceToNextFish))) == 0)
distanceToNextFish *= 2;
fish[i] = splitLeft;
int splitRightIndex = i + distanceToNextFish / 2;
fish[splitRightIndex] = splitRight;
fishBitset |= 1 << splitRightIndex;
// If the new left value is still greater than 10, we need to keep splitting it
if (splitLeft >= 10)
needsSplittingBitset |= 1 << i;
if (splitRight >= 10)
needsSplittingBitset |= 1 << splitRightIndex;
}
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static int GetMagnitude(Span<byte> snailFish)
{
static int GetMagnitudeInternal(Span<byte> snailFish, int firstNumber, int from, int to)
{
if (from + 1 == to)
return firstNumber;
int halfLen = from + (to - from) / 2;
int left = GetMagnitudeInternal(snailFish, firstNumber, from, halfLen);
int rightStart = snailFish[halfLen];
return rightStart == 255
? left
: 3 * left + 2 * GetMagnitudeInternal(snailFish, rightStart, halfLen, to);
}
return GetMagnitudeInternal(snailFish, snailFish[0], 0, 16);
}
}