From d968ed595b0e9c72417beaccd66e8e42d721ab24 Mon Sep 17 00:00:00 2001
From: Philippe Zwietering <philippe.zwietering@gmail.com>
Date: Wed, 22 Dec 2021 01:51:39 +0100
Subject: [PATCH] Advent of code day 6 finished - learned a lot today

---
 advent_of_code/d6/main.cpp | 111 +++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 advent_of_code/d6/main.cpp

diff --git a/advent_of_code/d6/main.cpp b/advent_of_code/d6/main.cpp
new file mode 100644
index 0000000..0abead5
--- /dev/null
+++ b/advent_of_code/d6/main.cpp
@@ -0,0 +1,111 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+typedef unsigned long long ll;
+
+size_t reproduction(int stage, int days){
+    vector<int> fishies(1, stage);
+    vector<int> new_fishies;
+
+    for(int i = 0; i < days; ++i){
+        new_fishies.clear();
+
+        for(auto& fishie : fishies){
+            if(fishie == 0){
+                new_fishies.push_back(6);
+                new_fishies.push_back(8);
+            } else{
+                new_fishies.push_back(--fishie);
+            }
+        }
+
+        fishies.clear();
+        fishies = new_fishies;
+
+        // cout << "Day " << i << ": ";
+        // for(auto fishie : new_fishies){
+        //     cout << fishie << " ";
+        // } cout << endl;
+    }
+
+    return fishies.size();
+}
+
+ll smart_reproduce(int days, const array<ll, 9> &start){
+    array<ll, 9> old_fishies = start;
+    array<ll, 9> new_fishies;
+
+    while(days > 0){
+        for(size_t i = 0; i < old_fishies.size(); ++i){
+            if(i == 8){
+                new_fishies[i] = old_fishies[0];
+            } else if(i == 6){
+                new_fishies[i] = old_fishies[i + 1] + old_fishies[0];
+            } else{
+                new_fishies[i] = old_fishies[i + 1];
+            }
+        }
+
+        old_fishies = new_fishies;
+        days--;
+    }
+
+    ll result = 0;
+    for(auto &fishie : old_fishies){
+        result += fishie;
+    }
+
+    return result;
+}
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+
+    vector<int> input_fishies = {5,1,1,3,1,1,5,1,2,1,5,2,5,1,1,1,4,1,1,5,1,1,4,1,1,1,3,5,1,1,1,1,1,1,1,1,1,4,4,4,1,1,1,1,1,4,1,1,1,1,1,5,1,1,1,4,1,1,1,1,1,3,1,1,4,1,4,1,1,2,3,1,1,1,1,4,1,2,2,1,1,1,1,1,1,3,1,1,1,1,1,2,1,1,1,1,1,1,1,4,4,1,4,2,1,1,1,1,1,4,3,1,1,1,1,2,1,1,1,2,1,1,3,1,1,1,2,1,1,1,3,1,3,1,1,1,1,1,1,1,1,1,3,1,1,1,1,3,1,1,1,1,1,1,2,1,1,2,3,1,2,1,1,4,1,1,5,3,1,1,1,2,4,1,1,2,4,2,1,1,1,1,1,1,1,2,1,1,1,1,1,1,1,1,4,3,1,2,1,2,1,5,1,2,1,1,5,1,1,1,1,1,1,2,2,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,4,1,1,1,1,1,3,1,1,5,1,1,1,1,5,1,4,1,1,1,4,1,3,4,1,4,1,1,1,1,1,1,1,1,1,3,5,1,3,1,1,1,1,4,1,5,3,1,1,1,1,1,5,1,1,1,2,2};
+    // vector<int> input_fishies = {3,4,3,1,2};
+    array<ll, 9> reproduction_numbers;
+    reproduction_numbers.fill(0);
+
+    ll result = 0;
+    for(auto &fishie : input_fishies){
+        if(reproduction_numbers[fishie] == 0){
+            int rn = reproduction(fishie, 80);
+            result += rn;
+            reproduction_numbers[fishie] = rn;
+        } else{
+            result += reproduction_numbers[fishie];
+        }
+    }
+
+    cout << result << endl;
+
+    // Now do the exact same but with a bigger number of days
+    
+    // Ok this doesn't really work out, it uses up too much memory, prolly because it's pretty exponential
+    // So it's probably better to do this with a bottoms up approach
+    
+    // Doing a problem analysis with example I came to the conclusion that you only need to calculate one 
+    // starting point, with 0 then being the obvious choice. The rest can be derived from that
+    
+    // Also, I realized that instead of keeping track of all lanternfish numbers in a vector,
+    // I can just use a map or array or vector tracking the number of fishes with the index indicating the stage
+    // This reduces the memory size drastically, while still being just iterative
+    
+    // First let's test the new method: works just dandy
+    // Only problem now is overflows I think
+
+    array<ll, 9> start = {0, };
+    for(auto &fish : input_fishies){
+        start[fish]++;
+    }
+
+    result = smart_reproduce(256, start);
+
+    cout << result << endl;
+
+    cout << flush;
+
+    return 0;
+}
\ No newline at end of file