From bb4164c89229c34d10dbc02a92d93ca648455f85 Mon Sep 17 00:00:00 2001
From: Patrick Collins <54278053+PatrickAlphaC@users.noreply.github.com>
Date: Wed, 29 Jun 2022 19:31:57 -0400
Subject: [PATCH] Added NFT Chance Array math

---
 chronological-updates.md | 42 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/chronological-updates.md b/chronological-updates.md
index 69fdde4..4149c16 100644
--- a/chronological-updates.md
+++ b/chronological-updates.md
@@ -88,3 +88,45 @@ If your keepers are not kicking off, run through this checklist to find out why.
 ## Metamask Troubleshooting while using Gitpod
 
 - Remember if you are using gitpod then you cannot connect your local hardhat node with metamask. To resolve this you can use vs code or testnets instead of local node.
+
+
+--------
+
+# Lesson 14
+
+## Math on NFT Chance Array
+
+The ```getBreedFromModdedRng()``` function in RandomIpfsNft.sol gets the math wrong.
+
+line 104,
+```solidity
+if (moddedRng >= cumulativeSum && moddedRng < cumulativeSum + chanceArray[i]) {
+    return Breed(i);
+}
+cumulativeSum = cumulativeSum + chanceArray[i];
+```
+needs to be changed to 
+```solidity
+if (moddedRng >= cumulativeSum && moddedRng < chanceArray[i]) {
+    return Breed(i);
+}
+cumulativeSum = chanceArray[i];
+```
+
+The chanceArray[] array already has a cumulative probability distribution, no need to keep adding cumulativeSum to it.
+
+According to the current algorithm:
+```
+moddedRng produced is between 0 and 99
+chanceArray=[10,30,100]
+```
+```
+PUG is produced if moddedRng is between [0,10)  = span of 10
+SHIBA is produce if moddedRng is between [10,40) = span of 30
+BERNARD is produced if moddedRng is between [40,140] = span of 60 (since moddedRng<100)
+```
+So the actual probabilities according to the current algo is
+``` [10%,30%,60%] ```
+and not
+``` [10%,20%,70%]```
+ as would be expected from a proper cumulative probability distribution function.