From 729aaf64275c61b8bc864ef9138eed078dea9cb2 Mon Sep 17 00:00:00 2001
From: Maxim Smolskiy <mithridatus@mail.ru>
Date: Thu, 4 Nov 2021 19:01:21 +0300
Subject: [PATCH] Improve Project Euler problem 014 solution 2 (#5744)

* Improve solution

* Uncomment code that has been commented due to slow execution affecting Travis

* Fix

* scikit-fuzzy is causing broken builds

* fuzz = None

* Update fuzzy_operations.py

Co-authored-by: Christian Clauss <cclauss@me.com>
---
 fuzzy_logic/fuzzy_operations.py   |  9 +++++++--
 project_euler/problem_014/sol2.py | 22 ++++++++++++----------
 requirements.txt                  |  2 +-
 3 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/fuzzy_logic/fuzzy_operations.py b/fuzzy_logic/fuzzy_operations.py
index 0f573f1..fbaca94 100644
--- a/fuzzy_logic/fuzzy_operations.py
+++ b/fuzzy_logic/fuzzy_operations.py
@@ -1,4 +1,5 @@
-"""README, Author - Jigyasa Gandhi(mailto:jigsgandhi97@gmail.com)
+"""
+README, Author - Jigyasa Gandhi(mailto:jigsgandhi97@gmail.com)
 Requirements:
   - scikit-fuzzy
   - numpy
@@ -7,7 +8,11 @@ Python:
   - 3.5
 """
 import numpy as np
-import skfuzzy as fuzz
+
+try:
+    import skfuzzy as fuzz
+except ImportError:
+    fuzz = None
 
 if __name__ == "__main__":
     # Create universe of discourse in Python using linspace ()
diff --git a/project_euler/problem_014/sol2.py b/project_euler/problem_014/sol2.py
index 0a58f8d..d2a1d9f 100644
--- a/project_euler/problem_014/sol2.py
+++ b/project_euler/problem_014/sol2.py
@@ -27,25 +27,27 @@ Which starting number, under one million, produces the longest chain?
 """
 from __future__ import annotations
 
+COLLATZ_SEQUENCE_LENGTHS = {1: 1}
+
 
 def collatz_sequence_length(n: int) -> int:
     """Returns the Collatz sequence length for n."""
-    sequence_length = 1
-    while n != 1:
-        if n % 2 == 0:
-            n //= 2
-        else:
-            n = 3 * n + 1
-        sequence_length += 1
+    if n in COLLATZ_SEQUENCE_LENGTHS:
+        return COLLATZ_SEQUENCE_LENGTHS[n]
+    if n % 2 == 0:
+        next_n = n // 2
+    else:
+        next_n = 3 * n + 1
+    sequence_length = collatz_sequence_length(next_n) + 1
+    COLLATZ_SEQUENCE_LENGTHS[n] = sequence_length
     return sequence_length
 
 
 def solution(n: int = 1000000) -> int:
     """Returns the number under n that generates the longest Collatz sequence.
 
-    # The code below has been commented due to slow execution affecting Travis.
-    # >>> solution(1000000)
-    # 837799
+    >>> solution(1000000)
+    837799
     >>> solution(200)
     171
     >>> solution(5000)
diff --git a/requirements.txt b/requirements.txt
index ef4e180..e01d87c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,7 +9,7 @@ pandas
 pillow
 qiskit
 requests
-scikit-fuzzy
+# scikit-fuzzy  # Causing broken builds
 sklearn
 statsmodels
 sympy
-- 
GitLab