From 263693e45cc6665e3c149cdcd4ba05875b8f9004 Mon Sep 17 00:00:00 2001
From: Simon Green <mail@simon.green>
Date: Fri, 14 Jan 2022 15:21:19 +1100
Subject: sgreen solutions to challenge 147

---
 challenge-147/sgreen/python/ch-1.py | 50 +++++++++++++++++++++++++++++++++++++
 challenge-147/sgreen/python/ch-2.py | 32 ++++++++++++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100755 challenge-147/sgreen/python/ch-1.py
 create mode 100755 challenge-147/sgreen/python/ch-2.py

(limited to 'challenge-147/sgreen/python')

diff --git a/challenge-147/sgreen/python/ch-1.py b/challenge-147/sgreen/python/ch-1.py
new file mode 100755
index 0000000000..1b77f58482
--- /dev/null
+++ b/challenge-147/sgreen/python/ch-1.py
@@ -0,0 +1,50 @@
+#!/usr/bin/env python
+
+import math
+
+
+def is_prime(number):
+    # Return true or false if the number is a prime
+    if number < 2:
+        return False
+
+    for i in range(2, int(math.sqrt(number)) + 1):
+        if number % i == 0:
+            return False
+
+    # It's a prime
+    return True
+
+
+def is_trunc_prime(number):
+    # Return true or false if the number and all it's left truncated
+    #  parts is a prime. Start with the smallest figure first
+    n = str(number)
+
+    if '0' in n:
+        # A left-truncatable prime cannot contain a zero
+        return False
+
+    for i in range(len(n), 0, -1):
+        if not is_prime(int(n[i - 1:])):
+            return False
+
+    return True
+
+
+def main():
+    solutions = []
+    number = 1
+
+    # Keep increasing number by one until we have twenty truncated prime
+    #  numbers
+    while len(solutions) < 20:
+        if is_trunc_prime(number):
+            solutions.append(number)
+        number += 1
+
+    print(*solutions, sep=', ')
+
+
+if __name__ == '__main__':
+    main()
diff --git a/challenge-147/sgreen/python/ch-2.py b/challenge-147/sgreen/python/ch-2.py
new file mode 100755
index 0000000000..c6424b9617
--- /dev/null
+++ b/challenge-147/sgreen/python/ch-2.py
@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+
+import math
+
+
+def pentagon_number(n):
+    return int(n * (3 * n - 1) / 2)
+
+
+def is_pentagon_number(x):
+    n = math.sqrt(24 * x + 1) + 1
+    return int(n / 6) if n % 6 == 0 else None
+
+
+def main():
+    n1 = 2
+    while True:
+        p1 = pentagon_number(n1)
+        for n2 in range(1, n1):
+            p2 = pentagon_number(n2)
+
+            s1 = is_pentagon_number(p1 + p2)
+            s2 = is_pentagon_number(p1 - p2)
+
+            if s1 is not None and s2 is not None:
+                print(f'P({n1}) + P({n2}) = {p1} + {p2} = { p1 + p2 } = P({s1})')
+                print(f'P({n1}) - P({n2}) = {p1} - {p2} = { p1 - p2 } = P({s2})')
+                return
+        n1 += 1
+
+
+main()
-- 
cgit