From bf3732794abc17096337d1a01e78334688b85f40 Mon Sep 17 00:00:00 2001
From: felixm <mail@felixm.de>
Date: Sun, 1 Oct 2023 18:10:27 +0200
Subject: [PATCH] Add own bitarray and Miller-Rabin primality test.

---
 python/lib_bitarray.py | 39 +++++++++++++++++++++++++++++++++++++++
 python/lib_prime.py    | 32 ++++++++++++++++++++++++++------
 2 files changed, 65 insertions(+), 6 deletions(-)
 create mode 100644 python/lib_bitarray.py

diff --git a/python/lib_bitarray.py b/python/lib_bitarray.py
new file mode 100644
index 0000000..b153e64
--- /dev/null
+++ b/python/lib_bitarray.py
@@ -0,0 +1,39 @@
+class Bitarray:
+    def __init__(self, num_bits):
+        self.word_size = 32
+        self.word_mask = 2**self.word_size - 1
+        num_words = num_bits // self.word_size
+        if num_bits % self.word_size != 0:
+            num_words += 1
+        self.num_bits = num_bits
+        self.num_words = num_words
+        self.array = [0] * num_words
+
+    def setall(self, target: bool):
+        if target:
+            self.array = [self.word_mask] * self.num_words
+        else:
+            self.array = [0] * self.num_words
+
+    def set(self, bit, target: bool):
+        assert(bit < self.num_bits)
+        word_index = bit // self.word_size
+        bit_index = bit % self.word_size
+        value = self.array[word_index]
+        if target:
+            value = (value | (1 << bit_index))
+        else:
+            value = (value & (~(1 << bit_index)))
+        self.array[word_index] = value
+
+    def get(self, bit) -> bool:
+        assert(bit < self.num_bits)
+        word_index = bit // self.word_size
+        bit_index = bit % self.word_size
+        return bool(self.array[word_index] & (1 << bit_index))
+
+    def __getitem__(self, bit: int) -> bool:
+        return self.get(bit)
+
+    def __setitem__(self, bit: int, value: bool):
+        self.set(bit, value)
diff --git a/python/lib_prime.py b/python/lib_prime.py
index 1cb33db..4d194c4 100644
--- a/python/lib_prime.py
+++ b/python/lib_prime.py
@@ -1,4 +1,5 @@
 from functools import lru_cache
+from lib_bitarray import Bitarray
 try:
     from lib_misc import get_item_counts
     from lib_misc import product
@@ -65,6 +66,29 @@ def is_prime(n):
     return True
 
 
+def is_prime_rabin_miller(number):
+    """ Rabin-Miller Primality Test """
+    if number % 6 not in [1,5]:
+        return False
+
+    r, s = 1, number - 1
+    while s % 2 == 0:
+        s //= 2
+        r += 1
+
+    for witness in [2, 3, 5, 7, 11, 13, 17, 23, 29, 31, 37, 41, 43, 47, 53]:
+        remainder = pow(witness, s, number)
+        if remainder == 1:
+            continue
+        for _ in range(1, r):
+            if remainder == number - 1:
+                break
+            remainder = pow(remainder, 2, number)
+        else:
+            return False
+    return True
+
+
 def prime_nth(n):
     """Returns the nth prime number. The first number
     is 1 indexed, i.e. n = 1 -> 2, n = 2 -> 3, etc.
@@ -97,12 +121,8 @@ def primes(n_max):
 
     :param n_max:
     """
-    try:
-        import bitarray
-        b = bitarray.bitarray(n_max)
-        b.setall(True)
-    except ModuleNotFoundError:
-        b = [True for _ in range(n_max)]
+    b = Bitarray(n_max)
+    b.setall(True)
     n = 1
     b[n - 1] = False
     while n * n <= n_max: