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bin/prime.py

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+#!/usr/bin/env python3
+# First time I use [[click]], it looks cool like that other [[pallets project]], [[flask]].
+# https://click.palletsprojects.com/en/8.1.x/arguments/
+# https://click.palletsprojects.com/en/8.1.x/options/
+
+import click
+import math
+
+# I'm not proud (I am a little bit?).
+PROOF = [] # :)
+
+def factor(n):
+    for i in range(2, n+1):
+        click.echo(f"Is {i} a factor of {n}, I wonder?")
+
+def is_prime(n):
+    is_prime = [True for n in range(0, n+1)]
+    upto = math.ceil(math.sqrt(n))
+    for i, _ in enumerate(is_prime):
+        # click.echo(f"i: {i}")
+        if i < 2:
+            continue
+        # If we're above the square root, we can stop considering the first factor, as we'll try higher numbers in the inner loop.
+        if i > upto:
+            break
+        # If this is a known composite, then we've already crossed off its multiples when we iterated over its primes.
+        if not is_prime[i]:
+            continue
+        # for j in range(2, math.ceil(math.sqrt(n) + 1)):
+        for j in range(2, n):
+            # I wrote >= here initially. That did *not* work ;)
+            if i*j > n:
+                break
+            PROOF.append(f"[[{i*j}]] is composite: {i} * {j}.")
+            try:
+                is_prime[i*j] = False
+            except IndexError:
+                continue
+    # click.echo(f"Sieve: {is_prime}.")
+    return is_prime[n]
+
+@click.command()
+@click.argument('n', type=click.INT)
+@click.option('-f', '--from', 'n0', type=click.INT, help="Lower bound of the range to factor.")
+@click.option('-t', '--to', 'n1', type=click.INT, help="Upper bound of the range to factor.")
+def prime(n, n0, n1):
+    """Simple program that factors a number using a [[Sieve of Eratosthenes]]."""
+    if is_prime(n):
+        click.echo(f"[[{n}]] is prime.")
+    else:
+        click.echo(f"{n} is not prime. Want proof? :)")
+        click.echo("\n".join([line for line in PROOF if f'[[{n}]]' in line]))
+
+if __name__ == '__main__':
+    prime()