commit b8775f36754a6fd6f80e9e60ea27a0941c903ac1
Author: theBreadCompany <contact@thebread.dev>
Date:   Tue Dec 3 23:27:00 2024 +0100

    add pollard-rho

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diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..84d9883
--- /dev/null
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diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..fd0ca05
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,7 @@
+mod pollard_rho;
+
+fn main() {
+    let n = rand::random::<u8>();
+    let n_primediv = pollard_rho::pollard_rho(n.into());
+    eprintln!("Generated random number {}, got prime divisor {}", n, n_primediv);
+}
diff --git a/src/pollard_rho.rs b/src/pollard_rho.rs
new file mode 100644
index 0000000..91ca656
--- /dev/null
+++ b/src/pollard_rho.rs
@@ -0,0 +1,53 @@
+use gcd::Gcd;
+
+/**
+ * Calculate the primedivisor for some number `n`
+*/
+pub fn pollard_rho(n: u16) -> u16 {
+    if n == 1 { return 1; } // 1 only has 1 as prime divisor
+    if n % 2 == 0 { return 2; } // even numbers have at least 2 as prime divisor
+    if is_prime(n) { return n; }
+
+    let mut x = rand::random::<u16>() % 2;
+    let mut y = x;
+
+    let c = rand::random::<u16>() % 2;
+    let mut div = 1;
+
+    while div == 1 {
+        x = (mod_pow(x, 2, n) + c + n) % n;
+        y = (mod_pow(y, 2, n) + c + n) % n;
+        y = (mod_pow(y, 2, n) + c + n) % n;
+        div = u16::try_from((i16::try_from(x).unwrap()-i16::try_from(y).unwrap()).abs()).unwrap().gcd(n);
+        eprintln!("Got div {}", div);
+        if div == n { return pollard_rho(n); }
+    }
+
+    return div;
+}
+
+/**
+ * Discrete/Modular exponentiation
+ * 
+ * Highly memory efficient because the full result is never stored, but shortened by defined modulo instead. 
+ * We can use that because the prime divisor required for our algorithm is guarenteed to be smaller
+ * than n.
+ * 
+ * Counterpart function to the discrete logarithm.
+ */
+fn mod_pow(base: u16, exp: u16, r#mod: u16) -> u16 {
+    let mut result = 1; 
+
+    for _ in 0..exp-1 {
+        result = (result * base) % r#mod;
+    }
+
+    result
+}
+
+fn is_prime(n: u16) -> bool {
+    for i in (3..=(n as f32).sqrt() as u16).step_by(2) {
+        if n % i == 0 {  return false; }
+    }
+    true
+}