factor: Move each factorisation method to its own module

Also decoupled the factorisation methods; now factor::factor contains
the logic that chains the different algorithms and aggregates results.

As a side-effect, rho::factor now performs extraneous allocations (as each
recursive step creates a new `Factors` value, which is then aggregated into
the previous one) but there is no significant performance impact.

src/uu/factor/src/factor.rs

@@ -18,35 +18,20 @@ extern crate rand;
 #[macro_use]
 extern crate uucore;
 
-use numeric::*;
-use rand::distributions::{Distribution, Uniform};
-use rand::rngs::SmallRng;
-use rand::{thread_rng, SeedableRng};
-use std::cmp::{max, min};
 use std::collections::HashMap;
 use std::fmt;
 use std::io::{stdin, BufRead};
-use std::mem::swap;
-use std::num::Wrapping;
 use std::ops;
 
 mod numeric;
-
-include!(concat!(env!("OUT_DIR"), "/prime_table.rs"));
+mod rho;
+mod table;
 
 static SYNTAX: &str = "[OPTION] [NUMBER]...";
 static SUMMARY: &str = "Print the prime factors of the given number(s).
  If none are specified, read from standard input.";
 static LONG_HELP: &str = "";
 
-fn gcd(mut a: u64, mut b: u64) -> u64 {
-    while b > 0 {
-        a %= b;
-        swap(&mut a, &mut b);
-    }
-    a
-}
-
 struct Factors {
     f: HashMap<u64, u8>,
 }
@@ -90,114 +75,36 @@ impl fmt::Display for Factors {
     }
 }
 
-
-fn rho_pollard_pseudorandom_function(x: u64, a: u64, b: u64, num: u64) -> u64 {
-    if num < 1 << 63 {
-        (sm_mul(a, sm_mul(x, x, num), num) + b) % num
-    } else {
-        big_add(big_mul(a, big_mul(x, x, num), num), b, num)
-    }
-}
-
-fn rho_pollard_find_divisor(num: u64) -> u64 {
-    #![allow(clippy::many_single_char_names)]
-    let range = Uniform::new(1, num);
-    let mut rng = SmallRng::from_rng(&mut thread_rng()).unwrap();
-    let mut x = range.sample(&mut rng);
-    let mut y = x;
-    let mut a = range.sample(&mut rng);
-    let mut b = range.sample(&mut rng);
-
-    loop {
-        x = rho_pollard_pseudorandom_function(x, a, b, num);
-        y = rho_pollard_pseudorandom_function(y, a, b, num);
-        y = rho_pollard_pseudorandom_function(y, a, b, num);
-        let d = gcd(num, max(x, y) - min(x, y));
-        if d == num {
-            // Failure, retry with different function
-            x = range.sample(&mut rng);
-            y = x;
-            a = range.sample(&mut rng);
-            b = range.sample(&mut rng);
-        } else if d > 1 {
-            return d;
-        }
-    }
-}
-
-fn rho_pollard_factor(num: u64, factors: &mut Factors) {
-    if is_prime(num) {
-        factors.push(num);
-        return;
-    }
-
-    let divisor = rho_pollard_find_divisor(num);
-    rho_pollard_factor(divisor, factors);
-    rho_pollard_factor(num / divisor, factors);
-}
-
-fn table_division(mut num: u64) -> Factors {
+fn factor(mut n: u64) -> Factors {
     let mut factors = Factors::new();
 
-    if num < 2 {
-        factors.push(num);
-        return factors
+    if n < 2 {
+        factors.push(n);
+        return factors;
     }
 
-    while num % 2 == 0 {
-        num /= 2;
+    while n % 2 == 0 {
+        n /= 2;
         factors.push(2);
     }
 
-    if num == 1 {
+    if n == 1 {
         return factors;
     }
 
-    if is_prime(num) {
-        factors.push(num);
+    if numeric::is_prime(n) {
+        factors.push(n);
         return factors;
     }
 
-    for &(prime, inv, ceil) in P_INVS_U64 {
-        if num == 1 {
-            break;
-        }
-
-        // inv = prime^-1 mod 2^64
-        // ceil = floor((2^64-1) / prime)
-        // if (num * inv) mod 2^64 <= ceil, then prime divides num
-        // See http://math.stackexchange.com/questions/1251327/
-        // for a nice explanation.
-        loop {
-            let Wrapping(x) = Wrapping(num) * Wrapping(inv); // x = num * inv mod 2^64
-            if x <= ceil {
-                num = x;
-                factors.push(prime);
-                if is_prime(num) {
-                    factors.push(num);
-                    return factors;
-                }
-            } else {
-                break;
-            }
-        }
-    }
-
-    // do we still have more factoring to do?
-    // Decide whether to use Pollard Rho or slow divisibility based on
-    // number's size:
-    //if num >= 1 << 63 {
-    // number is too big to use rho pollard without overflowing
-    //trial_division_slow(num, factors);
-    //} else if num > 1 {
-    // number is still greater than 1, but not so big that we have to worry
-    rho_pollard_factor(num, &mut factors);
+    let (f, n) = table::factor(n);
+    factors *= f;
+    factors *= rho::factor(n);
     factors
-    //}
 }
 
 fn print_factors(num: u64) {
-    print!("{}:{}", num, table_division(num));
+    print!("{}:{}", num, factor(num));
     println!();
 }
 

src/uu/factor/src/numeric.rs

@@ -9,9 +9,18 @@
 * that was distributed with this source code.
 */
 
+use std::mem::swap;
 use std::num::Wrapping;
 use std::u64::MAX as MAX_U64;
 
+pub fn gcd(mut a: u64, mut b: u64) -> u64 {
+    while b > 0 {
+        a %= b;
+        swap(&mut a, &mut b);
+    }
+    a
+}
+
 pub fn big_add(a: u64, b: u64, m: u64) -> u64 {
     let Wrapping(msb_mod_m) = Wrapping(MAX_U64) - Wrapping(m) + Wrapping(1);
     let msb_mod_m = msb_mod_m % m;

src/uu/factor/src/rho.rs

@@ -0,0 +1,54 @@
+use crate::Factors;
+use numeric::*;
+use rand::distributions::{Distribution, Uniform};
+use rand::rngs::SmallRng;
+use rand::{thread_rng, SeedableRng};
+use std::cmp::{max, min};
+
+fn pseudorandom_function(x: u64, a: u64, b: u64, num: u64) -> u64 {
+    if num < 1 << 63 {
+        (sm_mul(a, sm_mul(x, x, num), num) + b) % num
+    } else {
+        big_add(big_mul(a, big_mul(x, x, num), num), b, num)
+    }
+}
+
+fn find_divisor(num: u64) -> u64 {
+    #![allow(clippy::many_single_char_names)]
+    let range = Uniform::new(1, num);
+    let mut rng = SmallRng::from_rng(&mut thread_rng()).unwrap();
+    let mut x = range.sample(&mut rng);
+    let mut y = x;
+    let mut a = range.sample(&mut rng);
+    let mut b = range.sample(&mut rng);
+
+    loop {
+        x = pseudorandom_function(x, a, b, num);
+        y = pseudorandom_function(y, a, b, num);
+        y = pseudorandom_function(y, a, b, num);
+        let d = gcd(num, max(x, y) - min(x, y));
+        if d == num {
+            // Failure, retry with different function
+            x = range.sample(&mut rng);
+            y = x;
+            a = range.sample(&mut rng);
+            b = range.sample(&mut rng);
+        } else if d > 1 {
+            return d;
+        }
+    }
+}
+
+pub(crate) fn factor(num: u64) -> Factors {
+    let mut factors = Factors::new();
+    if num == 1 { return factors; }
+    if is_prime(num) {
+        factors.push(num);
+        return factors;
+    }
+
+    let divisor = find_divisor(num);
+    factors *= factor(divisor);
+    factors *= factor(num / divisor);
+    factors
+}

src/uu/factor/src/table.rs

@@ -0,0 +1,44 @@
+use crate::Factors;
+use numeric::*;
+use std::num::Wrapping;
+
+include!(concat!(env!("OUT_DIR"), "/prime_table.rs"));
+
+pub(crate) fn factor(mut num: u64) -> (Factors, u64) {
+    let mut factors = Factors::new();
+    for &(prime, inv, ceil) in P_INVS_U64 {
+        if num == 1 {
+            break;
+        }
+
+        // inv = prime^-1 mod 2^64
+        // ceil = floor((2^64-1) / prime)
+        // if (num * inv) mod 2^64 <= ceil, then prime divides num
+        // See http://math.stackexchange.com/questions/1251327/
+        // for a nice explanation.
+        loop {
+            let Wrapping(x) = Wrapping(num) * Wrapping(inv); // x = num * inv mod 2^64
+            if x <= ceil {
+                num = x;
+                factors.push(prime);
+                if is_prime(num) {
+                    factors.push(num);
+                    return (factors, 1);
+                }
+            } else {
+                break;
+            }
+        }
+    }
+
+    // do we still have more factoring to do?
+    // Decide whether to use Pollard Rho or slow divisibility based on
+    // number's size:
+    //if num >= 1 << 63 {
+    // number is too big to use rho pollard without overflowing
+    //trial_division_slow(num, factors);
+    //} else if num > 1 {
+    // number is still greater than 1, but not so big that we have to worry
+    (factors, num)
+    //}
+}