Touch up changes around advancing feature

* Add additional comments on how to use the advance() function
* Use clone() for clarity instead of reusing a seed
* Compare the generator as a whole as opposed the next value in unit
  tests

rand_pcg/src/pcg128.rs

@@ -49,6 +49,9 @@ impl Lcg128Xsl64 {
     ///
     /// Even though delta is an unsigned integer, we can pass a
     /// signed integer to go backwards, it just goes "the long way round".
+    ///
+    /// Using this function is equivalent to calling `next_64()` `delta`
+    /// number of times.
     #[inline]
     pub fn advance(&mut self, delta: u128) {
         let mut acc_mult: u128 = 1;
@@ -178,6 +181,9 @@ impl Mcg128Xsl64 {
     ///
     /// Even though delta is an unsigned integer, we can pass a
     /// signed integer to go backwards, it just goes "the long way round".
+    ///
+    /// Using this function is equivalent to calling `next_64()` `delta`
+    /// number of times.
     #[inline]
     pub fn advance(&mut self, delta: u128) {
         let mut acc_mult: u128 = 1;

rand_pcg/src/pcg64.rs

@@ -49,6 +49,9 @@ impl Lcg64Xsh32 {
     ///
     /// Even though delta is an unsigned integer, we can pass a
     /// signed integer to go backwards, it just goes "the long way round".
+    ///
+    /// Using this function is equivalent to calling `next_32()` `delta`
+    /// number of times.
     #[inline]
     pub fn advance(&mut self, delta: u64) {
         let mut acc_mult: u64 = 1;

rand_pcg/tests/lcg128xsl64.rs

@@ -5,12 +5,12 @@ use rand_pcg::{Lcg128Xsl64, Pcg64};
 fn test_lcg128xsl64_advancing() {
     let seed = Default::default();
     let mut rng1 = Lcg128Xsl64::from_seed(seed);
-    let mut rng2 = Lcg128Xsl64::from_seed(seed);
+    let mut rng2 = rng1.clone();
     for _ in 0..20 {
         rng1.next_u64();
     }
     rng2.advance(20);
-    assert_eq!(rng1.next_u64(), rng2.next_u64());
+    assert_eq!(rng1, rng2);
 }
 
 #[test]

rand_pcg/tests/lcg64xsh32.rs

@@ -5,12 +5,12 @@ use rand_pcg::{Lcg64Xsh32, Pcg32};
 fn test_lcg64xsh32_advancing() {
     let seed = Default::default();
     let mut rng1 = Lcg64Xsh32::from_seed(seed);
-    let mut rng2 = Lcg64Xsh32::from_seed(seed);
+    let mut rng2 = rng1.clone();
     for _ in 0..20 {
         rng1.next_u32();
     }
     rng2.advance(20);
-    assert_eq!(rng1.next_u32(), rng2.next_u32());
+    assert_eq!(rng1, rng2);
 }
 
 #[test]

rand_pcg/tests/mcg128xsl64.rs

@@ -5,12 +5,12 @@ use rand_pcg::{Mcg128Xsl64, Pcg64Mcg};
 fn test_mcg128xsl64_advancing() {
     let seed = Default::default();
     let mut rng1 = Mcg128Xsl64::from_seed(seed);
-    let mut rng2 = Mcg128Xsl64::from_seed(seed);
+    let mut rng2 = rng1.clone();
     for _ in 0..20 {
         rng1.next_u64();
     }
     rng2.advance(20);
-    assert_eq!(rng1.next_u64(), rng2.next_u64());
+    assert_eq!(rng1, rng2);
 }
 
 #[test]