modes:pair: Added pair type

Slightly more complex pattern since we've got two cards, but still
relatively trivial.

src/modes/pair.rs

@@ -0,0 +1,33 @@
+use crate::card::{Card, PlayingCard};
+use crate::helper::ordered;
+
+#[derive(Debug, Copy, Clone)]
+pub struct Pair(Card, Card);
+
+impl Pair {
+    fn new(c1: Card, c2: Card) -> Option<Pair> {
+        // Order the cards.  This means if xor(c1 is joker, c2 is joker) c1 will
+        // be that joker.
+        let (c1, c2) = ordered(c1, c2);
+
+        match (c1, c2) {
+            // Can't be a pair if you got two jokers homie.
+            (Card::Joker(_), Card::Joker(_)) => None,
+
+            // NOTE: c2 cannot be a joker because of prev condition.  If you've
+            // got a joker you're automatically a pair.
+            (Card::Joker(_), _) => Some(Pair(c1, c2)),
+
+            // NOTE: c1 and c2 cannot be jokers.  In which case, check their
+            // ranks are equivalent.
+            (
+                Card::PlayingCard(PlayingCard { rank: r1, .. }),
+                Card::PlayingCard(PlayingCard { rank: r2, .. }),
+            ) => (r1 == r2).then_some(Pair(c1, c2)),
+
+            // Not necessary since the previous patterns technically cover all
+            // cases.  But I love the compiler too much to tell them... 💔
+            _ => None,
+        }
+    }
+}