Day 25 Part 1

Signed-off-by: James Griffin <james@unsupervised.ca>

twentyfive/day_twentyfive.go

@@ -0,0 +1,65 @@
+package twentyfive
+
+import "fmt"
+
+type key struct {
+	public   int
+	sub      int
+	loopsize int
+}
+
+func (k *key) generatePublic() {
+	v := 1
+
+	for i := 0; i < k.loopsize; i++ {
+		v *= k.sub
+		v %= 20201227
+	}
+
+	k.public = v
+}
+
+func (k *key) encryption(public int) int {
+	v := 1
+
+	for i := 0; i < k.loopsize; i++ {
+		v *= public
+		v %= 20201227
+	}
+
+	return v
+}
+
+func (k *key) determineLoopSize() {
+	if k.public == 0 {
+		return
+	}
+
+	v := 1
+	loopsize := 0
+	for v != k.public {
+		v *= k.sub
+		v %= 20201227
+		loopsize++
+	}
+
+	k.loopsize = loopsize
+}
+
+// PartOne What encryption key is the handshake trying to establish?
+func PartOne() string {
+	card := key{
+		public: 2069194,
+		sub:    7,
+	}
+
+	door := key{
+		public: 16426071,
+		sub:    7,
+	}
+
+	card.determineLoopSize()
+	door.determineLoopSize()
+
+	return fmt.Sprintf("The handshake is trying to establish the key %d", door.encryption(card.public))
+}