Day 25 Part 1

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

main.go

@@ -3,7 +3,7 @@ package main
 import (
 	"fmt"
 
-	"github.com/thatguygriff/aoc2020/twentyfour"
+	"github.com/thatguygriff/aoc2020/twentyfive"
 )
 
 func main() {
@@ -100,6 +100,9 @@ func main() {
 	// fmt.Println(twentythree.PartTwo())
 
 	// Day 24
-	fmt.Println(twentyfour.PartOne())
+	// fmt.Println(twentyfour.PartOne())
-	fmt.Println(twentyfour.PartTwo())
+	// fmt.Println(twentyfour.PartTwo())
+
+	// Day 25
+	fmt.Println(twentyfive.PartOne())
 }

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))
+}

twentyfive/day_twentyfive_test.go

@@ -0,0 +1,63 @@
+package twentyfive
+
+import "testing"
+
+func Test_generate_encryption(t *testing.T) {
+	card := key{
+		public: 5764801,
+		sub:    7,
+	}
+
+	door := key{
+		public: 17807724,
+		sub:    7,
+	}
+
+	card.determineLoopSize()
+	door.determineLoopSize()
+
+	if card.loopsize != 8 {
+		t.Logf("expected loop size of 8, got %d", card.loopsize)
+		t.Fail()
+	}
+
+	if door.loopsize != 11 {
+		t.Logf("expected loop size of 11, got %d", door.loopsize)
+		t.Fail()
+	}
+
+	encryption1 := card.encryption(door.public)
+	encryption2 := door.encryption(card.public)
+
+	if encryption1 != 14897079 {
+		t.Logf("Expected encryption key of 14897079, got %d", encryption1)
+		t.FailNow()
+	}
+
+	if encryption1 != encryption2 {
+		t.Logf("Expected to generate the same encryption key, got %d and %d", encryption1, encryption2)
+		t.Fail()
+	}
+}
+
+func Test_part_one(t *testing.T) {
+	card := key{
+		public: 2069194,
+		sub:    7,
+	}
+
+	door := key{
+		public: 16426071,
+		sub:    7,
+	}
+
+	card.determineLoopSize()
+	door.determineLoopSize()
+
+	encryption1 := card.encryption(door.public)
+
+	if encryption1 != 11576351 {
+		t.Logf("Expected encryption key 11576351, got %d", encryption1)
+		t.FailNow()
+	}
+}

twentyfive/input.txt

@@ -0,0 +1,2 @@
+2069194
+16426071

twentyfive/sample.txt

@@ -0,0 +1,2 @@
+5764801
+17807724