puzzles/solutions/2015: Implement day01 part2

internal/puzzles/solutions/2015/day01/elevator.go

@@ -0,0 +1,41 @@
+package day01
+
+type move string
+
+const (
+	up   move = "("
+	down move = ")"
+
+	basement = -1
+)
+
+type elevator struct {
+	floor int
+}
+
+func newElevator() *elevator {
+	return &elevator{
+		floor: 0,
+	}
+}
+
+func (e *elevator) Up() {
+	e.floor++
+}
+
+func (e *elevator) Down() {
+	e.floor--
+}
+
+func (e elevator) Floor() int {
+	return e.floor
+}
+
+func (e *elevator) Move(m move) {
+	switch m {
+	case up:
+		e.Up()
+	case down:
+		e.Down()
+	}
+}

internal/puzzles/solutions/2015/day01/solution.go

@@ -20,15 +20,25 @@ type solution struct {
 	name string
 }
 
-const (
-	up   = "("
-	down = ")"
-)
+func (s solution) Name() string {
+	return s.name
+}
+
+func (s solution) Year() string {
+	return s.year
+}
+
+func init() {
+	puzzles.Register(solution{
+		year: year,
+		name: puzzleName,
+	})
+}
 
 func (s solution) Part1(in io.Reader) (string, error) {
 	reader := bufio.NewReader(in)
 
-	var floor int
+	e := newElevator()
 
 	for {
 		r, _, err := reader.ReadRune()
@@ -40,34 +50,33 @@ func (s solution) Part1(in io.Reader) (string, error) {
 			return "", fmt.Errorf("read rune: %w", err)
 		}
 
-		s := string(r)
-
-		switch s {
-		case up:
-			floor++
-		case down:
-			floor--
-		}
+		e.Move(move(r))
 	}
 
-	return strconv.Itoa(floor), nil
+	return strconv.Itoa(e.Floor()), nil
 }
 
-func (s solution) Part2(_ io.Reader) (string, error) {
-	return "", puzzles.ErrNotImplemented
-}
+func (s solution) Part2(in io.Reader) (string, error) {
+	reader := bufio.NewReader(in)
 
-func (s solution) Name() string {
-	return s.name
-}
+	e := newElevator()
 
-func (s solution) Year() string {
-	return s.year
-}
+	var pos int
 
-func init() {
-	puzzles.Register(solution{
-		year: year,
-		name: puzzleName,
-	})
+	for e.Floor() != basement {
+		r, _, err := reader.ReadRune()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				break
+			}
+
+			return "", fmt.Errorf("read rune: %w", err)
+		}
+
+		pos++
+
+		e.Move(move(r))
+	}
+
+	return strconv.Itoa(pos), nil
 }

internal/puzzles/solutions/2015/day01/solution_test.go

@@ -147,8 +147,6 @@ func Test_solution_Part1(t *testing.T) {
 }
 
 func Test_solution_Part2(t *testing.T) {
-	t.Skip()
-
 	type fields struct {
 		name string
 	}
@@ -170,9 +168,20 @@ func Test_solution_Part2(t *testing.T) {
 				name: "",
 			},
 			args: args{
-				input: strings.NewReader(""),
+				input: strings.NewReader(")"),
+			},
+			want:    "1",
+			wantErr: false,
+		},
+		{
+			name: "",
+			fields: fields{
+				name: "",
+			},
+			args: args{
+				input: strings.NewReader("()())"),
 			},
-			want:    "",
+			want:    "5",
 			wantErr: false,
 		},
 	}

internal/puzzles/solutions/2015/day01/spec.md

@@ -1,6 +1,6 @@
 # --- Day 1: Not Quite Lisp ---
 
-## Part1
+## Part One
 Santa was hoping for a white Christmas, but his weather machine's "snow" function is powered by stars, 
 and he's fresh out! To save Christmas, he needs you to collect fifty stars by December 25th.
 
@@ -20,10 +20,23 @@ The apartment building is very tall, and the basement is very deep; he will neve
 
 For example:
 
-    - `(())` and `()()` both result in floor `0`.
-    - `(((` and `(()(()(` both result in floor `3`.
-    - `))(((((` also results in floor `3`.
-    - `())` and `))(` both result in floor `-1` (the first basement level).
-    - `)))` and `)())())` both result in floor `-3`.
+- `(())` and `()()` both result in floor `0`.
+- `(((` and `(()(()(` both result in floor `3`.
+- `))(((((` also results in floor `3`.
+- `())` and `))(` both result in floor `-1` (the first basement level).
+- `)))` and `)())())` both result in floor `-3`.
+
+To what floor do the instructions take Santa?
+
+## --- Part Two ---
+Now, given the same instructions, find the position of the first character that causes him to enter the 
+basement (floor -1). 
+The first character in the instructions has position 1, the second character has position 2, and so on.
+
+For example:
+
+- `)` causes him to enter the basement at character position `1`.
+- `()())` causes him to enter the basement at character position `5`.
+
+What is the position of the character that causes Santa to first enter the basement?
 
-To what floor do the instructions take Santa?