Day 9.1 solution.

2018/internal/day09/marbles.go

@@ -0,0 +1,55 @@
+package day09
+
+type Marble struct {
+	Number int
+	CW     *Marble
+	CCW    *Marble
+}
+
+// Returns the new current marble and the score to add.
+func (m *Marble) PlaceNewMarble(number int) (*Marble, int) {
+	// if the marble that is about to be placed has a number which is a multiple
+	// of 23, the current player keeps the marble they would have placed, adding
+	// it to their score. In addition, the marble 7 marbles counter-clockwise
+	// from the current marble is removed from the circle and also added to the
+	// current player's score. The marble located immediately clockwise of the
+	// marble that was removed becomes the new current marble.
+	if number%23 == 0 {
+		score := number
+		cursor := m
+		for i := 0; i < 7; i++ {
+			cursor = cursor.CCW
+		}
+		score += cursor.Number
+		cursor = cursor.Remove()
+		return cursor, score
+	}
+
+	// [Insert the marble] between the marbles that are 1 and 2
+	// marbles clockwise of the current marble.
+	// The marble that was just placed then becomes the current marble.
+
+	// first create the marble and populate its fields correctly
+	newMarble := &Marble{Number: number}
+	cursor := m.CW
+	newMarble.CCW = cursor
+	newMarble.CW = cursor.CW
+
+	// now sync up the pointers in the adjacent marbles
+	newMarble.CCW.CW = newMarble
+	newMarble.CW.CCW = newMarble
+	return newMarble, 0
+}
+
+func (m *Marble) Remove() *Marble {
+	// removes the marble from the list, repairing links. Returns the
+	// marble immediately CW of this one.
+
+	// link the two newly-adjacent marbles.
+	ccw := m.CCW
+	cw := m.CW
+	ccw.CW = cw
+	cw.CCW = ccw
+
+	return cw
+}