day9, part2

day9/dayNine.go

@@ -24,6 +24,23 @@ func Run() int {
 	return result
 }
 
+func Run2() int {
+	log.Println("hello day 9, part2")
+	filename := "day9/input"
+	bytes, err := os.ReadFile(filename)
+	if err != nil {
+		panic(fmt.Sprintln("error reading file: ", err))
+	}
+	result := 0
+
+	for _, line := range strings.Split(string(bytes), "\n") {
+		seq := CreateSequence(line)
+		result += seq.Get(-1)
+	}
+
+	return result
+}
+
 type Cell struct {
 	IsCached   bool
 	Value      func() int
@@ -54,6 +71,7 @@ func CachedCell(val int) Cell {
 type Coord struct {
 	Row, Col int
 }
+
 func (c Coord) Equal(other Coord) bool {
 	return c.Col == other.Col && c.Row == other.Row
 }
@@ -105,9 +123,11 @@ func (s *Sequence) GetAny(coord Coord) Cell {
 		}
 
 		// log.Printf("creating new uncashed at %d %d\n", row, col)
-		cell = Cell{
-			IsCached: false,
-			Value: func() int {
+
+		var calcFunc func() int
+
+		if col > 0 {
+			calcFunc = func() int {
 				if cell.IsCached {
 					return cell.SavedValue
 				}
@@ -118,7 +138,26 @@ func (s *Sequence) GetAny(coord Coord) Cell {
 				cell.IsCached = true
 				cell.SavedValue = value
 				return value
-			},
+			}
+		} else {
+			// for part 2, propagate backward from known elements on the right
+			calcFunc = func() int {
+				if cell.IsCached {
+					return cell.SavedValue
+				}
+				// log.Printf("calc value for %d %d cell\n", row, col)
+				simplerCell := s.GetAny(Coord{row + 1, col})
+				nextCell := s.GetAny(Coord{row, col + 1})
+				value := nextCell.Value() - simplerCell.Value()
+				cell.IsCached = true
+				cell.SavedValue = value
+				return value
+			}
+		}
+
+		cell = Cell{
+			IsCached: false,
+			Value:    calcFunc,
 		}
 	}
 

main.go

@@ -9,17 +9,6 @@ import (
 func main() {
 	log.Print("> starting run:")
 
-	// line := "0 3 6 9 12 15"
-	// line := "1 3 6 10 15 21"
-	// line := "10 13 16 21 30 45"
-	// seq := day9.CreateSequence(line)
-	// fmt.Println(seq.String())
-
-	// fmt.Printf("got %+v \n", seq)
-	// fmt.Printf("%s\n", seq.String())
-
-	// fmt.Printf("next value is %d\n", seq.Next())
-
-	result := day9.Run()
+	result := day9.Run2()
 	log.Printf("day9 result: %d\n****\n", result)
 }