day14, example two

day14/dayFourteen.go

@@ -16,14 +16,41 @@ func Run() int {
 	// fmt.Printf("> lines for field %+v\n", field.UpIndices())
 	// field.Move(field.Height(), field.UpIndices())
 	cycles := 1000000000
-	for i := 0; i < cycles; i++ {
+	states := make(map[string]int)
+	// 2023/12/14 11:50:32 >>> found loop. known state after 10 equal to one after 3
+
+	var loopLen, initialStretch int
+
+	for i := 1; i <= cycles; i++ {
 		field.DoSpinCycle()
 		// fmt.Println(field.String())
+
+		stringRepr := field.String()
+		prevIter, known := states[stringRepr]
+		if known {
+			log.Printf(">>> found loop. known state after %d equal to one after %d", i, prevIter)
+			initialStretch = prevIter
+			loopLen = i - prevIter
+			break
+		}
+
+		states[stringRepr] = i
+
 		if i % 100000 == 0 {
 			log.Print("done ", i, " cycles")
 		}
 	}
 
+	// field is already in a 'loop' state.
+	// so we've already done 'initial stretch' so to make field in same state as after 'cycles'
+	// i only need to check rest of (cycles - initialStretch)
+	movesToMake := (cycles - initialStretch)%loopLen
+	log.Printf(">>> data: initial steps %d, loop len %d. to do same as %d iterations i need %d", initialStretch, loopLen, cycles, movesToMake)
+
+	for i := 1; i <= movesToMake; i++ {
+		field.DoSpinCycle()
+		// fmt.Println(field.String())
+	}
 
 	// north rock load
 	return field.NorthLoad()