diff --git a/day16/example b/day16/example
new file mode 100644
index 0000000..d6805ce
--- /dev/null
+++ b/day16/example
@@ -0,0 +1,10 @@
+.|...\....
+|.-.\.....
+.....|-...
+........|.
+..........
+.........\
+..../.\\..
+.-.-/..|..
+.|....-|.\
+..//.|....
diff --git a/day16/floorWillBeLava.go b/day16/floorWillBeLava.go
new file mode 100644
index 0000000..c21ac35
--- /dev/null
+++ b/day16/floorWillBeLava.go
@@ -0,0 +1,258 @@
+package day16
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"sync"
+)
+
+func Run() int {
+	fmt.Println("hello from day 16")
+	log.Println("starting")
+	field := ReadField("day16/example")
+	fmt.Println(field.String())
+	field.StartTraversal()
+
+	fmt.Println(field.ShowEnergyzed())
+
+	return field.CountEnergized()
+}
+
+// have shared field
+// running traversal recursive function per ray
+// exit if going out of field or visiting cell that already had light in this direction
+// (i.e encountering loop)
+
+type CellType rune
+
+const (
+	Empty           CellType = '.'
+	SplitterNS               = '|'
+	SplitterEW               = '-'
+	MirrorBackslash          = '\\'
+	MirrorSlash              = '/'
+)
+
+type Direction int
+
+const (
+	Upward Direction = iota
+	Downward
+	Leftward
+	Rightward
+)
+
+type Cell struct {
+	CellType   CellType
+	KnownBeams map[Direction]any
+}
+
+type Field struct {
+	cells [][]*Cell
+}
+
+func (f *Field)isValid(mp MovementPoint) bool {
+	if mp.Row < 0 || mp.Col < 0 {
+		return false
+	}
+	if mp.Row >= len(f.cells) || len(f.cells) == 0 || mp.Col >= len(f.cells[0]) {
+		return false
+	}
+	return true
+}
+
+func ReadField(filename string) Field {
+	result := Field{}
+	bytes, err := os.ReadFile(filename)
+	if err != nil {
+		panic(fmt.Sprint("cannot read file: ", filename))
+	}
+	text := string(bytes)
+	text = strings.TrimSpace(text)
+	for _, line := range strings.Split(text, "\n") {
+		rowCells := make([]*Cell, 0)
+		for _, symb := range line {
+			rowCells = append(rowCells, &Cell{
+				CellType: CellType(symb),
+				KnownBeams: make(map[Direction]any),
+			})
+		}
+		result.cells = append(result.cells, rowCells)
+	}
+	return result
+}
+
+func (f *Field)String() string {
+	result := "\n"
+	for _, row := range f.cells {
+		for _, cell := range row {
+			result += string(cell.CellType)
+		}
+		result += "\n"
+	}
+	return result
+}
+
+func (f *Field)ShowEnergyzed() string {
+	result := "\n"
+	for _, row := range f.cells {
+		for _, cell := range row {
+			if len(cell.KnownBeams) > 0 {
+				result += "#"
+			} else {
+				result += string(cell.CellType)
+			}
+
+		}
+		result += "\n"
+	}
+	return result
+
+}
+
+type MovementPoint struct {
+	Row, Col  int
+	Direction Direction
+}
+
+func (f *Field)StartTraversal() {
+	reportedVisits := make(chan MovementPoint)
+	var wg sync.WaitGroup
+
+	go f.RecordVisits(reportedVisits)
+	startPoint := MovementPoint {
+		Row: 0, Col: 0, Direction: Rightward,
+	}
+	wg.Add(1)
+	go f.TraverseFrom(startPoint, reportedVisits, &wg)
+
+	wg.Wait()
+	close(reportedVisits)
+}
+
+func (f *Field)CountEnergized() (result int) {
+	for _, row := range f.cells {
+		for _, cell := range row {
+			if len(cell.KnownBeams) > 0 {
+				result += 1
+			}
+		}
+	}
+	return
+}
+
+func (f *Field)RecordVisits(reportedPoints <-chan MovementPoint) {
+	for point := range reportedPoints {
+		cell := f.cells[point.Row][point.Col]
+		log.Printf("recording visit %+v to %+v at row %d col %d\n", point, cell, point.Row, point.Col)
+		cell.KnownBeams[point.Direction] = struct{}{}
+	}
+}
+
+// starting at point, mark as visited
+// move (concurrently if required) into next points
+// ends - when out of the field, or if encountering a cycle
+func (f *Field)TraverseFrom(current MovementPoint, reportVisits chan<- MovementPoint, wg *sync.WaitGroup) {
+	log.Printf("> starting traverse through %+v", current)
+	if !f.isValid(current) {
+		log.Println("invalid current ", current, " should be impossible")
+		wg.Done()
+		return
+	}
+	cell := f.cells[current.Row][current.Col]
+	_, knownDirection := cell.KnownBeams[current.Direction]
+	if knownDirection {
+		log.Printf("found cycle at %+v in %+v", current, cell)
+		wg.Done()
+		return
+	}
+
+	reportVisits <- current
+
+	nextPoints := NextPoints(f, current)
+	log.Printf("for current %+v next are: %+v\n", current, nextPoints)
+	switch len(nextPoints) {
+    case 0:
+		wg.Done()
+		return
+	case 1:
+		f.TraverseFrom(nextPoints[0], reportVisits, wg)
+		return
+	case 2:
+		wg.Add(1)
+		go f.TraverseFrom(nextPoints[0], reportVisits, wg)
+		f.TraverseFrom(nextPoints[1], reportVisits, wg)
+		return
+	}
+
+}
+
+func NextPoints(f *Field, current MovementPoint) []MovementPoint {
+	cell := f.cells[current.Row][current.Col]
+	nextDirections := cell.CellType.NextDirections(current.Direction)
+	nextCells := make([]MovementPoint, 0)
+	for _, direction := range nextDirections {
+		nextMovementPoint := current.ApplyDirection(direction)
+		if f.isValid(nextMovementPoint) {
+			nextCells = append(nextCells, nextMovementPoint)
+		}
+	}
+	return nextCells
+}
+
+// value receiver, can safely modify incoming mp
+// doesn't know about Field dimentions
+func (mp MovementPoint)ApplyDirection(d Direction) MovementPoint {
+	switch d {
+    case Upward:
+		mp.Row -= 1
+	case Downward:
+		mp.Row += 1
+	case Leftward:
+		mp.Col -= 1
+	case Rightward:
+		mp.Col += 1
+	}
+	mp.Direction = d
+	return mp
+}
+
+func (ct CellType) NextDirections(currentDirection Direction) (nextDirections []Direction) {
+	switch ct {
+	case Empty:
+		nextDirections = []Direction{currentDirection}
+	case SplitterNS:
+		if currentDirection == Rightward || currentDirection == Leftward {
+			nextDirections = []Direction{Upward, Downward}
+		} else {
+			nextDirections = []Direction{currentDirection}
+		}
+	case SplitterEW:
+		if currentDirection == Downward || currentDirection == Upward {
+			nextDirections = []Direction{Leftward, Rightward}
+		} else {
+			nextDirections = []Direction{currentDirection}
+		}
+	case MirrorBackslash:
+		// mirror symbol is \
+		directionMappings := map[Direction]Direction{
+			Leftward:  Upward,
+			Rightward: Downward,
+			Upward:    Leftward,
+			Downward:  Rightward,
+		}
+		nextDirections = []Direction{directionMappings[currentDirection]}
+	case MirrorSlash:
+		// mirrow symbol is /
+		directionMappings := map[Direction]Direction{
+			Leftward:  Downward,
+			Rightward: Upward,
+			Upward:    Rightward,
+			Downward:  Leftward,
+		}
+		nextDirections = []Direction{directionMappings[currentDirection]}
+	}
+	return
+}
diff --git a/main.go b/main.go
index ca58cb8..28e6220 100644
--- a/main.go
+++ b/main.go
@@ -3,12 +3,12 @@ package main
 import (
 	"log"
 
-	"sunshine.industries/aoc2023/day15"
+	"sunshine.industries/aoc2023/day16"
 )
 
 func main() {
 	log.Print("> starting run:")
 
-	result := day15.Run()
-	log.Printf("day15 result: %d\n****\n", result)
+	result := day16.Run()
+	log.Printf("\n\nday16 result: %d\n****\n", result)
 }