diff --git a/2017/day21/main.go b/2017/day21/main.go
index 04b07e2..92a3108 100644
--- a/2017/day21/main.go
+++ b/2017/day21/main.go
@@ -5,10 +5,8 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/alexchao26/advent-of-code-go/mathutil"
-
 	"github.com/alexchao26/advent-of-code-go/algos"
-
+	"github.com/alexchao26/advent-of-code-go/cast"
 	"github.com/alexchao26/advent-of-code-go/util"
 )
 
@@ -74,15 +72,7 @@ func parseInput(input string) map[string][][]string {
 		}
 		return str
 	}
-	mirrorImageGrid := func(grid [][]string) (flipped [][]string) {
-		for i := range grid {
-			flipped = append(flipped, []string{})
-			for j := len(grid[i]) - 1; j >= 0; j-- {
-				flipped[i] = append(flipped[i], grid[i][j])
-			}
-		}
-		return flipped
-	}
+
 	rules := map[string][][]string{}
 	for _, line := range strings.Split(input, "\n") {
 		parts := strings.Split(line, " => ")
@@ -92,7 +82,7 @@ func parseInput(input string) map[string][][]string {
 		for i := 0; i < 4; i++ {
 			keyGrid = algos.RotateStringGrid(keyGrid)
 			rules[stringifyGrid(keyGrid)] = resultGrid
-			rules[stringifyGrid((mirrorImageGrid(keyGrid)))] = resultGrid
+			rules[stringifyGrid((algos.MirrorStringGrid(keyGrid)))] = resultGrid
 		}
 	}
 	return rules
@@ -108,7 +98,7 @@ func tick(grid [][]string, rules map[string][][]string) [][]string {
 	} else if len(grid)%3 == 0 {
 		edgeSize = 3
 	} else {
-		panic("grid is not evenly divisible by 2 or 3, got " + mathutil.IntToStr(len(grid)))
+		panic("grid is not evenly divisible by 2 or 3, got " + cast.ToString(len(grid)))
 	}
 
 	// iterate over like a sudoku grid, r and c iterate over the top left corner
diff --git a/2020/day20/main.go b/2020/day20/main.go
index b2622c9..088dc76 100644
--- a/2020/day20/main.go
+++ b/2020/day20/main.go
@@ -70,7 +70,7 @@ func part2(input string) int {
 	// get the coordinates of all monsters by iterating over all possible
 	// orientations of the image
 	var monsterCoords [][2]int
-	for _, opt := range generateGridOrientations(image) {
+	for _, opt := range algos.AllGridOrientations(image) {
 		monsterCoords = findMonsterCoords(opt)
 		// assuming there's only one orientation of image with valid monsters
 		if len(monsterCoords) > 0 {
@@ -121,19 +121,6 @@ func parseTilesFromInput(input string) []*tile {
 	return ans
 }
 
-func generateGridOrientations(grid [][]string) [][][]string {
-	var options [][][]string
-	for i := 0; i < 2; i++ {
-		for j := 0; j < 4; j++ {
-			options = append(options, grid)
-			grid = algos.RotateStringGrid(grid)
-		}
-		grid = algos.MirrorStringGrid(grid)
-	}
-	// note: there will likely be duplicates in there... but that's fine...
-	return options
-}
-
 func backtrackAssemble(tiles []*tile, assembledTiles [][]*tile, usedIndices map[int]bool) [][]*tile {
 	// pray it's a square...
 	edgeSize := int(math.Sqrt(float64(len(tiles))))
@@ -153,7 +140,7 @@ func backtrackAssemble(tiles []*tile, assembledTiles [][]*tile, usedIndices map[
 				for i, t := range tiles {
 					if !usedIndices[i] {
 						// iterate over the OPTIONS for a particular tile, i.e. all 8 images of it...
-						for _, opt := range generateGridOrientations(t.contents) {
+						for _, opt := range algos.AllGridOrientations(t.contents) {
 							// check if setting this tile is okay with (if applicable) tiles above
 							// and to the left
 							if row != 0 { // check above
diff --git a/algos/grid-orientations.go b/algos/grid-orientations.go
new file mode 100644
index 0000000..0f2db0c
--- /dev/null
+++ b/algos/grid-orientations.go
@@ -0,0 +1,17 @@
+package algos
+
+// AllGridOrientations returns the 8 possible orientations of a given grid
+// i.e. rotated 4 times and the mirror image of all of those
+func AllGridOrientations(grid [][]string) [][][]string {
+	orientations := [][][]string{grid}
+	// add the 3 other rotations
+	for i := 0; i < 3; i++ {
+		orientations = append(orientations, RotateStringGrid(orientations[len(orientations)-1]))
+	}
+	// then add the mirror images of all 4 rotations
+	for i := 0; i < 4; i++ {
+		orientations = append(orientations, MirrorStringGrid(orientations[i]))
+	}
+
+	return orientations
+}