diff --git a/2021/day05/main.go b/2021/day05/main.go
new file mode 100644
index 0000000..af04c26
--- /dev/null
+++ b/2021/day05/main.go
@@ -0,0 +1,130 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := countIntersections(input, part)
+	util.CopyToClipboard(fmt.Sprintf("%v", ans))
+	fmt.Println("Output:", ans)
+}
+
+func countIntersections(input string, part int) int {
+	var coords [][4]int
+	for _, line := range strings.Split(input, "\n") {
+		var x1, y1, x2, y2 int
+		_, err := fmt.Sscanf(line, "%d,%d -> %d,%d", &x1, &y1, &x2, &y2)
+		if err != nil {
+			panic("parsing error: " + err.Error())
+		}
+		coords = append(coords, [4]int{x1, y1, x2, y2})
+	}
+
+	// find largest x and y
+	// Note: different x,y as prompt, x = rows, y = cols instead
+	var endRow, endCol int
+	for _, c := range coords {
+		if c[0] > endRow {
+			endRow = c[0]
+		}
+		if c[2] > endRow {
+			endRow = c[2]
+		}
+		if c[1] > endCol {
+			endCol = c[1]
+		}
+		if c[3] > endCol {
+			endCol = c[3]
+		}
+	}
+
+	grid := make([][]int, endRow+1)
+	for i := range grid {
+		grid[i] = make([]int, endCol+1)
+	}
+
+	for _, c := range coords {
+		if c[0] == c[2] {
+			// horiz line, row1 = row2
+			row := c[0]
+			start, end := c[1], c[3]
+			// swap start and end if needed
+			if c[1] > c[3] {
+				start, end = end, start
+			}
+			for col := start; col <= end; col++ {
+				grid[row][col]++
+			}
+		} else if c[1] == c[3] {
+			// vert line, col1 = col2
+			col := c[1]
+			start, end := c[0], c[2]
+			// swap start and end if needed
+			if c[0] > c[2] {
+				start, end = end, start
+			}
+			for row := start; row <= end; row++ {
+				grid[row][col]++
+			}
+		} else if part == 2 {
+			// check diagonals for part 2 only
+
+			// get left most pair first by comparing column coords
+			if c[1] > c[3] {
+				c = [4]int{
+					c[2], c[3],
+					c[0], c[1],
+				}
+			}
+
+			// compare row coords next, will be going rightwards regardless
+			// if going down and right
+			if c[0] < c[2] {
+				for row := c[0]; row <= c[2]; row++ {
+					col := c[1] + row - c[0]
+					grid[row][col]++
+				}
+			} else {
+				// going up and right
+				for row := c[0]; row >= c[2]; row-- {
+					col := c[1] + (c[0] - row)
+					grid[row][col]++
+				}
+			}
+		}
+	}
+
+	// count up intersections
+	var ans int
+	for _, rows := range grid {
+		for _, v := range rows {
+			if v >= 2 {
+				ans++
+			}
+		}
+	}
+
+	return ans
+}
diff --git a/2021/day05/main_test.go b/2021/day05/main_test.go
new file mode 100644
index 0000000..aff6e55
--- /dev/null
+++ b/2021/day05/main_test.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `0,9 -> 5,9
+8,0 -> 0,8
+9,4 -> 3,4
+2,2 -> 2,1
+7,0 -> 7,4
+6,4 -> 2,0
+0,9 -> 2,9
+3,4 -> 1,4
+0,0 -> 8,8
+5,5 -> 8,2`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			part:  1,
+			want:  5,
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  1,
+			want:  4993,
+		},
+		{
+			name:  "example",
+			input: example,
+			part:  2,
+			want:  12,
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  2,
+			want:  21101,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.part == 0 {
+				t.Error("part number cannot be zero")
+			}
+			if got := countIntersections(tt.input, tt.part); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}