diff --git a/src/resonant_collinearity.gleam b/src/resonant_collinearity.gleam
index 0f38d84..ac94881 100644
--- a/src/resonant_collinearity.gleam
+++ b/src/resonant_collinearity.gleam
@@ -12,6 +12,11 @@ type Point =
 type Grid =
   Dict(Point, String)
 
+type Direction {
+  Forward
+  Reverse
+}
+
 fn parse_column(line, y) {
   use grid, char, x <- list.index_fold(line, dict.new())
   dict.insert(grid, #(x, y), char)
@@ -24,7 +29,10 @@ fn square(input: Float) -> Float {
   }
 }
 
-fn find_collinears(point_a: Point, point_b: Point) -> List(Point) {
+fn find_collinear_vector(
+  point_a: Point,
+  point_b: Point,
+) -> #(Float, #(Float, Float)) {
   let #(x1, y1) = point_a
   let #(x2, y2) = point_b
 
@@ -43,18 +51,96 @@ fn find_collinears(point_a: Point, point_b: Point) -> List(Point) {
   let assert Ok(dx) = float.divide(float.subtract(x2f, x1f), distance)
   let assert Ok(dy) = float.divide(float.subtract(y2f, y1f), distance)
 
-  let x3f_forward = float.add(x2f, float.multiply(dx, distance))
-  let y3f_forward = float.add(y2f, float.multiply(dy, distance))
-  let x3f_reverse = float.subtract(x1f, float.multiply(dx, distance))
-  let y3f_reverse = float.subtract(y1f, float.multiply(dy, distance))
+  #(distance, #(dx, dy))
+}
+
+fn find_collinear_in_direction(
+  point_a: Point,
+  point_b: Point,
+  direction: Direction,
+) -> Point {
+  let #(distance, #(dx, dy)) = find_collinear_vector(point_a, point_b)
+  let #(x1, y1) = point_a
+  let #(x2, y2) = point_b
+
+  let x1f = int.to_float(x1)
+  let y1f = int.to_float(y1)
+
+  let x2f = int.to_float(x2)
+  let y2f = int.to_float(y2)
+
+  case direction {
+    Forward -> #(
+      float.round(float.subtract(x1f, float.multiply(dx, distance))),
+      float.round(float.subtract(y1f, float.multiply(dy, distance))),
+    )
+    Reverse -> #(
+      float.round(float.add(x2f, float.multiply(dx, distance))),
+      float.round(float.add(y2f, float.multiply(dy, distance))),
+    )
+  }
+}
+
+fn points_within_map(points: List(Point), grid: Grid) -> Bool {
+  list.all(points, point_within_map(_, grid))
+}
+
+fn find_forward_harmonic_collinears(
+  found: List(Point),
+  grid: Grid,
+) -> List(Point) {
+  let #(head, _rest) = list.split(found, 2)
+  let assert [p1, p2] = head
+  case points_within_map(head, grid) {
+    True -> {
+      find_forward_harmonic_collinears(
+        list.append(
+          list.wrap(find_collinear_in_direction(p1, p2, Forward)),
+          found,
+        ),
+        grid,
+      )
+    }
+    False -> found
+  }
+}
 
-  let x3_forward = float.round(x3f_forward)
-  let y3_forward = float.round(y3f_forward)
+fn find_reverse_harmonic_collinears(
+  found: List(Point),
+  grid: Grid,
+) -> List(Point) {
+  let #(head, _rest) = list.split(found, 2)
+  let assert [p1, p2] = head
+  case points_within_map(head, grid) {
+    True -> {
+      find_reverse_harmonic_collinears(
+        list.append(
+          list.wrap(find_collinear_in_direction(p1, p2, Reverse)),
+          found,
+        ),
+        grid,
+      )
+    }
+    False -> found
+  }
+}
 
-  let x3_reverse = float.round(x3f_reverse)
-  let y3_reverse = float.round(y3f_reverse)
+fn find_all_harmonic_collinears(
+  point_a: Point,
+  point_b: Point,
+  grid: Grid,
+) -> List(Point) {
+  list.append(
+    find_forward_harmonic_collinears([point_a, point_b], grid),
+    find_reverse_harmonic_collinears([point_a, point_b], grid),
+  )
+}
 
-  [#(x3_forward, y3_forward), #(x3_reverse, y3_reverse)]
+fn find_all_collinears(point_a: Point, point_b: Point) -> List(Point) {
+  [
+    find_collinear_in_direction(point_a, point_b, Forward),
+    find_collinear_in_direction(point_a, point_b, Reverse),
+  ]
 }
 
 fn is_not_empty(cell: String) -> Bool {
@@ -77,7 +163,20 @@ fn collinears_for_antenna(antenna: String, g: Grid) -> List(Point) {
   |> list.combinations(2)
   |> list.flat_map(fn(points) {
     case points {
-      [l1, l2] -> find_collinears(l1, l2)
+      [l1, l2] -> find_all_collinears(l1, l2)
+      _ -> []
+    }
+  })
+  |> list.filter(point_within_map(_, g))
+  |> list.unique
+}
+
+fn harmonic_collinears_for_antenna(antenna: String, g: Grid) -> List(Point) {
+  find_antenna_locations(antenna, g)
+  |> list.combinations(2)
+  |> list.flat_map(fn(points) {
+    case points {
+      [l1, l2] -> find_all_harmonic_collinears(l1, l2, g)
       _ -> []
     }
   })
@@ -105,6 +204,12 @@ pub fn solve_a(input: String) -> Int {
   |> list.length
 }
 
-pub fn solve_b(_inp: String) -> Int {
-  0
+pub fn solve_b(input: String) -> Int {
+  let grid = parse_input(input)
+
+  list.filter(list.unique(dict.values(grid)), is_not_empty)
+  |> list.map(harmonic_collinears_for_antenna(_, grid))
+  |> list.flatten
+  |> list.unique
+  |> list.length
 }
diff --git a/test/resonant_collinearity_test.gleam b/test/resonant_collinearity_test.gleam
index 9c73c27..57f2d9d 100644
--- a/test/resonant_collinearity_test.gleam
+++ b/test/resonant_collinearity_test.gleam
@@ -18,5 +18,5 @@ pub fn solve_a_test() {
 }
 
 pub fn solve_b_test() {
-  should.equal(day08.solve_b(test_data()), 0)
+  should.equal(day08.solve_b(test_data()), 34)
 }