1 files changed, 55 insertions, 0 deletions
diff --git a/src/image.gleam b/src/image.gleam
new file mode 100644
index 0000000..6672b46
--- /dev/null
+++ b/src/image.gleam
@@ -0,0 +1,55 @@
+import gleam/int
+
+pub type ImageDimensions {
+  ImageDimensions(width: Int, height: Int)
+}
+
+pub fn get_image_dimensions(image) {
+  case image {
+    <<0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A, _:bits>> ->
+      parse_png_chunks(image, 8)
+    <<
+      0x47,
+      0x49,
+      0x46,
+      _:16,
+      _:unsigned,
+      width_0:8,
+      width_1:8,
+      height_0:8,
+      height_1:8,
+      _rest:bits,
+    >> ->
+      Ok(ImageDimensions(
+        width_0
+          |> int.bitwise_or(
+          width_1
+          |> int.bitwise_shift_left(8),
+        ),
+        height_0
+          |> int.bitwise_or(
+          height_1
+          |> int.bitwise_shift_left(8),
+        ),
+      ))
+    _ -> Error("Invalid PNG signature")
+  }
+}
+
+fn parse_png_chunks(image, offset) {
+  let offset_bits = offset * 8
+
+  case image {
+    <<
+      _:size(offset_bits),
+      _length:32,
+      "IHDR":utf8,
+      width:32,
+      height:32,
+      _:bits,
+    >> -> Ok(ImageDimensions(width, height))
+    <<_:size(offset), length:32, _:4, _:bits>> ->
+      parse_png_chunks(image, offset + length + 12)
+    _ -> Error("Invalid PNG chunk")
+  }
+}