feat: add image format guessing functionality

native/src/api.rs

@@ -40,6 +40,19 @@ impl ImageFormatEnum {
             ImageFormatEnum::Tiff => ImageFormat::Tiff,
         }
     }
+
+    pub fn from_image_format(format: ImageFormat) -> Option<Self> {
+        match format {
+            ImageFormat::Png => Some(ImageFormatEnum::Png),
+            ImageFormat::Jpeg => Some(ImageFormatEnum::Jpeg),
+            ImageFormat::Gif => Some(ImageFormatEnum::Gif),
+            ImageFormat::WebP => Some(ImageFormatEnum::WebP),
+            ImageFormat::Bmp => Some(ImageFormatEnum::Bmp),
+            ImageFormat::Ico => Some(ImageFormatEnum::Ico),
+            ImageFormat::Tiff => Some(ImageFormatEnum::Tiff),
+            _ => None,
+        }
+    }
 }
 
 #[allow(dead_code)]
@@ -216,6 +229,18 @@ pub fn get_metadata(img: &DynamicImage) -> ImageMetadata {
     }
 }
 
+/// Guess image format from byte data
+pub fn guess_image_format(data: &[u8]) -> Result<ImageFormatEnum, ImageError> {
+    let format = image::guess_format(data)?;
+    ImageFormatEnum::from_image_format(format)
+        .ok_or_else(|| {
+            ImageError::Unsupported(image::error::UnsupportedError::from_format_and_kind(
+                image::error::ImageFormatHint::Unknown,
+                image::error::UnsupportedErrorKind::Format(image::error::ImageFormatHint::Unknown),
+            ))
+        })
+}
+
 /// Convert ImageError to error code
 pub fn error_to_code(err: &ImageError) -> ImageErrorCode {
     match err {

native/src/ffi.rs

@@ -206,6 +206,35 @@ pub extern "C" fn pixer_load_from_memory_with_format_and_error(
     }
 }
 
+// ============================================================================
+// Format Detection
+// ============================================================================
+
+/// Guess image format from byte data
+/// Returns the format enum value or ImageErrorCode on error
+#[unsafe(no_mangle)]
+pub extern "C" fn pixer_guess_format(
+    data: *const u8,
+    len: usize,
+    out_format: *mut u32,
+) -> ImageErrorCode {
+    if data.is_null() || len == 0 || out_format.is_null() {
+        return ImageErrorCode::InvalidPointer;
+    }
+
+    let buffer = unsafe { slice::from_raw_parts(data, len) };
+
+    match guess_image_format(buffer) {
+        Ok(format) => {
+            unsafe {
+                *out_format = format as u32;
+            }
+            ImageErrorCode::Success
+        }
+        Err(e) => error_to_code(&e),
+    }
+}
+
 // ============================================================================
 // Image Saving
 // ============================================================================

packages/pixer/lib/src/bindings/bindings.dart

@@ -22,6 +22,21 @@ external void pixer_free_buffer(ffi.Pointer<ffi.Uint8> ptr, int len);
 @ffi.Native<ffi.Void Function(ffi.Pointer<ImageHandle>)>(isLeaf: true)
 external void pixer_free(ffi.Pointer<ImageHandle> handle);
 
+/// Guess image format from byte data
+/// Returns the format enum value or ImageErrorCode on error
+@ffi.Native<
+  ImageErrorCode$1 Function(
+    ffi.Pointer<ffi.Uint8>,
+    ffi.UintPtr,
+    ffi.Pointer<ffi.Uint32>,
+  )
+>()
+external int pixer_guess_format(
+  ffi.Pointer<ffi.Uint8> data,
+  int len,
+  ffi.Pointer<ffi.Uint32> out_format,
+);
+
 /// Load an image from a file path
 /// Returns null on error
 @ffi.Native<ffi.Pointer<ImageHandle> Function(ffi.Pointer<ffi.Char>)>(

packages/pixer/lib/src/pixer_base.dart

@@ -40,6 +40,36 @@ final class Pixer {
   /// Whether the native resources have been disposed.
   bool get isDisposed => _isDisposed;
 
+  /// Guesses the image format from byte data.
+  ///
+  /// This is useful for detecting the format before encoding, to check
+  /// if the image is already in the desired format.
+  ///
+  /// Throws [PixerException] if the format cannot be detected.
+  static ImageFormatEnum guessFormat(Uint8List data) {
+    final dataPtr = malloc.allocate<ffi.Uint8>(data.length);
+    final outFormatPtr = malloc.allocate<ffi.Uint32>(ffi.sizeOf<ffi.Uint32>());
+
+    try {
+      dataPtr.asTypedList(data.length).setAll(0, data);
+      final errorCode = pixer_guess_format(
+        dataPtr,
+        data.length,
+        outFormatPtr,
+      );
+      final error = ImageErrorCode.fromValue(errorCode);
+      if (error != ImageErrorCode.Success) {
+        throw PixerException.fromCode(error);
+      }
+
+      final formatValue = outFormatPtr.value;
+      return ImageFormatEnum.fromValue(formatValue);
+    } finally {
+      malloc.free(dataPtr);
+      malloc.free(outFormatPtr);
+    }
+  }
+
   /// Loads an image from a file path
   ///
   /// Throws [InvalidPathException] if the path is empty or invalid.