add ptr+len decode dimens

src/pixie/fileformats/bmp.nim

@@ -228,19 +228,27 @@ proc decodeBmp*(data: string): Image {.raises: [PixieError].} =
   decodeDib(data[14].unsafeAddr, data.len - 14)
 
 proc decodeBmpDimensions*(
-  data: string
+  data: pointer, len: int
 ): ImageDimensions {.raises: [PixieError].} =
   ## Decodes the BMP dimensions.
-  if data.len < 26:
+  if len < 26:
     failInvalid()
 
+  let data = cast[ptr UncheckedArray[uint8]](data)
+
   # BMP Header
-  if data[0 .. 1] != "BM":
+  if data[0].char != 'B' or data[1].char != 'M': # Must start with BM
     failInvalid()
 
   result.width = data.readInt32(18).int
   result.height = abs(data.readInt32(22)).int
 
+proc decodeBmpDimensions*(
+  data: string
+): ImageDimensions {.raises: [PixieError].} =
+  ## Decodes the BMP dimensions.
+  decodeBmpDimensions(data.cstring, data.len)
+
 proc encodeDib*(image: Image): string {.raises: [].} =
   ## Encodes an image into a DIB.
 

src/pixie/fileformats/gif.nim

@@ -379,18 +379,29 @@ proc decodeGif*(data: string): Gif {.raises: [PixieError].} =
     result.duration += interval
 
 proc decodeGifDimensions*(
-  data: string
+  data: pointer, len: int
 ): ImageDimensions {.raises: [PixieError].} =
   ## Decodes the GIF dimensions.
-  if data.len < 10:
+  if len < 10:
     failInvalid()
 
-  if data[0 .. 5] notin gifSignatures:
+  let data = cast[ptr UncheckedArray[uint8]](data)
+
+  let startsWithSignature =
+    equalMem(data, gifSignatures[0].cstring, 6) or
+    equalMem(data, gifSignatures[1].cstring, 6)
+
+  if not startsWithSignature:
     raise newException(PixieError, "Invalid GIF file signature")
 
   result.width = data.readInt16(6).int
   result.height = data.readInt16(8).int
 
+proc decodeGifDimensions*(
+  data: string
+): ImageDimensions {.raises: [PixieError].} =
+  decodeGifDimensions(data.cstring, data.len)
+
 proc newImage*(gif: Gif): Image {.raises: [].} =
   gif.frames[0].copy()