1 files changed, 151 insertions, 0 deletions
diff --git a/external/construct/formats/graphics/gif.py b/external/construct/formats/graphics/gif.py
new file mode 100644
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+++ b/external/construct/formats/graphics/gif.py
@@ -0,0 +1,151 @@
+# Contributed by
+# Dany Zatuchna (danzat at gmail)
+""" Implementation of the following grammar for the GIF89a file format
+<GIF Data Stream> ::=     Header <Logical Screen> <Data>* Trailer
+
+<Logical Screen> ::=      Logical Screen Descriptor [Global Color Table]
+
+<Data> ::=                <Graphic Block>  |
+                          <Special-Purpose Block>
+
+<Graphic Block> ::=       [Graphic Control Extension] <Graphic-Rendering Block>
+
+<Graphic-Rendering Block> ::=  <Table-Based Image>  |
+                               Plain Text Extension
+
+<Table-Based Image> ::=   Image Descriptor [Local Color Table] Image Data
+
+<Special-Purpose Block> ::=    Application Extension  |
+                               Comment Extension
+"""
+from construct import *
+import six
+
+
+data_sub_block = Struct("data_sub_block",
+    ULInt8("size"),
+    String("data", lambda ctx: ctx["size"])
+)
+
+gif_logical_screen = Struct("logical_screen",
+    ULInt16("width"),
+    ULInt16("height"),
+    BitStruct("flags",
+        Bit("global_color_table"),
+        BitField("color_resolution", 3),
+        Bit("sort_flag"),
+        BitField("global_color_table_bpp", 3)
+    ),
+    ULInt8("bgcolor_index"),
+    ULInt8("pixel_aspect_ratio"),
+    If(lambda ctx: ctx["flags"]["global_color_table"],
+        Array(lambda ctx: 2**(ctx["flags"]["global_color_table_bpp"] + 1),
+            Struct("palette",
+                ULInt8("R"),
+                ULInt8("G"),
+                ULInt8("B")
+            )
+        )
+    )
+)
+
+gif_header = Struct("gif_header",
+    Const(String("signature", 3), six.b("GIF")),
+    Const(String("version", 3), six.b("89a")),
+)
+
+application_extension = Struct("application_extension",
+    Const(ULInt8("block_size"), 11),
+    String("application_identifier", 8),
+    String("application_auth_code", 3),
+    data_sub_block,
+    ULInt8("block_terminator")
+)
+
+comment_extension = Struct("comment_extension",
+    data_sub_block,
+    ULInt8("block_terminator")
+)
+
+graphic_control_extension = Struct("graphic_control_extension",
+    Const(ULInt8("block_size"), 4),
+    BitStruct("flags",
+        BitField("reserved", 3),
+        BitField("disposal_method", 3),
+        Bit("user_input_flag"),
+        Bit("transparent_color_flag"),
+    ),
+    ULInt16("delay"),
+    ULInt8("transparent_color_index"),
+    ULInt8("block_terminator")
+)
+
+plain_text_extension = Struct("plain_text_extension",
+    Const(ULInt8("block_size"), 12),
+    ULInt16("text_left"),
+    ULInt16("text_top"),
+    ULInt16("text_width"),
+    ULInt16("text_height"),
+    ULInt8("cell_width"),
+    ULInt8("cell_height"),
+    ULInt8("foreground_index"),
+    ULInt8("background_index"),
+    data_sub_block,
+    ULInt8("block_terminator")
+)
+
+extension = Struct("extension",
+    ULInt8("label"),
+    Switch("ext", lambda ctx: ctx["label"], {
+        0xFF: application_extension,
+        0xFE: comment_extension,
+        0xF9: graphic_control_extension,
+        0x01: plain_text_extension
+    })
+)
+
+image_descriptor = Struct("image_descriptor",
+    ULInt16("left"),
+    ULInt16("top"),
+    ULInt16("width"),
+    ULInt16("height"),
+    BitStruct("flags",
+        Bit("local_color_table"),
+        Bit("interlace"),
+        Bit("sort"),
+        BitField("reserved", 2),
+        BitField("local_color_table_bpp", 3)
+    ),
+    If(lambda ctx: ctx["flags"]["local_color_table"],
+        Array(lambda ctx: 2**(ctx["flags"]["local_color_table_bpp"] + 1),
+            Struct("palette",
+                ULInt8("R"),
+                ULInt8("G"),
+                ULInt8("B")
+            )
+        )
+    ),
+    ULInt8("LZW_minimum_code_size"),
+    RepeatUntil(lambda obj, ctx: obj.size == 0, data_sub_block)
+)
+
+gif_data = Struct("gif_data",
+    ULInt8("introducer"),
+    Switch("dat", lambda ctx: ctx["introducer"], {
+        0x21: extension,
+        0x2C: image_descriptor
+    })
+)
+
+gif_file = Struct("gif_file",
+    gif_header,
+    gif_logical_screen,
+    OptionalGreedyRange(gif_data),
+    #Const(ULInt8("trailer"), 0x3B)
+)
+
+if __name__ == "__main__":
+    f = open("../../../tests/sample.gif", "rb")
+    s = f.read()
+    f.close()
+    print(gif_file.parse(s))