LibDraw: Start work on a GIF decoder (not yet functional!)

Here comes the first part of a GIF decoder. It decodes up to the point
of gathering all the LZW-compressed data. The next step is to implement
decompression, and then turn the decompressed data into a bitmap using
the color maps, etc.

Libraries/LibDraw/GIFLoader.cpp

@@ -0,0 +1,235 @@
+#include <AK/BufferStream.h>
+#include <AK/ByteBuffer.h>
+#include <AK/FileSystemPath.h>
+#include <AK/NonnullOwnPtrVector.h>
+#include <LibDraw/GIFLoader.h>
+#include <stdio.h>
+
+static RefPtr<GraphicsBitmap> load_gif_impl(const u8*, size_t);
+
+RefPtr<GraphicsBitmap> load_gif(const StringView& path)
+{
+    MappedFile mapped_file(path);
+    if (!mapped_file.is_valid())
+        return nullptr;
+    auto bitmap = load_gif_impl((const u8*)mapped_file.data(), mapped_file.size());
+    if (bitmap)
+        bitmap->set_mmap_name(String::format("GraphicsBitmap [%dx%d] - Decoded GIF: %s", bitmap->width(), bitmap->height(), canonicalized_path(path).characters()));
+    return bitmap;
+}
+
+RefPtr<GraphicsBitmap> load_gif_from_memory(const u8* data, size_t length)
+{
+    auto bitmap = load_gif_impl(data, length);
+    if (bitmap)
+        bitmap->set_mmap_name(String::format("GraphicsBitmap [%dx%d] - Decoded GIF: <memory>", bitmap->width(), bitmap->height()));
+    return bitmap;
+}
+
+enum class GIFFormat {
+    GIF87a,
+    GIF89a,
+};
+
+struct RGB {
+    u8 r;
+    u8 g;
+    u8 b;
+};
+
+struct LogicalScreen {
+    u16 width;
+    u16 height;
+    RGB color_map[256];
+};
+
+struct ImageDescriptor {
+    u16 x;
+    u16 y;
+    u16 width;
+    u16 height;
+    bool use_global_color_map;
+    RGB color_map[256];
+    u8 lzw_min_code_size;
+    Vector<u8> lzw_encoded_bytes;
+};
+
+RefPtr<GraphicsBitmap> load_gif_impl(const u8* data, size_t data_size)
+{
+    if (data_size < 32)
+        return nullptr;
+
+    auto buffer = ByteBuffer::wrap(data, data_size);
+    BufferStream stream(buffer);
+
+    static const char valid_header_87[] = "GIF87a";
+    static const char valid_header_89[] = "GIF89a";
+
+    char header[6];
+    for (int i = 0; i < 6; ++i)
+        stream >> header[i];
+
+    GIFFormat format;
+    if (!memcmp(header, valid_header_87, sizeof(header)))
+        format = GIFFormat::GIF87a;
+    else if (!memcmp(header, valid_header_89, sizeof(header)))
+        format = GIFFormat::GIF89a;
+    else
+        return nullptr;
+
+    printf("Format is %s\n", format == GIFFormat::GIF89a ? "GIF89a" : "GIF87a");
+
+    LogicalScreen logical_screen;
+    stream >> logical_screen.width;
+    stream >> logical_screen.height;
+    if (stream.handle_read_failure())
+        return nullptr;
+
+    u8 gcm_info = 0;
+    stream >> gcm_info;
+
+    if (stream.handle_read_failure())
+        return nullptr;
+
+    bool global_color_map_follows_descriptor = gcm_info & 0x80;
+    u8 bits_per_pixel = (gcm_info & 7) + 1;
+    u8 bits_of_color_resolution = (gcm_info >> 4) & 7;
+
+    printf("LogicalScreen: %dx%d\n", logical_screen.width, logical_screen.height);
+    printf("global_color_map_follows_descriptor: %u\n", global_color_map_follows_descriptor);
+    printf("bits_per_pixel: %u\n", bits_per_pixel);
+    printf("bits_of_color_resolution: %u\n", bits_of_color_resolution);
+
+    u8 background_color = 0;
+    stream >> background_color;
+    if (stream.handle_read_failure())
+        return nullptr;
+
+    printf("background_color: %u\n", background_color);
+
+    u8 pixel_aspect_ratio = 0;
+    stream >> pixel_aspect_ratio;
+    if (stream.handle_read_failure())
+        return nullptr;
+
+    int color_map_entry_count = 1;
+    for (int i = 0; i < bits_per_pixel; ++i)
+        color_map_entry_count *= 2;
+
+    printf("color_map_entry_count: %d\n", color_map_entry_count);
+
+    for (int i = 0; i < color_map_entry_count; ++i) {
+        stream >> logical_screen.color_map[i].r;
+        stream >> logical_screen.color_map[i].g;
+        stream >> logical_screen.color_map[i].b;
+    }
+
+    if (stream.handle_read_failure())
+        return nullptr;
+
+    for (int i = 0; i < color_map_entry_count; ++i) {
+        auto& rgb = logical_screen.color_map[i];
+        printf("[%02x]: %s\n", i, Color(rgb.r, rgb.g, rgb.b).to_string().characters());
+    }
+
+    NonnullOwnPtrVector<ImageDescriptor> images;
+
+    for (;;) {
+        u8 sentinel = 0;
+        stream >> sentinel;
+        printf("Sentinel: %02x\n", sentinel);
+
+        if (sentinel == 0x21) {
+            u8 extension_type = 0;
+            stream >> extension_type;
+            if (stream.handle_read_failure())
+                return nullptr;
+
+            printf("Extension block of type %02x\n", extension_type);
+
+            u8 sub_block_length = 0;
+
+            for (;;) {
+                stream >> sub_block_length;
+
+                if (stream.handle_read_failure())
+                    return nullptr;
+
+                if (sub_block_length == 0)
+                    break;
+
+                u8 dummy;
+                for (u16 i = 0; i < sub_block_length; ++i)
+                    stream >> dummy;
+
+                if (stream.handle_read_failure())
+                    return nullptr;
+            }
+            continue;
+        }
+
+        if (sentinel == 0x2c) {
+            images.append(make<ImageDescriptor>());
+            auto& image = images.last();
+            u8 packed_fields;
+            stream >> image.x;
+            stream >> image.y;
+            stream >> image.width;
+            stream >> image.height;
+            stream >> packed_fields;
+            if (stream.handle_read_failure())
+                return nullptr;
+            printf("Image descriptor: %d,%d %dx%d, %02x\n", image.x, image.y, image.width, image.height, packed_fields);
+
+            stream >> image.lzw_min_code_size;
+
+            printf("min code size: %u\n", image.lzw_min_code_size);
+
+            u8 lzw_encoded_bytes_expected = 0;
+
+            for (;;) {
+                stream >> lzw_encoded_bytes_expected;
+
+                if (stream.handle_read_failure())
+                    return nullptr;
+
+                if (lzw_encoded_bytes_expected == 0)
+                    break;
+
+                u8 buffer[256];
+                for (int i = 0; i < lzw_encoded_bytes_expected; ++i) {
+                    stream >> buffer[i];
+                }
+
+                if (stream.handle_read_failure())
+                    return nullptr;
+
+                for (int i = 0; i < lzw_encoded_bytes_expected; ++i) {
+                    image.lzw_encoded_bytes.append(buffer[i]);
+                }
+            }
+            continue;
+        }
+
+        if (sentinel == 0x3b) {
+            printf("Trailer! Awesome :)\n");
+            break;
+        }
+
+        return nullptr;
+    }
+
+    // We exited the block loop after finding a trailer. We should have everything needed.
+    printf("Image count: %d\n", images.size());
+    if (images.is_empty())
+        return nullptr;
+
+    for (int i = 0; i < images.size(); ++i) {
+        auto& image = images.at(i);
+        printf("Image %d: %d,%d %dx%d  %d bytes LZW-encoded\n", i, image.x, image.y, image.width, image.height, image.lzw_encoded_bytes.size());
+
+        // FIXME: Decode the LZW-encoded bytes and turn them into an image.
+    }
+
+    return nullptr;
+}