feat: image variants engine

internal/domain/image/animatedGif.go

@@ -0,0 +1,114 @@
+package image
+
+import (
+	"bufio"
+	"errors"
+	"io"
+)
+
+var ErrNotAGif = errors.New("not a gif")
+
+const (
+	gifSignaturePrefix       = "GIF"
+	gifHeaderLength          = 6
+	gifScreenDescriptorSize  = 7
+	gifImageDescriptorSize   = 9
+	gifExtensionIntroducer   = 0x21
+	gifImageSeparator        = 0x2C
+	gifTrailer               = 0x3B
+	gifColorTableFlag        = 0x80
+	gifColorTableSizeMask    = 0x07
+	gifColorTableEntryBytes  = 3
+	gifFramesBeforeAnimation = 2
+)
+
+func IsAnimatedGIF(source io.Reader) (bool, error) {
+	reader := bufio.NewReader(source)
+	if err := skipGifPreamble(reader); err != nil {
+		return false, err
+	}
+	frames := 0
+	for {
+		marker, err := reader.ReadByte()
+		if err != nil {
+			return false, err
+		}
+		switch marker {
+		case gifTrailer:
+			return false, nil
+		case gifExtensionIntroducer:
+			err = skipGifExtension(reader)
+		case gifImageSeparator:
+			frames++
+			if frames >= gifFramesBeforeAnimation {
+				return true, nil
+			}
+			err = skipGifImage(reader)
+		default:
+			return false, ErrNotAGif
+		}
+		if err != nil {
+			return false, err
+		}
+	}
+}
+
+func skipGifPreamble(reader *bufio.Reader) error {
+	header := make([]byte, gifHeaderLength)
+	if _, err := io.ReadFull(reader, header); err != nil {
+		return err
+	}
+	if string(header[:len(gifSignaturePrefix)]) != gifSignaturePrefix {
+		return ErrNotAGif
+	}
+	screen := make([]byte, gifScreenDescriptorSize)
+	if _, err := io.ReadFull(reader, screen); err != nil {
+		return err
+	}
+	return skipGifColorTable(reader, screen[4])
+}
+
+func skipGifColorTable(reader *bufio.Reader, packed byte) error {
+	if packed&gifColorTableFlag == 0 {
+		return nil
+	}
+	entries := 1 << (int(packed&gifColorTableSizeMask) + 1)
+	_, err := reader.Discard(entries * gifColorTableEntryBytes)
+	return err
+}
+
+func skipGifExtension(reader *bufio.Reader) error {
+	if _, err := reader.ReadByte(); err != nil {
+		return err
+	}
+	return skipGifSubBlocks(reader)
+}
+
+func skipGifImage(reader *bufio.Reader) error {
+	descriptor := make([]byte, gifImageDescriptorSize)
+	if _, err := io.ReadFull(reader, descriptor); err != nil {
+		return err
+	}
+	if err := skipGifColorTable(reader, descriptor[8]); err != nil {
+		return err
+	}
+	if _, err := reader.ReadByte(); err != nil {
+		return err
+	}
+	return skipGifSubBlocks(reader)
+}
+
+func skipGifSubBlocks(reader *bufio.Reader) error {
+	for {
+		size, err := reader.ReadByte()
+		if err != nil {
+			return err
+		}
+		if size == 0 {
+			return nil
+		}
+		if _, err := reader.Discard(int(size)); err != nil {
+			return err
+		}
+	}
+}