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
		}
	}
}