package imaging

import (
	"bytes"
	"encoding/binary"
)

const (
	tiffOrientationTag = 0x0112
	tiffTypeShort      = 3
	tiffHeaderSize     = 8
	tiffEntrySize      = 12
	tiffMagic          = 42
	uprightOrientation = 1
	maxOrientation     = 8
)

var exifHeader = []byte("Exif\x00\x00")

func exifOrientation(payload []byte) (uint16, bool) {
	tiff := bytes.TrimPrefix(payload, exifHeader)
	if len(tiff) < tiffHeaderSize {
		return 0, false
	}
	var order binary.ByteOrder
	switch string(tiff[:2]) {
	case "II":
		order = binary.LittleEndian
	case "MM":
		order = binary.BigEndian
	default:
		return 0, false
	}
	if order.Uint16(tiff[2:]) != tiffMagic {
		return 0, false
	}
	ifd := int(order.Uint32(tiff[4:]))
	if ifd < tiffHeaderSize || ifd+2 > len(tiff) {
		return 0, false
	}
	count := int(order.Uint16(tiff[ifd:]))
	for i := range count {
		entry := ifd + 2 + i*tiffEntrySize
		if entry+tiffEntrySize > len(tiff) {
			return 0, false
		}
		if order.Uint16(tiff[entry:]) != tiffOrientationTag || order.Uint16(tiff[entry+2:]) != tiffTypeShort {
			continue
		}
		value := order.Uint16(tiff[entry+8:])
		if value < uprightOrientation || value > maxOrientation {
			return 0, false
		}
		return value, true
	}
	return 0, false
}

func needsOrientation(orientation uint16) bool {
	return orientation > uprightOrientation
}

func orientationOnlyTIFF(orientation uint16) []byte {
	tiff := make([]byte, 0, tiffHeaderSize+2+tiffEntrySize+4)
	tiff = append(tiff, 'M', 'M')
	tiff = binary.BigEndian.AppendUint16(tiff, tiffMagic)
	tiff = binary.BigEndian.AppendUint32(tiff, tiffHeaderSize)
	tiff = binary.BigEndian.AppendUint16(tiff, 1)
	tiff = binary.BigEndian.AppendUint16(tiff, tiffOrientationTag)
	tiff = binary.BigEndian.AppendUint16(tiff, tiffTypeShort)
	tiff = binary.BigEndian.AppendUint32(tiff, 1)
	tiff = binary.BigEndian.AppendUint16(tiff, orientation)
	tiff = binary.BigEndian.AppendUint16(tiff, 0)
	return binary.BigEndian.AppendUint32(tiff, 0)
}