package web

import (
	"bytes"
	"encoding/binary"

	"github.com/disintegration/imaging"
)

// normalizeOrientation returns image bytes that are physically upright.
//
// Phone cameras store a photo in the sensor's native orientation plus an EXIF
// "Orientation" tag telling viewers to rotate it. We bake that rotation into the
// pixels (and drop the tag) so the stored receipt is upright in every consumer —
// the browser, downloads, the AI classifier, a future PDF export — not just ones
// that happen to honor EXIF.
//
// It only acts when the orientation is actually KNOWN: a JPEG carrying an EXIF
// Orientation tag of 2..8. With no tag (the common case — there's no way to know
// which way is up) or an already-upright tag of 1, the original bytes are returned
// verbatim, so we neither guess nor needlessly recompress. PDFs and non-JPEG
// images pass through untouched, as does anything that fails to decode.
func normalizeOrientation(data []byte, mime string) []byte {
	if mime != "image/jpeg" {
		return data
	}
	switch exifOrientation(data) {
	case 0, 1:
		return data // no metadata to act on, or already upright
	}
	// imaging.AutoOrientation reads the EXIF tag and rotates/flips to upright.
	img, err := imaging.Decode(bytes.NewReader(data), imaging.AutoOrientation(true))
	if err != nil {
		return data // undecodable — keep the original rather than lose it
	}
	var buf bytes.Buffer
	if err := imaging.Encode(&buf, img, imaging.JPEG, imaging.JPEGQuality(90)); err != nil {
		return data
	}
	return buf.Bytes()
}

// exifOrientation returns the EXIF Orientation tag (1..8) of a JPEG, or 0 when the
// image has no such tag. It is a minimal, allocation-free scan: it walks the JPEG
// segment markers to the APP1 (Exif) block and reads tag 0x0112 from IFD0.
func exifOrientation(data []byte) int {
	if len(data) < 4 || data[0] != 0xFF || data[1] != 0xD8 { // SOI
		return 0
	}
	i := 2
	for i+4 <= len(data) {
		if data[i] != 0xFF {
			return 0 // not at a marker boundary — give up
		}
		marker := data[i+1]
		if marker == 0xDA || marker == 0xD9 { // SOS (image data begins) or EOI
			return 0
		}
		segLen := int(data[i+2])<<8 | int(data[i+3])
		if segLen < 2 {
			return 0
		}
		segStart, segEnd := i+4, i+2+segLen
		if segEnd > len(data) {
			return 0
		}
		if marker == 0xE1 { // APP1
			if o, ok := orientationFromAPP1(data[segStart:segEnd]); ok {
				return o
			}
		}
		i = segEnd
	}
	return 0
}

// orientationFromAPP1 reads the Orientation tag out of an APP1 segment's payload
// (the "Exif\0\0" header followed by a TIFF block). Returns (0, false) for any
// malformed or non-Exif segment.
func orientationFromAPP1(seg []byte) (int, bool) {
	const hdr = "Exif\x00\x00"
	if len(seg) < len(hdr)+8 || string(seg[:len(hdr)]) != hdr {
		return 0, false
	}
	tiff := seg[len(hdr):]
	var bo binary.ByteOrder
	switch {
	case tiff[0] == 'I' && tiff[1] == 'I':
		bo = binary.LittleEndian
	case tiff[0] == 'M' && tiff[1] == 'M':
		bo = binary.BigEndian
	default:
		return 0, false
	}
	ifdOff := int(bo.Uint32(tiff[4:8]))
	if ifdOff < 8 || ifdOff+2 > len(tiff) {
		return 0, false
	}
	count := int(bo.Uint16(tiff[ifdOff : ifdOff+2]))
	for k, entry := 0, ifdOff+2; k < count; k, entry = k+1, entry+12 {
		if entry+12 > len(tiff) {
			return 0, false
		}
		if bo.Uint16(tiff[entry:entry+2]) == 0x0112 { // Orientation
			val := int(bo.Uint16(tiff[entry+8 : entry+10]))
			if val >= 1 && val <= 8 {
				return val, true
			}
			return 0, false
		}
	}
	return 0, false
}