// Minimal Ogg decoder according to rfc3533
// (https://www.xiph.org/ogg/doc/rfc3533.txt). Whenever 'the spec' is mentioned,
// it is referring to rfc3533.
package vorbis

import (
	"bytes"
	"encoding/binary"
	"errors"
	"io"
)

var (
	ErrOggInvalidMagicNumber = errors.New("ogg: invalid magic number")
	ErrOggInvalidChecksum    = errors.New("ogg: invalid checksum")
)

const (
	headerSize = 27
	// Uncombined segments are meant here (see Page.Segments or the spec for
	// more details about segments).
	maxSegments    = 255
	maxSegmentSize = 255
	maxPageSize    = headerSize + maxSegments + maxSegments*maxSegmentSize
)

// Using polynomial 0x04c11db7 (see the spec).
var crcLookup = [256]uint32{
	0x00000000, 0x04c11db7, 0x09823b6e, 0x0d4326d9,
	0x130476dc, 0x17c56b6b, 0x1a864db2, 0x1e475005,
	0x2608edb8, 0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61,
	0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
	0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9,
	0x5f15adac, 0x5bd4b01b, 0x569796c2, 0x52568b75,
	0x6a1936c8, 0x6ed82b7f, 0x639b0da6, 0x675a1011,
	0x791d4014, 0x7ddc5da3, 0x709f7b7a, 0x745e66cd,
	0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
	0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5,
	0xbe2b5b58, 0xbaea46ef, 0xb7a96036, 0xb3687d81,
	0xad2f2d84, 0xa9ee3033, 0xa4ad16ea, 0xa06c0b5d,
	0xd4326d90, 0xd0f37027, 0xddb056fe, 0xd9714b49,
	0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
	0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1,
	0xe13ef6f4, 0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d,
	0x34867077, 0x30476dc0, 0x3d044b19, 0x39c556ae,
	0x278206ab, 0x23431b1c, 0x2e003dc5, 0x2ac12072,
	0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
	0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca,
	0x7897ab07, 0x7c56b6b0, 0x71159069, 0x75d48dde,
	0x6b93dddb, 0x6f52c06c, 0x6211e6b5, 0x66d0fb02,
	0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1, 0x53dc6066,
	0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
	0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e,
	0xbfa1b04b, 0xbb60adfc, 0xb6238b25, 0xb2e29692,
	0x8aad2b2f, 0x8e6c3698, 0x832f1041, 0x87ee0df6,
	0x99a95df3, 0x9d684044, 0x902b669d, 0x94ea7b2a,
	0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
	0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2,
	0xc6bcf05f, 0xc27dede8, 0xcf3ecb31, 0xcbffd686,
	0xd5b88683, 0xd1799b34, 0xdc3abded, 0xd8fba05a,
	0x690ce0ee, 0x6dcdfd59, 0x608edb80, 0x644fc637,
	0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
	0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f,
	0x5c007b8a, 0x58c1663d, 0x558240e4, 0x51435d53,
	0x251d3b9e, 0x21dc2629, 0x2c9f00f0, 0x285e1d47,
	0x36194d42, 0x32d850f5, 0x3f9b762c, 0x3b5a6b9b,
	0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
	0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623,
	0xf12f560e, 0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7,
	0xe22b20d2, 0xe6ea3d65, 0xeba91bbc, 0xef68060b,
	0xd727bbb6, 0xd3e6a601, 0xdea580d8, 0xda649d6f,
	0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
	0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7,
	0xae3afba2, 0xaafbe615, 0xa7b8c0cc, 0xa379dd7b,
	0x9b3660c6, 0x9ff77d71, 0x92b45ba8, 0x9675461f,
	0x8832161a, 0x8cf30bad, 0x81b02d74, 0x857130c3,
	0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
	0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c,
	0x7b827d21, 0x7f436096, 0x7200464f, 0x76c15bf8,
	0x68860bfd, 0x6c47164a, 0x61043093, 0x65c52d24,
	0x119b4be9, 0x155a565e, 0x18197087, 0x1cd86d30,
	0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
	0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088,
	0x2497d08d, 0x2056cd3a, 0x2d15ebe3, 0x29d4f654,
	0xc5a92679, 0xc1683bce, 0xcc2b1d17, 0xc8ea00a0,
	0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb, 0xdbee767c,
	0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
	0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4,
	0x89b8fd09, 0x8d79e0be, 0x803ac667, 0x84fbdbd0,
	0x9abc8bd5, 0x9e7d9662, 0x933eb0bb, 0x97ffad0c,
	0xafb010b1, 0xab710d06, 0xa6322bdf, 0xa2f33668,
	0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4,
}

type crc32Writer struct {
	sum uint32
}

func (w *crc32Writer) Write(p []byte) (n int, err error) {
	for _, v := range p {
		w.sum = (w.sum << 8) ^ crcLookup[uint8(w.sum>>24)^v]
	}
	return len(p), nil
}

var (
	FHeaderTypeContinuation = uint8(0x1)
	FHeaderTypeBOS          = uint8(0x2) // Beginning of stream
	FHeaderTypeEOS          = uint8(0x4) // End of stream
)

// See the spec for more info on the individual fields.
type OggPageHeader struct {
	MagicNumber        [4]uint8
	Version            uint8
	HeaderType         uint8
	GranulePosition    uint64
	BitstreamSerialNum uint32
	PageSequenceNum    uint32
	Checksum           uint32
	NumSegments        uint8 // Number of uncombined segments (see Page.Segments or the spec for more details).
}

type OggPage struct {
	Header OggPageHeader
	// These are the combined segments. The spec says that when a segment is
	// said to have a length of 255, it is to be combined with the next segment,
	// which is already done here.
	Segments [][]byte
}

// Decodes the given raw Ogg page according to rfc3533
// (https://www.xiph.org/ogg/doc/rfc3533.txt).
// The given reader `r` must be set to start at the beginning of the page, i.e.
// at the magic number "OggS".
// Leaves the reader position at the beginning of the next page (or EOF if
// the decoded page was the last page).
func OggDecode(r io.Reader) (OggPage, error) {
	var ret OggPage

	checksum := &crc32Writer{}

	// Read header into struct and into byte buffer.
	var rawHeaderBuf bytes.Buffer
	hdrR := io.TeeReader(r, &rawHeaderBuf)
	err := binary.Read(hdrR, binary.LittleEndian, &ret.Header)
	if err != nil {
		return ret, err
	}

	// Validate magic number.
	if string(ret.Header.MagicNumber[:]) != "OggS" {
		return ret, ErrOggInvalidMagicNumber
	}

	// Get raw header bytes, then set the header checksum field to 0 before
	// calculating the checksum (see the spec).
	rawHeader, err := io.ReadAll(&rawHeaderBuf)
	if err != nil {
		return ret, err
	}
	for i := 22; i <= 25; i++ {
		rawHeader[i] = 0
	}

	// Add raw header to checksum.
	checksum.Write(rawHeader) // Can't give an error (see implementation).

	// From now on, write everything that is read into the checksum.
	teeR := io.TeeReader(r, checksum)

	// Read the sizes of all segments in the page.
	segsizes := make([]byte, ret.Header.NumSegments)
	teeR.Read(segsizes)

	// Whether to append to the last segment. According to the spec, whenever
	// a segment length is specified as being 255, that means the next segment
	// should be appended to it.
	var app bool

	// Parse the segments, respecting combining segments (see the spec).
	for _, v := range segsizes {
		sz := int(v)
		content := make([]byte, sz)
		teeR.Read(content)

		if app {
			lastSeg := &ret.Segments[len(ret.Segments)-1]
			*lastSeg = append(*lastSeg, content...)
		} else {
			ret.Segments = append(ret.Segments, content)
		}
		app = sz == 255
	}

	// Verify the checksum.
	if checksum.sum != ret.Header.Checksum {
		return ret, ErrOggInvalidChecksum
	}

	return ret, nil
}