// RFC3640 RTP Payload Format for Transport of MPEG-4 Elementary Streams

#include <assert.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>

#include "rtp-packet.h"
#include "rtp-payload-helper.h"
#include "rtp-payload-internal.h"
#include "rtp-profile.h"

static int rtp_decode_mpeg4_generic(void *p, const void *packet, int bytes) {
    int i, size;
    int au_size;
    int au_numbers;
    int au_header_length;
    const uint8_t *ptr, *pau, *pend;
    struct rtp_packet_t pkt;
    struct rtp_payload_helper_t *helper;

    helper = (struct rtp_payload_helper_t *)p;
    // 1. 反序列化，把RTP 头解析出来
    if (!helper || 0 != rtp_packet_deserialize(&pkt, packet, bytes) || pkt.payloadlen < 4) return -EINVAL;

    rtp_payload_check(helper, &pkt);

    // save payload
    ptr = (const uint8_t *)pkt.payload;  // 这里还是包括了AU_header_length au_header占用的字节的
    pend = ptr + pkt.payloadlen;

    // AU-headers-length AU-headers-length是固定占用2个字节
    au_header_length = (ptr[0] << 8) + ptr[1];      // 读取长度
    au_header_length = (au_header_length + 7) / 8;  // bit -> byte   计算出来au_heade占用的字节情况

    if (ptr + au_header_length /*AU-size*/ > pend || au_header_length < 2) {
        assert(0);
        // helper->size = 0;
        helper->lost = 1;
        // helper->flags |= RTP_PAYLOAD_FLAG_PACKET_LOST;
        return -1;  // invalid packet
    }

    // 3.3.6. High Bit-rate AAC
    // SDP fmtp: sizeLength=13; indexLength=3; indexDeltaLength=3; 一般au header占用2字节，其他格式不支持
    au_size = 2;                              // only AU-size
    au_numbers = au_header_length / au_size;  // 计算au个数
    assert(0 == au_header_length % au_size);
    ptr += 2;                      // skip AU headers length section 2-bytes ; 此时ptr指向au header的起始位置
    pau = ptr + au_header_length;  // point to Access Unit 跳过AU header才是真正的数据; 此时pau指向的是au的起始位置

    for (i = 0; i < au_numbers; i++) {
        size = (ptr[0] << 8) | (ptr[1] & 0xF8);  // 获取au的size
        size = size >> 3;                        //  右移3bit 转成字节
        if (pau + size > pend) {
            assert(0);
            // helper->size = 0;
            helper->lost = 1;
            // helper->flags |= RTP_PAYLOAD_FLAG_PACKET_LOST;
            return -1;  // invalid packet
        }

        // TODO: add ADTS/ASC ???
        pkt.payload = pau;
        pkt.payloadlen = size;
        rtp_payload_write(helper, &pkt);

        ptr += au_size;  // 跳过au header size
        pau += size;     // 跳过au size

        if (au_numbers > 1 || pkt.rtp.m)  // 收到完整数据再发送给应用层
        {
            rtp_payload_onframe(helper);
        }
    }

    return helper->lost ? 0 : 1;  // packet handled
}

struct rtp_payload_decode_t *rtp_mpeg4_generic_decode() {
    static struct rtp_payload_decode_t unpacker = {
        rtp_payload_helper_create,
        rtp_payload_helper_destroy,
        rtp_decode_mpeg4_generic,
    };

    return &unpacker;
}