python - Scapy layer for MPEG2-TS, with conditionalField for adaptation -


i try create mpeg2-ts layer scapy.

i need "add" , parse additional fields based on adaptation field.

here code:

_adapt_type = { 0x0 : 'reserved', 0x1 : 'payload only', 0x2 : 'adaptation only', 0x3 : 'adaptation , payload', }  class mpeg2_ts(packet):     name = "mpeg2-ts"     fields_desc=[              byteenumfield('sync_byte', 0x47, _sync_type),             bitfield('trans_err',0x0, 1),             # transport error indicator (tei) if set 1 decoder ignore packet              bitfield('start_ind',0x0, 1),             bitfield('trans_pri',0x0, 1),             xbitfield('pid', 0xa, 13),             bitenumfield('trans_scramb', 0x0, 2, _scramble_type),             bitenumfield('adapt_ctrl', 0x0, 2, _adapt_type),             bitfield('cont_count', 0x0, 4),             ]

i need parse additional data if adapt_ctrl set to:

0x1 : 'payload only', 0x2 : 'adaptation only', 0x3 : 'adaptation , payload'

how can create condition able parse base on result of field ? if "payload only" bit set, there no adaptation fields after header, didn't need parse it. if "adaptation , payload" set, need parse adaptation fields using following structure:

_adapt_fields = [              # number of bytes in adaptation field following byte             bytefield('ada_len', 0x0),              # set 1 if current ts packet in discontinuity state respect either continuity counter or program clock reference             bitfield('dis', 0x0, 1),              # set 1 if pes packet in ts packet starts video/audio sequence             bitfield('ran_acc', 0x0, 1),              # 1 = higher priority             bitfield('es_str_pri', 0x0, 1),              # set 1 if adaptation field contains pcr field             bitfield('pcr_fla', 0x0, 1),                              # set 1 if adaptation field contains opcr field             bitfield('opcr_fla', 0x0, 1),              # set 1 if adaptation field contains splice countdown field             bitfield('spl_fla', 0x0, 1),              # set 1 if adaptation field contains private data bytes             bitfield('trs_pri_dat_fla', 0x0, 1),              # set 1 if adaptation field contains extension             bitfield('ada_ext_fla', 0x0, 1),                              # variable depends on flags             # bitfield('ada_ext_fla', 0x0, 1),                           # pcr   33+6+9      program clock reference, stored in 6 octets in big-endian 33 bits base, 6 bits padding, 9 bits extension.             # opcr  33+6+9      original program clock reference. helps when 1 ts copied             # splice countdown  8       indicates how many ts packets 1 splicing point occurs (may negative)             # stuffing bytes    variable                     ]

i tried conditionalfield without luck till now. help.


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