CSDS Reader

The Continuous Simulation Data Stream is a new output system for SWIFT designed to replace the snapshots in simulation using gravity. This is a submodule of SWIFT containing only the reader.

For details on the coverage, please see https://gitlab.cosma.dur.ac.uk/jenkins/job/CSDS-Pipeline/.

File Description

This description corresponds to the CSDS version 1.4. If this file is not up to date, please look at the following file src/csds_header.c and the function header_read. Except from the header, the file consists in a long list of records (see next section). In order to differentiate the records, a mask is present in the record's header. They are defined within the logfile's header and their value is given by 1 << i where i is the index according to the order given in the header. The two first masks are always reserved, in the order, for the special flags (e.g. deletion / creation of particles) and the timestamp (time information). The special flag has a size of 4 bytes. The first contains the special flag, the next two some related data and finally the particle type. The timestamp consists in a long long (integertime_t in SWIFT) and a double (time or scale factor).

A few variables needs to be defined first:

• STRING_SIZE set to 200. This variable is used for all the strings within the library and the file format name.
• CSDS_OFFSET_SIZE set to 6. This variable provides the number of bytes to use for any offset within the file.
• CSDS_MASK_SIZE set to 2. This variables provides the number of bytes to use for the masks in the records.
• swift_type_count set to 7. This variable contains the number of particle types available in SWIFT.

The structure of the file is the following one:

• The file format: "SWIFT_CSDS" (string of STRING_SIZE char)
• Major version number (int)
• Minor version number (int)
• Offset direction (int). This should be backward (0) after a simulation.
• Offset to the first record (unsigned int with a size of CSDS_OFFSET_SIZE bytes). This value provides directly the position of the first record from the beginning of the file.
• The length of the strings in the logfile (unsigned int): string_length
• The number of masks available (unsigned int).
• For each mask:
  • The name of the mask (string of string_length char)
  • Size of the mask in bytes (unsigned int)
• Read the number of fields per particle type (swift_type_count int): number_fields.
• For each particle type:
  • The order of the fields (number_fields int). The values correspond to the index within the mask read previously.
• A list of masks

Record Description

Each record has the following structure:

• The mask (CSDS_MASK_SIZE bytes). This describes the data contained in the tail of the record.
• The offset (CSDS_OFFSET_SIZE bytes). The distance in the file to the next (previous) corresponding record (depending on the offset direction in the logfile header).
• The data (variable size). Each field is given according to the particle type, the order of the fields and the mask in the logfile's header.