File information

Since Gadget outputs in binary format it can be quite tedious to see what is actually contained in the file. For this GadgetIO provides a number of helper functions.

Reading the header

The header block (HEAD) contains a number of informations about the file, for example how many particles of which kind are contained in the file, the current time or redshift, the cosmological parameters and so on.

The most convenient way to deal with this data is to read the header block into a SnapshotHeader struct by using

GadgetIO.read_header — Function

read_header(filename::String)

Reads the header of a snapshot file or file base (without .0, .1, etc.) and returns a SnapshotHeader object.

See also: head_to_struct

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the struct contains the following fields:

Name	Meaning
`npart::Vector{Int32}`	an array of particle numbers per type in this snapshot
`massarr::Vector{Float64}`	an array of particle masses per type in this snapshot - if zero: MASS block present
`time::Float64`	time / scale factor of the simulation
`z::Float64`	redshift of the simulation
`flag_sfr::Int32`	1 if simulation was run with star formation, else 0
`flag_feedback::Int32`	1 if simulation was run with stellar feedback, else 0
`nall::Vector{UInt32}`	total number of particles in the simulation
`flag_cooling::Int32`	1 if simulation was run with cooling, else 0
`num_files::Int32`	number of snapshots over which the simulation is distributed
`omega_0::Float64`	Omega matter
`boxsize::Float64`	total size of the simulation box
`omega_l::Float64`	Omega dark enery
`h0::Float64`	little h
`flag_stellarage::Int32`	1 if simulation was run with stellar age, else 0
`flag_metals::Int32`	1 if simulation was run with metals, else 0
`npartTotalHighWord::Vector{UInt32}`	If Npart > 1584^3 (>2^32) this contains a high bit: ntotal = npartTotalHighWord*2^32 + nall
`flag_entropy_instead_u::Int32`	1 if snapshot U field contains entropy instead of internal energy, else 0
`flag_doubleprecision::Int32`	1 if snapshot is in double precision, else 0
`flag_ic_info::Int32`	1 if initial snapshot file contains an info block, else 0
`lpt_scalingfactor::Float32`	factor to use second order ic generation
`fill::Vector{Int32}`	the HEAD block needs to be filled with zeros to have a size of 256 bytes

read_header is a wrapper around:

GadgetIO.head_to_struct — Function

head_to_struct(filename::String)

Returns the header of a snapshot as a SnapshotHeader object.

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If you want to read the header information into a dictionary you can use:

GadgetIO.head_to_dict — Function

head_to_dict(filename::String)

Returns the header of a snapshot as a dictionary.

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Getting the block names

GadgetIO.print_blocks — Function

print_blocks(filename::String; verbose::Bool=true)

Reads the block names of blocks in a snapshot and returns them in an array. Outputs them to console if verbose=true

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Checking if a block is present

GadgetIO.block_present — Function

block_present(filename::String, blockname::String, blocks::Vector{String}=[""])

Checks if a given block is present in the snapshot file, or in the supplied blocks Vector.

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Getting the block positions within a file

If you want to read in multiple blocks into individual variables you can speed the process up significantly by first getting the starting positions of the blocks with

GadgetIO.get_block_positions — Function

get_block_positions(filename::String)

Returns a dictionary with the starting positions of all blocks in a snapshot in bits.

Example

block_positions = get_block_positions(filename)
block_positions["POS"]

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This returns a dictionary with the block names as keys and the starting position in bytes as an Integer:

julia> block_pos["POS"]
1234567

and can be used with read_block by passing it to the keyword argument block_position:

gas_pos = read_block(filename, "POS", parttype=0, block_position=block_pos["POS"])
gas_vel = read_block(filename, "VEL", parttype=0, block_position=block_pos["VEL"])
gas_rho = read_block(filename, "RHO", parttype=0, block_position=block_pos["RHO"])

Please note that this only makes sense if you plan to read in multiple blocks. If the keyword argument is left out read_block will look for the block positions by itself.

Reading the INFO block

If you compiled Gadget with WRITE_INFO_BLOCK the snapshot contains a block INFO that holds information on the name, datatype, dimensionality and presence per particle for each block. This simplifies read-in greatly and is always recommended! You can obtain this block by using:

GadgetIO.read_info — Function

read_info(filename; verbose::Bool=false)

Reads the info block of a snapshot and returns the information in an array of InfoLine types. If verbose=true the blocknames are also printed to console.

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This returns an Array{InfoLine,1}, where InfoLine is:

struct InfoLine([  block_name="", data_type=Float32, n_dim=Int32(0),
                is_present=zeros(Int32, 6) ])

Contains the data of a single entry in the INFO block of a Gadget snapshot. The data can be accessed via the fields:

Name	Meaning
`block_name::String`	name of the data block, e.g. "POS"
`data_type::DataType`	datatype of the block, e.g. Float32 for single precision, Float64 for double
`n_dim::Int32`	number of dimensions of the block, usually 1 or 3
`is_present::Vector{Int32}`	array of flags for which particle type this block is present,
	e.g. gas only: [ 1, 0, 0, 0, 0, 0 ],
	or gas + BHs: [ 1, 0, 0, 0, 0, 1 ]

You can use this as well to speed up read-in by passing the relevant Array entry to the keyword argument info in read_block:

gas_pos = read_block(filename, "POS", parttype=0, info=info[1])

This can of course be combined with reading the block positions for additional speedup.

If there is no INFO block in the simulation you need to construct the InfoLine struct yourself and pass it to read_block like in the previous example.