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Writing MatlabOpaque objects to MAT-files #208

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dc7e37f
Rearrange methods for readability
foreverallama Nov 21, 2025
4026a63
Add some subsys methods to save objects
foreverallama Nov 24, 2025
dd1c995
Full subsystem code, some updates to writers to support writing subsy…
foreverallama Nov 25, 2025
32c678a
Some fixes to writing filewrapper metadata
foreverallama Nov 26, 2025
8416fc2
Fix dimensions of data in filewrapper
foreverallama Nov 27, 2025
7b846dc
Write nested objects in subsystem
foreverallama Nov 27, 2025
3c85b70
Improve MAT_subsys readability
foreverallama Nov 28, 2025
46c56aa
Fix: Write empty dicts as 0x0 structs in HDF5
foreverallama Nov 28, 2025
1d9e8bb
Update empty struct handling; Add tests
foreverallama Nov 29, 2025
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52 changes: 32 additions & 20 deletions src/MAT.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -204,31 +204,43 @@ Write a dictionary containing variable names as keys and values as values
to a Matlab file, opening and closing it automatically.
"""
function matwrite(filename::AbstractString, dict::AbstractDict{S, T}; compress::Bool = false, version::String ="v7.3") where {S, T}
if version == "v4"
file = open(filename, "w")
m = MAT_v4.Matlabv4File(file, false)
_write_dict(m, dict)
elseif version == "v7.3"
file = matopen(filename, "w"; compress = compress)
_write_dict(file, dict)
else
error("writing for \"$(version)\" is not supported")
file = nothing
try
if version == "v4"
file = open(filename, "w")
file = MAT_v4.Matlabv4File(file, false)
_write_dict(file, dict)
elseif version == "v7.3"
file = matopen(filename, "w"; compress = compress)
_write_dict(file, dict)
else
error("writing for \"$(version)\" is not supported")
end
finally
if file !== nothing
close(file)
end
end
end

function _write_dict(fileio, dict::AbstractDict)
try
for (k, v) in dict
local kstring
try
kstring = ascii(convert(String, k))
catch x
error("matwrite requires a Dict with ASCII keys")
end
write(fileio, kstring, v)

for (k, v) in dict
local kstring
try
kstring = ascii(convert(String, k))
catch x
error("matwrite requires a Dict with ASCII keys")
end
write(fileio, kstring, v)
end

if hasproperty(fileio, :subsystem) && fileio.subsystem !== nothing
# will always be nothing for MATv4 so we can ignore that case
subsys_data = MAT_subsys.set_subsystem_data!(fileio.subsystem)
if subsys_data !== nothing
MAT_HDF5.write_subsys(fileio, subsys_data)
end
finally
close(fileio)
end
end

Expand Down
76 changes: 69 additions & 7 deletions src/MAT_HDF5.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -36,7 +36,7 @@ import HDF5: Reference
import Dates
import Tables
import PooledArrays: PooledArray
import ..MAT_types: MatlabStructArray, StructArrayField, convert_struct_array, MatlabClassObject, MatlabOpaque, MatlabTable
import ..MAT_types: MatlabStructArray, StructArrayField, convert_struct_array, MatlabClassObject, MatlabOpaque, MatlabTable, EmptyStruct

const HDF5Parent = Union{HDF5.File, HDF5.Group}
const HDF5BitsOrBool = Union{HDF5.BitsType,Bool}
Expand Down Expand Up @@ -130,10 +130,12 @@ function matopen(filename::AbstractString, rd::Bool, wr::Bool, cr::Bool, tr::Boo
close(g)
end
subsys_refs = "#subsystem#"
if haskey(fid.plain, subsys_refs)
if rd && haskey(fid.plain, subsys_refs)
fid.subsystem.table_type = table
subsys_data = m_read(fid.plain[subsys_refs], fid.subsystem)
MAT_subsys.load_subsys!(fid.subsystem, subsys_data, endian_indicator)
elseif wr
MAT_subsys.init_save!(fid.subsystem)
end
fid
end
Expand Down Expand Up @@ -464,7 +466,7 @@ function _normalize_arr(x)
end

# Write a scalar or array
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, data::Union{T, Complex{T}, AbstractArray{T}, AbstractArray{Complex{T}}}) where {T<:HDF5BitsOrBool}
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, data::Union{T, Complex{T}, AbstractArray{T}, AbstractArray{Complex{T}}}, ) where {T<:HDF5BitsOrBool}
data = _normalize_arr(data)
if isempty(data)
m_writeempty(parent, name, data)
Expand Down Expand Up @@ -541,14 +543,19 @@ function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, c::Abs
end

# Write cell arrays
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, data::AbstractArray{T}) where T
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, data::AbstractArray{T}, object_decode::UInt32=UInt32(0)) where T
data = _normalize_arr(data)
refs = _write_references!(mfile, parent, data)
# Write the references as the chosen variable
cset, ctype = create_dataset(parent, name, refs)
try
write_dataset(cset, ctype, refs)
write_attribute(cset, name_type_attr_matlab, "cell")
if object_decode == UInt32(3)
write_attribute(cset, object_decode_attr_matlab, object_decode)
write_attribute(cset, name_type_attr_matlab, "FileWrapper__")
else
write_attribute(cset, name_type_attr_matlab, "cell")
end
finally
close(ctype)
close(cset)
Expand Down Expand Up @@ -680,8 +687,34 @@ function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, obj::M
end
end

# Write empty (zero-dimensional) structs with no fields
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, s::EmptyStruct)
dset, dtype = create_dataset(parent, name, s.dims)
try
write_attribute(dset, empty_attr_matlab, 0x01)
write_attribute(dset, name_type_attr_matlab, "struct")
write_dataset(dset, dtype, s.dims)
finally
close(dtype); close(dset)
end
end

# Write a struct from arrays of keys and values
function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, k::Vector{String}, v::Vector)
if length(k) == 0
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@foreverallama foreverallama Nov 28, 2025

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I added the ability to write empty dicts as 0x0 structs over here. The subsystem data works with 0x0 structs as well in MATLAB.

This eliminates the need for any new struct markers for now.

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@matthijscox matthijscox Nov 28, 2025

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Alright. Are 0x0 structs also read as empty dicts?

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@foreverallama foreverallama Nov 28, 2025

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Yup, all of 1x0, 0x1 and 0x0 are read as empty dicts already. But I hardcoded 0x0 for empty dict during write.

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@foreverallama foreverallama Nov 28, 2025

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So this might go against the point raised in #214 where an empty dict is being written as a 1x1 struct with no fields. Instead I guess we could either:

  • Write empty dicts as 1x1 structs with no fields. For 1x0, 0x0 dims we could maybe use a special key __dims__ that contains a tuple with the dimensions. I think its very unlikely to require reading 1x0, 0x0 structs (without fields), we just need write functionality
  • Create a new marker for empty structs with no fields
  • Update MatlabStructArray to handle empty structs

What do you suggest?

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@matthijscox matthijscox Nov 29, 2025

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It's a rather special corner case. I'm okay with a new type that we keep internal for the writing, and then we can always consider to refactor later:

struct EmptyStruct
   dims::Vector{Int}
end

# empty struct
adata = UInt64[1, 1]
dset, dtype = create_dataset(parent, name, adata)
try
write_attribute(dset, empty_attr_matlab, 0x01)
write_attribute(dset, name_type_attr_matlab, "struct")
write_dataset(dset, dtype, adata)
finally
close(dtype); close(dset)
end
return
end

g = create_group(parent, name)
try
write_attribute(g, name_type_attr_matlab, "struct")
Expand Down Expand Up @@ -719,11 +752,35 @@ function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, dat::D
end

function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, obj::MatlabOpaque)
error("writing of MatlabOpaque types is not yet supported")
if obj.class == "FileWrapper__"
m_write(mfile, parent, name, obj["__filewrapper__"], UInt32(3))
return
end

metadata = MAT_subsys.set_mcos_object_metadata(mfile.subsystem, obj)
dset, dtype = create_dataset(parent, name, metadata)
try
write_dataset(dset, dtype, metadata)
write_attribute(dset, name_type_attr_matlab, obj.class)
write_attribute(dset, object_type_attr_matlab, UInt32(3))
finally
close(dset)
close(dtype)
end
end

function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, obj::AbstractArray{MatlabOpaque})
error("writing of MatlabOpaque types is not yet supported")
metadata = MAT_subsys.set_mcos_object_metadata(mfile.subsystem, obj)
dset, dtype = create_dataset(parent, name, metadata)
try
# TODO: Handle empty array case
write_dataset(dset, dtype, metadata)
write_attribute(dset, name_type_attr_matlab, first(obj).class)
write_attribute(dset, object_type_attr_matlab, UInt32(3))
finally
close(dset)
close(dtype)
end
end

function m_write(mfile::MatlabHDF5File, parent::HDF5Parent, name::String, arr::PooledArray)
Expand All @@ -743,6 +800,11 @@ function write(parent::MatlabHDF5File, name::String, thing)
m_write(parent, parent.plain, name, thing)
end

function write_subsys(mfile::MatlabHDF5File, subsys_data::Dict{String,Any})
name = "#subsystem#"
m_write(mfile, mfile.plain, name, subsys_data)
end

## Type conversion operations ##

struct MatlabString end
Expand Down
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