XIR converter

[thisac]

Context: To fully support XIR in SF we need to have a way to convert back and forth between XIR and SF programs.

Description of the Change: Function to be able to convert between XIR and SF programs (both ways) are added.

• sf.io.to_program now recognizes whether an XIR program or a Blackbird program is being passed, and converts it properly into an SF program.
• A _from_xir function has been added taking an XIR program, returning the corresponding SF program.
• The Blackbird to SF program conversion has been moved to a function called _from_blackbird.
• A to_xir function has been added that takes an SF program and returns the corresponding XIR program. Keyword arguments add_decl, for adding declarations to the XIR program, and version, for passing a version string, are added.
• Both save, load and loads are now able to save and load both Blackbird and XIR scripts (determined by the ir keyword, which defaults to ir="blackbird" for backwards compatibility.
• Tests have been written for the XIR conversion, taken directly from the Blackbird tests in the former test_io.py and adapted to fit the XIR case. The tests have now been separated into test_io_xir.py, test_io_blackbird.py, and test_io, the latter containing all non-ir tests.

Benefits: SF supports serializing XIR!

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #643 (bc9653b) into master (4161918) will increase coverage by 0.00%. The diff coverage is 97.24%.

@@           Coverage Diff            @@
##           master     #643    +/-   ##
========================================
  Coverage   98.28%   98.29%            
========================================
  Files          72       75     +3     
  Lines        8824     9029   +205     
========================================
+ Hits         8673     8875   +202     
- Misses        151      154     +3     

Impacted Files	Coverage Δ
strawberryfields/io/blackbird_io.py	93.39% <93.39%> (ø)
strawberryfields/io/utils.py	98.24% <98.24%> (ø)
strawberryfields/io/xir_io.py	98.27% <98.27%> (ø)
strawberryfields/io/__init__.py	100.00% <100.00%> (ø)
strawberryfields/tdm/tdmprogram.py	98.29% <100.00%> (+0.01%)	:arrow_up:

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