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# Using MQSSCI as a Library

The [Passes](passes.md) page shows how to run passes through the `mqss-opt`/`mqss-cc` command-line
tools. This page is for the case where you don't want a separate command-line step at all — you want
your own C++ program to compile a circuit directly.

## Before You Start

Your project needs to link against the MQSSCI library first. That's a one-time CMake setup step —
see
[Integrating the MQSS Quantum Compilation Suite within your project](../develop-guide/integrate.md)
for how to fetch MQSSCI with `FetchContent` and link your target against `mqss-ci::mqss-ci`. The
rest of this page assumes that step is done and focuses purely on the C++ side.

## A Minimal Example

`mqss::mqssci::MQSSCompiler` is the simplest way to compile a circuit: one header, one class, one
call. It hides dialect registration, MLIR context setup, and pipeline assembly behind a single
`compile` call.

```cpp
#include "MQSSCIInterfaces/MQSSCompiler.h"

mqss::mqssci::MQSSCompiler compiler;

mqss::mqssci::CompilerOptions opts;
opts.optimization_level = mqss::mqssci::OptLevel::O1;        // O1, O2, or O3 — selects the preset pipeline
opts.result_format = mqss::mqssci::ResultFormat::OPENQASM2;  // or QIR, QIRBASE, QIRADAPTIVE, QIRFULL

std::optional<std::string> qasm = compiler.compile("path/to/circuit.qke", "planqc", opts); // Use compileSource() to parse source string
if (!qasm) {
  // Compilation failed. MQSSCompiler has already reported a diagnostic through
  // MLIR's diagnostic engine — it never throws or aborts the process.
  return 1;
}

llvm::outs() << *qasm;
```

Walking through it:

1. `#include "MQSSCIInterfaces/MQSSCompiler.h"` is the only header you need for this path.
2. `mqss::mqssci::CompilerOptions` configures the run: `optimization_level` selects the
   `O1`/`O2`/`O3` preset pipeline (see [Pass Pipelines](passes.md#pass-pipelines) for what each
   includes), and `result_format` selects the output format — one of the
   `mqss::mqssci::ResultFormat` enumerators: `OPENQASM2`, `QIR`, `QIRBASE`, `QIRADAPTIVE`, or
   `QIRFULL`.
3. The second argument to `compile` (`"planqc"` above) is a known backend name that selects a
   built-in native-gate set for decomposition. See [Choosing a Backend](#choosing-a-backend) below
   for the alternatives.
4. `compile` returns `std::optional<std::string>`, not the compiled circuit directly. On success, it
   holds the compiled output; on failure it's `std::nullopt`, and the reason has already been
   reported via `mlir::emitError` — MQSSCI is built without C++ exceptions, so a failed compile
   never throws or crashes your program, it just returns an empty optional.

## Choosing a Backend

`compile` has overloads for three ways to select the native-gate set used for decomposition: by a
known backend name, by an explicit gate list, or not at all:

```cpp
// By backend name: iqm, planqc, and wmi are recognized out of the box.
compiler.compile("path/to/circuit.qke", "iqm", opts);

// By an explicit native-gate set, when your target isn't one of the built-in backends.
compiler.compile("path/to/circuit.qke", {"rz", "rx", "cz"}, opts);

// Neither: skip native-gate decomposition entirely.
compiler.compile("path/to/circuit.qke", opts);
```

| `backend_name` | Native-gate set      |
| -------------- | -------------------- |
| `"iqm"`        | `phased_rx`, `cz`    |
| `"planqc"`     | `rx`, `cz`, `rz`     |
| `"wmi"`        | `cz`, `x`, `y`, `rz` |

The full signature (used in the examples above via its three shorthand overloads) also accepts a
`qubit_connectivity` map alongside the native-gate set, for targets with restricted qubit
connectivity. See `MQSSCIInterfaces/MQSSCompiler.h` for all four `compile` overloads.

## Discovering Supported Input Formats

`MQSSCompiler::getSupportedInputFormats()` is a static method that returns the display name of every
input format MQSSCI's `mqss::mqssci::InputFormat` enum defines:

```cpp
for (std::string_view name : mqss::mqssci::MQSSCompiler::getSupportedInputFormats()) {
  llvm::outs() << name << "\n";
}
```

It returns a `std::vector<std::string_view>` (currently `{"cudaq-quake", "catalyst-quantum"}`) —
useful for validating a user-supplied format name or listing the options in a CLI's `--help` output.
Note that `compile`/`compileSource` currently only accept the `cudaq-quake` dialect regardless of
what this list reports; see the note at the top of `MQSSCompiler.cpp`.

## Advanced: Building a Custom Pipeline Yourself

`MQSSCompiler` covers the common case: a preset optimization level, a fixed native-gate set, and one
of two output formats. If you need to run individual passes, pass pass-specific options (e.g.
`CommonGateCancellationPassOptions`), or assemble a pipeline `MQSSCompiler` doesn't support, drop
down to the same `mlir::OpPassManager`-based API `MQSSCompiler` itself is built on:

```cpp
#include "Passes/Transforms/Dialects.h"
#include "Passes/Transforms/Pipelines.h"
#include "Passes/CodeGen/CodeGenPasses.h"

// 1. Get an MLIRContext with every dialect MQSSCI's passes need already loaded.
std::unique_ptr<mlir::MLIRContext> context = mqss::mqssci::opt::createMQSSContext();

// 2. Parse your MLIR file into a module
auto module = mlir::parseSourceFile<mlir::ModuleOp>("path/to/file", context.get());
if (!module) {
  llvm::errs() << "failed to parse MLIR file\n";
}

// 3. Build a pass manager and add passes to the pipeline
mlir::PassManager pm(context.get());
CommonGateCancellationPassOptions CancelOpts;
CommonCommutePassOptions CommuteOpts;
CancelOpts.mode = "CancelGate";
CommuteOpts.mode = "CX-RX";

pm.addPass(CommonGateCancellationPass(CancelOpts));
pm.addPass(CommonCommutePass(CommuteOpts));

// Run the Pass Pipeline
if (mlir::failed(pm.run(*module))) {
  llvm::errs() << "Compiler: Pipeline failed\n";
}

// 4. Lower the optimized module to OpenQASM 2
pm.addPass(mqss::mqssci::codegen::QuakeToQASM2Pass());
if (mlir::failed(pm.run(*module))) {
  llvm::errs() << "Compiler: Conversion of Quake to QASM2 failed\n";
}
```

`mqss::mqssci::opt::createMQSSContext()` is a convenience that registers every MQSSCI dialect and
returns a ready-to-use `MLIRContext` in one call — equivalent to building a `DialectRegistry` with
`registerMQSSDialects`, constructing an `MLIRContext` from it, and calling
`loadAllAvailableDialects()` yourself.

If you'd rather capture the OpenQASM output in a string instead of printing it, `QuakeToQASM2Pass`
also accepts an `llvm::raw_ostream&`:

```cpp
std::string qasm;
llvm::raw_string_ostream os(qasm);
pm.addPass(mqss::mqssci::codegen::QuakeToQASM2Pass(os));
```

For the full list of available passes, their options, and valid option values, see
[Passes](passes.md).

## Testing the Interface

`MQSSCompiler` has its own testing framework at
`tests/unittests/MQSSCIInterfaces/test_Interface.cpp`. It builds automatically as part of the normal
build (via `scripts/build.sh` / `make target`) — run it with:

```sh
make test-interfaces
```

For what the interfaces testing framework covers and how to extend it, see
[Testing the Interface](../develop-guide/integrate.md#testing-the-interface) in the developer guide.