feat: Drop linear bits, improve pytket encoding/decoding (#420)

Removes the ad-hoc `LINEAR_BIT` used for decoding pytket circuits, and
uses non-linear `BOOL_T`s instead.
This now lets us encode guppy circuits with measurements;

```python
module = GuppyModule("test")
module.load(quantum)

@guppy(module)
def my_func(q0: qubit, q1: qubit) -> tuple[bool,]:
    q0 = phased_x(q0, py(math.pi / 2), py(-math.pi / 2))
    q0 = rz(q0, py(math.pi))
    q1 = phased_x(q1, py(math.pi / 2), py(-math.pi / 2))
    q1 = rz(q1, py(math.pi))
    q0, q1 = zz_max(q0, q1)
    _ = measure(q0)
    return (measure(q1),)

circ = guppy_to_circuit(my_func)
print(to_hugr_mermaid(circ))

tk1 = circ.to_tket1()
render_circuit_jupyter(tk1)

circ2 = Tk2Circuit(tk1)
print(to_hugr_mermaid(circ2))
```

Mermaid diagram (rooted on the `DataflowBlock`):
```mermaid
graph LR
    subgraph 0 ["(0) Module"]
        direction LR
        subgraph 7 ["(7) FuncDefn"]
            direction LR
            3["(3) Input"]
            3--"0:0<br>qubit"-->8
            3--"1:1<br>qubit"-->8
            6["(6) Output"]
            subgraph 8 ["(8) CFG"]
                direction LR
                subgraph 1 ["(1) DataflowBlock"]
                    direction LR
                    4["(4) Input"]
                    4--"0:0<br>qubit"-->13
                    4--"1:0<br>qubit"-->21
                    5["(5) Output"]
                    9["(9) const:custom:f64(1.5707963267948966)"]
                    9--"0:0<br>float64"-->10
                    10["(10) LoadConstant"]
                    10--"0:1<br>float64"-->13
                    11["(11) const:custom:f64(-1.5707963267948966)"]
                    11--"0:0<br>float64"-->12
                    12["(12) LoadConstant"]
                    12--"0:2<br>float64"-->13
                    13["(13) quantum.tket2.PhasedX"]
                    13--"0:0<br>qubit"-->16
                    14["(14) const:custom:f64(3.141592653589793)"]
                    14--"0:0<br>float64"-->15
                    15["(15) LoadConstant"]
                    15--"0:1<br>float64"-->16
                    16["(16) quantum.tket2.RzF64"]
                    16--"0:0<br>qubit"-->25
                    17["(17) const:custom:f64(1.5707963267948966)"]
                    17--"0:0<br>float64"-->18
                    18["(18) LoadConstant"]
                    18--"0:1<br>float64"-->21
                    19["(19) const:custom:f64(-1.5707963267948966)"]
                    19--"0:0<br>float64"-->20
                    20["(20) LoadConstant"]
                    20--"0:2<br>float64"-->21
                    21["(21) quantum.tket2.PhasedX"]
                    21--"0:0<br>qubit"-->24
                    22["(22) const:custom:f64(3.141592653589793)"]
                    22--"0:0<br>float64"-->23
                    23["(23) LoadConstant"]
                    23--"0:1<br>float64"-->24
                    24["(24) quantum.tket2.RzF64"]
                    24--"0:1<br>qubit"-->25
                    25["(25) quantum.tket2.ZZMax"]
                    25--"0:0<br>qubit"-->26
                    25--"1:1<br>qubit"-->26
                    26["(26) MakeTuple"]
                    26--"0:0<br>[qubit, qubit]"-->27
                    27["(27) UnpackTuple"]
                    27--"0:0<br>qubit"-->28
                    27--"1:0<br>qubit"-->30
                    28["(28) quantum.tket2.Measure"]
                    28--"0:0<br>qubit"-->29
                    29["(29) quantum.tket2.QFree"]
                    30["(30) quantum.tket2.Measure"]
                    30--"0:0<br>qubit"-->31
                    30--"1:0<br>[]+[]"-->32
                    31["(31) quantum.tket2.QFree"]
                    32["(32) MakeTuple"]
                    32--"0:0<br>[[]+[]]"-->33
                    33["(33) UnpackTuple"]
                    33--"0:1<br>[]+[]"-->5
                    34["(34) Tag"]
                    34--"0:0<br>[]"-->5
                end
                1-."0:0".->2
                2["(2) ExitBlock"]
            end
            8--"0:0<br>[]+[]"-->6
        end
    end
```
tket1 circuit:

![circuit](https://github.com/CQCL/tket2/assets/121866228/62877218-dd24-4e5e-a8f7-ce738e05662c)
Re-extracted circuit:
```mermaid
graph LR
    subgraph 0 ["(0) FuncDefn"]
        direction LR
        1["(1) Input"]
        1--"0:0<br>qubit"-->7
        1--"1:0<br>qubit"-->12
        2["(2) Output"]
        3["(3) const:custom:f64(1.5707963267948966)"]
        3--"0:0<br>float64"-->4
        4["(4) LoadConstant"]
        4--"0:1<br>float64"-->7
        5["(5) const:custom:f64(-1.5707963267948966)"]
        5--"0:0<br>float64"-->6
        6["(6) LoadConstant"]
        6--"0:2<br>float64"-->7
        7["(7) quantum.tket2.PhasedX"]
        7--"0:0<br>qubit"-->15
        8["(8) const:custom:f64(1.5707963267948966)"]
        8--"0:0<br>float64"-->9
        9["(9) LoadConstant"]
        9--"0:1<br>float64"-->12
        10["(10) const:custom:f64(-1.5707963267948966)"]
        10--"0:0<br>float64"-->11
        11["(11) LoadConstant"]
        11--"0:2<br>float64"-->12
        12["(12) quantum.tket2.PhasedX"]
        12--"0:0<br>qubit"-->18
        13["(13) const:custom:f64(3.141592653589793)"]
        13--"0:0<br>float64"-->14
        14["(14) LoadConstant"]
        14--"0:1<br>float64"-->15
        15["(15) quantum.tket2.RzF64"]
        15--"0:0<br>qubit"-->19
        16["(16) const:custom:f64(3.141592653589793)"]
        16--"0:0<br>float64"-->17
        17["(17) LoadConstant"]
        17--"0:1<br>float64"-->18
        18["(18) quantum.tket2.RzF64"]
        18--"0:1<br>qubit"-->19
        19["(19) quantum.tket2.ZZMax"]
        19--"0:0<br>qubit"-->21
        19--"1:0<br>qubit"-->20
        20["(20) quantum.tket2.Measure"]
        20--"0:1<br>qubit"-->2
        20--"1:2<br>[]+[]"-->2
        21["(21) quantum.tket2.Measure"]
        21--"0:0<br>qubit"-->2
        21--"1:3<br>[]+[]"-->2
    end
```


This required multiple improvements to the encoder/decoder logic,
including
- `Tk1Op::Native` operations (backed by a `Tk2Op`) can now have
different number of input/output qubit/bits.
- Added support for tket2 circuits with different input and output
signatures.
- Added support for `QAlloc`/`QFree` operations (generated by guppy) by
adding extra input/outputs to the circuit.
- Added support for pytket's implicit permutations, and recalculates the
value when encoding a tket2 circuit.
- Preserve the `opgroup` value from decoded pytket operations.
- Improved error reporting.

Closes #379

---------

Co-authored-by: Seyon Sivarajah <seyon.sivarajah@cambridgequantum.com>

tket2-py/src/circuit.rs

@@ -15,7 +15,7 @@ use pyo3::prelude::*;
 use std::fmt;
 
 use hugr::{type_row, Hugr, HugrView, PortIndex};
-use tket2::extension::{LINEAR_BIT, REGISTRY};
+use tket2::extension::REGISTRY;
 use tket2::rewrite::CircuitRewrite;
 use tket2::serialize::TKETDecode;
 use tket_json_rs::circuit_json::SerialCircuit;
@@ -315,11 +315,6 @@ impl PyHugrType {
         Self(QB_T)
     }
 
-    #[staticmethod]
-    fn linear_bit() -> Self {
-        Self(LINEAR_BIT.to_owned())
-    }
-
     #[staticmethod]
     fn bool() -> Self {
         Self(BOOL_T)

tket2-py/test/test_guppy.py

@@ -65,3 +65,10 @@ def my_func(
 
     # The 7 operations in the function, plus two implicit QFree
     assert circ.num_operations() == 9
+
+    tk1 = circ.to_tket1()
+    assert tk1.n_gates == 7
+    assert tk1.n_qubits == 2
+
+    gates = list(tk1)
+    assert gates[4].op.type == pytket.circuit.OpType.ZZMax

tket2-py/tket2/_tket2/circuit.pyi

@@ -157,10 +157,6 @@ class HugrType:
     def qubit() -> HugrType:
         """Qubit type from HUGR prelude."""
 
-    @staticmethod
-    def linear_bit() -> HugrType:
-        """Linear bit type from TKET1 extension."""
-
     @staticmethod
     def bool() -> HugrType:
         """Boolean type (HUGR 2-ary unit sum)."""

tket2-py/tket2/circuit/build.py

@@ -3,7 +3,6 @@
 from dataclasses import dataclass
 
 QB_T = HugrType.qubit()
-LB_T = HugrType.linear_bit()
 BOOL_T = HugrType.bool()
 
 
@@ -22,7 +21,7 @@ def bits(self) -> list[int]:
 
     @classmethod
     def op(cls) -> CustomOp:
-        types = [QB_T] * cls.n_qb + [LB_T] * cls.n_lb
+        types = [QB_T] * cls.n_qb + [BOOL_T] * cls.n_lb
         return CustomOp(cls.extension_name, cls.gate_name, types, types)
 
 

tket2/src/circuit.rs

@@ -681,8 +681,8 @@ mod tests {
         assert_eq!(circ.operations().count(), 3);
 
         assert_eq!(circ.units().count(), qubits + bits);
-        assert_eq!(circ.nonlinear_units().count(), 0);
-        assert_eq!(circ.linear_units().count(), qubits + bits);
+        assert_eq!(circ.nonlinear_units().count(), bits);
+        assert_eq!(circ.linear_units().count(), qubits);
         assert_eq!(circ.qubits().count(), qubits);
     }
 

tket2/src/circuit/command.rs

@@ -7,7 +7,7 @@ use std::collections::{HashMap, HashSet};
 use std::iter::FusedIterator;
 
 use hugr::hugr::views::{HierarchyView, SiblingGraph};
-use hugr::hugr::NodeType;
+use hugr::hugr::{NodeMetadata, NodeType};
 use hugr::ops::{OpTag, OpTrait};
 use hugr::{HugrView, IncomingPort, OutgoingPort};
 use itertools::Either::{self, Left, Right};
@@ -161,6 +161,12 @@ impl<'circ, T: HugrView> Command<'circ, T> {
             .port_kind(port)
             .map_or(false, |kind| kind.is_linear())
     }
+
+    /// Returns a metadata value associated with the command's node.
+    #[inline]
+    pub fn metadata(&self, key: impl AsRef<str>) -> Option<&NodeMetadata> {
+        self.circ.hugr().get_metadata(self.node, key)
+    }
 }
 
 impl<'a, 'circ, T: HugrView> UnitLabeller for &'a Command<'circ, T> {

tket2/src/extension.rs

@@ -10,7 +10,7 @@ use hugr::extension::{CustomSignatureFunc, ExtensionId, ExtensionRegistry, Signa
 use hugr::hugr::IdentList;
 use hugr::std_extensions::arithmetic::float_types::{EXTENSION as FLOAT_EXTENSION, FLOAT64_TYPE};
 use hugr::types::type_param::{TypeArg, TypeParam};
-use hugr::types::{CustomType, FunctionType, PolyFuncType, Type, TypeBound};
+use hugr::types::{CustomType, FunctionType, PolyFuncType, TypeBound};
 use hugr::{type_row, Extension};
 use lazy_static::lazy_static;
 use smol_str::SmolStr;
@@ -21,9 +21,6 @@ pub mod angle;
 /// The ID of the TKET1 extension.
 pub const TKET1_EXTENSION_ID: ExtensionId = IdentList::new_unchecked("TKET1");
 
-/// The name for the linear bit custom type.
-pub const LINEAR_BIT_NAME: SmolStr = SmolStr::new_inline("LBit");
-
 /// The name for opaque TKET1 operations.
 pub const TKET1_OP_NAME: SmolStr = SmolStr::new_inline("TKET1 Json Op");
 
@@ -39,8 +36,6 @@ pub static ref TKET1_OP_PAYLOAD : CustomType =
 pub static ref TKET1_EXTENSION: Extension = {
     let mut res = Extension::new(TKET1_EXTENSION_ID);
 
-    res.add_type(LINEAR_BIT_NAME, vec![], "A linear bit.".into(), TypeBound::Any.into()).unwrap();
-
     let tket1_op_payload_def = res.add_type(TKET1_PAYLOAD_NAME, vec![], "Opaque TKET1 operation metadata.".into(), TypeBound::Eq.into()).unwrap();
     let tket1_op_payload = TypeParam::Opaque{ty:tket1_op_payload_def.instantiate([]).unwrap()};
     res.add_op(
@@ -52,15 +47,6 @@ pub static ref TKET1_EXTENSION: Extension = {
     res
 };
 
-/// The type for linear bits. Part of the TKET1 extension.
-pub static ref LINEAR_BIT: Type = {
-    Type::new_extension(TKET1_EXTENSION
-        .get_type(&LINEAR_BIT_NAME)
-        .unwrap()
-        .instantiate([])
-        .unwrap())
-    };
-
 /// Extension registry including the prelude, TKET1 and Tk2Ops extensions.
 pub static ref REGISTRY: ExtensionRegistry = ExtensionRegistry::try_new([
     TKET1_EXTENSION.clone(),

tket2/src/serialize/pytket.rs

@@ -6,39 +6,47 @@ mod op;
 
 use hugr::types::Type;
 
+use hugr::Node;
+use itertools::Itertools;
 // Required for serialising ops in the tket1 hugr extension.
 pub(crate) use op::serialised::OpaqueTk1Op;
 
 #[cfg(test)]
 mod tests;
 
+use std::collections::hash_map::DefaultHasher;
+use std::hash::{Hash, Hasher};
 use std::path::Path;
 use std::{fs, io};
 
-use hugr::ops::{OpType, Value};
+use hugr::ops::{NamedOp, OpType, Value};
 use hugr::std_extensions::arithmetic::float_types::ConstF64;
 
 use thiserror::Error;
-use tket_json_rs::circuit_json::SerialCircuit;
+use tket_json_rs::circuit_json::{self, SerialCircuit};
 use tket_json_rs::optype::OpType as SerialOpType;
 
 use crate::circuit::Circuit;
 
-use self::decoder::JsonDecoder;
-use self::encoder::JsonEncoder;
+use self::decoder::Tk1Decoder;
+use self::encoder::Tk1Encoder;
 
 pub use crate::passes::pytket::lower_to_pytket;
 
 /// Prefix used for storing metadata in the hugr nodes.
-pub const METADATA_PREFIX: &str = "TKET1_JSON";
+pub const METADATA_PREFIX: &str = "TKET1";
 /// The global phase specified as metadata.
-const METADATA_PHASE: &str = "TKET1_JSON.phase";
-/// The implicit permutation of qubits.
-const METADATA_IMPLICIT_PERM: &str = "TKET1_JSON.implicit_permutation";
+const METADATA_PHASE: &str = "TKET1.phase";
 /// Explicit names for the input qubit registers.
-const METADATA_Q_REGISTERS: &str = "TKET1_JSON.qubit_registers";
+const METADATA_Q_REGISTERS: &str = "TKET1.qubit_registers";
+/// The reordered qubit registers in the output, if an implicit permutation was applied.
+const METADATA_Q_OUTPUT_REGISTERS: &str = "TKET1.qubit_output_registers";
 /// Explicit names for the input bit registers.
-const METADATA_B_REGISTERS: &str = "TKET1_JSON.bit_registers";
+const METADATA_B_REGISTERS: &str = "TKET1.bit_registers";
+/// The reordered bit registers in the output, if an implicit permutation was applied.
+const METADATA_B_OUTPUT_REGISTERS: &str = "TKET1.bit_output_registers";
+/// A tket1 operation "opgroup" field.
+const METADATA_OPGROUP: &str = "TKET1.opgroup";
 
 /// A serialized representation of a [`Circuit`].
 ///
@@ -59,7 +67,7 @@ impl TKETDecode for SerialCircuit {
     type EncodeError = TK1ConvertError;
 
     fn decode(self) -> Result<Circuit, Self::DecodeError> {
-        let mut decoder = JsonDecoder::try_new(&self)?;
+        let mut decoder = Tk1Decoder::try_new(&self)?;
 
         if !self.phase.is_empty() {
             // TODO - add a phase gate
@@ -68,18 +76,18 @@ impl TKETDecode for SerialCircuit {
         }
 
         for com in self.commands {
-            decoder.add_command(com);
+            decoder.add_command(com)?;
         }
         Ok(decoder.finish().into())
     }
 
     fn encode(circ: &Circuit) -> Result<Self, Self::EncodeError> {
-        let mut encoder = JsonEncoder::new(circ)?;
+        let mut encoder = Tk1Encoder::new(circ)?;
         for com in circ.commands() {
             let optype = com.optype();
             encoder.add_command(com.clone(), optype)?;
         }
-        Ok(encoder.finish())
+        Ok(encoder.finish(circ))
     }
 }
 
@@ -156,13 +164,83 @@ pub enum OpConvertError {
     /// The serialized operation is not supported.
     #[error("Cannot serialize tket2 operation: {0:?}")]
     UnsupportedOpSerialization(OpType),
+    /// The operation has non-serializable inputs.
+    #[error("Operation {} in {node} has an unsupported input of type {typ}.", optype.name())]
+    UnsupportedInputType {
+        /// The unsupported type.
+        typ: Type,
+        /// The operation name.
+        optype: OpType,
+        /// The node.
+        node: Node,
+    },
+    /// The operation has non-serializable outputs.
+    #[error("Operation {} in {node} has an unsupported output of type {typ}.", optype.name())]
+    UnsupportedOutputType {
+        /// The unsupported type.
+        typ: Type,
+        /// The operation name.
+        optype: OpType,
+        /// The node.
+        node: Node,
+    },
+    /// A parameter input could not be evaluated.
+    #[error("The {typ} parameter input for operation {} in {node} could not be resolved.", optype.name())]
+    UnresolvedParamInput {
+        /// The parameter type.
+        typ: Type,
+        /// The operation with the missing input param.
+        optype: OpType,
+        /// The node.
+        node: Node,
+    },
+    /// The operation has output-only qubits.
+    /// This is not currently supported by the encoder.
+    #[error("Operation {} in {node} has more output qubits than inputs.", optype.name())]
+    TooManyOutputQubits {
+        /// The unsupported type.
+        typ: Type,
+        /// The operation name.
+        optype: OpType,
+        /// The node.
+        node: Node,
+    },
     /// The opaque tket1 operation had an invalid type parameter.
     #[error("Opaque TKET1 operation had an invalid type parameter. {error}")]
     InvalidOpaqueTypeParam {
         /// The serialization error.
         #[from]
         error: serde_yaml::Error,
     },
+    /// Tried to decode a tket1 operation with not enough parameters.
+    #[error(
+        "Operation {} is missing encoded parameters. Expected at least {expected} but only \"{}\" were specified.",
+        optype.name(),
+        params.iter().join(", "),
+    )]
+    MissingSerialisedParams {
+        /// The operation name.
+        optype: OpType,
+        /// The expected number of parameters.
+        expected: usize,
+        /// The given of parameters.
+        params: Vec<String>,
+    },
+    /// Tried to decode a tket1 operation with not enough qubit/bit arguments.
+    #[error(
+        "Operation {} is missing encoded arguments. Expected {expected_qubits} and {expected_bits}, but only \"{args:?}\" were specified.",
+        optype.name(),
+    )]
+    MissingSerialisedArguments {
+        /// The operation name.
+        optype: OpType,
+        /// The expected number of qubits.
+        expected_qubits: usize,
+        /// The expected number of bits.
+        expected_bits: usize,
+        /// The given of parameters.
+        args: Vec<circuit_json::Register>,
+    },
 }
 
 /// Error type for conversion between `Op` and `OpType`.
@@ -234,3 +312,20 @@ fn try_constant_to_param(val: &Value) -> Option<String> {
     let half_turns = radians / std::f64::consts::PI;
     Some(half_turns.to_string())
 }
+
+/// A hashed register, used to identify registers in the [`Tk1Decoder::register_wire`] map,
+/// avoiding string and vector clones on lookup.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+struct RegisterHash {
+    hash: u64,
+}
+
+impl From<&circuit_json::Register> for RegisterHash {
+    fn from(reg: &circuit_json::Register) -> Self {
+        let mut hasher = DefaultHasher::new();
+        reg.hash(&mut hasher);
+        Self {
+            hash: hasher.finish(),
+        }
+    }
+}