From aa55ccbd862aebaf35e4df35c4b9a89bd11f9ddc Mon Sep 17 00:00:00 2001
From: Luca Mondada <72734770+lmondada@users.noreply.github.com>
Date: Wed, 30 Aug 2023 15:28:17 +0200
Subject: [PATCH] feat!: CircuitPattern::from_circuit may fail. (#62)

BREAKING CHANGE: CircuitPattern::from_circuit is now CircuitPattern::try_from_circuit and returns a Result type.
---
 Cargo.toml                  |  2 +-
 src/portmatching/matcher.rs |  2 +-
 src/portmatching/pattern.rs | 35 +++++++++++++++++++++++++++++------
 src/portmatching/pyo3.rs    |  4 +++-
 4 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index edf12f980..c97366c5b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,7 +26,7 @@ itertools = "0.11.0"
 petgraph = { version = "0.6.3", default-features = false }
 serde_yaml = "0.9.22"
 # portmatching = { version = "0.2.0", optional = true, features = ["serde"]}
-portmatching = { optional = true, git = "https://github.com/lmondada/portmatching", rev = "219f53d" }
+portmatching = { optional = true, git = "https://github.com/lmondada/portmatching", rev = "61ef939" }
 derive_more = "0.99.17"
 quantinuum-hugr = { workspace = true }
 portgraph = { workspace = true }
diff --git a/src/portmatching/matcher.rs b/src/portmatching/matcher.rs
index dd7c61e60..ca3f217dd 100644
--- a/src/portmatching/matcher.rs
+++ b/src/portmatching/matcher.rs
@@ -284,7 +284,7 @@ mod tests {
         let hugr = h_cx();
         let circ: DescendantsGraph<'_, DfgID> = DescendantsGraph::new(&hugr, hugr.root());
 
-        let p = CircuitPattern::from_circuit(&circ);
+        let p = CircuitPattern::try_from_circuit(&circ).unwrap();
         let m = CircuitMatcher::from_patterns(vec![p]);
 
         let matches = m.find_matches(&circ);
diff --git a/src/portmatching/pattern.rs b/src/portmatching/pattern.rs
index 885bac0d9..b8f5903b7 100644
--- a/src/portmatching/pattern.rs
+++ b/src/portmatching/pattern.rs
@@ -2,8 +2,9 @@
 
 use hugr::{ops::OpTrait, Node, Port};
 use itertools::Itertools;
-use portmatching::{HashMap, Pattern, SinglePatternMatcher};
+use portmatching::{patterns::NoRootFound, HashMap, Pattern, SinglePatternMatcher};
 use std::fmt::Debug;
+use thiserror::Error;
 
 use super::{
     matcher::{validate_unweighted_edge, validate_weighted_node},
@@ -27,7 +28,12 @@ pub struct CircuitPattern {
 
 impl CircuitPattern {
     /// Construct a pattern from a circuit.
-    pub fn from_circuit<'circ, C: Circuit<'circ>>(circuit: &'circ C) -> Self {
+    pub fn try_from_circuit<'circ, C: Circuit<'circ>>(
+        circuit: &'circ C,
+    ) -> Result<Self, InvalidPattern> {
+        if circuit.num_gates() == 0 {
+            return Err(InvalidPattern::EmptyCircuit);
+        }
         let mut pattern = Pattern::new();
         for cmd in circuit.commands() {
             let op = circuit.command_optype(&cmd).clone();
@@ -41,7 +47,7 @@ impl CircuitPattern {
                 }
             }
         }
-        pattern.set_any_root().unwrap();
+        pattern.set_any_root()?;
         let (inp, out) = (circuit.input(), circuit.output());
         let inp_ports = circuit.get_optype(inp).signature().output_ports();
         let out_ports = circuit.get_optype(out).signature().input_ports();
@@ -57,11 +63,11 @@ impl CircuitPattern {
                     .expect("invalid circuit")
             })
             .collect();
-        Self {
+        Ok(Self {
             pattern,
             inputs,
             outputs,
-        }
+        })
     }
 
     /// Compute the map from pattern nodes to circuit nodes in `circ`.
@@ -88,6 +94,23 @@ impl Debug for CircuitPattern {
     }
 }
 
+/// Conversion error from circuit to pattern.
+#[derive(Debug, Error)]
+pub enum InvalidPattern {
+    /// An empty circuit cannot be a pattern.
+    #[error("empty circuit is invalid pattern")]
+    EmptyCircuit,
+    /// Patterns must be connected circuits.
+    #[error("pattern is not connected")]
+    NotConnected,
+}
+
+impl From<NoRootFound> for InvalidPattern {
+    fn from(_: NoRootFound) -> Self {
+        InvalidPattern::NotConnected
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use hugr::extension::prelude::QB_T;
@@ -119,7 +142,7 @@ mod tests {
         let hugr = h_cx();
         let circ: DescendantsGraph<'_, DfgID> = DescendantsGraph::new(&hugr, hugr.root());
 
-        let p = CircuitPattern::from_circuit(&circ);
+        let p = CircuitPattern::try_from_circuit(&circ).unwrap();
 
         let edges = p
             .pattern
diff --git a/src/portmatching/pyo3.rs b/src/portmatching/pyo3.rs
index e32d334a6..72cfba70b 100644
--- a/src/portmatching/pyo3.rs
+++ b/src/portmatching/pyo3.rs
@@ -14,6 +14,7 @@ use crate::json::TKETDecode;
 
 create_exception!(pyrs, PyValidateError, PyException);
 create_exception!(pyrs, PyInvalidReplacement, PyException);
+create_exception!(pyrs, PyInvalidPattern, PyException);
 
 #[pymethods]
 impl CircuitPattern {
@@ -22,7 +23,8 @@ impl CircuitPattern {
     pub fn py_from_circuit(circ: PyObject) -> PyResult<CircuitPattern> {
         let hugr = pyobj_as_hugr(circ)?;
         let circ = hugr_as_view(&hugr);
-        Ok(CircuitPattern::from_circuit(&circ))
+        CircuitPattern::try_from_circuit(&circ)
+            .map_err(|e| PyInvalidPattern::new_err(e.to_string()))
     }
 
     /// A string representation of the pattern.