More decoders2 (#253)

* add relay-bp

Author: ciaranra

Cargo.lock

@@ -167,9 +167,21 @@ pecos-chromobius = { version = "0.1.1", path = "crates/pecos-chromobius" }
 # Fusion Blossom decoder wrapper (pure Rust MWPM)
 pecos-fusion-blossom = { version = "0.1.1", path = "crates/pecos-fusion-blossom" }
 
+# Relay BP decoder wrapper (pure Rust BP for qLDPC codes)
+pecos-relay-bp = { version = "0.1.1", path = "crates/pecos-relay-bp" }
+
 # Fusion Blossom library (pure Rust MWPM decoder)
 fusion-blossom = "0.2"
 
+# Relay BP library (pure Rust belief propagation decoder)
+relay-bp = "0.2"
+
+# ndarray 0.16 for relay-bp compatibility (relay-bp uses ndarray 0.16, PECOS uses 0.17)
+ndarray-016 = { package = "ndarray", version = "0.16" }
+
+# sprs for sparse matrix construction (pinned to 0.11.3 for ndarray 0.16 compat with relay-bp)
+sprs = "=0.11.3"
+
 # petgraph for graph algorithms (used by pymatching)
 petgraph = "0.8"
 

Cargo.toml

@@ -18,6 +18,7 @@ pecos-fusion-blossom = { workspace = true, optional = true }
 pecos-pymatching = { workspace = true, optional = true }
 pecos-tesseract = { workspace = true, optional = true }
 pecos-chromobius = { workspace = true, optional = true }
+pecos-relay-bp = { workspace = true, optional = true }
 
 [features]
 default = []
@@ -26,7 +27,8 @@ fusion-blossom = ["dep:pecos-fusion-blossom"]
 pymatching = ["dep:pecos-pymatching"]
 tesseract = ["dep:pecos-tesseract"]
 chromobius = ["dep:pecos-chromobius"]
-all = ["ldpc", "fusion-blossom", "pymatching", "tesseract", "chromobius"]
+relay-bp = ["dep:pecos-relay-bp"]
+all = ["ldpc", "fusion-blossom", "pymatching", "tesseract", "chromobius", "relay-bp"]
 
 [lints]
 workspace = true

crates/pecos-decoders/Cargo.toml

@@ -15,6 +15,7 @@ Enable the appropriate features to include specific decoder families:
 - `pymatching` - PyMatching MWPM decoder
 - `tesseract` - Tesseract search-based decoder
 - `chromobius` - Chromobius color code decoder
+- `relay-bp` - Relay BP decoder for qLDPC codes
 - `all` - Enable all decoders
 
 ## Key Types

crates/pecos-decoders/README.md

@@ -10,6 +10,7 @@
 //! - `pymatching` - `PyMatching` MWPM decoder (C++ FFI)
 //! - `tesseract` - Tesseract search-based decoder (C++ FFI)
 //! - `chromobius` - Chromobius color code decoder (C++ FFI)
+//! - `relay-bp` - Relay BP decoder for qLDPC codes (pure Rust)
 //! - `all` - Enable all decoders
 
 // Re-export core traits
@@ -80,3 +81,10 @@ pub use pecos_chromobius::{
     ChromobiusConfig, ChromobiusDecoder, ChromobiusError,
     DecodingResult as ChromobiusDecodingResult,
 };
+
+// Re-export Relay BP decoder when feature is enabled
+#[cfg(feature = "relay-bp")]
+pub use pecos_relay_bp::{
+    DecodingResult as RelayBpDecodingResult, MinSumBpBuilder, MinSumBpDecoder, MinSumConfig,
+    RelayBpBuilder, RelayBpDecoder, RelayBpError, RelayConfig, StoppingCriterion,
+};

crates/pecos-decoders/src/lib.rs

@@ -15,7 +15,7 @@ Wraps the Fusion Blossom minimum-weight perfect matching decoder for quantum err
 
 ## Acknowledgements
 
-This crate wraps [Fusion Blossom](https://github.com/yuewuo/fusion-blossom), a fast MWPM decoder developed by Yue Wu, Namitha Liyanage, and Lin Zhong at Yale University.
+This crate wraps [Fusion Blossom](https://github.com/yuewuo/fusion-blossom), a fast MWPM decoder developed by Yue Wu and Lin Zhong at Yale University.
 
 **Paper:**
-- Wu, Y., Liyanage, N., & Zhong, L. (2023). "Fusion Blossom: Fast MWPM Decoders for QEC." [arXiv:2305.08307](https://arxiv.org/abs/2305.08307)
+- Wu, Y. & Zhong, L. (2023). "Fusion Blossom: Fast MWPM Decoders for QEC." [arXiv:2305.08307](https://arxiv.org/abs/2305.08307)

crates/pecos-fusion-blossom/README.md

@@ -21,4 +21,4 @@ Provides LDPC (Low-Density Parity-Check) based decoders including belief propaga
 This crate wraps [ldpc](https://github.com/quantumgizmos/ldpc), a high-performance LDPC decoder library developed by Joschka Roffe and collaborators.
 
 **Paper:**
-- Roffe, J., White, D. R., Burton, S., & Campbell, E. (2020). "Decoding across the quantum low-density parity-check code landscape." Physical Review Research, 2(4), 043423. [arXiv:2005.07016](https://arxiv.org/abs/2005.07016)
+- Roffe, J., White, D. R., Burton, S., & Campbell, E. T. (2020). "Decoding Across the Quantum LDPC Code Landscape." Physical Review Research, 2(4), 043423. [arXiv:2005.07016](https://arxiv.org/abs/2005.07016)

crates/pecos-ldpc-decoders/README.md

@@ -0,0 +1,27 @@
+[package]
+name = "pecos-relay-bp"
+version.workspace = true
+edition.workspace = true
+readme = "README.md"
+authors.workspace = true
+homepage.workspace = true
+repository.workspace = true
+license.workspace = true
+keywords.workspace = true
+categories.workspace = true
+description = "Relay BP decoder for PECOS"
+
+[dependencies]
+pecos-decoder-core.workspace = true
+ndarray.workspace = true
+ndarray-016.workspace = true
+thiserror.workspace = true
+relay-bp.workspace = true
+sprs.workspace = true
+rayon.workspace = true
+
+[lib]
+name = "pecos_relay_bp"
+
+[lints]
+workspace = true

crates/pecos-relay-bp/Cargo.toml

@@ -0,0 +1,22 @@
+# pecos-relay-bp
+
+Relay BP decoder for PECOS.
+
+## Purpose
+
+Wraps the Relay BP decoder for quantum low-density parity-check (qLDPC) code decoding. Relay BP enhances standard min-sum belief propagation with disordered memory strengths, ensembling, and relaying for improved convergence on codes like bivariate bicycle codes.
+
+## Key Types
+
+- `RelayBpDecoder` - Relay BP ensemble decoder
+- `MinSumBpDecoder` - Plain min-sum BP decoder
+- `RelayConfig` - Relay ensemble configuration
+- `MinSumConfig` - Min-sum BP configuration
+- `RelayBpBuilder` / `MinSumBpBuilder` - Builder patterns for decoder construction
+
+## Acknowledgements
+
+This crate wraps [relay-bp](https://github.com/trmue/relay), a Relay BP decoder developed by Tristan Mueller, Thomas Alexander, Michael E. Beverland, Markus Buehler, Blake R. Johnson, Thilo Maurer, and Drew Vandeth.
+
+**Paper:**
+- Mueller, T., Alexander, T., Beverland, M. E., Buehler, M., Johnson, B. R., Maurer, T., & Vandeth, D. (2025). "Improved Belief Propagation Is Sufficient for Real-Time Decoding of Quantum Memory." [arXiv:2506.01779](https://arxiv.org/abs/2506.01779)