Implement context biasing with a Aho Corasick automata (#145)

* Implement context graph

* Modify the interface to support context biasing

* Support context biasing in modified beam search; add python wrapper

* Support context biasing in python api example

* Minor fixes

* Fix context graph

* Minor fixes

* Fix tests

* Fix style

* Fix style

* Fix comments

* Minor fixes

* Add missing header

* Replace std::shared_ptr with std::unique_ptr for effciency

* Build graph in constructor

* Fix comments

* Minor fixes

* Fix docs

sherpa-onnx/csrc/context-graph.h

@@ -0,0 +1,66 @@
+// sherpa-onnx/csrc/context-graph.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_CONTEXT_GRAPH_H_
+#define SHERPA_ONNX_CSRC_CONTEXT_GRAPH_H_
+
+#include <memory>
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+#include "sherpa-onnx/csrc/log.h"
+
+namespace sherpa_onnx {
+
+class ContextGraph;
+using ContextGraphPtr = std::shared_ptr<ContextGraph>;
+
+struct ContextState {
+  int32_t token;
+  float token_score;
+  float node_score;
+  float local_node_score;
+  bool is_end;
+  std::unordered_map<int32_t, std::unique_ptr<ContextState>> next;
+  const ContextState *fail = nullptr;
+  const ContextState *output = nullptr;
+
+  ContextState() = default;
+  ContextState(int32_t token, float token_score, float node_score,
+               float local_node_score, bool is_end)
+      : token(token),
+        token_score(token_score),
+        node_score(node_score),
+        local_node_score(local_node_score),
+        is_end(is_end) {}
+};
+
+class ContextGraph {
+ public:
+  ContextGraph() = default;
+  ContextGraph(const std::vector<std::vector<int32_t>> &token_ids,
+               float context_score)
+      : context_score_(context_score) {
+    root_ = std::make_unique<ContextState>(-1, 0, 0, 0, false);
+    root_->fail = root_.get();
+    Build(token_ids);
+  }
+
+  std::pair<float, const ContextState *> ForwardOneStep(
+      const ContextState *state, int32_t token_id) const;
+  std::pair<float, const ContextState *> Finalize(
+      const ContextState *state) const;
+
+  const ContextState *Root() const { return root_.get(); }
+
+ private:
+  float context_score_;
+  std::unique_ptr<ContextState> root_;
+  void Build(const std::vector<std::vector<int32_t>> &token_ids) const;
+  void FillFailOutput() const;
+};
+
+}  // namespace sherpa_onnx
+#endif  // SHERPA_ONNX_CSRC_CONTEXT_GRAPH_H_