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-test.cc

@@ -0,0 +1,43 @@
+// sherpa-onnx/csrc/context-graph-test.cc
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#include "sherpa-onnx/csrc/context-graph.h"
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+
+namespace sherpa_onnx {
+
+TEST(ContextGraph, TestBasic) {
+  std::vector<std::string> contexts_str(
+      {"S", "HE", "SHE", "SHELL", "HIS", "HERS", "HELLO", "THIS", "THEM"});
+  std::vector<std::vector<int32_t>> contexts;
+  for (int32_t i = 0; i < contexts_str.size(); ++i) {
+    contexts.emplace_back(contexts_str[i].begin(), contexts_str[i].end());
+  }
+  auto context_graph = ContextGraph(contexts, 1);
+
+  auto queries = std::map<std::string, float>{
+      {"HEHERSHE", 14}, {"HERSHE", 12}, {"HISHE", 9},   {"SHED", 6},
+      {"HELL", 2},      {"HELLO", 7},   {"DHRHISQ", 4}, {"THEN", 2}};
+
+  for (const auto &iter : queries) {
+    float total_scores = 0;
+    auto state = context_graph.Root();
+    for (auto q : iter.first) {
+      auto res = context_graph.ForwardOneStep(state, q);
+      total_scores += res.first;
+      state = res.second;
+    }
+    auto res = context_graph.Finalize(state);
+    EXPECT_EQ(res.second->token, -1);
+    total_scores += res.first;
+    EXPECT_EQ(total_scores, iter.second);
+  }
+}
+
+}  // namespace sherpa_onnx