byLine(istream), with partial consumption

Summary:
Without this, `folly::gen`'s other `byLine()` implementations only
support a single read pass (usually through the whole input file) without
support for different formats through the file.

Reviewed By: yfeldblum

Differential Revision: D9540396

fbshipit-source-id: 6539851a70aed9a88d3c61986a459f60dfb7be5b

folly/gen/IStream.h

+/*
+ * Copyright 2018-present Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include <istream>
+#include <string>
+
+#include <folly/gen/Core.h>
+
+namespace folly {
+namespace gen {
+namespace detail {
+
+/**
+ * Generates lines by calling std::getline() on a given istream.
+ */
+class IStreamByLine : public GenImpl<std::string&&, IStreamByLine> {
+ public:
+  IStreamByLine(std::istream& in) : in_(in) {}
+
+  template <class Body>
+  bool apply(Body&& body) const {
+    for (std::string line; std::getline(in_, line);) {
+      if (!body(std::move(line))) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  // Technically, there could be infinite files (e.g. /dev/random), but people
+  // who open those can do so at their own risk.
+  static constexpr bool infinite = false;
+
+ private:
+  std::istream& in_;
+};
+
+} // namespace detail
+
+inline detail::IStreamByLine byLine(std::istream& in) {
+  return detail::IStreamByLine(in);
+}
+
+} // namespace gen
+} // namespace folly

folly/gen/test/IStreamTest.cpp

+/*
+ * Copyright 2014-present Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <folly/gen/IStream.h>
+
+#include <sstream>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include <folly/gen/Base.h>
+#include <folly/gen/String.h>
+#include <folly/portability/GTest.h>
+
+namespace folly {
+namespace gen {
+
+// Test mixed consumption of std::istream with and without folly::gen, and
+// demonstrates partial consumption of input.
+TEST(IStream, ByLine) {
+  std::stringstream in(
+      "2\n" // Count applies to both below groups.
+      "1\tred\n"
+      "3\tblue\n"
+      "3.4\t5.6\t7.8\n"
+      "1.1\t2.2\t3.3\n");
+  size_t n;
+  in >> n;
+  in.get(); // Consume trailing newline
+  auto colors = byLine(in) | take(n) | eachToPair<int, std::string>('\t') |
+      as<std::vector>();
+  auto coords = byLine(in) | take(n) | eachToTuple<float, float, float>('\t') |
+      as<std::vector>();
+  EXPECT_EQ(
+      colors,
+      (std::vector<std::pair<int, std::string>>{
+          {1, "red"},
+          {3, "blue"},
+      }));
+  EXPECT_EQ(
+      coords,
+      (std::vector<std::tuple<float, float, float>>({
+          std::tuple<float, float, float>{3.4f, 5.6f, 7.8f},
+          std::tuple<float, float, float>{1.1f, 2.2f, 3.3f},
+      })));
+}
+
+} // namespace gen
+} // namespace folly