folly::gen::splitByLine

Summary:
I made it work, but please send your feedback how to improve code quality.

splitByLine will split on \r, \n, and \r\n.

Test Plan: add new test, arc unit

Reviewed By: tjackson@fb.com

Subscribers: folly@lists, crawler-diffs@

FB internal diff: D1322212

folly/gen/String-inl.h

@@ -26,6 +26,39 @@ namespace folly {
 namespace gen {
 namespace detail {
 
+inline bool splitPrefix(StringPiece& in,
+                        StringPiece& prefix,
+                        StringPiece delimiter) {
+  auto p = in.find(delimiter);
+  if (p != std::string::npos) {
+    prefix.assign(in.data(), in.data() + p);
+    in.advance(p + delimiter.size());
+    return true;
+  }
+  prefix.clear();
+  return false;
+}
+
+/**
+ * Split by any of the EOL terms: \r, \n, or \r\n.
+ */
+inline bool splitPrefix(StringPiece& in,
+                        StringPiece& prefix,
+                        MixedNewlines) {
+  auto newline = "\r\n";
+  auto p = in.find_first_of(newline);
+  if (p != std::string::npos) {
+    prefix.assign(in.data(), in.data() + p);
+    in.advance(p);
+    if (!in.removePrefix(newline)) {
+      in.advance(1);
+    }
+    return true;
+  }
+  prefix.clear();
+  return false;
+}
+
 inline bool splitPrefix(StringPiece& in, StringPiece& prefix, char delimiter) {
   auto p = static_cast<const char*>(memchr(in.data(), delimiter, in.size()));
   if (p) {
@@ -128,14 +161,16 @@ class StringResplitter : public Operator<StringResplitter> {
   }
 };
 
-class SplitStringSource : public GenImpl<StringPiece, SplitStringSource> {
+template <class DelimiterType = char>
+class SplitStringSource
+    : public GenImpl<StringPiece, SplitStringSource<DelimiterType>> {
   StringPiece source_;
-  char delimiter_;
+  DelimiterType delimiter_;
  public:
   SplitStringSource(const StringPiece& source,
-                    char delimiter)
+                    DelimiterType delimiter)
     : source_(source)
-    , delimiter_(delimiter) { }
+    , delimiter_(std::move(delimiter)) { }
 
   template <class Body>
   bool apply(Body&& body) const {
@@ -166,7 +201,7 @@ template<class Delimiter,
 class Unsplit : public Operator<Unsplit<Delimiter, Output>> {
   Delimiter delimiter_;
  public:
-  Unsplit(const Delimiter& delimiter)
+  explicit Unsplit(const Delimiter& delimiter)
     : delimiter_(delimiter) {
   }
 

folly/gen/String.h

 /*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2014 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,8 @@ namespace gen {
 
 namespace detail {
 class StringResplitter;
+
+template<class Delimiter>
 class SplitStringSource;
 
 template<class Delimiter, class Output>
@@ -54,11 +56,30 @@ S resplit(char delimiter) {
   return S(delimiter);
 }
 
-template <class S=detail::SplitStringSource>
+template <class S = detail::SplitStringSource<char>>
 S split(const StringPiece& source, char delimiter) {
   return S(source, delimiter);
 }
 
+template <class S = detail::SplitStringSource<StringPiece>>
+S split(StringPiece source, StringPiece delimiter) {
+  return S(source, delimiter);
+}
+
+/**
+ * EOL terms ("\r", "\n", or "\r\n").
+ */
+class MixedNewlines {};
+
+/**
+ * Split by EOL ("\r", "\n", or "\r\n").
+ * @see split().
+ */
+template <class S = detail::SplitStringSource<MixedNewlines>>
+S lines(StringPiece source) {
+  return S(source, MixedNewlines{});
+}
+
 /*
  * Joins a sequence of tokens into a string, with the chosen delimiter.
  *

folly/gen/test/StringTest.cpp

@@ -82,13 +82,36 @@ TEST(StringGen, Split) {
   }
 
   {
-    auto pieces = split("hello,, world, goodbye, meow", ',')
+    auto pieces = split("hello,, world, goodbye, meow", ",")
                 | take(5) | collect;
     EXPECT_EQ(5, pieces.size());
     EXPECT_EQ("hello", pieces[0]);
     EXPECT_EQ("", pieces[1]);
     EXPECT_EQ(" world", pieces[2]);
   }
+
+  {
+    auto pieces = split("hello,, world, goodbye, meow", ", ")
+                | collect;
+    EXPECT_EQ(4, pieces.size());
+    EXPECT_EQ("hello,", pieces[0]);
+    EXPECT_EQ("world", pieces[1]);
+    EXPECT_EQ("goodbye", pieces[2]);
+    EXPECT_EQ("meow", pieces[3]);
+  }
+}
+
+TEST(StringGen, SplitByNewLine) {
+  auto collect = eachTo<std::string>() | as<vector>();
+  {
+    auto pieces = lines("hello\n\n world\r\n goodbye\r meow") | collect;
+    EXPECT_EQ(5, pieces.size());
+    EXPECT_EQ("hello", pieces[0]);
+    EXPECT_EQ("", pieces[1]);
+    EXPECT_EQ(" world", pieces[2]);
+    EXPECT_EQ(" goodbye", pieces[3]);
+    EXPECT_EQ(" meow", pieces[4]);
+  }
 }
 
 TEST(StringGen, EmptyResplit) {