Optimize FileReadStream and BasicIStreamWrapper.

On (my) linux, perftest reports:
- ~40% gain for FileReadStream (Take() loop),
- ~10% gain for ReaderParse_DummyHandler_FileReadStream.

With the same logic applied to BasicIStreamWrapper, which thus can now
also be created with a user buffer, performances align with those of
FileReadStream (same buffer size).

The "unbuffered" versions (added for FileReadStream) work solely with
the internal peekBuffer (Ch[4]) and are measured in perftest.  When
performances don't matter much, they can avoid the use of large
stack/heap buffers.

include/rapidjson/filereadstream.h

@@ -17,6 +17,7 @@
 
 #include "stream.h"
 #include <cstdio>
+#include <cstring>
 
 #ifdef __clang__
 RAPIDJSON_DIAG_PUSH
@@ -35,21 +36,42 @@ class FileReadStream {
 public:
     typedef char Ch;    //!< Character type (byte).
 
+    //! Constructor.
+    /*!
+        \param fp File pointer opened for read.
+    */
+    FileReadStream(std::FILE* fp) : fp_(fp), buffer_(peekBuffer_), size_(sizeof(peekBuffer_) / sizeof(Ch)), pos_(), len_(), count_()
+    {
+        RAPIDJSON_ASSERT(fp_ != 0);
+    }
+
     //! Constructor.
     /*!
         \param fp File pointer opened for read.
         \param buffer user-supplied buffer.
         \param bufferSize size of buffer in bytes. Must >=4 bytes.
     */
-    FileReadStream(std::FILE* fp, char* buffer, size_t bufferSize) : fp_(fp), buffer_(buffer), bufferSize_(bufferSize), bufferLast_(0), current_(buffer_), readCount_(0), count_(0), eof_(false) { 
-        RAPIDJSON_ASSERT(fp_ != 0);
-        RAPIDJSON_ASSERT(bufferSize >= 4);
-        Read();
+    FileReadStream(std::FILE* fp, Ch *buffer, size_t size) : fp_(fp), buffer_(buffer), size_(size), pos_(), len_(), count_() {
+        RAPIDJSON_ASSERT(fp_ != 0 && buffer_ != 0 && size_ > 0);
+        if (RAPIDJSON_UNLIKELY(size_ < sizeof(peekBuffer_) / sizeof(Ch))) {
+            size_ = sizeof(peekBuffer_) / sizeof(Ch);
+            buffer_ = peekBuffer_;
+        }
     }
 
-    Ch Peek() const { return *current_; }
-    Ch Take() { Ch c = *current_; Read(); return c; }
-    size_t Tell() const { return count_ + static_cast<size_t>(current_ - buffer_); }
+    Ch Peek() const {
+        if (RAPIDJSON_UNLIKELY(pos_ == len_) && !Read())
+            return static_cast<Ch>('\0');
+        return buffer_[pos_];
+    }
+
+    Ch Take() {
+        if (RAPIDJSON_UNLIKELY(pos_ == len_) && !Read())
+            return static_cast<Ch>('\0');
+        return buffer_[pos_++];
+    }
+
+    size_t Tell() const { return count_ + pos_; }
 
     // Not implemented
     void Put(Ch) { RAPIDJSON_ASSERT(false); }
@@ -59,35 +81,36 @@ public:
 
     // For encoding detection only.
     const Ch* Peek4() const {
-        return (current_ + 4 <= bufferLast_) ? current_ : 0;
+        if (len_ - pos_ < 4) {
+            if (pos_) {
+                len_ -= pos_;
+                std::memmove(buffer_, buffer_ + pos_, len_);
+                count_ += pos_;
+                pos_ = 0;
             }
-
-private:
-    void Read() {
-        if (current_ < bufferLast_)
-            ++current_;
-        else if (!eof_) {
-            count_ += readCount_;
-            readCount_ = std::fread(buffer_, 1, bufferSize_, fp_);
-            bufferLast_ = buffer_ + readCount_ - 1;
-            current_ = buffer_;
-
-            if (readCount_ < bufferSize_) {
-                buffer_[readCount_] = '\0';
-                ++bufferLast_;
-                eof_ = true;
+            len_ += std::fread(buffer_ + len_, sizeof(Ch), size_ - len_, fp_);
+            if (len_ < 4)
+                return 0;
         }
+        return &buffer_[pos_];
     }
+
+private:
+    FileReadStream();
+    FileReadStream(const FileReadStream&);
+    FileReadStream& operator=(const FileReadStream&);
+
+    size_t Read() const {
+        count_ += pos_;
+        pos_ = 0;
+        len_ = std::fread(buffer_, sizeof(Ch), size_, fp_);
+        return len_;
     }
 
     std::FILE* fp_;
-    Ch *buffer_;
-    size_t bufferSize_;
-    Ch *bufferLast_;
-    Ch *current_;
-    size_t readCount_;
-    size_t count_;  //!< Number of characters read
-    bool eof_;
+    Ch peekBuffer_[4], *buffer_;
+    size_t size_;
+    mutable size_t pos_, len_, count_;
 };
 
 RAPIDJSON_NAMESPACE_END

include/rapidjson/istreamwrapper.h

@@ -17,6 +17,7 @@
 
 #include "stream.h"
 #include <iosfwd>
+#include <cstring>
 
 #ifdef __clang__
 RAPIDJSON_DIAG_PUSH
@@ -48,26 +49,33 @@ template <typename StreamType>
 class BasicIStreamWrapper {
 public:
     typedef typename StreamType::char_type Ch;
-    BasicIStreamWrapper(StreamType& stream) : stream_(stream), count_(), peekBuffer_() {}
+
+    BasicIStreamWrapper(StreamType& stream) : stream_(stream), buffer_(peekBuffer_), size_(sizeof(peekBuffer_) / sizeof(Ch)), pos_(), len_(), count_() {}
+
+    BasicIStreamWrapper(StreamType& stream, Ch *buffer, size_t size) : stream_(stream), buffer_(buffer), size_(size), pos_(), len_(), count_() {
+        RAPIDJSON_ASSERT(buffer_ != 0 && static_cast<std::streamsize>(size_) > 0);
+        if (RAPIDJSON_UNLIKELY(size_ < sizeof(peekBuffer_) / sizeof(Ch))) {
+            size_ = sizeof(peekBuffer_) / sizeof(Ch);
+            buffer_ = peekBuffer_;
+        }
+    }
 
     Ch Peek() const {
-        typename StreamType::int_type c = stream_.peek();
-        return RAPIDJSON_LIKELY(c != StreamType::traits_type::eof()) ? static_cast<Ch>(c) : static_cast<Ch>('\0');
+        if (RAPIDJSON_UNLIKELY(pos_ == len_) && !Read())
+            return static_cast<Ch>('\0');
+        return buffer_[pos_];
     }
 
     Ch Take() {
-        typename StreamType::int_type c = stream_.get();
-        if (RAPIDJSON_LIKELY(c != StreamType::traits_type::eof())) {
-            count_++;
-            return static_cast<Ch>(c);
-        }
-        else
-            return '\0';
+        if (RAPIDJSON_UNLIKELY(pos_ == len_) && !Read())
+            return static_cast<Ch>('\0');
+        return buffer_[pos_++];
     }
 
     // tellg() may return -1 when failed. So we count by ourself.
-    size_t Tell() const { return count_; }
+    size_t Tell() const { return count_ + pos_; }
 
+    // Not implemented
     Ch* PutBegin() { RAPIDJSON_ASSERT(false); return 0; }
     void Put(Ch) { RAPIDJSON_ASSERT(false); }
     void Flush() { RAPIDJSON_ASSERT(false); }
@@ -76,29 +84,42 @@ public:
     // For encoding detection only.
     const Ch* Peek4() const {
         RAPIDJSON_ASSERT(sizeof(Ch) == 1); // Only usable for byte stream.
-        int i;
-        bool hasError = false;
-        for (i = 0; i < 4; ++i) {
-            typename StreamType::int_type c = stream_.get();
-            if (c == StreamType::traits_type::eof()) {
-                hasError = true;
-                stream_.clear();
-                break;
+        if (len_ - pos_ < 4) {
+            if (pos_) {
+                len_ -= pos_;
+                std::memmove(buffer_, buffer_ + pos_, len_);
+                count_ += pos_;
+                pos_ = 0;
             }
-            peekBuffer_[i] = static_cast<Ch>(c);
+            if (!stream_.read(buffer_ + len_, static_cast<std::streamsize>(size_ - len_))) {
+                len_ += static_cast<size_t>(stream_.gcount());
+                if (len_ < 4)
+                    return 0;
             }
-        for (--i; i >= 0; --i)
-            stream_.putback(peekBuffer_[i]);
-        return !hasError ? peekBuffer_ : 0;
+            else
+                len_ = size_;
+        }
+        return &buffer_[pos_];
     }
 
 private:
     BasicIStreamWrapper(const BasicIStreamWrapper&);
     BasicIStreamWrapper& operator=(const BasicIStreamWrapper&);
 
+    size_t Read() const {
+        count_ += pos_;
+        pos_ = 0;
+        if (!stream_.read(buffer_, static_cast<std::streamsize>(size_)))
+            len_ = static_cast<size_t>(stream_.gcount());
+        else
+            len_ = size_;
+        return len_;
+    }
+
     StreamType& stream_;
-    size_t count_;  //!< Number of characters read. Note:
-    mutable Ch peekBuffer_[4];
+    Ch peekBuffer_[4], *buffer_;
+    size_t size_;
+    mutable size_t pos_, len_, count_;
 };
 
 typedef BasicIStreamWrapper<std::istream> IStreamWrapper;

test/perftest/rapidjsontest.cpp

@@ -21,9 +21,12 @@
 #include "rapidjson/prettywriter.h"
 #include "rapidjson/stringbuffer.h"
 #include "rapidjson/filereadstream.h"
+#include "rapidjson/istreamwrapper.h"
 #include "rapidjson/encodedstream.h"
 #include "rapidjson/memorystream.h"
 
+#include <fstream>
+
 #ifdef RAPIDJSON_SSE2
 #define SIMD_SUFFIX(name) name##_SSE2
 #elif defined(RAPIDJSON_SSE42)
@@ -451,6 +454,16 @@ TEST_F(RapidJson, FileReadStream) {
     }
 }
 
+TEST_F(RapidJson, FileReadStream_Unbuffered) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        FILE *fp = fopen(filename_, "rb");
+        FileReadStream s(fp);
+        while (s.Take() != '\0')
+            ;
+        fclose(fp);
+    }
+}
+
 TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_FileReadStream)) {
     for (size_t i = 0; i < kTrialCount; i++) {
         FILE *fp = fopen(filename_, "rb");
@@ -463,6 +476,88 @@ TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_FileReadStream)) {
     }
 }
 
+TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_FileReadStream_Unbuffered)) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        FILE *fp = fopen(filename_, "rb");
+        FileReadStream s(fp);
+        BaseReaderHandler<> h;
+        Reader reader;
+        reader.Parse(s, h);
+        fclose(fp);
+    }
+}
+
+TEST_F(RapidJson, IStreamWrapper) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is(filename_);
+        char buffer[65536];
+        IStreamWrapper isw(is, buffer, sizeof(buffer));
+        while (isw.Take() != '\0')
+            ;
+        is.close();
+    }
+}
+
+TEST_F(RapidJson, IStreamWrapper_Unbuffered) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is(filename_);
+        IStreamWrapper isw(is);
+        while (isw.Take() != '\0')
+            ;
+        is.close();
+    }
+}
+
+TEST_F(RapidJson, IStreamWrapper_Setbuffered) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is;
+        char buffer[65536];
+        is.rdbuf()->pubsetbuf(buffer, sizeof(buffer));
+        is.open(filename_);
+        IStreamWrapper isw(is);
+        while (isw.Take() != '\0')
+            ;
+        is.close();
+    }
+}
+
+TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_IStreamWrapper)) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is(filename_);
+        char buffer[65536];
+        IStreamWrapper isw(is, buffer, sizeof(buffer));
+        BaseReaderHandler<> h;
+        Reader reader;
+        reader.Parse(isw, h);
+        is.close();
+    }
+}
+
+TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_IStreamWrapper_Unbuffered)) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is(filename_);
+        IStreamWrapper isw(is);
+        BaseReaderHandler<> h;
+        Reader reader;
+        reader.Parse(isw, h);
+        is.close();
+    }
+}
+
+TEST_F(RapidJson, SIMD_SUFFIX(ReaderParse_DummyHandler_IStreamWrapper_Setbuffered)) {
+    for (size_t i = 0; i < kTrialCount; i++) {
+        std::ifstream is;
+        char buffer[65536];
+        is.rdbuf()->pubsetbuf(buffer, sizeof(buffer));
+        is.open(filename_);
+        IStreamWrapper isw(is);
+        BaseReaderHandler<> h;
+        Reader reader;
+        reader.Parse(isw, h);
+        is.close();
+    }
+}
+
 TEST_F(RapidJson, StringBuffer) {
     StringBuffer sb;
     for (int i = 0; i < 32 * 1024 * 1024; i++)