Add new private class JSON::Writer

Create a simple utility class for writing JSON to a pipeline.

include/qpdf/JSON.hh

@@ -290,8 +290,11 @@ class JSON
     QPDF_DLL
     qpdf_offset_t getEnd() const;
 
+    // The following class does not form part of the public API and is for internal use only.
+
+    class Writer;
+
   private:
-    static std::string encode_string(std::string const& utf8);
     static void writeClose(Pipeline* p, bool first, size_t depth, char const* delimeter);
 
     enum value_type_e {

libqpdf/JSON.cc

@@ -1,5 +1,7 @@
 #include <qpdf/JSON.hh>
 
+#include <qpdf/JSON_writer.hh>
+
 #include <qpdf/BufferInputSource.hh>
 #include <qpdf/Pl_Base64.hh>
 #include <qpdf/Pl_Concatenate.hh>
@@ -119,7 +121,7 @@ JSON::JSON_array::write(Pipeline* p, size_t depth) const
 JSON::JSON_string::JSON_string(std::string const& utf8) :
     JSON_value(vt_string),
     utf8(utf8),
-    encoded(encode_string(utf8))
+    encoded(Writer::encode_string(utf8))
 {
 }
 
@@ -211,7 +213,7 @@ JSON::unparse() const
 }
 
 std::string
-JSON::encode_string(std::string const& str)
+JSON::Writer::encode_string(std::string const& str)
 {
     static auto constexpr hexchars = "0123456789abcdef";
 
@@ -279,7 +281,7 @@ JSON
 JSON::addDictionaryMember(std::string const& key, JSON const& val)
 {
     if (auto* obj = m ? dynamic_cast<JSON_dictionary*>(m->value.get()) : nullptr) {
-        return obj->members[encode_string(key)] = val.m ? val : makeNull();
+        return obj->members[Writer::encode_string(key)] = val.m ? val : makeNull();
     } else {
         throw std::runtime_error("JSON::addDictionaryMember called on non-dictionary");
     }

libqpdf/qpdf/JSON_writer.hh

@@ -0,0 +1,125 @@
+#ifndef JSON_WRITER_HH
+#define JSON_WRITER_HH
+
+#include <qpdf/JSON.hh>
+#include <qpdf/Pipeline.hh>
+
+#include <string_view>
+
+// Writer is a small utility class to aid writing JSON to a pipeline. Methods are designed to allow
+// chaining of calls.
+//
+// Some uses of the class have a significant performance impact. The class is intended purely for
+// internal use to allow it to be adapted as needed to maintain performance.
+class JSON::Writer
+{
+  public:
+    Writer(Pipeline* p, size_t depth) :
+        p(p),
+        indent(2 * depth)
+    {
+    }
+
+    Writer&
+    write(char const* data, size_t len)
+    {
+        p->write(reinterpret_cast<unsigned char const*>(data), len);
+        return *this;
+    }
+
+    Writer&
+    writeNext()
+    {
+        auto n = indent;
+        if (first) {
+            first = false;
+            write(&spaces[1], n % n_spaces + 1);
+        } else {
+            write(&spaces[0], n % n_spaces + 2);
+        }
+        while (n >= n_spaces) {
+            write(&spaces[2], n_spaces);
+            n -= n_spaces;
+        }
+        return *this;
+    }
+
+    Writer&
+    writeStart(char const& c)
+    {
+        write(&c, 1);
+        first = true;
+        indent += 2;
+        return *this;
+    }
+
+    Writer&
+    writeEnd(char const& c)
+    {
+        if (indent > 1) {
+            indent -= 2;
+        }
+        if (!first) {
+            first = true;
+            writeNext();
+        }
+        first = false;
+        write(&c, 1);
+        return *this;
+    }
+
+    Writer&
+    operator<<(std::string_view sv)
+    {
+        p->write(reinterpret_cast<unsigned char const*>(sv.data()), sv.size());
+        return *this;
+    }
+
+    Writer&
+    operator<<(char const* s)
+    {
+        *this << std::string_view{s};
+        return *this;
+    }
+
+    Writer&
+    operator<<(bool val)
+    {
+        *this << (val ? "true" : "false");
+        return *this;
+    }
+
+    Writer&
+    operator<<(int val)
+    {
+        *this << std::to_string(val);
+        return *this;
+    }
+
+    Writer&
+    operator<<(size_t val)
+    {
+        *this << std::to_string(val);
+        return *this;
+    }
+
+    Writer&
+    operator<<(JSON&& j)
+    {
+        j.write(p, indent / 2);
+        return *this;
+    }
+
+    static std::string encode_string(std::string const& utf8);
+
+  private:
+    Pipeline* p;
+    bool first{true};
+    size_t indent;
+
+    static constexpr std::string_view spaces =
+        ",\n                                                  ";
+    static constexpr auto n_spaces = spaces.size() - 2;
+};
+
+#endif // JSON_WRITER_HH