json_view: fewer allocations when parsing small documents

- Size the buffer of decoded strings on its first growth from the input (a
  quarter of it plus 16 bytes, at most 4 KiB) instead of 272 bytes, so that a
  document with escapes does not grow it twice, with a zero-filled copy each
  time.
- Allocate the node index of documents with an estimate of up to 1024 nodes
  (was 256) together with the header, which saves one allocation. When the
  estimate is too low (dense numbers), that block stays unused: at most 16 KiB
  instead of 4 KiB. shrink_to_fit() cannot release it separately; the
  documentation says so.

Fresh parse of a 2.5 KB minified tweet: 993 to 958 ns (-3.5 %), traverse and
select -3 %, with no row consistently slower (Apple M1 Max, 30 rounds, three
runs). Dense numeric documents parse about 9 % faster and keep up to 16 KiB
more memory.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view.cpp

@@ -666,6 +666,61 @@ TEST_CASE("json_view")
         CHECK(small.root().materialize() == json::parse("[1,[2,3],{\"a\":\"b\\n\"}]"));
     }
 
+    SECTION("block sizes")
+    {
+        // documents whose node estimate lies around the size that is allocated
+        // with the header, and whose decoded strings need the first growth of
+        // the buffer, in all sizes around the points where the policies change
+        for (const int count :
+                {
+                    1, 2, 10, 50, 100, 255, 256, 257, 500, 1000, 1023, 1024, 1025, 2000, 5000
+                })
+        {
+            // strings with escapes (decoded text, minified)
+            std::string escaped = "[";
+            for (int i = 0; i < count; ++i)
+            {
+                escaped += (i != 0 ? ",\"a\\n" : "\"a\\n") + std::to_string(i) + "\\u00e4\"";
+            }
+            escaped += ']';
+            json_document e = json_document::parse(escaped);
+            CHECK(e.root().materialize() == json::parse(escaped));
+            e.shrink_to_fit();
+            CHECK(e.root().materialize() == json::parse(escaped));
+
+            // dense numbers (more nodes per byte than the estimate expects)
+            std::string dense = "[";
+            for (int i = 0; i < count; ++i)
+            {
+                dense += (i != 0 ? ",1" : "1");
+            }
+            dense += ']';
+            json_document n = json_document::parse(dense);
+            CHECK(n.node_count() == static_cast<std::size_t>(count) + 1);
+            CHECK(n.root().materialize() == json::parse(dense));
+            n.shrink_to_fit();
+            CHECK(n.root().size() == static_cast<std::size_t>(count));
+
+            // the same documents, pretty-printed
+            const std::string pretty = json::parse(escaped).dump(2);
+            json_document p = json_document::parse(pretty);
+            CHECK(p.root().materialize() == json::parse(escaped));
+        }
+
+        // a large document with a single escape does not need a buffer of the
+        // size of its input
+        std::string large = "[\"a\\n\"";
+        for (int i = 0; i < 100000; ++i)
+        {
+            large += ",1";
+        }
+        large += ']';
+        const json_document l = json_document::parse(large);
+        CHECK(l.root().size() == 100001);
+        CHECK(l.root()[0].get_string() == "a\n");
+        CHECK(l.root()[100000].get<int>() == 1);
+    }
+
     SECTION("source offsets")
     {
         const std::string text = R"(  {"key": "value", "escaped": "a\nb", "n": 42})";