Scan the get area of an input stream in bulk

Parsing from a std::istream read one character at a time through the
streambuf, so none of the parser's fast paths applied: it was 1.8 to 2.5
times slower than reading the file into a string and parsing that.

input_stream_adapter now exposes the streambuf's get area (gptr() to
egptr()) to the lexer as a window. The protected get-area members are
reached through member pointers formed in a class derived from
std::streambuf, which is ordinary, standard-conforming member access. The
lexer scans the window with the same bulk paths as a string, and asks
the streambuf for the next character as before when the window is used
up. Nothing is read ahead, copied, or put back, so the stream is left
exactly where it was left before, with and without
JSON_PRECISE_STREAM_POSITION, and a streambuf without a get area is still
read one character at a time.

The adapter also keeps the current token for error messages as a pointer
into the get area, and copies it out only before the get area is
refilled, instead of the lexer copying every character.

On an Apple M1 Max (Apple Clang, -O3), parsing from an std::ifstream is
1.6 to 2.1 times as fast (twitter.json 4.08 -> 2.30 ms, citm_catalog.json
8.75 -> 4.08 ms), and reading 100,000 small values with operator>> 1.4
(objects) to 4.0 (integers) times as fast. With GCC 13 and libstdc++ on
x86-64 Linux, std::ifstream is 1.5 to 2.2 times as fast. Parsing from a
stream is now within 11 to 18 % of reading the file into a string first.

JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time,
for streambufs that do not follow the usual get-area contract. The new
tests compare every observable result (values, exception messages, stream
state, remaining characters) for get areas of 1 to 4096 bytes against a
streambuf without a get area, and run again with the macro and with
JSON_PRECISE_STREAM_POSITION.

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

docs/mkdocs/docs/api/basic_json/accept.md

@@ -132,6 +132,8 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
 - [operator>>](../operator_gtgt.md) - deserialize from stream
 - [`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
   instead of treating it as end of input
+- [`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character
+  at a time
 
 ## Version history
 
@@ -143,6 +145,8 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
   it as end of input; planned to become the default in version 4.0.0.
+- Parsing from an `std::istream` scans the characters in its stream buffer in bulk in version 3.13.0
+  ([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md)).
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/parse.md

@@ -112,6 +112,14 @@ By default, a `'\0'` (NUL) byte anywhere in the input is treated as end of input
 outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-bytes-in-the-input) for details and the
 [`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) macro to opt into rejecting it instead.
 
+!!! tip "Performance"
+
+    Contiguous inputs (strings, string views, vectors, arrays, pointer ranges) are the fastest to parse. A
+    `#!cpp std::istream` comes close: the parser scans the characters its stream buffer holds in bulk, without reading
+    ahead, so the stream is left exactly where it would be after reading one character at a time. See
+    [input sources and performance](../../features/parsing/index.md#input-sources-and-performance) and
+    [`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md).
+
 ## Examples
 
 ??? example "Example: (1) parse from a character array"
@@ -247,6 +255,8 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
 - [operator>>](../operator_gtgt.md) - deserialize from stream
 - [`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
   instead of treating it as end of input
+- [`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character
+  at a time
 
 ## Version history
 
@@ -261,6 +271,9 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
   it as end of input; planned to become the default in version 4.0.0.
 - Extended empty-input detection to also cover an `std::istream` without a stream buffer, and fixed a crash
   (`std::terminate`) when parsing from an `std::istream` with `eofbit` in its exception mask, in version 3.13.0.
+- Parsing from an `std::istream` scans the characters in its stream buffer in bulk in version 3.13.0
+  ([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) restores reading one character at a
+  time).
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/sax_parse.md

@@ -137,6 +137,8 @@ A UTF-8 byte order mark is silently ignored.
 
 - [parse](parse.md) - deserialize from a compatible input
 - [accept](accept.md) - check if the input is valid JSON
+- [`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md) - read a `std::istream` one character
+  at a time
 
 ## Version history
 
@@ -148,6 +150,8 @@ A UTF-8 byte order mark is silently ignored.
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave a `#!cpp std::istream` positioned right
   after the parsed value when `strict` is `#!cpp false`.
+- Parsing from an `std::istream` scans the characters in its stream buffer in bulk in version 3.13.0
+  ([`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md)).
 
 !!! warning "Deprecation"