docs: output growth with nesting depth and ordered_json objects with many keys (#6118)

untrusted_input.md did not say that indented dump() and flatten() produce
results that grow with the nesting depth times the size of the value (80 KB
of input give 600 MB of dump(1)), nor that the binary readers take
quadratic time for objects with many keys when they read into
ordered_json. Add a warning to "Nesting depth", a section "Objects with
many keys in ordered_json", and both points to the checklist.

The warning in ordered_map.md still said that parsing JSON text into
ordered_json is quadratic; that is no longer true. Name the cases that
still are (filling an object key by key, the binary readers) and replace
the table with current measurements.

dump.md and flatten.md called their complexity linear in the value; it is
linear in the result, which can be larger by a factor of the depth.

Fixes #6108. Refs #6104.

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

docs/mkdocs/docs/api/basic_json/dump.md

@@ -58,7 +58,9 @@ is not UTF-8 encoded and `error_handler` is set to `strict`
 
 ## Complexity
 
-Linear.
+Linear in the size of the result. With an `indent`, every line starts with up to `indent` characters per nesting level,
+so the result can be larger than the value by a factor of its nesting depth (see
+[Parsing untrusted input](../../features/parsing/untrusted_input.md#nesting-depth)).
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json/flatten.md

@@ -18,7 +18,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Complexity
 
-Linear in the size of the JSON value.
+Linear in the size of the result. Every key repeats the path of its value, so the keys can be larger than the value by a
+factor of its nesting depth (see [Parsing untrusted input](../../features/parsing/untrusted_input.md#nesting-depth)).
 
 ## Notes
 

docs/mkdocs/docs/api/ordered_map.md

@@ -89,19 +89,21 @@ This differs from `#!cpp std::map`, where the same operations are O(log n).
 !!! warning "Quadratic cost of building large objects"
 
     Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs
-    **O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key as well as to
-    parsing one, since the parser inserts each key as it is read.
+    **O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key (with
+    `operator[]`, `emplace`, or `insert`) and, for now, to reading one with a binary reader such as
+    [`from_cbor`](basic_json/from_cbor.md) ([#6104](https://github.com/nlohmann/json/issues/6104)). Parsing JSON text
+    is not affected: the parser finds the keys of an object with many members in a hash table and takes linear time.
 
     The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows
-    steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat
-    object of `n` keys, relative to `#!cpp nlohmann::json` (which uses `#!cpp std::map`):
+    steeply for machine-generated objects with many thousands of keys. Measured with Apple clang 21, `-O2 -DNDEBUG`,
+    arm64, for a flat object of `n` keys (best of seven runs):
 
-    | `n`    | `json` | `ordered_json` | factor |
-    |--------|--------|----------------|--------|
-    | 2000   | 0.7 ms | 3.6 ms         | 5×     |
-    | 4000   | 0.8 ms | 14.0 ms        | 19×    |
-    | 8000   | 1.6 ms | 67.8 ms        | 43×    |
-    | 16 000 | 3.3 ms | 181.6 ms       | 54×    |
+    | `n`    | `json::parse` | `ordered_json::parse` | `ordered_json`, key by key | `ordered_json::from_cbor` |
+    |--------|---------------|-----------------------|----------------------------|---------------------------|
+    | 2000   | 0.3 ms        | 0.1 ms                | 4.5 ms                     | 4.4 ms                    |
+    | 4000   | 0.5 ms        | 0.2 ms                | 17.1 ms                    | 16.8 ms                   |
+    | 8000   | 0.9 ms        | 0.3 ms                | 67.0 ms                    | 66.8 ms                   |
+    | 16 000 | 2.0 ms        | 0.7 ms                | 282.4 ms                   | 282.4 ms                  |
 
     If key order matters for objects of that size, consider a container with a lookup index, such as
     [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map)

docs/mkdocs/docs/features/parsing/untrusted_input.md

@@ -81,6 +81,20 @@ result. See the [documentation of the macro](../../api/macros/json_recursion_dep
 the speed, and the rules for choosing a value. Destroying a value (its destructor) recurses over the same number of
 levels and then continues without the call stack, and needs 32 KiB or less at every limit.
 
+!!! warning "Indented output and flattened keys grow with the depth"
+
+    Some results are larger than the value by a factor of its nesting depth.
+    [`dump`](../../api/basic_json/dump.md) with an `indent` of `n` (and likewise `operator<<` with `std::setw(n)` and
+    [`basic_json_view::dump`](../../api/basic_json_view/dump.md)) writes up to `n` characters per nesting level in
+    front of every line, and [`flatten`](../../api/basic_json/flatten.md) repeats the path of every value in its key.
+    For the 80,001-byte array `[1,[1,...[1,1]...]]` with 20,000 levels, `dump(1)` returns 600 MB, `dump(4)` 2.4 GB,
+    and the keys of `flatten()` hold 400 MB. A compact `dump()` without indentation stays the size of the value. If you
+    pretty-print, log, or flatten untrusted input, limit its nesting depth while parsing, for example with a
+    [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback): with a limit of `d`, every line has
+    at most `n * d` indentation characters, and every flattened key has at most `d` reference tokens.
+    [`JSON_RECURSION_DEPTH_LIMIT`](../../api/macros/json_recursion_depth_limit.md) does not help here: it changes only
+    how much of the call stack a walk uses, never its result, and it does not limit the depth that `parse()` accepts.
+
 ## Input size
 
 The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory
@@ -140,6 +154,20 @@ values for a repeated key ends up in the parsed object. If your application must
 silently resolving them one way or another, see the
 [parser callback recipe for rejecting duplicate keys](parser_callbacks.md#recipe-rejecting-duplicate-object-keys).
 
+## Objects with many keys in `ordered_json`
+
+[`ordered_json`](../../api/ordered_json.md) keeps the members of an object in a vector in insertion order
+([`ordered_map`](../../api/ordered_map.md)) and finds a key by searching all members. Parsing JSON text into an
+`ordered_json` avoids that search for objects with many keys and takes linear time. The binary readers
+([`from_cbor`](../../api/basic_json/from_cbor.md), [`from_msgpack`](../../api/basic_json/from_msgpack.md),
+[`from_ubjson`](../../api/basic_json/from_ubjson.md), [`from_bjdata`](../../api/basic_json/from_bjdata.md),
+[`from_bson`](../../api/basic_json/from_bson.md), and [`from_bon8`](../../api/basic_json/from_bon8.md)) do not yet:
+reading an object of `n` keys into an `ordered_json` costs time proportional to `n²`. A flat object of 40,000 keys
+(about 300 KB of CBOR) takes about 1.3 s, against a few milliseconds as JSON text or into a `json`
+([#6104](https://github.com/nlohmann/json/issues/6104)). `json` is not affected. If you read untrusted binary input
+into `ordered_json`, bound the input size more tightly, read it into a `json`, or reject objects with many keys in a
+[SAX handler](sax_interface.md).
+
 ## Numbers
 
 A number whose value cannot be represented — for instance `1E1000`, which overflows `double` — is rejected while
@@ -171,9 +199,11 @@ indices (or the pointer as a whole) before using them, or use [`at`](../../api/b
 
 - Wrap parsing in a `#!cpp try`/`#!cpp catch` block, or use `allow_exceptions=false`/`accept()` if your environment
   cannot use exceptions; never let `JSON_NOEXCEPTION`'s `abort()` be the first time you think about error handling.
-- If the input's nesting depth matters to you, enforce your own limit with a
-  [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback) or a
+- If the input's nesting depth matters to you (for example because you pretty-print, log, or flatten it), enforce your
+  own limit with a [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback) or a
   [SAX handler](sax_interface.md); the library bounds the call stack but not memory use.
+- If you read untrusted binary input into `ordered_json`, mind that objects with many keys are
+  [slow to read](#objects-with-many-keys-in-ordered_json).
 - Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary
   format lengths.
 - Decide up front how you want a string with invalid UTF-8 — from any source, not only parsing — to be serialized