Check each distinct string and float range once in the full image check

The full check scanned the contents of every string node and every float token over its own range, so many nodes pointing to one large range made load() quadratic. The structural walk now only collects the ranges; after it, each kind is sorted and checked once per distinct range. Ranges of one kind that overlap without being identical are rejected, as save() never writes them (nodes that share a value share the whole range). The cost is linear in the size of the image plus sorting the ranges.

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

docs/mkdocs/docs/api/basic_json_document/index.md

@@ -48,6 +48,7 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
 
 - **view_type** - the type of view returned by [`root()`](root.md) (`#!cpp basic_json_view<BasicJsonType, Editable>`)
 - **value_t** - the JSON type enumeration, see [`basic_json::value_t`](../basic_json/value_t.md)
+- **image_check** - how [`load()`](load.md) validates an image (`full`, `bounds`, `none`), see [`image_check`](load.md#image_check)
 
 ## Member functions
 

docs/mkdocs/docs/api/basic_json_document/load.md

@@ -85,9 +85,13 @@ Otherwise throws [`parse_error.116`](../../home/exceptions.md#jsonexceptionparse
 
 ## Complexity
 
-Linear in the number of nodes, which are always copied into the document. With `#!cpp check == image_check::full`,
-additionally linear in the combined length of the text and the decoded strings; `#!cpp image_check::bounds` and
-`#!cpp image_check::none` do not read them.
+Linear in the number of nodes, which are always copied into the document. `#!cpp image_check::none` does not read
+the text or the decoded strings. `#!cpp image_check::bounds` reads one byte of the text for each float token (its sign,
+to check the recorded digits against the token's length), and nothing else of the text or the decoded strings.
+
+With `#!cpp image_check::full`, linear in the size of the image (the nodes, the text, and the decoded strings), plus
+sorting the ranges of the strings and of the float tokens (at most one per node): the contents of each distinct range
+are checked once, however many nodes refer to it.
 
 ## Notes
 
@@ -108,7 +112,7 @@ How thoroughly `load()` validates `image` before trusting it.
 
 | value    | checks                                                                                                    | guarantees |
 |----------|--------------------------------------------------------------------------------------------------------------|------------|
-| `full`   | everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a string still in the source text, that it contains no quote, backslash, or control character); and that every number token is well-formed and matches the value stored for it | reading and serializing a checked image is safe and always produces valid JSON, exactly as for a parsed document |
+| `full`   | everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a string still in the source text, that it contains no quote, backslash, or control character); and that every number token is well-formed and matches the value stored for it; strings and float tokens may share a range only if the ranges are identical (as nodes that share a value do), and never overlap otherwise | reading and serializing a checked image is safe and always produces valid JSON, exactly as for a parsed document |
 | `bounds` | structure and bounds only -- that every offset and count in the node index stays inside the image            | reading and serializing stay memory-safe, but a crafted image can hold strings that are not valid UTF-8 or that serialize to invalid JSON ([`dump()`](../basic_json_view/dump.md) writes them unchanged or throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316)), and numbers whose values differ from their text |
 | `none`   | nothing                                                                                                       | images from a trusted source only -- reading a damaged image is undefined behavior |