Decode the CBOR item at the start of a raw vector

Decodes the one CBOR data item that x starts with and reports how many bytes it used, for CBOR inside binary framing: a COSE key in the middle of WebAuthn authData, or a payload after a fixed header. The item is checked and decoded exactly as by cbor_decode(), with the same arguments; what follows it is not read at all.

Usage

cbor_decode_prefix(
  x,
  simplify = c("preserve", "none"),
  map_keys = c("auto", "map", "string"),
  tags = c("convert", "keep"),
  big_integers = c("bigint", "double", "error"),
  duplicate_keys = FALSE,
  deterministic = FALSE,
  max_depth = 256L,
  max_size = 64 * 1024^2,
  max_items = 1e+06,
  tag_handlers = NULL,
  data_frame = FALSE,
  max_cells = 1e+07
)

Arguments

x

A raw vector starting with a CBOR data item.

simplify

"preserve" simplifies arrays whose elements agree to atomic vectors; "none" makes every array a list.

map_keys

"auto" gives a named list when every key is a non-empty, unique text string, and a cbor_map otherwise. "map" always gives a cbor_map. "string" always gives a named list, naming each entry by its text key, or by the key's diagnostic notation when it is not text; it is lossy, and refuses a map whose keys collide once stringified.

tags

"convert" turns the tags in the table into R values and keeps the rest as cbor_tag; "keep" makes every tag a cbor_tag.

big_integers

What to do with an integer beyond 2^53, which a double cannot hold exactly: "bigint" returns a cbor_bigint, "double" the nearest double, and "error" refuses the input.

duplicate_keys

If FALSE (the default), a map with the same key twice is invalid.

deterministic

If TRUE, also require RFC 8949 section 4.2.1 core deterministic encoding: shortest integers, lengths and floats, definite lengths, map keys in bytewise order, and bignums in preferred form.

max_depth

Deepest nesting allowed, counting arrays, maps and tags. At most zucbor_info()$max_depth_cap.

max_size

Largest input allowed, in bytes, or Inf.

max_items

Most data items allowed, counting every tag and every chunk of an indefinite-length string, or Inf.

tag_handlers

NULL, or a list of functions of one argument, named by tag number, such as list("37" = function(value) ...). See "Tag handlers".

data_frame

If TRUE, an array whose elements are all maps with text keys becomes a data frame. See "Data frames".

max_cells

With data_frame = TRUE, the most cells (rows times columns) one data frame may have, or Inf. Checked before the frame is allocated: rows that share no keys make a frame quadratic in the input.

Value

A list: value, the decoded item, and consumed, the number of bytes it took, a double.

Details

That also means nothing about the rest is known: it may be more CBOR, or the next field of the framing, or garbage. It is the caller's to make sense of, as x[-seq_len(consumed)].

max_size applies to x as a whole, since all of it is in memory already. An empty x is zucbor_parse_error, as for cbor_decode(); so is an item cut short by the end of x.

See also

cbor_decode() for one item and nothing else, and cbor_decode_seq() for items all the way to the end.

Examples

# [1, 2] followed by four bytes of something else.
x <- as.raw(c(0x82, 0x01, 0x02, 0xde, 0xad, 0xbe, 0xef))
r <- cbor_decode_prefix(x)
r$value
#> [1] 1 2
x[-seq_len(r$consumed)]
#> [1] de ad be ef