Sniff a stream chunk by chunk

mime_sniffer() starts an incremental sniffer; mime_push() hands it the next chunk and returns NULL while the bytes seen cannot yet decide the type, or the result row once they can. mime_finish() forces an answer from what has been seen, as the end of a short stream would. The answer is the one mime_sniff() gives for the whole stream with the same limit. A stream exactly limit bytes long is read as one that goes on, since the sniffer cannot know it has ended, so a character cut at the end does not count against UTF-8.

Usage

mime_sniffer(
  type = NULL,
  no_sniff = FALSE,
  context = c("none", "browsing", "image", "audio_video", "font"),
  limit = 4096L
)

mime_push(s, chunk)

mime_finish(s)

Arguments

type

The supplied type, such as a Content-Type header value, or NULL when there is none. A value that does not parse as a MIME type is treated as NULL.

no_sniff

TRUE for the no-sniff flag (X-Content-Type-Options: nosniff): a supplied type is then returned as is.

context

One of "none", "browsing", "image", "audio_video" or "font".

limit

The number of bytes to read; 0 reads them all. A raw vector longer than limit is sniffed on its first limit bytes.

s

A sniffer from mime_sniffer().

chunk

A raw vector, the next bytes of the stream.

Value

mime_sniffer() returns a zumime_sniffer. mime_push() returns NULL or a one-row zumime_result; mime_finish() always returns the result.

Details

Once the sniffer has decided, further pushes ignore their chunks and return the same row. Once it has finished, a push raises a zumime_state_error; mime_finish() can still be called and returns the same row.

The bytes decide as soon as one of three things holds. No longer signature could still override the match. The container reader for the type has what it needs. Or limit bytes have arrived. Text decides only at limit or at the finish, since a later byte can still change it. The tail-reading column, complete, is NA for a stream.

The sniffer keeps its bytes in an R raw vector, grown as they arrive up to limit. With limit = 0 text never decides before the finish, so the sniffer holds the whole stream until mime_finish(). A sniffer saved and loaded again keeps those bytes but forgets its decision, and is undecided until the next push.

See also

mime_sniff() for bytes in hand.

Examples

s <- mime_sniffer()
mime_push(s, charToRaw("GIF8"))
#> NULL
mime_push(s, charToRaw("9a"))
#> <zumime_result: 1 row>
#>   path      type confidence charset inner encrypted complete newline
#> 1 <NA> image/gif   definite    <NA>  <NA>        NA       NA    <NA>
#>   interpreter
#> 1        <NA>

# Text waits for the limit or the finish.
t <- mime_sniffer(limit = 64)
mime_push(t, charToRaw("hello, "))
#> NULL
mime_finish(t)
#> <zumime_result: 1 row>
#>   path       type confidence charset inner encrypted complete newline
#> 1 <NA> text/plain   probable   UTF-8  <NA>        NA       NA    <NA>
#>   interpreter
#> 1        <NA>