fs-readstream-seek

A
fs.ReadStream
that supports seeking to arbtrary locations within a file.

![Build
Status](https://travis-ci.org/isaacs/fs-readstream-seek)

Note that this stream is _only_ appropriate for files where positioned
reads are supported. For abstract filesystem objects where you wish
to do ordered asynhronous reads without specifying position (for
example, FIFO devices), use fs.ReadStream instead.

USAGE

``js
const ReadStream = require('fs-readstream-seek')
const s = new ReadStream('some-filename.db')
s.seek(123)
s.once('data', chunk => {
console.log('the data at position 123 is %s', chunk)
})
`

API

Everything on fs.ReadStream is supported, plus:

* stream.seek(n) Seek to a position in the file. If the position is
within the portion of the file that has already been read into
memory, no new read is triggered, and the in-memory buffer is
updated. If the position is beyond the end of the buffer, or before
the beginning of the buffer, then the buffer is discarded a new
fs.read() is made at the apporpriate location.

* stream.readPos Read-only indication of where in the file the next
read() will occur at. This is always updated when
stream.seek(n) is called.

Note that this is _not_ the position where
the current buffer in a 'data' event was found, but rather the
position where the _next_ data chunk will be read from. You can,
however, get that value by subtracting the chunk length from the
stream.readPos value.

`javascript
stream.on('data', chunk => {
console.error('position=%d data=%j',
stream.readPos - chunk.length,
chunk.toString())
})
`

* stream.filePos Read-only indication of where the read buffer is
currently filled up to, and thus where the next fs.read() will
occur within the file. This may be updated by stream.seek(n), if
necessary, and will naturally increase as more data is pulled into
the buffer.

Caveat re Stream Conventions

By convention, when a Readable stream emits an 'end' event, it is an
indication that no more data will be made available. Thus 'end' is
always a single-time event per-stream. Likewise, close and open
events on fs streams are generally unique in the lifetime of a
stream.

However, when you seek to a new location within a file, it resets the
EOF handling. If the end of the file was read into the buffer, and
thus automatically closed, then it will be re-opened if necessary when
your program calls stream.seek(n).

So you can do this to read a file and print to stdout repeatedly:

`js
const ReadStream = require('fs-readstream-seek')
const s = new ReadStream('some-filename.txt')
s.on('end', _ => {
s.seek(0)
})
s.on('data', c => {
process.stdout.write(c)
})
`

In this case, end will be emitted every time the stream gets to the
end of the data. When s.seek(0) is called, the file is re-opened
and starts reading from the beginning again.

Because it's a very common convention, 'end' and 'close' events
cause a readable.pipe(writable) chain to be disassembled. If this
is a thing that your program will be triggering by seek()-ing
backwards in the file after it has emitted 'end', then you are
strongly advised _not_ to pipe() that data anywhere, and instead
consume it directly using 'data' events or read()` method calls.