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* Each base channel is either seekable or not
* Each transformation can use one of several seek policies while attached
to a channel.
- 'transform position'
Each seek/tell request is passed down to the channel below, and
the specified/returned position is converted as necessary (one
additional vector in the transformation driver).
- 'pass'
Is subsumed by the policy above, with position transformation
= identity.
- 'unseekable'
The transformation does a running count of the current position
as it sees the world. Returns the information upon request
(tell). Real seeking is disallowed.
* The actual seek policy used by a transformation is computed from
several sources:
- The 'natural seek policy' of the transformation.
- The seekability of the base channel.
- The seek policies used by the transformations below it.
(how to determine ?)
- Policy requests by the user.
Rules:
1) An unseekable base channel enforces 'unseekable' in all
transformations above it.
2) An unseekable transformation enforces 'unseekable' in all
transformations above it.
3) The user is able to overide 'unseekable' with pass or
transform, but only for the transformation currently at
the top, and not if lower transformations or the base
channel enforce unseekable. I.e. the rules (1) and (2)
have precedence.
In case of 'transform position' the user has to specify the
conversion he wishes for the location in the form of a tcl
command. An empty command causes the system to use 'identity'.
This way the user is able to enforce the current behaviour of
simply passing the position without changing it. The user has
to know the effects of this on the various transformations in
the stack.
* The user specifies his requests by using the new standard options
@ -seekpolicy transform|unseekable
@ -seektransform command
('-seektransform' command implies '-seekpolicy tranform')
* A transformation specifies its natural policy through a new attribute
and a new function vector in its driver structure.
One argument to the vector specifies the direction of the conversion
(pass position down to channel, get position from channel below), the
instance configuration provides additional context (mode encode/decode,
etc.).
* A position transformation returning an error causes the system to
reject the seek.
An error during up-conversion (tell) implies that an
'identity loc. transform' was in effect for some time allowing the
fractional placement of the down-location with respect to up-location.
Handling: ?
* The general 'transform' command uses additional keywords while calling
back into the tcl level to query and implement its natural seek policy.
* Questions:
- Is it possible to determine the seekability of a channel
by simply using the public API's ?
(Without destroying the state of the channel ?)
Answer: Yes, by checking the channel type for a non-NULL
pointer for the SeekProc.
- Is it possible to determine the seekability of a
transformation ?
Yes, if the seekability of a channel is checkable via Tcl_Seek
itself. But not necessarily without disturbing the state of
the transformation and/or channel below (Because we will have
to try to seek to get either success or error. On success the
test seek has to be reversed immediately. But a transformation
below may have already discarded its input buffer !).
Answer: Not generally. As the notion of an unseekable
transformation is slightly different from unseekable
channel we have to provide a SeekProc at all times,
rendering the check above for channels unusable.
If we don't want to disturb the state of other
transformations we are reduced to check for
SeekProc == TrfSeek first, to filter out any non-Trf
transforms and then to look directly into the instance
structure. For non-Trf transforms we have to make
conservative assumptions.
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