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# Summary

Reads a file in chunks - perfect for when you have a small buffer or when you
don't know the file size. Copes with files up to 4,294,967,295 bytes long.

# Code

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@file-read-chunks ( func* udata* buf* size* filename* -- func* udata'* buf* size* filename* )

	#0000 DUP2                 ( F* U* B* SZ* FN* OL* OH* / )
	&resume
	ROT2 STH2                  ( F* U* B* SZ* OL* OH*     / FN* )
	ROT2                       ( F* U* B* OL* OH* SZ*     / FN* )

	&loop
	STH2kr .File/name DEO2     ( F* U* B* OL* OH* SZ*     / FN* )
	STH2k .File/length DEO2    ( F* U* B* OL* OH*         / FN* SZ* )
	STH2k .File/offset-hs DEO2 ( F* U* B* OL*             / FN* SZ* OH* )
	STH2k .File/offset-ls DEO2 ( F* U* B*                 / FN* SZ* OH* OL* )
	SWP2                       ( F* B* U*                 / FN* SZ* OH* OL* )
	ROT2k NIP2                 ( F* B* U* B* F*           / FN* SZ* OH* OL* )
	OVR2 .File/load DEO2       ( F* B* U* B* F*           / FN* SZ* OH* OL* )
	.File/success DEI2 SWP2    ( F* B* U* B* length* F*   / FN* SZ* OH* OL* )
	JSR2                       ( F* B* U'* done-up-to*    / FN* SZ* OH* OL* )
	ROT2 SWP2                  ( F* U'* B* done-up-to*    / FN* SZ* OH* OL* )
	SUB2k NIP2                 ( F* U'* B* -done-length*  / FN* SZ* OH* OL* )
	ORAk ,&not-end JCN         ( F* U'* B* -done-length*  / FN* SZ* OH* OL* )

	POP2 POP2r POP2r           ( F* U'* B*                / FN* SZ* )
	STH2r STH2r                ( F* U'* B* SZ* FN*        / )
	JMP2r

	&not-end
	STH2r SWP2                 ( F* U'* B* OL* -done-length* / FN* SZ* OH* )
	LTH2k JMP INC2r            ( F* U'* B* OL* -done-length* / FN* SZ* OH'* )
	SUB2                       ( F* U'* B* OL'*              / FN* SZ* OH'* )
	STH2r STH2r                ( F* U'* B* OL'* OH'* SZ*     / FN* )
	,&loop JMP

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# Arguments

* func*     - address of callback routine
* udata*    - userdata to pass to callback routine
* buf*      - address of first byte of buffer of file's contents
* size*     - size in bytes of buffer
* filename* - address of filename string (zero-terminated)

All of the arguments are shorts (suffixed by asterisks *).

# Callback routine

If you make use of userdata, the signature of the callback routine is:
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	( udata* buf* length* -- udata'* done-up-to* )
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* udata* and buf* are as above.
* length* is the length of the chunk being worked on, which could be less than
  size* when near the end of the file, and func* is called with zero length* to
  signify end of file.
* udata'* is the (potentially) modified userdata, to be passed on to the next
  callback routine call and returned by file-read-chunks after the last chunk.
* done-up-to* is the pointer to the first unprocessed byte in the buffer, or
  buf* + length* if the whole chunk was processed.

If you don't make use of any userdata, feel free to pretend the signature is:
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	( buf* length* -- done-up-to* )
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# Userdata

The udata* parameter is not processed by file-read-chunks, except to keep the
one returned from one callback to the next. The meaning of its contents is up
to you - it could simply be a short integer or a pointer to a region of memory.

# Operation

file-read-chunks reads a file into the buffer you provide and calls func* with
JSR2 with each chunk of data, finishing with an empty chunk at end of file.

file-read-chunks loops until done-up-to* equals buf*, equivalent to when no
data is processed by func*. This could be because processing cannot continue
without a larger buffer, an error is detected in the data and further
processing is pointless, or because the end-of-file empty chunk leaves the
callback routine with no other choice.

# Return values

Since file-read-chunks's input parameters remain available throughout its
operation, they are not automatically discarded in case they are useful to the
caller.

# Discussion about done-up-to*

file-read-chunks is extra flexible because it doesn't just give you one chance
to process each part of the file. Consider a func* routine that splits the
chunk's contents into words separated by whitespace. If the buffer ends with a
letter, you can't assume that letter is the end of that word - it's more likely
to be the in the middle of a word that continues on. If func* returns the
address of the first letter of the word so far, it will be called again with
that first letter as the first character of the next chunk's buffer. There's no
need to remember the earlier part of the word because you get presented with
the whole lot again to give parsing another try.

That said, func* must make at least _some_ progress through the chunk: if it
returns the address at the beginning of the buffer, buf*, file-read-chunks will
terminate and return to its caller. With our word example, a buffer of ten
bytes will be unable to make progress with words that are ten or more letters
long. Depending on your application, either make the buffer big enough so that
progress should always be possible, or find a way to discern this error
condition from everything working fine.

# Discussion about recursion

Since all of file-read-chunks's data is on the working and return stacks, it
can be called recursively by code running in the callback routine. For example,
a code assembler can process the phrase "include library.tal" by calling
file-read-chunks again with library.tal as the filename. There are a couple of
caveats:

* the filename string must not reside inside file-read-chunk's working buffer,
  otherwise it gets overwritten by the file's contents and subsequent chunks
  will fail to be read properly; and

* if the buffer is shared with the parent file-read-chunk, the callback routine
  should stop further processing and return with done-up-to* straight away,
  since the buffer contents have already been replaced by the child
  file-read-chunk.

# Resuming / starting operation from an arbitrary offset

You can call file-read-chunks/resume instead of the main routine if you'd like
to provide your own offset shorts rather than beginning at the start of the
file. The effective signature for file-read-chunks/resume is:
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	( func* udata* buf* size* filename* offset-ls* offset-hs* -- func* udata'* buf* size* filename* )
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# Example callback routines

This minimal routine is a no-op that "processes" the entire buffer each time
and returns a valid done-up-to*:

	@quick-but-useless
		ADD2 JMP2r

This extremely inefficient callback routine simply prints a single character
from the buffer and asks for the next one. It operates with a buffer that is
just one byte long, but for extra inefficiency you can assign a much larger
buffer and it will ignore everything after the first byte each time. If the
buffer is zero length it returns done-up-to* == buf* so that file-read-chunks
returns properly.

	@one-at-a-time
		#0000 NEQ2 JMP JMP2r
		LDAk .Console/write DEO
		INC2 JMP2r

This more efficient example writes the entire chunk to the console before
requesting the next one by returning. How short can you make a routine that
does the same?

	@chunk-at-a-time
		&loop
		ORAk ,&not-eof JCN
		POP2 JMP2r

		&not-eof
		STH2
		LDAk .Console/write DEO
		INC2 STH2r #0001 SUB2
		,&loop JMP

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