/*
 * jdatasrc.c
 *
 * Copyright (C) 1994, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains decompression data source routines for the case of
 * reading JPEG data from a file (or any stdio stream).  While these routines
 * are sufficient for most applications, some will want to use a different
 * source manager.
 * IMPORTANT: we assume that fread() will correctly transcribe an array of
 * JOCTETs from 8-bit-wide elements on external storage.  If char is wider
 * than 8 bits on your machine, you may need to do some tweaking.
 */


/* this is not a core library module, so it doesn't define JPEG_INTERNALS */
#include "jinclude.h"
#include "jpeglib.h"
#include "jerror.h"


/* Expanded data source object for stdio input */

typedef struct {
	struct jpeg_source_mgr pub; /* public fields */

	unsigned char *infile;      /* source stream */
	JOCTET * buffer;    /* start of buffer */
	boolean start_of_file;  /* have we gotten any data yet? */
} my_source_mgr;

typedef my_source_mgr * my_src_ptr;

#define INPUT_BUF_SIZE  4096    /* choose an efficiently fread'able size */


/*
 * Initialize source --- called by jpeg_read_header
 * before any data is actually read.
 */

METHODDEF void
init_source( j_decompress_ptr cinfo ) {
	my_src_ptr src = (my_src_ptr) cinfo->src;

	/* We reset the empty-input-file flag for each image,
	 * but we don't clear the input buffer.
	 * This is correct behavior for reading a series of images from one source.
	 */
	src->start_of_file = TRUE;
}


/*
 * Fill the input buffer --- called whenever buffer is emptied.
 *
 * In typical applications, this should read fresh data into the buffer
 * (ignoring the current state of next_input_byte & bytes_in_buffer),
 * reset the pointer & count to the start of the buffer, and return TRUE
 * indicating that the buffer has been reloaded.  It is not necessary to
 * fill the buffer entirely, only to obtain at least one more byte.
 *
 * There is no such thing as an EOF return.  If the end of the file has been
 * reached, the routine has a choice of ERREXIT() or inserting fake data into
 * the buffer.  In most cases, generating a warning message and inserting a
 * fake EOI marker is the best course of action --- this will allow the
 * decompressor to output however much of the image is there.  However,
 * the resulting error message is misleading if the real problem is an empty
 * input file, so we handle that case specially.
 *
 * In applications that need to be able to suspend compression due to input
 * not being available yet, a FALSE return indicates that no more data can be
 * obtained right now, but more may be forthcoming later.  In this situation,
 * the decompressor will return to its caller (with an indication of the
 * number of scanlines it has read, if any).  The application should resume
 * decompression after it has loaded more data into the input buffer.  Note
 * that there are substantial restrictions on the use of suspension --- see
 * the documentation.
 *
 * When suspending, the decompressor will back up to a convenient restart point
 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
 * indicate where the restart point will be if the current call returns FALSE.
 * Data beyond this point must be rescanned after resumption, so move it to
 * the front of the buffer rather than discarding it.
 */

METHODDEF boolean
fill_input_buffer( j_decompress_ptr cinfo ) {
	my_src_ptr src = (my_src_ptr) cinfo->src;

	memcpy( src->buffer, src->infile, INPUT_BUF_SIZE );

	src->infile += INPUT_BUF_SIZE;

	src->pub.next_input_byte = src->buffer;
	src->pub.bytes_in_buffer = INPUT_BUF_SIZE;
	src->start_of_file = FALSE;

	return TRUE;
}


/*
 * Skip data --- used to skip over a potentially large amount of
 * uninteresting data (such as an APPn marker).
 *
 * Writers of suspendable-input applications must note that skip_input_data
 * is not granted the right to give a suspension return.  If the skip extends
 * beyond the data currently in the buffer, the buffer can be marked empty so
 * that the next read will cause a fill_input_buffer call that can suspend.
 * Arranging for additional bytes to be discarded before reloading the input
 * buffer is the application writer's problem.
 */

METHODDEF void
skip_input_data( j_decompress_ptr cinfo, long num_bytes ) {
	my_src_ptr src = (my_src_ptr) cinfo->src;

	/* Just a dumb implementation for now.  Could use fseek() except
	 * it doesn't work on pipes.  Not clear that being smart is worth
	 * any trouble anyway --- large skips are infrequent.
	 */
	if ( num_bytes > 0 ) {
		while ( num_bytes > (long) src->pub.bytes_in_buffer ) {
			num_bytes -= (long) src->pub.bytes_in_buffer;
			(void) fill_input_buffer( cinfo );
			/* note we assume that fill_input_buffer will never return FALSE,
			 * so suspension need not be handled.
			 */
		}
		src->pub.next_input_byte += (size_t) num_bytes;
		src->pub.bytes_in_buffer -= (size_t) num_bytes;
	}
}


/*
 * An additional method that can be provided by data source modules is the
 * resync_to_restart method for error recovery in the presence of RST markers.
 * For the moment, this source module just uses the default resync method
 * provided by the JPEG library.  That method assumes that no backtracking
 * is possible.
 */


/*
 * Terminate source --- called by jpeg_finish_decompress
 * after all data has been read.  Often a no-op.
 *
 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
 * application must deal with any cleanup that should happen even
 * for error exit.
 */

METHODDEF void
term_source( j_decompress_ptr cinfo ) {
	/* no work necessary here */
}


/*
 * Prepare for input from a stdio stream.
 * The caller must have already opened the stream, and is responsible
 * for closing it after finishing decompression.
 */

GLOBAL void
jpeg_stdio_src( j_decompress_ptr cinfo, unsigned char *infile ) {
	my_src_ptr src;

	/* The source object and input buffer are made permanent so that a series
	 * of JPEG images can be read from the same file by calling jpeg_stdio_src
	 * only before the first one.  (If we discarded the buffer at the end of
	 * one image, we'd likely lose the start of the next one.)
	 * This makes it unsafe to use this manager and a different source
	 * manager serially with the same JPEG object.  Caveat programmer.
	 */
	if ( cinfo->src == NULL ) { /* first time for this JPEG object? */
		cinfo->src = (struct jpeg_source_mgr *)
				  ( *cinfo->mem->alloc_small ) ( (j_common_ptr) cinfo, JPOOL_PERMANENT,
												 SIZEOF( my_source_mgr ) );
		src = (my_src_ptr) cinfo->src;
		src->buffer = ( JOCTET * )
					  ( *cinfo->mem->alloc_small )( (j_common_ptr) cinfo, JPOOL_PERMANENT,
													INPUT_BUF_SIZE * SIZEOF( JOCTET ) );
	}

	src = (my_src_ptr) cinfo->src;
	src->pub.init_source = init_source;
	src->pub.fill_input_buffer = fill_input_buffer;
	src->pub.skip_input_data = skip_input_data;
	src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
	src->pub.term_source = term_source;
	src->infile = infile;
	src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
	src->pub.next_input_byte = NULL; /* until buffer loaded */
}
