/*
 * memory.c
 *
 * Code and data caching routines
 *
 */

#include "ztypes.h"

/* A cache entry */

typedef struct cache_entry {
    struct cache_entry *flink;
    int page_number;
    zbyte_t data[PAGE_SIZE];
} cache_entry_t;

/* Cache chain anchor */

static cache_entry_t *cache = NULL;

/* Pseudo translation buffer, one entry each for code and data pages */

static unsigned int current_code_page = 0;
static cache_entry_t *current_code_cachep = NULL;
static unsigned int current_data_page = 0;
static cache_entry_t *current_data_cachep = NULL;

#ifdef __STDC__
static unsigned int calc_data_pages (void);
static cache_entry_t *update_cache (int);
#else
static unsigned int calc_data_pages ();
static cache_entry_t *update_cache ();
#endif

/*
 * load_cache
 *
 * Initialise the cache and any other dynamic memory objects. The memory
 * required can be split into two areas. Firstly, three buffers are required for
 * input, output and status line. Secondly, two data areas are required for
 * writeable data and read only data. The writeable data is the first chunk of
 * the file and is put into non-paged cache. The read only data is the remainder
 * of the file which can be paged into the cache as required. Writeable data has
 * to be memory resident because it cannot be written out to a backing store.
 *
 */

#ifdef __STDC__
void load_cache (void)
#else
void load_cache ()
#endif
{
    unsigned long file_size;
    unsigned int i, file_pages, data_pages;
    cache_entry_t *cachep;

    /* Allocate output and status line buffers */

    line = (char *) malloc (screen_cols + 1);
    if (line == NULL)
        fatal ("Insufficient memory to play game");
    status_line = (char *) malloc (screen_cols + 1);
    if (status_line == NULL)
        fatal ("Insufficient memory to play game");

    /* Must have at least one cache page for memory calculation */

    cachep = (cache_entry_t *) malloc (sizeof (cache_entry_t));
    if (cachep == NULL)
        fatal ("Insufficient memory to play game");
    cachep->flink = cache;
    cachep->page_number = 0;
    cache = cachep;

    /* Calculate dynamic cache pages required */

    if (h_config & CONFIG_MAX_DATA)
        data_pages = calc_data_pages ();
    else
        data_pages = (h_data_size + PAGE_MASK) >> PAGE_SHIFT;
    data_size = data_pages * PAGE_SIZE;
    file_size = (unsigned long) h_file_size * story_scaler;
    file_pages = (unsigned int) ((file_size + PAGE_MASK) >> PAGE_SHIFT);

    /* Allocate static data area and initialise it */

    datap = (zbyte_t *) malloc (data_size);
    if (datap == NULL)
        fatal ("Insufficient memory to play game");
    for (i = 0; i < data_pages; i++)
        read_page (i, &datap[i * PAGE_SIZE]);

    /* Allocate memory for undo */

    undo_datap = (zbyte_t *) malloc (data_size);

    /* Allocate cache pages and initialise them */

    for (i = data_pages; cachep != NULL && i < file_pages; i++) {
        cachep = (cache_entry_t *) malloc (sizeof (cache_entry_t));
        if (cachep != NULL) {
            cachep->flink = cache;
            cachep->page_number = i;
            read_page (cachep->page_number, cachep->data);
            cache = cachep;
        }
    }

}/* load_cache */

/*
 * unload_cache
 *
 * Deallocate cache and other memory objects.
 *
 */

#ifdef __STDC__
void unload_cache (void)
#else
void unload_cache ()
#endif
{
    cache_entry_t *cachep, *nextp;

    /* Make sure all output has been flushed */

    new_line ();

    /* Free output buffer, status line and data memory */

    free (line);
    free (status_line);
    free (datap);
    free (undo_datap);

    /* Free cache memory */

    for (cachep = cache; cachep->flink != NULL; cachep = nextp) {
        nextp = cachep->flink;
        free (cachep);
    }

}/* unload_cache */

/*
 * read_code_word
 *
 * Read a word from the instruction stream.
 *
 */

#ifdef __STDC__
zword_t read_code_word (void)
#else
zword_t read_code_word ()
#endif
{
    zword_t w;

    w = (zword_t) read_code_byte () << 8;
    w |= (zword_t) read_code_byte ();

    return (w);

}/* read_code_word */

/*
 * read_code_byte
 *
 * Read a byte from the instruction stream.
 *
 */

#ifdef __STDC__
zbyte_t read_code_byte (void)
#else
zbyte_t read_code_byte ()
#endif
{
    unsigned int page_number, page_offset;

    /* Calculate page and offset values */

    page_number = (unsigned int) (pc >> PAGE_SHIFT);
    page_offset = (unsigned int) pc & PAGE_MASK;

    /* Load page into translation buffer */

    if (page_number != current_code_page) {
        current_code_cachep = update_cache (page_number);
        current_code_page = page_number;
    }

    /* Update the PC */

    pc++;

    /* Return byte from page offset */

    return (current_code_cachep->data[page_offset]);

}/* read_code_byte */

/*
 * read_data_word
 *
 * Read a word from the data area.
 *
 */

#ifdef __STDC__
zword_t read_data_word (unsigned long *addr)
#else
zword_t read_data_word (addr)
unsigned long *addr;
#endif
{
    zword_t w;

    w = (zword_t) read_data_byte (addr) << 8;
    w |= (zword_t) read_data_byte (addr);

    return (w);

}/* read_data_word */

/*
 * read_data_byte
 *
 * Read a byte from the data area.
 *
 */

#ifdef __STDC__
zbyte_t read_data_byte (unsigned long *addr)
#else
zbyte_t read_data_byte (addr)
unsigned long *addr;
#endif
{
    unsigned int page_number, page_offset;
    zbyte_t value;

    /* Check if byte is in non-paged cache */

    if (*addr < (unsigned long) data_size)
        value = datap[*addr];
    else {

        /* Calculate page and offset values */

        page_number = (int) (*addr >> PAGE_SHIFT);
        page_offset = (int) *addr & PAGE_MASK;

        /* Load page into translation buffer */

        if (page_number != current_data_page) {
            current_data_cachep = update_cache (page_number);
            current_data_page = page_number;
        }

        /* Fetch byte from page offset */

        value = current_data_cachep->data[page_offset];
    }

    /* Update the address */

    (*addr)++;

    return (value);

}/* read_data_byte */

/*
 * calc_data_pages
 *
 * Compute the best size for the data area cache. Some games have the data size
 * header parameter set too low. This causes a write outside of data area on
 * some games. To alleviate this problem the data area size is set to the
 * maximum of the restart size, the data size and the end of the dictionary. An
 * attempt is made to put the dictionary in the data area to stop paging during
 * a dictionary lookup. Some games have the dictionary end very close to the
 * 64K limit which may cause problems for machines that allocate memory in
 * 64K chunks.
 *
 */

#ifdef __STDC__
static unsigned int calc_data_pages (void)
#else
static unsigned int calc_data_pages ()
#endif
{
    unsigned long offset, data_end, dictionary_end;
    int separator_count, word_size, word_count;
    unsigned int data_pages;

    /* Calculate end of data area, use restart size if data size is too low */

    if (h_data_size > h_restart_size)
        data_end = h_data_size;
    else
        data_end = h_restart_size;

    /* Calculate end of dictionary table */

    offset = h_words_offset;
    separator_count = read_data_byte (&offset);
    offset += separator_count;
    word_size = read_data_byte (&offset);
    word_count = read_data_word (&offset);
    dictionary_end = offset + (word_size * word_count);

    /* If data end is too low then use end of dictionary instead */

    if (dictionary_end > data_end)
        data_pages = (unsigned int) ((dictionary_end + PAGE_MASK) >> PAGE_SHIFT);
    else
        data_pages = (unsigned int) ((data_end + PAGE_MASK) >> PAGE_SHIFT);

    return (data_pages);

}/* calc_data_pages */

/*
 * update_cache
 *
 * Called on a code or data page cache miss to find the page in the cache or
 * read the page in from disk. The chain is kept as a simple LRU chain. If a
 * page cannot be found then the page on the end of the chain is reused. If the
 * page is found, or reused, then it is moved to the front of the chain.
 *
 */

#ifdef __STDC__
static cache_entry_t *update_cache (int page_number)
#else
static cache_entry_t *update_cache (page_number)
int page_number;
#endif
{
    cache_entry_t *cachep, *lastp;

    /* Search the cache chain for the page */

    for (lastp = cache, cachep = cache;
         cachep->flink != NULL &&
         cachep->page_number &&
         cachep->page_number != page_number;
         lastp = cachep, cachep = cachep->flink)
        ;

    /* If no page in chain then read it from disk */

    if (cachep->page_number != page_number) {

        /* Reusing last cache page, so invalidate cache if page was in use */

        if (cachep->flink == NULL && cachep->page_number) {
            if (current_code_page == (unsigned int) cachep->page_number)
                current_code_page = 0;
            if (current_data_page == (unsigned int) cachep->page_number)
                current_data_page = 0;
        }

        /* Load the new page number and the page contents from disk */

        cachep->page_number = page_number;
        read_page (page_number, cachep->data);
    }

    /* If page is not at front of cache chain then move it there */

    if (lastp != cache) {
        lastp->flink = cachep->flink;
        cachep->flink = cache;
        cache = cachep;
    }

    return (cachep);

}/* update_cache */