post-scarcity/src/conspage.c

/**
 * conspage.c
 *
 * Setup and tear down cons pages, and (FOR NOW) do primitive 
 * allocation/deallocation of cells.
 * NOTE THAT before we go multi-threaded, these functions must be
 * aggressively
 * thread safe.
 *
 * (c) 2017 Simon Brooke <simon@journeyman.cc>
 * Licensed under GPL version 2.0, or, at your option, any later version.
 */

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "consspaceobject.h"
#include "conspage.h"



/**
 * Flag indicating whether conspage initialisation has been done.
 */
bool conspageinitihasbeencalled = false;

/**
 * the number of cons pages which have thus far been initialised.
 */
int initialised_cons_pages = 0;

/**
 * The (global) pointer to the (global) freelist. Not sure whether this ultimately 
 * belongs in this file.
 */
struct cons_pointer freelist = NIL;

/**
 * An array of pointers to cons pages.
 */
struct cons_page* conspages[NCONSPAGES];


/**
 * Make a cons page whose serial number (i.e. index in the conspages directory) is pageno.
 * Initialise all cells and prepend each to the freelist; if pageno is zero, do not prepend
 * cells 0 and 1 to the freelist but initialise them as NIL and T respectively.
 */
void make_cons_page() {
  struct cons_page* result = malloc( sizeof( struct cons_page));

  if ( result != NULL) {
    conspages[initialised_cons_pages] = result;

    for (int i = 0; i < CONSPAGESIZE; i++) {
      struct cons_space_object * cell = &conspages[initialised_cons_pages]->cell[i];
      if ( initialised_cons_pages == 0 && i < 2) {
        if ( i == 0) {
          /* initialise cell as NIL */
          strncpy( &cell->tag.bytes[0], NILTAG, TAGLENGTH);
          cell->count = MAXREFERENCE;
          cell->payload.free.car = NIL;
          cell->payload.free.cdr = NIL;
	  fprintf( stderr, "Allocated special cell NIL\n");
        } else if ( i == 1) {
          /* initialise cell as T */
          strncpy( &cell->tag.bytes[0], TRUETAG, TAGLENGTH);
          cell->count = MAXREFERENCE;
          cell->payload.free.car = (struct cons_pointer){ 0, 1};
          cell->payload.free.cdr = (struct cons_pointer){ 0, 1};
	  fprintf( stderr, "Allocated special cell T\n");
        }
      } else {
	/* otherwise, standard initialisation */
	strncpy( &cell->tag.bytes[0], FREETAG, TAGLENGTH);
	cell->payload.free.car = NIL;
	cell->payload.free.cdr = freelist;
	freelist.page = initialised_cons_pages;
	freelist.offset = i;
      }
    }

    initialised_cons_pages ++;
  } else {
    fprintf( stderr, "FATAL: Failed to allocate memory for cons page %d\n", initialised_cons_pages);
    exit(1);
  }

}


/**
 * dump the allocated pages to this output stream.
 */
void dump_pages( FILE* output) {
  for ( int i = 0; i < initialised_cons_pages; i++) {
    fprintf( output, "\nDUMPING PAGE %d\n", i);

    for ( int j = 0; j < CONSPAGESIZE; j++) {
      dump_object( output, (struct cons_pointer){i, j});
    }
  }
}


/**
 * Frees the cell at the specified pointer. Dangerous, primitive, low
 * level.
 *
 * @pointer the cell to free
 */
void free_cell(struct cons_pointer pointer) {
  struct cons_space_object* cell = &pointer2cell( pointer);

  if ( !check_tag(pointer, FREETAG)) {
      if ( cell->count == 0) {
        strncpy( &cell->tag.bytes[0], FREETAG, 4);
	cell->payload.free.car = NIL;
        cell->payload.free.cdr = freelist;
        freelist = pointer;
      } else {
        fprintf( stderr,
		 "Attempt to free cell with %d dangling references at page %d, offset %d\n",
                 cell->count, pointer.page, pointer.offset);
      }
    } else {
      fprintf( stderr,
	       "Attempt to free cell which is already FREE at page %d, offset %d\n",
               pointer.page, pointer.offset);
    }      
}


/**
 * Allocates a cell with the specified tag. Dangerous, primitive, low
 * level.
 *
 * @param tag the tag of the cell to allocate - must be a valid cons space tag.
 * @return the cons pointer which refers to the cell allocated.
 */
struct cons_pointer allocate_cell( char* tag) {
  struct cons_pointer result = freelist;

  if ( result.page == NIL.page && result.offset == NIL.offset) {
    make_cons_page();
    result = allocate_cell( tag);
  } else {
    struct cons_space_object* cell = &pointer2cell(result);

    if ( strncmp( &cell->tag.bytes[0], FREETAG, TAGLENGTH) == 0) {
      freelist = cell->payload.free.cdr;

      strncpy( &cell->tag.bytes[0], tag, 4);
    
      cell->count = 0;
      cell->payload.cons.car = NIL;
      cell->payload.cons.cdr = NIL;

      fprintf( stderr, "Allocated cell of type '%s' at %d, %d \n",
	       tag, result.page, result.offset);
      dump_object( stderr, result);
    } else {
      fprintf( stderr, "WARNING: Allocating non-free cell!");
    }
  }

  return result;
}


/**
 * initialise the cons page system; to be called exactly once during startup.
 */
void initialise_cons_pages() {
  if ( conspageinitihasbeencalled == false) {
    for (int i = 0; i < NCONSPAGES; i++) {
      conspages[i] = (struct cons_page *) NULL;
    }

    make_cons_page();
    conspageinitihasbeencalled = true;
  } else {
    fprintf( stderr, "WARNING: conspageinit() called a second or subsequent time\n");
  }
}