gb_save.w

% This file is part of the Stanford GraphBase (c) Stanford University 1993
@i boilerplate.w %<< legal stuff: PLEASE READ IT BEFORE MAKING ANY CHANGES!
@i gb_types.w

\def\title{GB\_\,SAVE}

\prerequisites{GB\_\,GRAPH}{GB\_\,IO}
@* Introduction. This GraphBase module contains the code for
two special utility routines, |save_graph| and |restore_graph|, which
convert graphs back and forth between the internal representation that is
described in {\sc GB\_\,GRAPH} and a symbolic file format that is described
below. Researchers can use these routines to transmit graphs between
computers in a machine-independent way, or to use GraphBase graphs with other
graph manipulation software that supports the same symbolic format.

All kinds of tricks are possible in the \CEE/ language, so it is
easy to abuse the GraphBase conventions and to create data structures that
make sense only on a particular machine. But if users follow the
recommended ground rules, |save_graph| will be able to transform their
graphs into files that any other GraphBase installation will be able
to read with |restore_graph|. The graphs created on remote machines will
then be semantically equivalent to the originals.

Restrictions: Strings must contain only standard printable characters, not
including \.\\ or \." or newline, and must be at most 4095 characters long;
the |g->id| string should be at most 154 characters long. All
pointers to vertices and arcs must be confined to blocks within the
|g->data| area; blocks within |g->aux_data| are not saved or restored.
Storage blocks in |g->data| must be ``pure''; that is,
each block must be entirely
devoted either to |Vertex| records, or to |Arc| records, or to
characters of strings. The |save_graph| procedure places all
|Vertex| records into a single |Vertex| block and
all |Arc| records into a single |Arc| block, preserving the
relative order of the original records where possible; but it does not
preserve the relative order of string data in memory. For example, if
|u->name| and |v->name| point to the same memory location in the saved
graph, they will point to different memory locations (representing equal
strings) in the restored graph. All utility fields must conform to
the conventions of the graph's |util_types| string; the \.G option, which
leads to graphs within graphs, is not permitted in that string.

@d MAX_SV_STRING 4095 /* longest strings supported */
@d MAX_SV_ID 154 /* longest |id| supported, is less than |ID_FIELD_SIZE| */

@(gb_save.h@>=
extern long save_graph();
extern Graph *restore_graph();

@ Here is an overview of the \CEE/ code, \.{gb\_save.c}, for this module:

@p
#include "gb_io.h" /* we use the input/output conventions of {\sc GB\_\,IO} */
#include "gb_graph.h"
 /* and, of course, the data structures of {\sc GB\_\,GRAPH} */
@h@#
@<Type declarations@>@;
@<Private variables@>@;
@<Private functions@>@;
@<External functions@>

@* External representation of graphs. The internal representation of
graphs has been described in {\sc GB\_\,GRAPH}. We now need to supplement
that description by devising an alternative format suitable for
human-and-machine-readable files.

The following somewhat contrived example illustrates the simple conventions
that we shall follow:
$$\let\par=\cr \obeylines %
\vbox{\halign{\.{#}\hfil
* GraphBase graph (util\_types IZAZZZZVZZZZSZ,3V,4A)
"somewhat\_contrived\_example(3.14159265358979323846264338327\\
9502884197169399375105820974944592307816406286208998628)",1,
3,"pi"
* Vertices
"look",A0,15,A1
"feel",0,-9,A1
"",0,0,0
* Arcs
V0,A2,3,V1
V1,0,5,0
V1,0,-8,1
0,0,0,0
* Checksum 271828
}}$$
The first line specifies the 14 characters of |util_types| and the total number
of |Vertex| and |Arc| records; in this case there are 3 vertices and
4~arcs. The next line or lines specify the |id|,
|n|, and |m| fields of the |Graph| record, together with any utility
fields that are not being ignored. In this case, the |id| is a rather
long string; a string may be broken into parts by ending the initial parts
with a backslash, so that no line of the file has more than 79 characters.
The last six characters of |util_types| refer to the utility fields of the
|Graph| record, and in this case they are \.{ZZZZSZ}; so all utility
fields are ignored except the second-to-last, |yy|, which is of type
string. The |restore_graph| routine will construct a |Graph| record~|g| from
this example in which |g->n=1|, |g->m=3|, and |g->yy.S="pi"|.

Notice that the individual field values for a record are separated by commas.
If a line ends with a comma, the following line contains
additional fields of the same record.

After the |Graph| record fields have been specified, there's a special line
`\.{*\ Vertices}', after which we learn the fields of each vertex in turn.
First comes the |name| field, then the |arcs| field, and then any
non-ignored utility fields. In this example the |util_types|
for |Vertex| records are \.{IZAZZZ}, so the utility field values are
|u.I| and |w.A|. Let |v| point to the first |Vertex| record (which incidentally
is also pointed to by |g->vertices|), and let |a| point to the first
|Arc| record. Then in this example we will have |v->name="look"|,
|v->arcs=a|, |v->u.I=15|, and |v->w.A=(a+1)|.

After the |Vertex| records comes a special line `\.{*\ Arcs}', followed by
the fields of each |Arc| record in an entirely analogous way. First
comes the |tip| field, then the |next| field, then the |len|, and finally
the utility fields (if any). In this example the |util_types|
for |Arc| utility fields are \.{ZV}; hence field |a| is ignored, and
field~|b| is a pointer to a |Vertex|. We will have |a->tip=v|, |a->next=(a+2)|,
|a->len=3|, and |a->b.V=(v+1)|.

The null pointer |NULL| is denoted by \.0. Furthermore, a |Vertex| pointer
is allowed to have the special value \.1, because of conventions
explained in {\sc GB\_\,GATES}. (This special value appears in the fourth
field of the third arc in the example above.) The |restore_graph| procedure
does not allow |Vertex| pointers to take on constant values
greater than~1, nor does it permit the value `\.1' where an |Arc|
pointer ought to be.

There should be exactly as many |Vertex| and |Arc| specifications as
indicated after the utility types at the beginning of the file.  The
final |Arc| should then be followed by a special checksum line, which
must contain either a number consistent with the data on all the previous
lines or a negative value (which is not checked).
All information after the checksum line is ignored.

Users should not edit the files produced by |save_graph|, because an
incorrect checksum is liable to ruin everything. However, additional
lines beginning with `\.*' may be placed as comments at the very
beginning of the file; such lines are immune to checksumming.

@ We can establish these conventions firmly in mind by writing the
|restore_graph| routine before we write |save_graph|. The subroutine
call |restore_graph("foo.gb")| produces a pointer to the graph
defined in file |"foo.gb"|, or a null pointer in case that file
is unreadable or incorrect. In the latter case, |panic_code|
indicates the problem.

@<External functions@>=
Graph *restore_graph(f)
  char *f; /* the file name */
{@+Graph *g=NULL; /* the graph being restored */
  register char *p; /* register for string manipulation */
  long m; /* the number of |Arc| records to allocate */
  long n; /* the number of |Vertex| records to allocate */
  @<Open the file and parse the first line; |goto sorry| if there's trouble@>;
  @<Create the |Graph| record |g| and fill in its fields@>;
  @<Fill in the fields of all |Vertex| records@>;
  @<Fill in the fields of all |Arc| records@>;
  @<Check the checksum and close the file@>;
  return g;
sorry: gb_raw_close();@+gb_recycle(g);@+return NULL;
}

@ As mentioned above, users can add comment lines at the beginning
of the file, if they put a \.* at the beginning of every such line.
But the line that precedes the data proper must adhere to
strict standards.

@d panic(c)@+{@+panic_code=c;@+goto sorry;@+}

@<Open the file...@>=
gb_raw_open(f);
if (io_errors) panic(early_data_fault); /* can't open the file */
while (1) {
  gb_string(str_buf,')');
  if (sscanf(str_buf,"* GraphBase graph (util_types %14[ZIVSA],%ldV,%ldA",
       str_buf+80,&n,&m)==3 && strlen(str_buf+80)==14) break;
  if (str_buf[0]!='*') panic(syntax_error); /* first line is unreadable */
}

@ The previous code has placed the graph's |util_types| into
location |str_buf+80| and verified that it contains precisely
14 characters, all belonging to the set $\{\.Z,\.I,\.V,\.S,\.A\}$.

@<Create the |Graph| record |g| and fill in its fields@>=
g=gb_new_graph(0L);
if (g==NULL) panic(no_room); /* out of memory before we're even started */
gb_free(g->data);
g->vertices=verts=gb_typed_alloc(n==0?1:n,Vertex,g->data);
last_vert=verts+n;
arcs=gb_typed_alloc(m==0?1:m,Arc,g->data);
last_arc=arcs+m;
if (gb_trouble_code) panic(no_room+1);
   /* not enough room for vertices and arcs */
strcpy(g->util_types,str_buf+80);
gb_newline();
if (gb_char()!='"') panic(syntax_error+1);
 /* missing quotes before graph |id| string */
p=gb_string(g->id,'"');
if (*(p-2)=='\n' && *(p-3)=='\\' && p>g->id+2) {
  gb_newline(); gb_string(p-3,'"');
}
if (gb_char()!='"') panic(syntax_error+2);
 /* missing quotes after graph |id| string */
@<Fill in |g->n|, |g->m|, and |g|'s utility fields@>;

@ The |util_types| and |id| fields are slightly different from other string
fields, because we store them directly in the |Graph| record instead of
storing a pointer. The other fields to be filled by |restore_graph|
can all be done by a macro called |fillin|, which invokes a subroutine
called |fill_field|. The first parameter
to |fillin| is the address of a field in a record; the second parameter
is one of the codes $\{\.Z,\.I,\.V,\.S,\.A\}$. A global variable
|comma_expected| is nonzero when this field is not the first in its record.

The value returned by |fill_field| is nonzero if something goes wrong.

We assume here that a utility field takes exactly as much space as
a field of any of its constituent types.
@^system dependencies@>

@d fillin(l,t) if (fill_field((util*)&(l),t)) goto sorry

@<Private f...@>=
static long fill_field(l,t)
  util *l; /* location of field to be filled in */
  char t; /* its type code */
{@+register char c; /* character just read */
  if (t!='Z'&&comma_expected) {
    if (gb_char()!=',') return (panic_code=syntax_error-1); /* missing comma */
    if (gb_char()=='\n') gb_newline();
    else gb_backup();
  }
  else comma_expected=1;
  c=gb_char();
  switch (t) {
  case 'I': @<Fill in a numeric field@>;
  case 'V': @<Fill in a vertex pointer@>;
  case 'S': @<Fill in a string pointer@>;
  case 'A': @<Fill in an arc pointer @>;
  default: gb_backup();@+break;
  }
  return panic_code;
}    

@ Some of the communication between |restore_graph| and |fillin| is best
done via global variables.

@<Private v...@>=
static long comma_expected; /* should |fillin| look for a comma? */
static Vertex *verts; /* beginning of the block of |Vertex| records */
static Vertex *last_vert; /* end of the block of |Vertex| records */
static Arc *arcs; /* beginning of the block of |Arc| records */
static Arc *last_arc; /* end of the block of |Arc| records */

@ @<Fill in a numeric field@>=
if (c=='-') l->I=-gb_number(10);
else {
  gb_backup();
  l->I=gb_number(10);
}
break;

@ @<Fill in a vertex pointer@>=
if (c=='V') {
  l->V=verts+gb_number(10);
  if (l->V>=last_vert || l->V<verts)
    panic_code=syntax_error-2; /* vertex address too big */
}@+else if (c=='0' || c=='1') l->I=c-'0';
else panic_code=syntax_error-3; /* vertex numeric address illegal */
break;

@ @<Fill in an arc pointer@>=
if (c=='A') {
  l->A=arcs+gb_number(10);
  if (l->A>=last_arc || l->A<arcs)
    panic_code=syntax_error-4; /* arc address too big */
}@+else if (c=='0') l->A=NULL;
else panic_code=syntax_error-5; /* arc numeric address illegal */
break;

@ We can restore a string slightly longer than the strings we can save.

@<Fill in a string pointer@>=
if (c!='"')
  panic_code=syntax_error-6; /* missing quotes at beginning of string */
else {@+register char* p;
  p=gb_string(item_buf,'"');
  while (*(p-2)=='\n' && *(p-3)=='\\' && p>item_buf+2 && p<=buffer) {
    gb_newline(); p=gb_string(p-3,'"'); /* splice a broken string together */
  }
  if (gb_char()!='"')
    panic_code=syntax_error-7; /* missing quotes at end of string */
  else if (item_buf[0]=='\0') l->S=null_string;
  else l->S=gb_save_string(item_buf);
}
break;

@ @d buffer (&item_buf[MAX_SV_STRING+3]) /* the last 81 chars of |item_buf| */

@<Private v...@>=
static char item_buf[MAX_SV_STRING+3+81]; /* an item to be output */

@ When all fields of a record have been filled in, we call |finish_record|
and hope that it returns~0.

@<Private f...@>=
static long finish_record()
{
  if (gb_char()!='\n')
    return (panic_code=syntax_error-8); /* garbage present */
  gb_newline();
  comma_expected=0;
  return 0;
}

@ @<Fill in |g->n|, |g->m|, and |g|'s utility fields@>=
panic_code=0;
comma_expected=1;
fillin(g->n,'I');
fillin(g->m,'I');
fillin(g->uu,g->util_types[8]);
fillin(g->vv,g->util_types[9]);
fillin(g->ww,g->util_types[10]);
fillin(g->xx,g->util_types[11]);
fillin(g->yy,g->util_types[12]);
fillin(g->zz,g->util_types[13]);
if (finish_record()) goto sorry;

@ The rest is easy.

@<Fill in the fields of all |Vertex| records@>=
{@+register Vertex* v;
  gb_string(str_buf,'\n');
  if (strcmp(str_buf,"* Vertices")!=0)@/
    panic(syntax_error+3); /* introductory line for vertices is missing */
  gb_newline();
  for (v=verts;v<last_vert;v++) {
    fillin(v->name,'S');
    fillin(v->arcs,'A');
    fillin(v->u,g->util_types[0]);
    fillin(v->v,g->util_types[1]);
    fillin(v->w,g->util_types[2]);
    fillin(v->x,g->util_types[3]);
    fillin(v->y,g->util_types[4]);
    fillin(v->z,g->util_types[5]);
    if (finish_record()) goto sorry;
  }
}

@ @<Fill in the fields of all |Arc| records@>=
{@+register Arc* a;
  gb_string(str_buf,'\n');
  if (strcmp(str_buf,"* Arcs")!=0)
    panic(syntax_error+4); /* introductory line for arcs is missing */
  gb_newline();
  for (a=arcs;a<last_arc;a++) {
    fillin(a->tip,'V');
    fillin(a->next,'A');
    fillin(a->len,'I');
    fillin(a->a,g->util_types[6]);
    fillin(a->b,g->util_types[7]);
    if (finish_record()) goto sorry;
  }
}

@ @<Check the checksum and close the file@>=
{@+long s;
  gb_string(str_buf,'\n');
  if (sscanf(str_buf,"* Checksum %ld",&s)!=1)
    panic(syntax_error+5); /* checksum line is missing */
  if (gb_raw_close()!=s && s>=0)
    panic(late_data_fault); /* checksum does not match */
}

@* Saving a graph. Now that we know how to restore a graph, once it has
been saved, we are ready to write the |save_graph| routine.

Users say |save_graph(g,"foo.gb")|; our job is to create a file
|"foo.gb"| from which the subroutine call |restore_graph("foo.gb")|
will be able to reconstruct a graph equivalent to~|g|, assuming that
|g| meets the restrictions stated earlier.  If nothing goes wrong,
|save_graph| should return the value zero.  Otherwise it should return
an encoded trouble report.

We will set things up so that |save_graph| produces
a syntactically correct file |"foo.gb"| in almost
every case, with explicit error indications written at the end of the file
whenever certain aspects of the given graph have had to be changed.
The value |-1| will be returned if |g==NULL|; the value
|-2| will be returned if |g!=NULL| but the file |"foo.gb"| could not
be opened for output; the value |-3| will be returned if memory is
exhausted. In other cases a file |"foo.gb"| will be created.

Here is a list of things that might go wrong, and the corresponding
corrective actions to be taken in each case, assuming that
|save_graph| does create a file:

@d bad_type_code 0x1 /* illegal character, is changed to |'Z'| */
@d string_too_long 0x2 /* extralong string, is truncated */
@d addr_not_in_data_area 0x4 /* address out of range, is changed to |NULL| */
@d addr_in_mixed_block 0x8 /* address not in pure block, is |NULL|ified */
@d bad_string_char 0x10 /* illegal string character, is changed to |'?'| */
@d ignored_data 0x20 /* nonzero value in |'Z'| format, is not output */ 

@<Private v...@>=
static long anomalies; /* problems accumulated by |save_graph| */
static FILE *save_file; /* the file being written */

@ @<External f...@>=
long save_graph(g,f)
  Graph *g; /* graph to be saved */
  char *f; /* name of the file to be created */
{@+@<Local variables for |save_graph|@>@;@#
  if (g==NULL || g->vertices==NULL) return -1; /* where is |g|? */
  anomalies=0;
  @<Figure out the extent of |g|'s internal records@>;
  save_file=fopen(f,"w");
  if (!save_file) return -2; /* oops, the operating system won't cooperate */
  @<Translate |g| into external format@>;
  @<Make notes at the end of the file about any changes that were necessary@>;
  fclose(save_file);
  gb_free(working_storage);
  return anomalies;
}

@ The main difficulty faced by |save_graph| is the problem of
translating vertex and arc pointers into symbolic form. A graph's
vertices usually appear in a single block, |g->vertices|, but its arcs
usually appear in separate blocks that were created whenever the
|gb_new_arc| routine needed more space. Other blocks, created by
|gb_save_string|, are usually also present in the |g->data| area.  We
need to classify the various data blocks. We also want to be able
to handle graphs that have been created with homegrown methods of
memory allocation, because GraphBase structures need not conform to
the conventions of |gb_new_arc| and |gb_save_string|.

A simple data structure based on \&{block\_rep} records will
facilitate our task.  Each \&{block\_rep} will be set up to contain
the information we need to know about a particular block of data
accessible from |g->data|. Such blocks are classified into four
categories, identified by the |cat| field in a \&{block\_rep}:

@d unk 0 /* |cat| value for blocks of unknown nature */
@d ark 1 /* |cat| value for blocks assumed to hold |Arc| records */
@d vrt 2 /* |cat| value for blocks assumed to hold |Vertex| records */
@d mxt 3 /* |cat| value for blocks being used for more than one purpose */

@<Type...@>=
typedef struct {
  char *start_addr; /* starting address of a data block */
  char *end_addr; /* ending address of a data block */
  long offset; /* index number of first record in the block, if known */
  long cat; /* |cat| code for the block */
  long expl; /* have we finished exploring this block? */
} block_rep;

@ The |block_rep| records don't need to be linked together in any fancy way,
because there usually aren't very many of them. We will simply create
an array, organized in decreasing order of |start_addr| and |end_addr|, with a
dummy record standing as a sentinel at the end.

A system-dependent change might be necessary in the following code,
if pointer values can be longer than 32 bits, or if comparisons between
pointers are undefined.
@^system dependencies@>

@<Private v...@>=
static block_rep* blocks; /* beginning of table of block representatives */
static Area working_storage;

@ Initially we set the |end_addr| field to the location following a
block's data area. Later we will change it as explained below.

The code in this section uses the fact that all bits of storage blocks
are zero until set nonzero. In particular, the |cat| field of each
|block_rep| will initially be |unk|, and the |expl| will be zero;
the |start_addr| and |end_addr| of the sentinel record will be zero.

@<Initialize the |blocks| array@>=
{@+Area t; /* variable that runs through |g->data| */
  for (*t=*(g->data),block_count=0;*t;*t=(*t)->next) block_count++;
  blocks=gb_typed_alloc(block_count+1,block_rep,working_storage);
  if (blocks==NULL) return -3; /* out of memory */
  for (*t=*(g->data),block_count=0;*t;*t=(*t)->next,block_count++) {
    cur_block=blocks+block_count;
    while (cur_block>blocks&&(cur_block-1)->start_addr<(*t)->first) {
      cur_block->start_addr=(cur_block-1)->start_addr;
      cur_block->end_addr=(cur_block-1)->end_addr;
      cur_block--;
    }
    cur_block->start_addr=(*t)->first;
    cur_block->end_addr=(char*)*t;
  }
}

@ @<Local variables for |save...@>=
register block_rep *cur_block; /* the current block of interest */
long block_count; /* how many blocks have we processed? */

@ The |save_graph| routine makes two passes over the graph. The
goal of the first pass is reconnaissance: We try to see where everything
is, and we prune off parts that don't conform to the restrictions.
When we get to the second pass, our task will then be almost trivial.
We will be able to march through the known territory and spew out a copy
of what we encounter. (Items that are ``pruned'' are not actually
removed from |g| itself, only from the portion of~|g| that is saved.)

The first pass is essentially a sequence of calls of the |lookup| macro,
which looks at one field of one record and notes whether
the existence of this field extends the known boundaries of the graph.
The |lookup| macro is a shorthand notation for calling the |classify|
subroutine. We make the same assumption about field sizes as the
|fill_field| routine did above.
@^system dependencies@>

@d lookup(l,t) classify((util*)&(l),t) /* explore field |l| of type |t| */

@<Private f...@>=
static void classify(l,t)
  util *l; /* location of field to be classified */
  char t; /* its type code, from the set $\{\.Z,\.I,\.V,\.S,\.A\}$ */
{@+register block_rep *cur_block;
  register char* loc;
  register long tcat; /* category corresponding to |t| */
  register long tsize; /* record size corresponding to |t| */
  switch (t) {
  default: return;
  case 'V': if (l->I==1) return;
    tcat=vrt;
    tsize=sizeof(Vertex);
    break;
  case 'A': tcat=ark;
    tsize=sizeof(Arc);
    break;
  }
  if (l->I==0) return;
  @<Classify a pointer variable@>;
}

@ Here we know that |l| points to a |Vertex| or
to an |Arc|, according as |tcat| is |vrt| or |ark|. We need to check that
this doesn't violate any assumptions about all such pointers lying
in pure blocks within the |g->data| area.

@<Classify a pointer variable@>=
loc=(char*)l->V;
for (cur_block=blocks; cur_block->start_addr>loc; cur_block++) ;
if (loc<cur_block->end_addr) {
  if ((loc-cur_block->start_addr)%tsize!=0 || loc+tsize>cur_block->end_addr)
    cur_block->cat=mxt;
  if (cur_block->cat==unk) cur_block->cat=tcat;
  else if (cur_block->cat!=tcat) cur_block->cat=mxt;
}

@ We go through the list of blocks repeatedly until we reach a stable
situation in which every |vrt| or |ark| block has been explored.

@<Figure out the extent of |g|'s internal records@>=
{@+long activity;
  @<Initialize the |blocks| array@>;
  lookup(g->vertices,'V');
  lookup(g->uu,g->util_types[8]);
  lookup(g->vv,g->util_types[9]);
  lookup(g->ww,g->util_types[10]);
  lookup(g->xx,g->util_types[11]);
  lookup(g->yy,g->util_types[12]);
  lookup(g->zz,g->util_types[13]);
  do@+{@+activity=0;
    for(cur_block=blocks;cur_block->end_addr;cur_block++) {
      if (cur_block->cat==vrt && !cur_block->expl)
        @<Explore a block of supposed vertex records@>@;
      else if (cur_block->cat==ark && !cur_block->expl)
        @<Explore a block of supposed arc records@>@;
      else continue;
      cur_block->expl=activity=1;
   }
  }@+while (activity);
}

@ While we are exploring a block, the |lookup| routine might classify
a previously explored block (or even the current block) as |mxt|.
Therefore some data we assumed would be accessible will actually be
removed from the graph; contradictions that arose might no longer exist.
But we plunge ahead anyway, because we aren't going to try especially
hard to ``save'' portions of graphs that violate our ground rules.

@<Explore a block of supposed vertex records@>=
{@+register Vertex*v;
  for (v=(Vertex*)cur_block->start_addr;@|
      (char*)(v+1)<=cur_block->end_addr && cur_block->cat==vrt;v++) {
    lookup(v->arcs,'A');
    lookup(v->u,g->util_types[0]);
    lookup(v->v,g->util_types[1]);
    lookup(v->w,g->util_types[2]);
    lookup(v->x,g->util_types[3]);
    lookup(v->y,g->util_types[4]);
    lookup(v->z,g->util_types[5]);
  }
}

@ @<Explore a block of supposed arc records@>=
{@+register Arc*a;
  for (a=(Arc*)cur_block->start_addr;@|
      (char*)(a+1)<=cur_block->end_addr && cur_block->cat==ark;a++) {
    lookup(a->tip,'V');
    lookup(a->next,'A');
    lookup(a->a,g->util_types[6]);
    lookup(a->b,g->util_types[7]);
  }
}

@ OK, the first pass is complete. And the second pass is routine:

@<Translate |g| into external format@>=
@<Orient the |blocks| table for translation@>;
@<Initialize the output buffer mechanism and output the first line@>;
@<Translate the |Graph| record@>;
@<Translate the |Vertex| records@>;
@<Translate the |Arc| records@>;
@<Output the checksum line@>;

@ During this pass we decrease the |end_addr| field of a |block_rep|,
so that it points to the first byte of
the final record in a |vrt| or |ark| block.

The variables |m| and |n| are set to the number of arc records and
vertex records, respectively.

@<Local variables for |save...@>=
long m; /* total number of |Arc| records to be translated */
long n; /* total number of |Vertex| records to be translated */
register long s; /* accumulator register for arithmetic calculations */

@ One tricky point needs to be observed, in the unusual case that
there are two or more blocks of \&{Vertex} records: The base block
|g->vertices| must come first in the final ordering. (This is the only
exception to the rule that \&{Vertex} and \&{Arc} records each retain
their relative order with respect to less-than and greater-than.)

@<Orient the |blocks| table for translation@>=
m=0;@+@<Set |n| to the size of the block that starts with |g->vertices|@>;
for (cur_block=blocks+block_count-1;cur_block>=blocks;cur_block--) {
  if (cur_block->cat==vrt) {
    s=(cur_block->end_addr-cur_block->start_addr)/sizeof(Vertex);
    cur_block->end_addr=cur_block->start_addr+((s-1)*sizeof(Vertex));
    if (cur_block->start_addr!=(char*)g->vertices) {
      cur_block->offset=n;@+ n+=s;
    } /* otherwise |cur_block->offset| remains zero */
  }@+else if (cur_block->cat==ark) {
    s=(cur_block->end_addr-cur_block->start_addr)/sizeof(Arc);
    cur_block->end_addr=cur_block->start_addr+((s-1)*sizeof(Arc));
    cur_block->offset=m;
    m+=s;
  }
}

@ @<Set |n| to the size of the block that starts with |g->vertices|@>=
n=0;
for (cur_block=blocks+block_count-1;cur_block>=blocks;cur_block--)
  if (cur_block->start_addr==(char *)g->vertices) {
    n=(cur_block->end_addr-cur_block->start_addr)/sizeof(Vertex);
    break;
  }

@ We will store material to be output in the |buffer| array,
so that we can compute the correct checksum.

@<Private v...@>=
static char *buf_ptr; /* the first unfilled position in |buffer| */
static long magic; /* the checksum */

@ @<Private f...@>=
static void flushout() /* output the buffer to |save_file| */
{
  *buf_ptr++='\n';
  *buf_ptr='\0';
  magic=new_checksum(buffer,magic);
  fputs(buffer,save_file);
  buf_ptr=buffer;
}

@ If a supposed string pointer is zero, we output the null string.
(This case arises when a string field has not been initialized,
for example in vertices and arcs that have been allocated but not used.)

@<Private f...@>=
static void prepare_string(s)
  char *s; /* string that is moved to |item_buf| */
{@+register char *p,*q;
  item_buf[0]='"';
  p=&item_buf[1];
  if (s==0) goto sready;
  for (q=s;*q&&p<=&item_buf[MAX_SV_STRING];q++,p++)
    if (*q=='"'||*q=='\n'||*q=='\\'||imap_ord(*q)==unexpected_char) {
      anomalies |= bad_string_char;
      *p='?';
    }@+else *p=*q;
  if (*q) anomalies |= string_too_long;
sready:  *p='"';
  *(p+1)='\0';
}

@ The main idea of this part of the program is to format an item into
|item_buf|, then move it to |buffer|, making sure that there is always
room for a comma.

@d append_comma *buf_ptr++=','

@<Private f...@>=
static void move_item()
{@+register long l=strlen(item_buf);
  if (buf_ptr+l>&buffer[78]) {
    if (l<=78) flushout();
    else {@+register char *p=item_buf;
      if (buf_ptr>&buffer[77]) flushout();
           /* no room for initial \.{\char`\"} */
      do@+{
        for (;buf_ptr<&buffer[78];buf_ptr++,p++,l--) *buf_ptr=*p;
        *buf_ptr++='\\';
        flushout();
      }@+while(l>78);
    strcpy(buffer,p);
    buf_ptr=&buffer[l];
    return;
    }
  }
  strcpy(buf_ptr,item_buf);
  buf_ptr+=l;
}  
 
@ @<Initialize the output buffer mechanism and output the first line@>=
buf_ptr=buffer;
magic=0;
fputs("* GraphBase graph (util_types ",save_file);
{@+register char*p;
  for (p=g->util_types;p<g->util_types+14;p++)
    if (*p=='Z'||*p=='I'||*p=='V'||*p=='S'||*p=='A') fputc(*p,save_file);
    else fputc('Z',save_file);
}
fprintf(save_file,",%ldV,%ldA)\n",n,m);

@ A macro called |trans|, which is sort of an inverse to |fillin|,
takes care of the main work in the second pass.

@d trans(l,t) translate_field((util*)&(l),t)

@<Private f...@>=
static void translate_field(l,t)
  util *l; /* address of field to be output in symbolic form */
  char t; /* type of formatting desired */
{@+register block_rep *cur_block;
  register char* loc;
  register long tcat; /* category corresponding to |t| */
  register long tsize; /* record size corresponding to |t| */
  if (comma_expected) append_comma;
  else comma_expected=1;
  switch (t) {
 default: anomalies|=bad_type_code;
    /* fall through to case \.Z */
 case 'Z': buf_ptr--; /* forget spurious comma */
  if (l->I) anomalies|=ignored_data;
  return;
 case 'I': numeric: sprintf(item_buf,"%ld",l->I);@+goto ready;
 case 'S': prepare_string(l->S);@+goto ready;
 case 'V': if (l->I==1) goto numeric;
    tcat=vrt;@+tsize=sizeof(Vertex);@+break;
 case 'A': tcat=ark;@+tsize=sizeof(Arc);@+break;
  }
  @<Translate a pointer variable@>;
ready:move_item();
}

@ @<Translate a pointer variable@>=
loc=(char*)l->V;
item_buf[0]='0';@+item_buf[1]='\0'; /* |NULL| will be the default */
if (loc==NULL) goto ready;
for (cur_block=blocks; cur_block->start_addr>loc; cur_block++) ;
if (loc>cur_block->end_addr) {
  anomalies|=addr_not_in_data_area;
  goto ready;
}
if (cur_block->cat!=tcat||(loc-cur_block->start_addr)%tsize!=0) {
  anomalies|=addr_in_mixed_block;
  goto ready;
}
sprintf(item_buf,"%c%ld",t,
  cur_block->offset+((loc-cur_block->start_addr)/tsize));

@ @<Translate the |Graph| record@>=
prepare_string(g->id);
if (strlen(g->id)>MAX_SV_ID) {
  strcpy(item_buf+MAX_SV_ID+1,"\"");
  anomalies|=string_too_long;
}
move_item();
comma_expected=1;
trans(g->n,'I');
trans(g->m,'I');
trans(g->uu,g->util_types[8]);
trans(g->vv,g->util_types[9]);
trans(g->ww,g->util_types[10]);
trans(g->xx,g->util_types[11]);
trans(g->yy,g->util_types[12]);
trans(g->zz,g->util_types[13]);
flushout();

@ @<Translate the |Vertex| records@>=
{@+register Vertex* v;
  fputs("* Vertices\n",save_file);
  for (cur_block=blocks+block_count-1;cur_block>=blocks;cur_block--)
    if (cur_block->cat==vrt && cur_block->offset==0)
      @<Translate all |Vertex| records in |cur_block|@>;
  for (cur_block=blocks+block_count-1;cur_block>=blocks;cur_block--)
    if (cur_block->cat==vrt && cur_block->offset!=0)
      @<Translate all |Vertex| records in |cur_block|@>;
}

@ @<Translate all |Vertex| records in |cur_block|@>=
for (v=(Vertex*)cur_block->start_addr;
     v<=(Vertex*)cur_block->end_addr;v++) {
  comma_expected=0;
  trans(v->name,'S');
  trans(v->arcs,'A');
  trans(v->u,g->util_types[0]);
  trans(v->v,g->util_types[1]);
  trans(v->w,g->util_types[2]);
  trans(v->x,g->util_types[3]);
  trans(v->y,g->util_types[4]);
  trans(v->z,g->util_types[5]);
  flushout();
}

@ @<Translate the |Arc| records@>=
{@+register Arc* a;
  fputs("* Arcs\n",save_file);
  for (cur_block=blocks+block_count-1;cur_block>=blocks;cur_block--)
    if (cur_block->cat==ark)
      for (a=(Arc*)cur_block->start_addr;a<=(Arc*)cur_block->end_addr;a++) {
        comma_expected=0;
        trans(a->tip,'V');
        trans(a->next,'A');
        trans(a->len,'I');
        trans(a->a,g->util_types[6]);
        trans(a->b,g->util_types[7]);
        flushout();
      }
}

@ @<Output the checksum line@>=
fprintf(save_file,"* Checksum %ld\n",magic);

@ @<Make notes at the end of the file about any changes that were necessary@>=
if (anomalies) {
  fputs("> WARNING: I had trouble making this file from the given graph!\n",
    save_file);
  if (anomalies&bad_type_code)
    fputs(">> The original util_types had to be corrected.\n",save_file);
  if (anomalies&ignored_data)
    fputs(">> Some data suppressed by Z format was actually nonzero.\n",
      save_file);
  if (anomalies&string_too_long)
    fputs(">> At least one long string had to be truncated.\n",
      save_file);
  if (anomalies&bad_string_char)
    fputs(">> At least one string character had to be changed to '?'.\n",
      save_file);
  if (anomalies&addr_not_in_data_area)
    fputs(">> At least one pointer led out of the data area.\n",save_file);
  if (anomalies&addr_in_mixed_block)
    fputs(">> At least one data block had an illegal mixture of records.\n",
      save_file);
  if (anomalies&(addr_not_in_data_area+addr_in_mixed_block))
    fputs(">>  (Pointers to improper data have been changed to 0.)\n",
      save_file);
  fputs("> You should be able to read this file with restore_graph,\n",
      save_file);
  fputs("> but the graph you get won't be exactly like the original.\n",
      save_file);
}

@* Index. Here is a list that shows where the identifiers of this program are
defined and used.