casereader: Make parameter to casereader_count_cases const.

[pspp-builds.git] / src / data / casereader.c

diff --git a/src/data/casereader.c b/src/data/casereader.c
index 5ae5a0a8ea4431019660c60e1d56d9d68ed8c3fa..97b7ceff3d1b7eb19e8ac6243aef08eb961a0f6b 100644 (file)
--- a/src/data/casereader.c
+++ b/src/data/casereader.c
@@ -257,6 +257,20 @@ casereader_get_case_cnt (struct casereader *reader)
   return reader->case_cnt;
 }
 
+static casenumber
+casereader_count_cases__ (const struct casereader *reader,
+                          casenumber max_cases)
+{
+  struct casereader *clone;
+  casenumber n_cases;
+
+  clone = casereader_clone (reader);
+  n_cases = casereader_advance (clone, max_cases);
+  casereader_destroy (clone);
+
+  return n_cases;
+}
+
 /* Returns the number of cases that will be read by successive
    calls to casereader_read for READER, assuming that no errors
    occur.  Upon an error condition, the case count drops to 0, so
@@ -267,25 +281,30 @@ casereader_get_case_cnt (struct casereader *reader)
    of the contents of a clone of READER.  Thus, the return value
    is always correct in the absence of I/O errors. */
 casenumber
-casereader_count_cases (struct casereader *reader)
+casereader_count_cases (const struct casereader *reader)
 {
   if (reader->case_cnt == CASENUMBER_MAX)
     {
-      casenumber n_cases = 0;
-      struct ccase *c;
-
-      struct casereader *clone = casereader_clone (reader);
-
-      for (; (c = casereader_read (clone)) != NULL; case_unref (c))
-        n_cases++;
-
-      casereader_destroy (clone);
-      reader->case_cnt = n_cases;
+      struct casereader *reader_rw = CONST_CAST (struct casereader *, reader);
+      reader_rw->case_cnt = casereader_count_cases__ (reader, CASENUMBER_MAX);
     }
-
   return reader->case_cnt;
 }
 
+/* Truncates READER to at most N cases. */
+void
+casereader_truncate (struct casereader *reader, casenumber n)
+{
+  /* This could be optimized, if it ever becomes too expensive, by adding a
+     "max_cases" member to struct casereader.  We could also add a "truncate"
+     function to the casereader implementation, to allow the casereader to
+     throw away data that cannot ever be read. */
+  if (reader->case_cnt == CASENUMBER_MAX)
+    reader->case_cnt = casereader_count_cases__ (reader, n);
+  if (reader->case_cnt > n)
+    reader->case_cnt = n;
+}
+
 /* Returns the prototype for the cases in READER.  The caller
    must not unref the returned prototype. */
 const struct caseproto *
@@ -294,6 +313,26 @@ casereader_get_proto (const struct casereader *reader)
   return reader->proto;
 }
 
+/* Skips past N cases in READER, stopping when the last case in
+   READER has been read or on an input error.  Returns the number
+   of cases successfully skipped. */
+casenumber
+casereader_advance (struct casereader *reader, casenumber n)
+{
+  casenumber i;
+
+  for (i = 0; i < n; i++)
+    {
+      struct ccase *c = casereader_read (reader);
+      if (c == NULL)
+        break;
+      case_unref (c);
+    }
+
+  return i;
+}
+
+
 /* Copies all the cases in READER to WRITER, propagating errors
    appropriately. */
 void
@@ -664,3 +703,42 @@ static const struct casereader_random_class shim_class =
     shim_destroy,
     shim_advance,
   };
+\f
+static const struct casereader_class casereader_null_class;
+
+/* Returns a casereader with no cases.  The casereader has the prototype
+   specified by PROTO.  PROTO may be specified as a null pointer, in which case
+   the casereader has no variables. */
+struct casereader *
+casereader_create_empty (const struct caseproto *proto_)
+{
+  struct casereader *reader;
+  struct caseproto *proto;
+
+  proto = proto_ != NULL ? caseproto_ref (proto_) : caseproto_create ();
+  reader = casereader_create_sequential (NULL, proto, 0,
+                                         &casereader_null_class, NULL);
+  caseproto_unref (proto);
+
+  return reader;
+}
+
+static struct ccase *
+casereader_null_read (struct casereader *reader UNUSED, void *aux UNUSED)
+{
+  return NULL;
+}
+
+static void
+casereader_null_destroy (struct casereader *reader UNUSED, void *aux UNUSED)
+{
+  /* Nothing to do. */
+}
+
+static const struct casereader_class casereader_null_class =
+  {
+    casereader_null_read,
+    casereader_null_destroy,
+    NULL,                       /* clone */
+    NULL,                       /* peek */
+  };