locale: Implement struct grouping_iterator

The iterator allows grouping while scanning forward through
the digits.  This enables emitting digits as they are processed.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdio-common/grouping_iterator.c

@@ -0,0 +1,126 @@
+/* Iterator for inserting thousands separators into numbers.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <grouping_iterator.h>
+
+#include <assert.h>
+#include <limits.h>
+#include <locale/localeinfo.h>
+#include <stdint.h>
+#include <string.h>
+
+/* Initializes *IT with no grouping information for a string of length
+   DIGITS, and return false to indicate no grouping.  */
+bool
+__grouping_iterator_init_none (struct grouping_iterator *it,
+                               unsigned int digits)
+{
+  memset (it, 0, sizeof (*it));
+  it->remaining_in_current_group = digits;
+  it->remaining = digits;
+  return false;
+}
+
+static bool
+grouping_iterator_setup (struct grouping_iterator *it, unsigned int digits,
+                         const char *grouping)
+{
+  /* We treat all negative values like CHAR_MAX.  */
+
+  if (*grouping == CHAR_MAX || *grouping <= 0)
+    /* No grouping should be done.  */
+    return __grouping_iterator_init_none (it, digits);
+
+  unsigned int remaining_to_group = digits;
+  unsigned int non_repeating_groups = 0;
+  unsigned int groups = 0;
+  while (true)
+    {
+      non_repeating_groups += *grouping;
+      if (remaining_to_group <= (unsigned int) *grouping)
+        break;
+
+      ++groups;
+      remaining_to_group -= *grouping++;
+
+      if (*grouping == CHAR_MAX
+#if CHAR_MIN < 0
+          || *grouping < 0
+#endif
+          )
+          /* No more grouping should be done.  */
+        break;
+      else if (*grouping == 0)
+        {
+          /* Same grouping repeats.  */
+          --grouping;
+          non_repeating_groups -= *grouping; /* Over-counted.  */
+          unsigned int repeats = (remaining_to_group - 1) / *grouping;
+          groups += repeats;
+          remaining_to_group -= repeats * *grouping;
+          break;
+        }
+    }
+
+  it->remaining_in_current_group = remaining_to_group;
+  it->remaining = digits;
+  it->groupings = grouping;
+  it->non_repeating_groups = non_repeating_groups;
+  it->separators = groups;
+  return it->separators > 0;
+}
+
+/* Returns the appropriate grouping item in LOC depending on CATEGORY
+   (which must be LC_MONETARY or LC_NUMERIC).  */
+static const char *
+get_grouping (int category, locale_t loc)
+{
+  return _nl_lookup (loc, category,
+                     category == LC_MONETARY ? MON_GROUPING : GROUPING);
+}
+
+
+bool
+__grouping_iterator_init (struct grouping_iterator *it,
+                          int category, locale_t loc, unsigned int digits)
+{
+  if (digits <= 1)
+    return __grouping_iterator_init_none (it, digits);
+  else
+    return grouping_iterator_setup (it, digits, get_grouping (category, loc));
+}
+
+bool
+__grouping_iterator_next (struct grouping_iterator *it)
+{
+  assert (it->remaining > 0);
+  --it->remaining;
+
+  if (it->remaining_in_current_group > 0)
+    {
+      --it->remaining_in_current_group;
+      return false;
+    }
+
+  /* If we are in the non-repeating part, switch group.  */
+  if (it->remaining < it->non_repeating_groups)
+    --it->groupings;
+
+  it->remaining_in_current_group = *it->groupings - 1;
+  return true;
+}