Replace executable’s fingerprint in place

* admin/merge-gnulib (GNULIB_MODULES): Add memmem-simple.
(AVOIDED_MODULES): Add memchr.
* configure.ac (HAVE_PDUMPER): AC_SUBST it, too, for use in makefiles.
* lib/Makefile.in (libgnu_a_OBJECTS): Add fingerprint.o.
* lib/fingerprint.c: New file.
* lib/memmem.c, lib/str-two-way.h, m4/memmem.m4: New files,
copied from Gnulib.
* lib/fingerprint.h: Rename from src/fingerprint.h.
* lib-src/make-fingerprint.c: Include limits.h, sys/stat.h,
fingerprint.h, intprops.h, min-max.h.
(SSIZE_MAX): New macro, if not already defined.
(main): Without -r, Replace the fingerprint in the input file
instead of generating a fingerprint.c.
* lib/Makefile.in (libgnu_a_OBJECTS): Add fingerprint.o.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/Makefile.in (HAVE_PDUMPER, MAKE_PDUMPER_FINGERPRINT):
New macros.
(temacs$(EXEEXT)): Use them to replace the fingerprint instead
of precalculating it.
(mostlyclean, ctagsfiles1): Do not worry about fingerprint.c.

6 files changed, 632 insertions, 2 deletions

diff --git a/lib/Makefile.in b/lib/Makefile.in
index f2d203564ac..ac32c7070f3 100644
--- a/lib/Makefile.in
+++ b/lib/Makefile.in
@@ -84,7 +84,7 @@ Makefile: ../config.status $(srcdir)/Makefile.in
 # and building it would just waste time.
 not_emacs_OBJECTS = regex.o
 
-libgnu_a_OBJECTS = $(gl_LIBOBJS) \
+libgnu_a_OBJECTS = fingerprint.o $(gl_LIBOBJS) \
   $(patsubst %.c,%.o,$(filter %.c,$(libgnu_a_SOURCES)))
 for_emacs_OBJECTS = $(filter-out $(not_emacs_OBJECTS),$(libgnu_a_OBJECTS))
 libegnu_a_OBJECTS = $(patsubst %.o,e-%.o,$(for_emacs_OBJECTS))
diff --git a/lib/fingerprint.c b/lib/fingerprint.c
new file mode 100644
index 00000000000..e55de9c6da3
--- /dev/null
+++ b/lib/fingerprint.c
@@ -0,0 +1,66 @@
+/* Placeholder fingerprint for Emacs
+
+Copyright 2019 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at
+your option) any later version.
+
+GNU Emacs 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "fingerprint.h"
+
+/* This random fingerprint was generated by the shell command:
+
+   shuf -i 0-255 -n 32 -r | awk '{printf "   0x%.02X,\n", $0}'
+
+   In the final Emacs executable, this random fingerprint is replaced
+   by a fingerprint of the temporary Emacs executable that was built
+   along the way.  */
+
+unsigned char const fingerprint[] =
+  {
+   0xDE,
+   0x86,
+   0xBB,
+   0x99,
+   0xFF,
+   0xF5,
+   0x46,
+   0x9A,
+   0x9E,
+   0x3F,
+   0x9F,
+   0x5D,
+   0x9A,
+   0xDF,
+   0xF0,
+   0x91,
+   0xBD,
+   0xCD,
+   0xC1,
+   0xE8,
+   0x0C,
+   0x16,
+   0x1E,
+   0xAF,
+   0xB8,
+   0x6C,
+   0xE2,
+   0x2B,
+   0xB1,
+   0x24,
+   0xCE,
+   0xB0,
+  };
diff --git a/lib/fingerprint.h b/lib/fingerprint.h
new file mode 100644
index 00000000000..0b195fd0ca7
--- /dev/null
+++ b/lib/fingerprint.h
@@ -0,0 +1,29 @@
+/* Header file for the Emacs build fingerprint.
+
+Copyright (C) 2016, 2018-2019 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at
+your option) any later version.
+
+GNU Emacs 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef EMACS_FINGERPRINT_H
+#define EMACS_FINGERPRINT_H
+
+/* We generate fingerprint.c and fingerprint.o from all the sources in
+   Emacs.  This way, we have a unique value that we can use to pair
+   data files (like a portable dump image) with a specific build of
+   Emacs.  */
+extern unsigned char const fingerprint[32];
+
+#endif
diff --git a/lib/gnulib.mk.in b/lib/gnulib.mk.in
index 1450df9f638..03160340c8e 100644
--- a/lib/gnulib.mk.in
+++ b/lib/gnulib.mk.in
@@ -44,6 +44,7 @@
 #  --avoid=malloc-posix \
 #  --avoid=mbrtowc \
 #  --avoid=mbsinit \
+#  --avoid=memchr \
 #  --avoid=mkdir \
 #  --avoid=msvc-inval \
 #  --avoid=msvc-nothrow \
@@ -111,6 +112,7 @@
 #  largefile \
 #  lstat \
 #  manywarnings \
+#  memmem-simple \
 #  memrchr \
 #  minmax \
 #  mkostemp \
@@ -1053,7 +1055,6 @@ gl_GNULIB_ENABLED_03e0aaad4cb89ca757653bd367a6ccb7 = @gl_GNULIB_ENABLED_03e0aaad
 gl_GNULIB_ENABLED_2049e887c7e5308faad27b3f894bb8c9 = @gl_GNULIB_ENABLED_2049e887c7e5308faad27b3f894bb8c9@
 gl_GNULIB_ENABLED_21ee726a3540c09237a8e70c0baf7467 = @gl_GNULIB_ENABLED_21ee726a3540c09237a8e70c0baf7467@
 gl_GNULIB_ENABLED_260941c0e5dc67ec9e87d1fb321c300b = @gl_GNULIB_ENABLED_260941c0e5dc67ec9e87d1fb321c300b@
-gl_GNULIB_ENABLED_37f71b604aa9c54446783d80f42fe547 = @gl_GNULIB_ENABLED_37f71b604aa9c54446783d80f42fe547@
 gl_GNULIB_ENABLED_5264294aa0a5557541b53c8c741f7f31 = @gl_GNULIB_ENABLED_5264294aa0a5557541b53c8c741f7f31@
 gl_GNULIB_ENABLED_6099e9737f757db36c47fa9d9f02e88c = @gl_GNULIB_ENABLED_6099e9737f757db36c47fa9d9f02e88c@
 gl_GNULIB_ENABLED_682e609604ccaac6be382e4ee3a4eaec = @gl_GNULIB_ENABLED_682e609604ccaac6be382e4ee3a4eaec@
@@ -1963,6 +1964,17 @@ EXTRA_libgnu_a_SOURCES += lstat.c
 endif
 ## end   gnulib module lstat
 
+## begin gnulib module memmem-simple
+ifeq (,$(OMIT_GNULIB_MODULE_memmem-simple))
+
+
+EXTRA_DIST += memmem.c str-two-way.h
+
+EXTRA_libgnu_a_SOURCES += memmem.c
+
+endif
+## end   gnulib module memmem-simple
+
 ## begin gnulib module memrchr
 ifeq (,$(OMIT_GNULIB_MODULE_memrchr))
 
diff --git a/lib/memmem.c b/lib/memmem.c
new file mode 100644
index 00000000000..12ae24f41b4
--- /dev/null
+++ b/lib/memmem.c
@@ -0,0 +1,71 @@
+/* Copyright (C) 1991-1994, 1996-1998, 2000, 2004, 2007-2019 Free Software
+   Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3, or (at your option)
+   any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+/* This particular implementation was written by Eric Blake, 2008.  */
+
+#ifndef _LIBC
+# include <config.h>
+#endif
+
+/* Specification of memmem.  */
+#include <string.h>
+
+#define RETURN_TYPE void *
+#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
+#include "str-two-way.h"
+
+/* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
+   if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
+   HAYSTACK.  */
+void *
+memmem (const void *haystack_start, size_t haystack_len,
+        const void *needle_start, size_t needle_len)
+{
+  /* Abstract memory is considered to be an array of 'unsigned char' values,
+     not an array of 'char' values.  See ISO C 99 section 6.2.6.1.  */
+  const unsigned char *haystack = (const unsigned char *) haystack_start;
+  const unsigned char *needle = (const unsigned char *) needle_start;
+
+  if (needle_len == 0)
+    /* The first occurrence of the empty string is deemed to occur at
+       the beginning of the string.  */
+    return (void *) haystack;
+
+  /* Sanity check, otherwise the loop might search through the whole
+     memory.  */
+  if (__builtin_expect (haystack_len < needle_len, 0))
+    return NULL;
+
+  /* Use optimizations in memchr when possible, to reduce the search
+     size of haystack using a linear algorithm with a smaller
+     coefficient.  However, avoid memchr for long needles, since we
+     can often achieve sublinear performance.  */
+  if (needle_len < LONG_NEEDLE_THRESHOLD)
+    {
+      haystack = memchr (haystack, *needle, haystack_len);
+      if (!haystack || __builtin_expect (needle_len == 1, 0))
+        return (void *) haystack;
+      haystack_len -= haystack - (const unsigned char *) haystack_start;
+      if (haystack_len < needle_len)
+        return NULL;
+      return two_way_short_needle (haystack, haystack_len, needle, needle_len);
+    }
+  else
+    return two_way_long_needle (haystack, haystack_len, needle, needle_len);
+}
+
+#undef LONG_NEEDLE_THRESHOLD
diff --git a/lib/str-two-way.h b/lib/str-two-way.h
new file mode 100644
index 00000000000..9155e6b5607
--- /dev/null
+++ b/lib/str-two-way.h
@@ -0,0 +1,452 @@
+/* Byte-wise substring search, using the Two-Way algorithm.
+   Copyright (C) 2008-2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Written by Eric Blake <ebb9@byu.net>, 2008.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3, or (at your option)
+   any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, see <https://www.gnu.org/licenses/>.  */
+
+/* Before including this file, you need to include <config.h> and
+   <string.h>, and define:
+     RESULT_TYPE             A macro that expands to the return type.
+     AVAILABLE(h, h_l, j, n_l)
+                             A macro that returns nonzero if there are
+                             at least N_L bytes left starting at H[J].
+                             H is 'unsigned char *', H_L, J, and N_L
+                             are 'size_t'; H_L is an lvalue.  For
+                             NUL-terminated searches, H_L can be
+                             modified each iteration to avoid having
+                             to compute the end of H up front.
+
+  For case-insensitivity, you may optionally define:
+     CMP_FUNC(p1, p2, l)     A macro that returns 0 iff the first L
+                             characters of P1 and P2 are equal.
+     CANON_ELEMENT(c)        A macro that canonicalizes an element right after
+                             it has been fetched from one of the two strings.
+                             The argument is an 'unsigned char'; the result
+                             must be an 'unsigned char' as well.
+
+  This file undefines the macros documented above, and defines
+  LONG_NEEDLE_THRESHOLD.
+*/
+
+#include <limits.h>
+#include <stdint.h>
+
+/* We use the Two-Way string matching algorithm (also known as
+   Chrochemore-Perrin), which guarantees linear complexity with
+   constant space.  Additionally, for long needles, we also use a bad
+   character shift table similar to the Boyer-Moore algorithm to
+   achieve improved (potentially sub-linear) performance.
+
+   See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260,
+   https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm,
+   https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.34.6641&rep=rep1&type=pdf
+*/
+
+/* Point at which computing a bad-byte shift table is likely to be
+   worthwhile.  Small needles should not compute a table, since it
+   adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a
+   speedup no greater than a factor of NEEDLE_LEN.  The larger the
+   needle, the better the potential performance gain.  On the other
+   hand, on non-POSIX systems with CHAR_BIT larger than eight, the
+   memory required for the table is prohibitive.  */
+#if CHAR_BIT < 10
+# define LONG_NEEDLE_THRESHOLD 32U
+#else
+# define LONG_NEEDLE_THRESHOLD SIZE_MAX
+#endif
+
+#ifndef MAX
+# define MAX(a, b) ((a < b) ? (b) : (a))
+#endif
+
+#ifndef CANON_ELEMENT
+# define CANON_ELEMENT(c) c
+#endif
+#ifndef CMP_FUNC
+# define CMP_FUNC memcmp
+#endif
+
+/* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN.
+   Return the index of the first byte in the right half, and set
+   *PERIOD to the global period of the right half.
+
+   The global period of a string is the smallest index (possibly its
+   length) at which all remaining bytes in the string are repetitions
+   of the prefix (the last repetition may be a subset of the prefix).
+
+   When NEEDLE is factored into two halves, a local period is the
+   length of the smallest word that shares a suffix with the left half
+   and shares a prefix with the right half.  All factorizations of a
+   non-empty NEEDLE have a local period of at least 1 and no greater
+   than NEEDLE_LEN.
+
+   A critical factorization has the property that the local period
+   equals the global period.  All strings have at least one critical
+   factorization with the left half smaller than the global period.
+   And while some strings have more than one critical factorization,
+   it is provable that with an ordered alphabet, at least one of the
+   critical factorizations corresponds to a maximal suffix.
+
+   Given an ordered alphabet, a critical factorization can be computed
+   in linear time, with 2 * NEEDLE_LEN comparisons, by computing the
+   shorter of two ordered maximal suffixes.  The ordered maximal
+   suffixes are determined by lexicographic comparison while tracking
+   periodicity.  */
+static size_t
+critical_factorization (const unsigned char *needle, size_t needle_len,
+                        size_t *period)
+{
+  /* Index of last byte of left half, or SIZE_MAX.  */
+  size_t max_suffix, max_suffix_rev;
+  size_t j; /* Index into NEEDLE for current candidate suffix.  */
+  size_t k; /* Offset into current period.  */
+  size_t p; /* Intermediate period.  */
+  unsigned char a, b; /* Current comparison bytes.  */
+
+  /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered
+     out 0-length needles.  */
+  if (needle_len < 3)
+    {
+      *period = 1;
+      return needle_len - 1;
+    }
+
+  /* Invariants:
+     0 <= j < NEEDLE_LEN - 1
+     -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed)
+     min(max_suffix, max_suffix_rev) < global period of NEEDLE
+     1 <= p <= global period of NEEDLE
+     p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j]
+     1 <= k <= p
+  */
+
+  /* Perform lexicographic search.  */
+  max_suffix = SIZE_MAX;
+  j = 0;
+  k = p = 1;
+  while (j + k < needle_len)
+    {
+      a = CANON_ELEMENT (needle[j + k]);
+      b = CANON_ELEMENT (needle[max_suffix + k]);
+      if (a < b)
+        {
+          /* Suffix is smaller, period is entire prefix so far.  */
+          j += k;
+          k = 1;
+          p = j - max_suffix;
+        }
+      else if (a == b)
+        {
+          /* Advance through repetition of the current period.  */
+          if (k != p)
+            ++k;
+          else
+            {
+              j += p;
+              k = 1;
+            }
+        }
+      else /* b < a */
+        {
+          /* Suffix is larger, start over from current location.  */
+          max_suffix = j++;
+          k = p = 1;
+        }
+    }
+  *period = p;
+
+  /* Perform reverse lexicographic search.  */
+  max_suffix_rev = SIZE_MAX;
+  j = 0;
+  k = p = 1;
+  while (j + k < needle_len)
+    {
+      a = CANON_ELEMENT (needle[j + k]);
+      b = CANON_ELEMENT (needle[max_suffix_rev + k]);
+      if (b < a)
+        {
+          /* Suffix is smaller, period is entire prefix so far.  */
+          j += k;
+          k = 1;
+          p = j - max_suffix_rev;
+        }
+      else if (a == b)
+        {
+          /* Advance through repetition of the current period.  */
+          if (k != p)
+            ++k;
+          else
+            {
+              j += p;
+              k = 1;
+            }
+        }
+      else /* a < b */
+        {
+          /* Suffix is larger, start over from current location.  */
+          max_suffix_rev = j++;
+          k = p = 1;
+        }
+    }
+
+  /* Choose the shorter suffix.  Return the index of the first byte of
+     the right half, rather than the last byte of the left half.
+
+     For some examples, 'banana' has two critical factorizations, both
+     exposed by the two lexicographic extreme suffixes of 'anana' and
+     'nana', where both suffixes have a period of 2.  On the other
+     hand, with 'aab' and 'bba', both strings have a single critical
+     factorization of the last byte, with the suffix having a period
+     of 1.  While the maximal lexicographic suffix of 'aab' is 'b',
+     the maximal lexicographic suffix of 'bba' is 'ba', which is not a
+     critical factorization.  Conversely, the maximal reverse
+     lexicographic suffix of 'a' works for 'bba', but not 'ab' for
+     'aab'.  The shorter suffix of the two will always be a critical
+     factorization.  */
+  if (max_suffix_rev + 1 < max_suffix + 1)
+    return max_suffix + 1;
+  *period = p;
+  return max_suffix_rev + 1;
+}
+
+/* Return the first location of non-empty NEEDLE within HAYSTACK, or
+   NULL.  HAYSTACK_LEN is the minimum known length of HAYSTACK.  This
+   method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD.
+   Performance is guaranteed to be linear, with an initialization cost
+   of 2 * NEEDLE_LEN comparisons.
+
+   If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
+   most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
+   If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
+   HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.  */
+static RETURN_TYPE
+two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
+                      const unsigned char *needle, size_t needle_len)
+{
+  size_t i; /* Index into current byte of NEEDLE.  */
+  size_t j; /* Index into current window of HAYSTACK.  */
+  size_t period; /* The period of the right half of needle.  */
+  size_t suffix; /* The index of the right half of needle.  */
+
+  /* Factor the needle into two halves, such that the left half is
+     smaller than the global period, and the right half is
+     periodic (with a period as large as NEEDLE_LEN - suffix).  */
+  suffix = critical_factorization (needle, needle_len, &period);
+
+  /* Perform the search.  Each iteration compares the right half
+     first.  */
+  if (CMP_FUNC (needle, needle + period, suffix) == 0)
+    {
+      /* Entire needle is periodic; a mismatch in the left half can
+         only advance by the period, so use memory to avoid rescanning
+         known occurrences of the period in the right half.  */
+      size_t memory = 0;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Scan for matches in right half.  */
+          i = MAX (suffix, memory);
+          while (i < needle_len && (CANON_ELEMENT (needle[i])
+                                    == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (memory < i + 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i + 1 < memory + 1)
+                return (RETURN_TYPE) (haystack + j);
+              /* No match, so remember how many repetitions of period
+                 on the right half were scanned.  */
+              j += period;
+              memory = needle_len - period;
+            }
+          else
+            {
+              j += i - suffix + 1;
+              memory = 0;
+            }
+        }
+    }
+  else
+    {
+      /* The two halves of needle are distinct; no extra memory is
+         required, and any mismatch results in a maximal shift.  */
+      period = MAX (suffix, needle_len - suffix) + 1;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Scan for matches in right half.  */
+          i = suffix;
+          while (i < needle_len && (CANON_ELEMENT (needle[i])
+                                    == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
+                                       == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i == SIZE_MAX)
+                return (RETURN_TYPE) (haystack + j);
+              j += period;
+            }
+          else
+            j += i - suffix + 1;
+        }
+    }
+  return NULL;
+}
+
+/* Return the first location of non-empty NEEDLE within HAYSTACK, or
+   NULL.  HAYSTACK_LEN is the minimum known length of HAYSTACK.  This
+   method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN.
+   Performance is guaranteed to be linear, with an initialization cost
+   of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations.
+
+   If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
+   most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching,
+   and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible.
+   If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
+   HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and
+   sublinear performance is not possible.  */
+static RETURN_TYPE
+two_way_long_needle (const unsigned char *haystack, size_t haystack_len,
+                     const unsigned char *needle, size_t needle_len)
+{
+  size_t i; /* Index into current byte of NEEDLE.  */
+  size_t j; /* Index into current window of HAYSTACK.  */
+  size_t period; /* The period of the right half of needle.  */
+  size_t suffix; /* The index of the right half of needle.  */
+  size_t shift_table[1U << CHAR_BIT]; /* See below.  */
+
+  /* Factor the needle into two halves, such that the left half is
+     smaller than the global period, and the right half is
+     periodic (with a period as large as NEEDLE_LEN - suffix).  */
+  suffix = critical_factorization (needle, needle_len, &period);
+
+  /* Populate shift_table.  For each possible byte value c,
+     shift_table[c] is the distance from the last occurrence of c to
+     the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE.
+     shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0.  */
+  for (i = 0; i < 1U << CHAR_BIT; i++)
+    shift_table[i] = needle_len;
+  for (i = 0; i < needle_len; i++)
+    shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1;
+
+  /* Perform the search.  Each iteration compares the right half
+     first.  */
+  if (CMP_FUNC (needle, needle + period, suffix) == 0)
+    {
+      /* Entire needle is periodic; a mismatch in the left half can
+         only advance by the period, so use memory to avoid rescanning
+         known occurrences of the period in the right half.  */
+      size_t memory = 0;
+      size_t shift;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Check the last byte first; if it does not match, then
+             shift to the next possible match location.  */
+          shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
+          if (0 < shift)
+            {
+              if (memory && shift < period)
+                {
+                  /* Since needle is periodic, but the last period has
+                     a byte out of place, there can be no match until
+                     after the mismatch.  */
+                  shift = needle_len - period;
+                }
+              memory = 0;
+              j += shift;
+              continue;
+            }
+          /* Scan for matches in right half.  The last byte has
+             already been matched, by virtue of the shift table.  */
+          i = MAX (suffix, memory);
+          while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len - 1 <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (memory < i + 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i + 1 < memory + 1)
+                return (RETURN_TYPE) (haystack + j);
+              /* No match, so remember how many repetitions of period
+                 on the right half were scanned.  */
+              j += period;
+              memory = needle_len - period;
+            }
+          else
+            {
+              j += i - suffix + 1;
+              memory = 0;
+            }
+        }
+    }
+  else
+    {
+      /* The two halves of needle are distinct; no extra memory is
+         required, and any mismatch results in a maximal shift.  */
+      size_t shift;
+      period = MAX (suffix, needle_len - suffix) + 1;
+      j = 0;
+      while (AVAILABLE (haystack, haystack_len, j, needle_len))
+        {
+          /* Check the last byte first; if it does not match, then
+             shift to the next possible match location.  */
+          shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
+          if (0 < shift)
+            {
+              j += shift;
+              continue;
+            }
+          /* Scan for matches in right half.  The last byte has
+             already been matched, by virtue of the shift table.  */
+          i = suffix;
+          while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
+                                        == CANON_ELEMENT (haystack[i + j])))
+            ++i;
+          if (needle_len - 1 <= i)
+            {
+              /* Scan for matches in left half.  */
+              i = suffix - 1;
+              while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
+                                       == CANON_ELEMENT (haystack[i + j])))
+                --i;
+              if (i == SIZE_MAX)
+                return (RETURN_TYPE) (haystack + j);
+              j += period;
+            }
+          else
+            j += i - suffix + 1;
+        }
+    }
+  return NULL;
+}
+
+#undef AVAILABLE
+#undef CANON_ELEMENT
+#undef CMP_FUNC
+#undef MAX
+#undef RETURN_TYPE