New module 'integer_length'.

* lib/integer_length.h: New file.
* lib/integer_length.c: New file.
* modules/integer_length: New file.

diff --git a/ChangeLog b/ChangeLog
index 7530b73..03a78f7 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2011-10-14  Bruno Haible  <bruno@clisp.org>
+
+       New module 'integer_length'.
+       * lib/integer_length.h: New file.
+       * lib/integer_length.c: New file.
+       * modules/integer_length: New file.
+
 2011-10-14  Daniel Richard G.  <skunk@iskunk.org>  (tiny change)
 
        popen: Fix dependency conditions.

diff --git a/lib/integer_length.c b/lib/integer_length.c
new file mode 100644 (file)
index 0000000..e5ab665
--- /dev/null
+++ b/lib/integer_length.c
@@ -0,0 +1,141 @@
+/* integer_length - find most significant bit in an 'unsigned int'.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+
+   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 of the License, 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 <http://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2011.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "integer_length.h"
+
+#include <limits.h>
+
+#include "float+.h"
+
+#define NBITS (sizeof (unsigned int) * CHAR_BIT)
+
+int
+integer_length (unsigned int x)
+{
+#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
+  if (x == 0)
+    return 0;
+  else
+    return NBITS - __builtin_clz (x);
+#else
+# if defined DBL_EXPBIT0_WORD && defined DBL_EXPBIT0_BIT
+  if (NBITS <= DBL_MANT_BIT)
+    {
+      /* Use 'double' operations.
+         Assumes an IEEE 754 'double' implementation.  */
+#  define DBL_EXP_MASK ((DBL_MAX_EXP - DBL_MIN_EXP) | 7)
+#  define DBL_EXP_BIAS (DBL_EXP_MASK / 2 - 1)
+#  define NWORDS \
+    ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
+      typedef union { double value; unsigned int word[NWORDS]; }
+              memory_double;
+
+      if (x == 0)
+        return 0;
+      else
+        {
+          memory_double m;
+          unsigned int exponent;
+
+          if (1)
+            {
+              /* Use a single integer to floating-point conversion.  */
+              m.value = x;
+            }
+          else
+            {
+              /* Use a single floating-point subtraction.  */
+              /* 2^(DBL_MANT_DIG-1).  */
+              static const double TWO_DBL_MANT_DIG =
+                /* Assume DBL_MANT_DIG <= 5 * 31.
+                   Use the identity
+                   n = floor(n/5) + floor((n+1)/5) + ... + floor((n+4)/5).  */
+                (double) (1U << ((DBL_MANT_DIG - 1) / 5))
+                * (double) (1U << ((DBL_MANT_DIG - 1 + 1) / 5))
+                * (double) (1U << ((DBL_MANT_DIG - 1 + 2) / 5))
+                * (double) (1U << ((DBL_MANT_DIG - 1 + 3) / 5))
+                * (double) (1U << ((DBL_MANT_DIG - 1 + 4) / 5));
+
+              /* Construct 2^(DBL_MANT_DIG-1) + x by hand.  */
+              m.word[DBL_EXPBIT0_WORD] =
+                (DBL_MANT_DIG + DBL_EXP_BIAS) << DBL_EXPBIT0_BIT;
+              m.word[1 - DBL_EXPBIT0_WORD] = x;
+
+              /* Subtract 2^(DBL_MANT_DIG-1).  */
+              m.value = m.value - TWO_DBL_MANT_DIG;
+            }
+
+          exponent =
+            (m.word[DBL_EXPBIT0_WORD] >> DBL_EXPBIT0_BIT) & DBL_EXP_MASK;
+          return exponent - DBL_EXP_BIAS;
+        }
+    }
+  else
+# endif
+    if (NBITS == 32)
+      {
+        /* 6 comparisons.  */
+        if (x != 0)
+          {
+            int result = 1;
+            if (x >= 0x10000)
+              {
+                x = x >> 16;
+                result += 16;
+              }
+            if (x >= 0x100)
+              {
+                x = x >> 8;
+                result += 8;
+              }
+            if (x >= 0x10)
+              {
+                x = x >> 4;
+                result += 4;
+              }
+            if (x >= 0x4)
+              {
+                x = x >> 2;
+                result += 2;
+              }
+            if (x >= 0x2)
+              {
+                x = x >> 1;
+                result += 1;
+              }
+            return result;
+          }
+        else
+          return 0;
+      }
+    else
+      {
+        /* Naive loop.
+           Works for any value of NBITS.  */
+        int j;
+
+        for (j = NBITS - 1; j >= 0; j--)
+          if (x & (1U << j))
+            return j + 1;
+        return 0;
+      }
+#endif
+}

diff --git a/lib/integer_length.h b/lib/integer_length.h
new file mode 100644 (file)
index 0000000..d51130f
--- /dev/null
+++ b/lib/integer_length.h
@@ -0,0 +1,49 @@
+/* integer_length - find most significant bit in an unsigned integer.
+   Copyright (C) 2011 Free Software Foundation, Inc.
+
+   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 of the License, 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 <http://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2011.  */
+
+#ifndef _INTEGER_LENGTH_H
+#define _INTEGER_LENGTH_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* These functions return the minimum number of bits required to represent
+   the given unsigned integer.
+   For non-zero values, this is the position of the most significant bit
+   that is set, plus one.  For zero, it is 0.  */
+
+/* Returns the integer length of x.
+   The result is >= 0, <= sizeof (unsigned int) * CHAR_BIT.  */
+extern int integer_length (unsigned int x);
+
+/* Returns the integer length of x.
+   The result is >= 0, <= sizeof (unsigned long) * CHAR_BIT.  */
+extern int integer_length_l (unsigned long x);
+
+#if HAVE_UNSIGNED_LONG_LONG_INT
+/* Returns the integer length of x.
+   The result is >= 0, <= sizeof (unsigned long long) * CHAR_BIT.  */
+extern int integer_length_ll (unsigned long long x);
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _INTEGER_LENGTH_H */

diff --git a/modules/integer_length b/modules/integer_length
new file mode 100644 (file)
index 0000000..e757772
--- /dev/null
+++ b/modules/integer_length
@@ -0,0 +1,28 @@
+Description:
+Finds the most significant bit in an 'unsigned int'.
+
+Files:
+lib/integer_length.h
+lib/integer_length.c
+lib/float+.h
+m4/longlong.m4
+m4/exponentd.m4
+
+Depends-on:
+float
+
+configure.ac:
+AC_REQUIRE([AC_TYPE_UNSIGNED_LONG_LONG_INT])
+AC_REQUIRE([gl_DOUBLE_EXPONENT_LOCATION])
+
+Makefile.am:
+lib_SOURCES += integer_length.c
+
+Include:
+"integer_length.h"
+
+License:
+LGPLv2+
+
+Maintainer:
+Bruno Haible