diff --git a/foo.h b/foo.h
new file mode 100644
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--- /dev/null
+++ b/foo.h
@@ -0,0 +1,158 @@
+/* bits.h - bits functions.
+ *
+ * Copyright (C) 2021 Bruno Raoult ("br")
+ * Licensed under the GNU General Public License v3.0 or later.
+ * Some rights reserved. See COPYING.
+ *
+ * You should have received a copy of the GNU General Public License along with this
+ * program. If not, see <https://www.gnu.org/licenses/gpl-3.0-standalone.htmlL>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later <https://spdx.org/licenses/GPL-3.0-or-later.html>
+ *
+ */
+#ifndef BITS_H
+#define BITS_H
+
+#include <stdint.h>
+
+/* next include will define __WORDSIZE: 32 or 64
+ */
+#include <bits/wordsize.h>
+#include "debug.h"
+
+#ifndef __has_builtin
+#define __has_builtin(x) 0
+#endif
+
+/* no plan to support 32bits for now...
+ */
+#if __WORDSIZE != 64
+ERROR_64_BYTES_WORDSIZE_ONLY
+#endif
+
+typedef uint64_t u64;
+typedef uint32_t u32;
+typedef uint16_t u16;
+typedef uint8_t  u8;
+typedef unsigned int uint;
+typedef unsigned char uchar;
+
+/* count trailing zeroes : 00101000 -> 3
+ *                              ^^^
+ */
+static inline int ctz64(u64 n)
+{
+#   if __has_builtin(__builtin_ctzl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin ctzl.\n");
+#   endif
+    return __builtin_ctzl(n);
+
+#   elif __has_builtin(__builtin_clzl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin clzl.\n");
+#   endif
+    return __WORDSIZE - (__builtin_clzl(n & -n) + 1);
+
+#   else
+#   ifdef DEBUG_BITS
+    log_f(1, "emulated.\n");
+#   endif
+    return popcount64((n & −n) − 1);
+#   endif
+}
+
+/* count leading zeroes : 00101000 -> 2
+ *                        ^^
+ */
+static inline int clz64(u64 n)
+{
+#   if __has_builtin(__builtin_clzl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin.\n");
+#   endif
+    return __builtin_clzl(n);
+
+#   else
+#   ifdef DEBUG_BITS
+    log_f(1, "emulated.\n");
+#   endif
+    u64 r, q;
+
+    r = (n > 0xFFFFFFFF) << 5; n >>= r;
+    q = (n > 0xFFFF)     << 4; n >>= q; r |= q;
+    q = (n > 0xFF  )     << 3; n >>= q; r |= q;
+    q = (n > 0xF   )     << 2; n >>= q; r |= q;
+    q = (n > 0x3   )     << 1; n >>= q; r |= q;
+    r |= (n >> 1);
+    return __WORDSIZE - r - 1;
+#   endif
+}
+
+/* find first set :  00101000 -> 4
+ *                       ^
+ */
+static inline uint ffs64(u64 n)
+{
+#   if __has_builtin(__builtin_ffsl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin ffsl.\n");
+#   endif
+    return __builtin_ffsll(n);
+
+#   elif __has_builtin(__builtin_ctzl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin ctzl.\n");
+#   endif
+    if (n == 0)
+        return (0);
+    return __builtin_ctzl(n) + 1;
+
+#   else
+#   ifdef DEBUG_BITS
+    log_f(1, "emulated.\n");
+#   endif
+    return popcount64(n ^ ~-n);
+#   endif
+}
+
+static inline int popcount64(u64 n)
+{
+#   if __has_builtin(__builtin_popcountl)
+#   ifdef DEBUG_BITS
+    log_f(1, "builtin.\n");
+#   endif
+    return __builtin_popcountl(n);
+
+#   else
+#   ifdef DEBUG_BITS
+    log_f(1, "emulated.\n");
+#   endif
+    int count = 0;
+    while (n) {
+        count++;
+        n &= (n - 1);
+    }
+    return count;
+#   endif
+}
+
+/** bit_for_each64 - iterate over an u64 bits
+ * @pos:        an int used as current bit
+ * @tmp:        a temp u64 used as temporary storage
+ * @ul:         the u64 to loop over
+ *
+ * Usage:
+ * u64 u=139, _t;     //  u=b10001011
+ * int cur;
+ * bit_for_each64(cur, _t, u) {
+ *     printf("%d\n", cur);
+ * }
+ * This will display the position of each bit in u: 1, 2, 4, 8
+ *
+ * I should probably re-think the implementation...
+ */
+#define bit_for_each64(pos, tmp, ul)                                    \
+    for (tmp = ul, pos = ffs64(tmp); pos; tmp &= (tmp - 1),  pos = ffs64(tmp))
+
+#endif  /* BITS_H */