1 files changed, 158 insertions, 0 deletions

diff --git a/lib/bitmap.c b/lib/bitmap.c
index a6939e18d7bb..c4cb48f77f0c 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -714,6 +714,164 @@ int bitmap_bitremap(int oldbit, const unsigned long *old,
 }
 EXPORT_SYMBOL(bitmap_bitremap);
 
+/**
+ * bitmap_onto - translate one bitmap relative to another
+ *      @dst: resulting translated bitmap
+ *      @orig: original untranslated bitmap
+ *      @relmap: bitmap relative to which translated
+ *      @bits: number of bits in each of these bitmaps
+ *
+ * Set the n-th bit of @dst iff there exists some m such that the
+ * n-th bit of @relmap is set, the m-th bit of @orig is set, and
+ * the n-th bit of @relmap is also the m-th _set_ bit of @relmap.
+ * (If you understood the previous sentence the first time your
+ * read it, you're overqualified for your current job.)
+ *
+ * In other words, @orig is mapped onto (surjectively) @dst,
+ * using the the map { <n, m> | the n-th bit of @relmap is the
+ * m-th set bit of @relmap }.
+ *
+ * Any set bits in @orig above bit number W, where W is the
+ * weight of (number of set bits in) @relmap are mapped nowhere.
+ * In particular, if for all bits m set in @orig, m >= W, then
+ * @dst will end up empty.  In situations where the possibility
+ * of such an empty result is not desired, one way to avoid it is
+ * to use the bitmap_fold() operator, below, to first fold the
+ * @orig bitmap over itself so that all its set bits x are in the
+ * range 0 <= x < W.  The bitmap_fold() operator does this by
+ * setting the bit (m % W) in @dst, for each bit (m) set in @orig.
+ *
+ * Example [1] for bitmap_onto():
+ *  Let's say @relmap has bits 30-39 set, and @orig has bits
+ *  1, 3, 5, 7, 9 and 11 set.  Then on return from this routine,
+ *  @dst will have bits 31, 33, 35, 37 and 39 set.
+ *
+ *  When bit 0 is set in @orig, it means turn on the bit in
+ *  @dst corresponding to whatever is the first bit (if any)
+ *  that is turned on in @relmap.  Since bit 0 was off in the
+ *  above example, we leave off that bit (bit 30) in @dst.
+ *
+ *  When bit 1 is set in @orig (as in the above example), it
+ *  means turn on the bit in @dst corresponding to whatever
+ *  is the second bit that is turned on in @relmap.  The second
+ *  bit in @relmap that was turned on in the above example was
+ *  bit 31, so we turned on bit 31 in @dst.
+ *
+ *  Similarly, we turned on bits 33, 35, 37 and 39 in @dst,
+ *  because they were the 4th, 6th, 8th and 10th set bits
+ *  set in @relmap, and the 4th, 6th, 8th and 10th bits of
+ *  @orig (i.e. bits 3, 5, 7 and 9) were also set.
+ *
+ *  When bit 11 is set in @orig, it means turn on the bit in
+ *  @dst corresponding to whatever is the twelth bit that is
+ *  turned on in @relmap.  In the above example, there were
+ *  only ten bits turned on in @relmap (30..39), so that bit
+ *  11 was set in @orig had no affect on @dst.
+ *
+ * Example [2] for bitmap_fold() + bitmap_onto():
+ *  Let's say @relmap has these ten bits set:
+ *              40 41 42 43 45 48 53 61 74 95
+ *  (for the curious, that's 40 plus the first ten terms of the
+ *  Fibonacci sequence.)
+ *
+ *  Further lets say we use the following code, invoking
+ *  bitmap_fold() then bitmap_onto, as suggested above to
+ *  avoid the possitility of an empty @dst result:
+ *
+ *      unsigned long *tmp;     // a temporary bitmap's bits
+ *
+ *      bitmap_fold(tmp, orig, bitmap_weight(relmap, bits), bits);
+ *      bitmap_onto(dst, tmp, relmap, bits);
+ *
+ *  Then this table shows what various values of @dst would be, for
+ *  various @orig's.  I list the zero-based positions of each set bit.
+ *  The tmp column shows the intermediate result, as computed by
+ *  using bitmap_fold() to fold the @orig bitmap modulo ten
+ *  (the weight of @relmap).
+ *
+ *      @orig           tmp            @dst
+ *      0                0             40
+ *      1                1             41
+ *      9                9             95
+ *      10               0             40 (*)
+ *      1 3 5 7          1 3 5 7       41 43 48 61
+ *      0 1 2 3 4        0 1 2 3 4     40 41 42 43 45
+ *      0 9 18 27        0 9 8 7       40 61 74 95
+ *      0 10 20 30       0             40
+ *      0 11 22 33       0 1 2 3       40 41 42 43
+ *      0 12 24 36       0 2 4 6       40 42 45 53
+ *      78 102 211       1 2 8         41 42 74 (*)
+ *
+ * (*) For these marked lines, if we hadn't first done bitmap_fold()
+ *     into tmp, then the @dst result would have been empty.
+ *
+ * If either of @orig or @relmap is empty (no set bits), then @dst
+ * will be returned empty.
+ *
+ * If (as explained above) the only set bits in @orig are in positions
+ * m where m >= W, (where W is the weight of @relmap) then @dst will
+ * once again be returned empty.
+ *
+ * All bits in @dst not set by the above rule are cleared.
+ */
+void bitmap_onto(unsigned long *dst, const unsigned long *orig,
+                        const unsigned long *relmap, int bits)
+{
+        int n, m;               /* same meaning as in above comment */
+
+        if (dst == orig)        /* following doesn't handle inplace mappings */
+                return;
+        bitmap_zero(dst, bits);
+
+        /*
+         * The following code is a more efficient, but less
+         * obvious, equivalent to the loop:
+         *      for (m = 0; m < bitmap_weight(relmap, bits); m++) {
+         *              n = bitmap_ord_to_pos(orig, m, bits);
+         *              if (test_bit(m, orig))
+         *                      set_bit(n, dst);
+         *      }
+         */
+
+        m = 0;
+        for (n = find_first_bit(relmap, bits);
+             n < bits;
+             n = find_next_bit(relmap, bits, n + 1)) {
+                /* m == bitmap_pos_to_ord(relmap, n, bits) */
+                if (test_bit(m, orig))
+                        set_bit(n, dst);
+                m++;
+        }
+}
+EXPORT_SYMBOL(bitmap_onto);
+
+/**
+ * bitmap_fold - fold larger bitmap into smaller, modulo specified size
+ *      @dst: resulting smaller bitmap
+ *      @orig: original larger bitmap
+ *      @sz: specified size
+ *      @bits: number of bits in each of these bitmaps
+ *
+ * For each bit oldbit in @orig, set bit oldbit mod @sz in @dst.
+ * Clear all other bits in @dst.  See further the comment and
+ * Example [2] for bitmap_onto() for why and how to use this.
+ */
+void bitmap_fold(unsigned long *dst, const unsigned long *orig,
+                        int sz, int bits)
+{
+        int oldbit;
+
+        if (dst == orig)        /* following doesn't handle inplace mappings */
+                return;
+        bitmap_zero(dst, bits);
+
+        for (oldbit = find_first_bit(orig, bits);
+             oldbit < bits;
+             oldbit = find_next_bit(orig, bits, oldbit + 1))
+                set_bit(oldbit % sz, dst);
+}
+EXPORT_SYMBOL(bitmap_fold);
+
 /*
  * Common code for bitmap_*_region() routines.
  *      bitmap: array of unsigned longs corresponding to the bitmap