Ticket #9352: dynamic_bitset.patch

dynamic_bitset.hpp

@@ -295 +295 @@
     // lookup
     size_type find_first() const;
     size_type find_next(size_type pos) const;
+    size_type find_last() const;
+    size_type find_prev(size_type pos) const;
 
 
 #if !defined BOOST_DYNAMIC_BITSET_DONT_USE_FRIENDS
@@ -336 +338 @@
     bool m_check_invariants() const;
 
     size_type m_do_find_from(size_type first_block) const;
+    size_type m_do_find_downto(size_type last_block) const;
 
     block_width_type count_extra_bits() const { return bit_index(size()); }
     static size_type block_index(size_type pos) { return pos / bits_per_block; }
@@ -1234 +1237 @@
 
 }
 
+// look for the nearest bit "on", starting
+// from the block with index last_block downto first block
+//
+template <typename Block, typename Allocator>
+typename dynamic_bitset<Block, Allocator>::size_type
+dynamic_bitset<Block, Allocator>::m_do_find_downto(size_type last_block) const
+{
+    size_type i = last_block;
+
+    // skip null blocks
+    while (i >= 0 && i != npos && m_bits[i] == 0)
+        --i;
+
+    if (i < 0 || i == npos)
+        return npos; // not found
+
+    return i * bits_per_block + boost::integer_log2(m_bits[i]);
+
+}
+
 
 template <typename Block, typename Allocator>
 typename dynamic_bitset<Block, Allocator>::size_type
@@ -1267 +1290 @@
 
 }
 
+template <typename Block, typename Allocator>
+typename dynamic_bitset<Block, Allocator>::size_type
+dynamic_bitset<Block, Allocator>::find_last() const
+{
+    return m_do_find_downto(num_blocks()-1);
+}
+
+template <typename Block, typename Allocator>
+typename dynamic_bitset<Block, Allocator>::size_type
+dynamic_bitset<Block, Allocator>::find_prev(size_type pos) const
+{
+
+    const size_type sz = size();
+    if (pos <= 0 || pos == npos || pos >= sz || sz == 0)
+        return npos;
+
+    --pos;
+
+    const size_type blk = block_index(pos);
+    const block_width_type ind = bit_index(pos);
+    const size_type mask =  ind == 0 ? 1 : (1 << ind) | ((1 << ind)-1);
+    // mask out bits after pos
+    const Block fore = m_bits[blk] & mask;
+
+    return fore?
+        blk * bits_per_block + boost::integer_log2(fore)
+        :
+        m_do_find_downto(blk - 1);
+
+}
 
 
 //-----------------------------------------------------------------------------