diff --git a/Makefile.in b/Makefile.in
index 0dd6c99..d09c873 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -31,6 +31,7 @@ SOURCE=\
 	base/error_state.cc \
 	base/error_string.cc \
 	base/progress_monitor.cc \
+	base/rolling_hash.cc \
 	base/xml_utils.cc \
 	block-cache/block_cache.cc \
 	caching/cache_check.cc \
@@ -72,6 +73,7 @@ SOURCE=\
 	persistent-data/transaction_manager.cc \
 	persistent-data/validators.cc \
 	thin-provisioning/cache_stream.cc \
+	thin-provisioning/chunk_stream.cc \
 	thin-provisioning/device_tree.cc \
 	thin-provisioning/human_readable_format.cc \
 	thin-provisioning/mapping_tree.cc \
@@ -92,6 +94,7 @@ SOURCE=\
 	thin-provisioning/thin_rmap.cc \
 	thin-provisioning/thin_show_duplicates.cc \
 	thin-provisioning/thin_trim.cc \
+	thin-provisioning/variable_chunk_stream.cc \
 	thin-provisioning/xml_format.cc
 
 CC:=@CC@
diff --git a/base/rolling_hash.cc b/base/rolling_hash.cc
new file mode 100644
index 0000000..1ea362f
--- /dev/null
+++ b/base/rolling_hash.cc
@@ -0,0 +1,148 @@
+#include "base/rolling_hash.h"
+
+using namespace base;
+using namespace boost;
+using namespace std;
+
+//----------------------------------------------------------------
+
+namespace {
+	uint32_t MULTIPLIER = 4294967291UL;
+	uint32_t SEED = 123;
+}
+
+rolling_hash::rolling_hash(unsigned window_size)
+	: a_(MULTIPLIER),
+	  a_to_k_minus_1_(a_),
+	  window_size_(window_size) {
+
+	for (unsigned i = 1; i < window_size_ - 1; i++)
+		a_to_k_minus_1_ *= a_;
+
+	reset();
+}
+
+void
+rolling_hash::reset()
+{
+	// prime with zeroes
+	chars_.clear();
+
+	hash_ = 0;
+	for (unsigned i = 0; i < window_size_; i++) {
+		hash_ = (hash_ * a_) + SEED;
+		chars_.push_back(0);
+	}
+}
+
+uint32_t
+rolling_hash::step(uint8_t byte)
+{
+	update_hash(byte);
+	return hash_;
+}
+
+uint32_t
+rolling_hash::get_hash() const
+{
+	return hash_;
+}
+
+void
+rolling_hash::update_hash(uint8_t byte)
+{
+	hash_ -= a_to_k_minus_1_ * (chars_.front() + SEED);
+	chars_.pop_front();
+	chars_.push_back(byte);
+	hash_ = (hash_ * a_) + byte + SEED;
+}
+
+//--------------------------------
+
+content_based_hash::content_based_hash(unsigned window_size)
+	: rhash_(window_size),
+
+	  // FIXME: hard coded values
+	  backup_div_((window_size / 4) - 1),
+	  div_((window_size / 2) - 1),
+	  min_len_(window_size / 8),
+	  max_len_(window_size),
+	  len_(0)
+{
+}
+
+void
+content_based_hash::reset()
+{
+	len_ = 0;
+	backup_break_.reset();
+	rhash_.reset();
+}
+
+optional<unsigned>
+content_based_hash::step(uint8_t byte)
+{
+#if 0
+	optional<unsigned> r;
+
+	rhash_.step(byte);
+	len_++;
+
+	if (len_ < min_len_)
+		return r;
+
+	if (hit_break(backup_div_))
+		backup_break_ = len_;
+
+	if (hit_break(div_)) {
+		// found a break
+		r = len_;
+		len_ = 0;
+		backup_break_.reset();
+
+	} else if (len_ >= max_len_) {
+		// too big, is there a backup?
+		if (backup_break_) {
+			len_ -= *backup_break_;
+			r = backup_break_;
+			backup_break_.reset();
+
+		} else {
+			r = len_;
+			len_ = 0;
+		}
+	}
+
+	return r;
+#else
+	optional<unsigned> r;
+
+	rhash_.step(byte);
+	len_++;
+
+	if (len_ < min_len_)
+		return r;
+
+	if (hit_break(div_)) {
+		// found a break
+		r = len_;
+		len_ = 0;
+		backup_break_.reset();
+
+	} else if (len_ >= max_len_) {
+		r = len_;
+		len_ = 0;
+	}
+
+	return r;
+#endif
+}
+
+bool
+content_based_hash::hit_break(uint32_t mask) const
+{
+	uint32_t h = rhash_.get_hash() >> 8;
+	return !(h & mask);
+}
+
+//----------------------------------------------------------------
diff --git a/base/rolling_hash.h b/base/rolling_hash.h
new file mode 100644
index 0000000..d44012a
--- /dev/null
+++ b/base/rolling_hash.h
@@ -0,0 +1,61 @@
+#ifndef BASE_ROLLING_HASH_H
+#define BASE_ROLLING_HASH_H
+
+#include <list>
+#include <stdint.h>
+#include <boost/optional.hpp>
+
+//----------------------------------------------------------------
+
+namespace base {
+	class rolling_hash {
+	public:
+		rolling_hash(unsigned window_size);
+
+		void reset();
+
+		// Returns the current hash
+		uint32_t step(uint8_t byte);
+
+		uint32_t get_hash() const;
+
+	private:
+		void update_hash(uint8_t byte);
+
+		uint32_t a_;
+		uint32_t a_to_k_minus_1_;
+
+		// FIXME: use a ring buffer
+		std::list<uint8_t> chars_;
+
+		uint32_t hash_;
+		uint32_t window_size_;
+	};
+
+	class content_based_hash {
+	public:
+		content_based_hash(unsigned window_size);
+		void reset();
+
+		// Returns a break point relative to the last reset/break.
+		boost::optional<unsigned> step(uint8_t byte);
+
+	private:
+		bool hit_break(uint32_t div) const;
+
+		rolling_hash rhash_;
+
+		uint32_t backup_div_;
+		uint32_t div_;
+
+		unsigned min_len_;
+		unsigned max_len_;
+
+		unsigned len_;
+		boost::optional<unsigned> backup_break_;
+	};
+}
+
+//----------------------------------------------------------------
+
+#endif
diff --git a/thin-provisioning/cache_stream.cc b/thin-provisioning/cache_stream.cc
index 956fc4e..8fbcc72 100644
--- a/thin-provisioning/cache_stream.cc
+++ b/thin-provisioning/cache_stream.cc
@@ -32,9 +32,8 @@ cache_stream::cache_stream(string const &path,
 	  v_(new bcache::noop_validator()),
 	  cache_(new block_cache(fd_, block_size / 512, nr_blocks_, cache_mem)),
 	  current_index_(0) {
-	load(0);
-	for (block_address i = 1; i < min(cache_blocks_, nr_blocks_); i++)
-		cache_->prefetch(i);
+
+	rewind();
 }
 
 block_address
@@ -46,19 +45,27 @@ cache_stream::nr_chunks() const
 void
 cache_stream::rewind()
 {
-	load(0);
+	current_index_ = 0;
+
+	for (block_address i = 1; i < min(cache_blocks_, nr_blocks_); i++)
+		cache_->prefetch(i);
 }
 
 bool
-cache_stream::advance(block_address count)
+cache_stream::next(block_address count)
 {
-	if (current_index_ + count >= nr_blocks_)
-		return false;
+	current_index_ = min(current_index_ + count, nr_blocks_);
 
-	current_index_ += count;
+	if (current_index_ + cache_blocks_ < nr_blocks_)
+		cache_->prefetch(current_index_ + cache_blocks_);
 
-	load(current_index_);
-	return true;
+	return !eof();
+}
+
+bool
+cache_stream::eof() const
+{
+	return current_index_ >= nr_blocks_;
 }
 
 block_address
@@ -68,24 +75,26 @@ cache_stream::index() const
 }
 
 chunk const &
-cache_stream::get() const
+cache_stream::get()
 {
-	return current_chunk_;
+	chunk_wrapper *w = new chunk_wrapper(*this);
+	return w->c_;
 }
 
 void
-cache_stream::load(block_address b)
+cache_stream::put(chunk const &c)
 {
-	current_index_ = b;
-	current_block_ = cache_->get(current_index_, 0, v_);
+	chunk_wrapper *w = container_of(const_cast<chunk *>(&c), chunk_wrapper, c_);
+	delete w;
+}
 
-	current_chunk_.offset_sectors_ = (b * block_size_) / 512;
-	current_chunk_.mem_.clear();
-	current_chunk_.mem_.push_back(mem(static_cast<uint8_t *>(current_block_.get_data()),
-					  static_cast<uint8_t *>(current_block_.get_data()) + block_size_));
-
-	if (current_index_ + cache_blocks_ < nr_blocks_)
-		cache_->prefetch(current_index_ + cache_blocks_);
+cache_stream::chunk_wrapper::chunk_wrapper(cache_stream &parent)
+	: block_(parent.cache_->get(parent.current_index_, 0, parent.v_))
+{
+	c_.offset_ = parent.current_index_ * parent.block_size_;
+	c_.len_ = parent.block_size_;
+	c_.mem_.push_back(mem(static_cast<uint8_t *>(block_.get_data()),
+			      static_cast<uint8_t *>(block_.get_data()) + parent.block_size_));
 }
 
 //----------------------------------------------------------------
diff --git a/thin-provisioning/cache_stream.h b/thin-provisioning/cache_stream.h
index cfe6ca8..65c81b1 100644
--- a/thin-provisioning/cache_stream.h
+++ b/thin-provisioning/cache_stream.h
@@ -14,25 +14,36 @@ namespace thin_provisioning {
 			     block_address block_size,
 			     size_t cache_mem);
 
-		virtual block_address nr_chunks() const;
+		block_address nr_chunks() const;
+
 		virtual void rewind();
-		virtual bool advance(block_address count = 1ull);
 		virtual block_address index() const;
-		virtual chunk const &get() const;
+
+		virtual bool next(block_address count = 1ull);
+		virtual bool eof() const;
+
+		virtual chunk const &get();
+		virtual void put(chunk const &c);
 
 	private:
-		void load(block_address b);
+		struct chunk_wrapper {
+			chunk_wrapper(cache_stream &parent);
+
+			block_cache::auto_block block_;
+			chunk c_;
+		};
+
+		friend class chunk_wrapper;
 
 		block_address block_size_;
 		block_address nr_blocks_;
 		block_address cache_blocks_;
+
 		int fd_;
 		validator::ptr v_;
 		std::auto_ptr<block_cache> cache_;
 
 		block_address current_index_;
-		block_cache::auto_block current_block_;
-		chunk current_chunk_;
 	};
 }
 
diff --git a/thin-provisioning/chunk_stream.cc b/thin-provisioning/chunk_stream.cc
new file mode 100644
index 0000000..4ac99ff
--- /dev/null
+++ b/thin-provisioning/chunk_stream.cc
@@ -0,0 +1,23 @@
+#include "thin-provisioning/chunk_stream.h"
+
+using namespace std;
+using namespace thin_provisioning;
+
+//----------------------------------------------------------------
+
+uint8_t
+chunk::operator[](uint64_t n) const
+{
+	std::deque<mem>::const_iterator it;
+	for (it = mem_.begin(); it != mem_.end(); it++) {
+		uint64_t mem_len = it->end - it->begin;
+		if (n > mem_len)
+			n -= mem_len;
+		else
+			return it->begin[n];
+	}
+
+	throw runtime_error("chunk out of bounds");
+}
+
+//----------------------------------------------------------------
diff --git a/thin-provisioning/chunk_stream.h b/thin-provisioning/chunk_stream.h
index d87ee9c..4e1ea96 100644
--- a/thin-provisioning/chunk_stream.h
+++ b/thin-provisioning/chunk_stream.h
@@ -37,21 +37,24 @@ namespace thin_provisioning {
 	};
 
 	struct chunk {
-		// FIXME: switch to bytes rather than sectors
-		// FIXME: add length too
-		uint64_t offset_sectors_;
+		uint64_t offset_, len_;
 		std::deque<mem> mem_;
+
+		uint8_t operator[](uint64_t n) const;
 	};
 
 	class chunk_stream {
 	public:
 		virtual ~chunk_stream() {}
 
-		virtual bcache::block_address nr_chunks() const = 0;
 		virtual void rewind() = 0;
-		virtual bool advance(bcache::block_address count = 1ull) = 0;
 		virtual bcache::block_address index() const = 0;
-		virtual chunk const &get() const = 0;
+
+		virtual bool next(bcache::block_address count = 1ull) = 0;
+		virtual bool eof() const = 0;
+
+		virtual chunk const &get() = 0;
+		virtual void put(chunk const &c) = 0;
 	};
 }
 
diff --git a/thin-provisioning/pool_stream.cc b/thin-provisioning/pool_stream.cc
index 0191698..21964f9 100644
--- a/thin-provisioning/pool_stream.cc
+++ b/thin-provisioning/pool_stream.cc
@@ -60,7 +60,7 @@ pool_stream::rewind()
 }
 
 bool
-pool_stream::advance(block_address count)
+pool_stream::next(block_address count)
 {
 	while (count--)
 		if (!advance_one())
@@ -69,6 +69,12 @@ pool_stream::advance(block_address count)
 	return true;
 }
 
+bool
+pool_stream::eof() const
+{
+	return stream_.eof();
+}
+
 block_address
 pool_stream::index() const
 {
@@ -76,12 +82,16 @@ pool_stream::index() const
 }
 
 chunk const &
-pool_stream::get() const
+pool_stream::get()
 {
 	return stream_.get();
 }
 
-
+void
+pool_stream::put(chunk const &c)
+{
+	stream_.put(c);
+}
 
 // FIXME: too big to return by value
 vector<pool_stream::rmap_region>
@@ -140,7 +150,7 @@ pool_stream::advance_one()
 	if (new_index >= nr_chunks())
 		return false;
 
-	return stream_.advance(new_index - index());
+	return stream_.next(new_index - index());
 }
 
 //----------------------------------------------------------------
diff --git a/thin-provisioning/pool_stream.h b/thin-provisioning/pool_stream.h
index 1fa5c51..71576ed 100644
--- a/thin-provisioning/pool_stream.h
+++ b/thin-provisioning/pool_stream.h
@@ -34,16 +34,20 @@ namespace thin_provisioning {
 
 		block_address nr_chunks() const;
 		void rewind();
-		bool advance(block_address count = 1ull);
+		bool next(block_address count = 1ull);
+		bool eof() const;
 		block_address index() const;
-		chunk const &get() const;
+
+		chunk const &get();
+		void put(chunk const &c);
 
 	private:
 		typedef rmap_visitor::region region;
 		typedef rmap_visitor::rmap_region rmap_region;
 
 		// FIXME: too big to return by value
-		vector<rmap_region> read_rmap(transaction_manager::ptr tm, superblock_detail::superblock const &sb,
+		vector<rmap_region> read_rmap(transaction_manager::ptr tm,
+					      superblock_detail::superblock const &sb,
 					      block_address nr_blocks);
 		void init_rmap(transaction_manager::ptr tm, superblock_detail::superblock const &sb,
 			       block_address nr_blocks);
diff --git a/thin-provisioning/thin_show_duplicates.cc b/thin-provisioning/thin_show_duplicates.cc
index 7819ca1..dafcd34 100644
--- a/thin-provisioning/thin_show_duplicates.cc
+++ b/thin-provisioning/thin_show_duplicates.cc
@@ -36,6 +36,7 @@
 #include "thin-provisioning/mapping_tree.h"
 #include "thin-provisioning/rmap_visitor.h"
 #include "thin-provisioning/superblock.h"
+#include "thin-provisioning/variable_chunk_stream.h"
 
 #include <boost/uuid/sha1.hpp>
 #include <boost/lexical_cast.hpp>
@@ -98,25 +99,21 @@ namespace {
 		unsigned cache_mem;
 	};
 
-	// FIXME: introduce abstraction for a stream of segments
-
 	using namespace mapping_tree_detail;
 
 	class duplicate_counter {
 	public:
-		duplicate_counter(block_address nr_blocks)
-		: counts_(nr_blocks),
-		  non_zero_dups_(0),
+		duplicate_counter()
+		: non_zero_dups_(0),
 		  zero_dups_(0) {
 		}
 
-		void add_duplicate(block_address b1, block_address b2) {
-			non_zero_dups_++;
-			counts_[b1]++;
+		void add_duplicate(block_address len) {
+			non_zero_dups_ += len;
 		}
 
-		void add_zero_duplicate(block_address b) {
-			zero_dups_++;
+		void add_zero_duplicate(block_address len) {
+			zero_dups_ += len;
 		}
 
 		block_address get_total() const {
@@ -132,45 +129,35 @@ namespace {
 		}
 
 	private:
-		vector<block_address> counts_;
 		block_address non_zero_dups_;
 		block_address zero_dups_;
 	};
 
 	class duplicate_detector {
 	public:
-		duplicate_detector(unsigned block_size, block_address nr_blocks)
-			: block_size_(block_size),
-			  results_(nr_blocks),
-			  zero_fingerprint_(5, 0ull) {
-			calc_zero_fingerprint();
-		}
-
 		void examine(chunk const &c) {
-			digestor_.reset();
-
-			for (deque<mem>::const_iterator it = c.mem_.begin(); it != c.mem_.end(); it++)
-				digestor_.process_bytes(it->begin, it->end - it->begin);
-
-			unsigned int digest[5];
-			digestor_.get_digest(digest);
-
-			// hack
-			vector<unsigned int> v(5);
-			for (unsigned i = 0; i < 5; i++)
-				v[i] = digest[i];
-
-			block_address index = (c.offset_sectors_ * 512) / block_size_;
-
-			if (v == zero_fingerprint_)
-				results_.add_zero_duplicate(index);
+			if (all_zeroes(c))
+				results_.add_zero_duplicate(c.len_);
 
 			else {
+				digestor_.reset();
+
+				for (deque<mem>::const_iterator it = c.mem_.begin(); it != c.mem_.end(); it++)
+					digestor_.process_bytes(it->begin, it->end - it->begin);
+
+				unsigned int digest[5];
+				digestor_.get_digest(digest);
+
+				// hack
+				vector<unsigned int> v(5);
+				for (unsigned i = 0; i < 5; i++)
+					v[i] = digest[i];
+
 				fingerprint_map::const_iterator it = fm_.find(v);
 				if (it != fm_.end()) {
-					results_.add_duplicate(it->second, index);
+					results_.add_duplicate(c.len_);
 				} else
-					fm_.insert(make_pair(v, index));
+					fm_.insert(make_pair(v, c.offset_));
 			}
 		}
 
@@ -178,30 +165,24 @@ namespace {
 			return results_;
 		}
 
-		void calc_zero_fingerprint() {
-			auto_ptr<uint8_t> bytes(new uint8_t[block_size_]);
-			memset(bytes.get(), 0, block_size_);
+	private:
+		bool all_zeroes(chunk const &c) const {
+			for (deque<mem>::const_iterator it = c.mem_.begin(); it != c.mem_.end(); it++) {
+				for (uint8_t *ptr = it->begin; ptr != it->end; ptr++) {
+					if (*ptr != 0)
+						return false;
+				}
+			}
 
-			digestor_.reset();
-			digestor_.process_bytes(bytes.get(), block_size_);
-
-			unsigned int digest[5];
-			digestor_.get_digest(digest);
-
-			// hack
-			for (unsigned i = 0; i < 5; i++)
-				zero_fingerprint_[i] = digest[i];
+			return true;
 		}
 
-	private:
 		typedef map<vector<unsigned int>, block_address> fingerprint_map;
 
 		unsigned block_size_;
 		boost::uuids::detail::sha1 digestor_;
 		fingerprint_map fm_;
 		duplicate_counter results_;
-
-		vector<unsigned int> zero_fingerprint_;
 	};
 
 	int show_dups_pool(flags const &fs) {
@@ -218,15 +199,16 @@ namespace {
 		cache_stream stream(fs.data_dev, block_size, fs.cache_mem);
 		pool_stream pstream(stream, tm, sb, nr_blocks);
 
-		duplicate_detector detector(block_size, nr_blocks);
+		duplicate_detector detector;
 		auto_ptr<progress_monitor> pbar = create_progress_bar("Examining data");
 
 		do {
 			chunk const &c = pstream.get();
 			detector.examine(c);
+			pstream.put(c);
 			pbar->update_percent((pstream.index() * 100) / pstream.nr_chunks());
 
-		} while (pstream.advance());
+		} while (pstream.next());
 		pbar->update_percent(100);
 
 		cout << "\n\ntotal dups: " << detector.get_results().get_total() << endl;
@@ -247,24 +229,27 @@ namespace {
 		cerr << "nr_blocks = " << nr_blocks << "\n";
 		cerr << "block size = " << block_size << "\n";
 
-		cache_stream stream(fs.data_dev, block_size, fs.cache_mem);
-		duplicate_detector detector(block_size, nr_blocks);
+		cache_stream low_level_stream(fs.data_dev, block_size, fs.cache_mem);
+		variable_chunk_stream stream(low_level_stream, 4096);
+		duplicate_detector detector;
 
 		auto_ptr<progress_monitor> pbar = create_progress_bar("Examining data");
 		do {
+			// FIXME: use a wrapper class to automate the put()
 			chunk const &c = stream.get();
 			detector.examine(c);
-			pbar->update_percent((stream.index() * 100) / stream.nr_chunks());
+			stream.put(c);
+//			pbar->update_percent((stream.index() * 100) / stream.nr_chunks());
 
-		} while (stream.advance());
+		} while (stream.next());
 		pbar->update_percent(100);
 
-		cout << "\n\ntotal dups: " << detector.get_results().get_total() << endl;
-		cout << (detector.get_results().get_total() * 100) / nr_blocks << "% duplicates\n";
-
 		duplicate_counter r = detector.get_results();
-		cout << "\n\nchunks\tnon zero dups\tzero dups\n"
-		     << nr_blocks << "\t" << r.get_non_zeroes() << "\t" << r.get_zeroes() << "\n";
+		block_address meg = 1024 * 1024;
+		cout << "\n\n"
+		     << (nr_blocks * block_size) / meg << "m examined, "
+		     << r.get_non_zeroes() / meg << "m duplicates, "
+		     << r.get_zeroes() / meg << "m zeroes\n";
 
 		return 0;
 	}
diff --git a/thin-provisioning/variable_chunk_stream.cc b/thin-provisioning/variable_chunk_stream.cc
new file mode 100644
index 0000000..9a9d11e
--- /dev/null
+++ b/thin-provisioning/variable_chunk_stream.cc
@@ -0,0 +1,152 @@
+#include "thin-provisioning/variable_chunk_stream.h"
+
+using namespace boost;
+using namespace std;
+using namespace thin_provisioning;
+
+//----------------------------------------------------------------
+
+variable_chunk_stream::variable_chunk_stream(chunk_stream &stream, unsigned window_size)
+	: index_(0),
+	  h_(window_size),
+	  stream_(stream),
+	  big_chunk_(0) {
+	next_big_chunk();
+}
+
+variable_chunk_stream::~variable_chunk_stream()
+{
+	put_big_chunk();
+}
+
+void
+variable_chunk_stream::rewind()
+{
+	// FIXME: not complete
+	index_ = 0;
+	stream_.rewind();
+	h_.reset();
+}
+
+bool
+variable_chunk_stream::next(bcache::block_address count)
+{
+	while (count--) {
+		index_++;
+		advance_one();
+	}
+
+	return !eof();
+}
+
+bool
+variable_chunk_stream::eof() const
+{
+	return stream_.eof();
+}
+
+bcache::block_address
+variable_chunk_stream::index() const
+{
+	return index_;
+}
+
+chunk const &
+variable_chunk_stream::get()
+{
+	assert(big_chunk_);
+
+	little_chunk_.len_ = little_e_ - little_b_;
+	little_chunk_.offset_ = big_chunk_->offset_ + little_chunk_.len_;
+
+	little_chunk_.mem_.clear();
+	little_chunk_.mem_.push_back(mem(little_b_, little_e_));
+
+	return little_chunk_;
+}
+
+void
+variable_chunk_stream::put(chunk const &c)
+{
+	// noop
+}
+
+bool
+variable_chunk_stream::next_big_chunk()
+{
+	put_big_chunk();
+
+	if (!stream_.next())
+		return false;
+
+	big_chunk_ = &stream_.get();
+	little_b_ = little_e_ = big_chunk_->mem_.front().begin;
+	h_.reset();
+
+	return true;
+}
+
+bool
+variable_chunk_stream::advance_one()
+{
+	uint8_t *big_e;
+
+	assert(big_chunk_);
+
+	big_e = big_chunk_->mem_.front().end;
+	little_b_ = little_e_;
+
+	if (little_b_ == big_e) {
+		if (next_big_chunk())
+			big_e = big_chunk_->mem_.front().end;
+		else
+			return false;
+	}
+
+	assert(little_e_ >= big_chunk_->mem_.front().begin);
+	assert(little_b_ >= big_chunk_->mem_.front().begin);
+#if 1
+	if (little_e_ > big_e) {
+		cerr << "before -- little_e_: " << (void *) little_e_ << ", big_e: " << (void *) big_e << "\n";
+	}
+#endif
+	assert(little_e_ <= big_e);
+	assert(little_b_ <= big_e);
+
+
+	while (little_e_ != big_e) {
+		optional<unsigned> maybe_break = h_.step(*little_e_);
+
+		if (maybe_break) {
+			// The break is not neccessarily at the current
+			// byte.
+			little_e_ = little_b_ + *maybe_break;
+			break;
+		}
+
+		little_e_++;
+	}
+
+	assert(little_e_ >= big_chunk_->mem_.front().begin);
+	assert(little_b_ >= big_chunk_->mem_.front().begin);
+#if 1
+	if (little_e_ > big_e) {
+		cerr << "after -- little_e_: " << (void *) little_e_ << ", big_e: " << (void *) big_e << "\n";
+	}
+#endif
+	assert(little_e_ <= big_e);
+	assert(little_b_ <= big_e);
+
+	return true;
+}
+
+void
+variable_chunk_stream::put_big_chunk()
+{
+	if (big_chunk_)
+		stream_.put(*big_chunk_);
+
+	big_chunk_ = 0;
+}
+
+//----------------------------------------------------------------
diff --git a/thin-provisioning/variable_chunk_stream.h b/thin-provisioning/variable_chunk_stream.h
new file mode 100644
index 0000000..0327f1d
--- /dev/null
+++ b/thin-provisioning/variable_chunk_stream.h
@@ -0,0 +1,42 @@
+#ifndef THIN_PROVISIONING_VARIABLE_CHUNK_STREAM_H
+#define THIN_PROVISIONING_VARIABLE_CHUNK_STREAM_H
+
+#include "base/rolling_hash.h"
+#include "thin-provisioning/chunk_stream.h"
+
+//----------------------------------------------------------------
+
+namespace thin_provisioning {
+	class variable_chunk_stream : public chunk_stream {
+	public:
+		// window_size must be a power of 2
+		variable_chunk_stream(chunk_stream &stream, unsigned window_size);
+		~variable_chunk_stream();
+
+		// FIXME: we don't know in advance how many chunks we will have
+		virtual void rewind();
+		virtual bool next(bcache::block_address count = 1ull);
+		virtual bool eof() const;
+		virtual bcache::block_address index() const;
+		virtual chunk const &get();
+		virtual void put(chunk const &c);
+
+	private:
+		bool next_big_chunk();
+		bool advance_one();
+		void put_big_chunk();
+
+		bcache::block_address index_;
+		base::content_based_hash h_;
+
+		chunk_stream &stream_;
+		chunk const *big_chunk_;
+
+		uint8_t *little_b_, *little_e_;
+		chunk little_chunk_;
+	};
+}
+
+//----------------------------------------------------------------
+
+#endif
diff --git a/unit-tests/Makefile.in b/unit-tests/Makefile.in
index 9307e5f..38f3d04 100644
--- a/unit-tests/Makefile.in
+++ b/unit-tests/Makefile.in
@@ -59,6 +59,7 @@ TEST_SOURCE=\
 	unit-tests/endian_t.cc \
 	unit-tests/error_state_t.cc \
 	unit-tests/rmap_visitor_t.cc \
+	unit-tests/rolling_hash_t.cc \
 	unit-tests/run_set_t.cc \
 	unit-tests/space_map_t.cc \
 	unit-tests/span_iterator_t.cc \
diff --git a/unit-tests/rolling_hash_t.cc b/unit-tests/rolling_hash_t.cc
new file mode 100644
index 0000000..c25b650
--- /dev/null
+++ b/unit-tests/rolling_hash_t.cc
@@ -0,0 +1,153 @@
+#include "gmock/gmock.h"
+
+#include "base/rolling_hash.h"
+
+using namespace base;
+using namespace boost;
+using namespace std;
+using namespace testing;
+
+//----------------------------------------------------------------
+
+namespace {
+	class RollingHashTests : public Test {
+	public:
+		RollingHashTests()
+			: window_size_(4096),
+			  rhash_(window_size_) {
+		}
+
+		typedef vector<uint8_t> bytes;
+		bytes random_bytes(unsigned count) {
+			bytes v(count, 0);
+
+			for (unsigned i = 0; i < count; i++)
+				v[i] = random_byte();
+
+			return v;
+		}
+
+		uint8_t random_byte() const {
+			return random() % 256;
+		}
+
+		void apply_bytes(bytes const &bs) {
+			for (unsigned i = 0; i < bs.size(); i++)
+				rhash_.step(bs[i]);
+		}
+
+		unsigned window_size_;
+		rolling_hash rhash_;
+	};
+
+	class ContentBasedHashTests : public Test {
+	public:
+		ContentBasedHashTests()
+			: window_size_(8192),
+			  h_(window_size_) {
+		}
+
+		typedef vector<uint8_t> bytes;
+		bytes random_bytes(unsigned count) {
+			bytes v(count, 0);
+
+			for (unsigned i = 0; i < count; i++)
+				v[i] = random_byte();
+
+			return v;
+		}
+
+		uint8_t random_byte() const {
+			return random() % 256;
+		}
+
+		unsigned window_size_;
+		content_based_hash h_;
+	};
+}
+
+//----------------------------------------------------------------
+
+TEST_F(RollingHashTests, ctr)
+{
+}
+
+//--------------------------------
+
+TEST_F(RollingHashTests, hash_changes)
+{
+	bytes bs = random_bytes(window_size_ * 100);
+
+	uint32_t prev = rhash_.get_hash();
+	for (unsigned i = 0; i < bs.size(); i++) {
+		rhash_.step(bs[i]);
+		ASSERT_NE(rhash_.get_hash(), prev);
+		prev = rhash_.get_hash();
+	}
+}
+
+TEST_F(RollingHashTests, hash_repeats)
+{
+	bytes bs = random_bytes(window_size_);
+
+	apply_bytes(bs);
+	uint32_t h1 = rhash_.get_hash();
+	apply_bytes(bs);
+
+	ASSERT_EQ(rhash_.get_hash(), h1);
+}
+
+TEST_F(RollingHashTests, reset_is_deterministic)
+{
+	uint8_t bytes[] = "lksdfuwerh,sdg";
+
+	for (unsigned i = 0; i < sizeof(bytes) - 1; i++)
+		rhash_.step(bytes[i]);
+
+	uint32_t h1 = rhash_.get_hash();
+
+	rhash_.reset();
+
+	for (unsigned i = 0; i < sizeof(bytes) - 1; i++)
+		rhash_.step(bytes[i]);
+
+	uint32_t h2 = rhash_.get_hash();
+
+	ASSERT_EQ(h1, h2);
+}
+
+//----------------------------------------------------------------
+
+TEST_F(ContentBasedHashTests, ctr)
+{
+}
+
+TEST_F(ContentBasedHashTests, chunk_limits_respected)
+{
+	unsigned min = 100000, max = 0;
+
+	bytes bs = random_bytes(1024 * 1024 * 100);
+	vector<unsigned> counts(window_size_, 0);
+
+	for (unsigned i = 0; i < bs.size(); i++) {
+		optional<unsigned> b = h_.step(bs[i]);
+		if (b) {
+			counts[*b]++;
+
+			if (*b < min)
+				min = *b;
+
+			if (*b > max)
+				max = *b;
+		}
+	}
+
+#if 1
+	for (unsigned i = 0; i < counts.size(); i++)
+		cerr << i << ": " << counts[i] << "\n";
+
+	cerr << "min: " << min << ", max: " << max << "\n";
+#endif
+}
+
+//----------------------------------------------------------------