diff --git a/Makefile b/Makefile
index 5046a8c..02b621a 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,8 @@ SOURCE=\
 	metadata.cc
 
 PROGRAM_SOURCE=\
-	block_t.cc
+	block_t.cc \
+	space_map_t.cc
 
 OBJECTS=$(subst .cc,.o,$(SOURCE))
 CPPFLAGS=-Wall -std=c++0x
@@ -25,5 +26,8 @@ multisnap_display: $(OBJECTS) main.o
 block_t: block_t.o
 	g++ -o $@ $+ $(LIBS)
 
+space_map_t: space_map_t.o
+	g++ -o $@ $+ $(LIBS)
+
 include $(subst .cc,.d,$(SOURCE))
 include $(subst .cc,.d,$(PROGRAM_SOURCE))
\ No newline at end of file
diff --git a/core-map.h b/core_map.h
similarity index 66%
rename from core-map.h
rename to core_map.h
index a759d2d..fcdf374 100644
--- a/core-map.h
+++ b/core_map.h
@@ -9,15 +9,16 @@ namespace persistent_data {
 	class core_map : public space_map {
 	public:
 		core_map(block_address nr_blocks)
-			: counts_(nr_blocks, 0) {
+			: counts_(nr_blocks, 0),
+			  nr_free_(nr_blocks) {
 		}
 
 		block_address get_nr_blocks() const {
-			counts_.size();
+			return counts_.size();
 		}
 
 		block_address get_nr_free() const {
-			nr_free_;
+			return nr_free_;
 		}
 
 		ref_t get_count(block_address b) const {
@@ -25,34 +26,50 @@ namespace persistent_data {
 		}
 
 		void set_count(block_address b, ref_t c) {
+			if (counts_[b] == 0 && c > 0)
+				nr_free_--;
+
+			else if (counts_[b] > 0 && c == 0)
+				nr_free_++;
+
 			counts_[b] = c;
 		}
 
 		void commit() {
 		}
 
-		void inc_block(block_address b) {
+		void inc(block_address b) {
+			if (counts_[b] == 0)
+				nr_free_--;
+
 			counts_[b]++;
 		}
 
-		void dec_block(block_address b) {
+		void dec(block_address b) {
 			counts_[b]--;
+
+			if (counts_[b] == 0)
+				nr_free_++;
 		}
 
 		block_address new_block() {
 			for (block_address i = 0; i < counts_.size(); i++)
 				if (counts_[i] == 0) {
 					counts_[i] = 1;
+					nr_free_--;
 					return i;
 				}
+
+			throw std::runtime_error("no space");
 		}
 
 		bool count_possibly_greater_than_one(block_address b) const {
-			return counts_[i] > 1;
+			return counts_[b] > 1;
 		}
 
 	private:
 		std::vector<ref_t> counts_;
+		unsigned nr_free_;
 	};
 }
 
diff --git a/space_map.h b/space_map.h
index f66644c..e1f9478 100644
--- a/space_map.h
+++ b/space_map.h
@@ -22,8 +22,8 @@ namespace persistent_data {
 		virtual void set_count(block_address b, ref_t c) = 0;
 		virtual void commit() = 0;
 
-		virtual void inc_block(block_address b) = 0;
-		virtual void dec_block(block_address b) = 0;
+		virtual void inc(block_address b) = 0;
+		virtual void dec(block_address b) = 0;
 		virtual block_address new_block() = 0;
 
 		virtual bool count_possibly_greater_than_one(block_address b) const = 0;
diff --git a/space_map_t.cc b/space_map_t.cc
new file mode 100644
index 0000000..3024629
--- /dev/null
+++ b/space_map_t.cc
@@ -0,0 +1,83 @@
+#include "space_map.h"
+#include "core_map.h"
+
+#define BOOST_TEST_MODULE SpaceMapTests
+#include <boost/test/included/unit_test.hpp>
+
+using namespace std;
+using namespace boost;
+
+unsigned const NR_BLOCKS = 1024;
+
+//----------------------------------------------------------------
+
+namespace {
+}
+
+//----------------------------------------------------------------
+
+BOOST_AUTO_TEST_CASE(test_get_nr_blocks)
+{
+	core_map sm(NR_BLOCKS);
+	BOOST_CHECK_EQUAL(sm.get_nr_blocks(), NR_BLOCKS);
+}
+
+BOOST_AUTO_TEST_CASE(test_get_nr_free)
+{
+	core_map sm(NR_BLOCKS);
+	BOOST_CHECK_EQUAL(sm.get_nr_free(), NR_BLOCKS);
+
+	for (unsigned i = 0; i < NR_BLOCKS; i++) {
+		sm.new_block();
+		BOOST_CHECK_EQUAL(sm.get_nr_free(), NR_BLOCKS - i - 1);
+	}
+
+	for (unsigned i = 0; i < NR_BLOCKS; i++) {
+		sm.dec(i);
+		BOOST_CHECK_EQUAL(sm.get_nr_free(), i + 1);
+	}
+}
+
+BOOST_AUTO_TEST_CASE(test_throws_no_space)
+{
+	core_map sm(NR_BLOCKS);
+	for (unsigned i = 0; i < NR_BLOCKS; i++)
+		sm.new_block();
+
+	BOOST_CHECK_THROW(sm.new_block(), std::runtime_error);
+}
+
+BOOST_AUTO_TEST_CASE(test_inc_and_dec)
+{
+	core_map sm(NR_BLOCKS);
+	block_address b = 63;
+
+	for (unsigned i = 0; i < 50; i++) {
+		BOOST_CHECK_EQUAL(sm.get_count(b), i);
+		sm.inc(b);
+	}
+
+	for (unsigned i = 50; i > 0; i--) {
+		BOOST_CHECK_EQUAL(sm.get_count(b), i);
+		sm.dec(b);
+	}
+}
+
+BOOST_AUTO_TEST_CASE(test_not_allocated_twice)
+{
+	core_map sm(NR_BLOCKS);
+	block_address b = sm.new_block();
+
+	try {
+		for (;;)
+			BOOST_CHECK(sm.new_block() != b);
+	} catch (...) {
+	}
+}
+
+BOOST_AUTO_TEST_CASE(test_set_count)
+{
+	core_map sm(NR_BLOCKS);
+	sm.set_count(43, 5);
+	BOOST_CHECK_EQUAL(sm.get_count(43), 5);
+}