Implement try_unreserve() on Array to reduce underlying memory buffer inplace.

3 files changed, 47 insertions, 0 deletions

diff --git a/vespalib/src/tests/array/array_test.cpp b/vespalib/src/tests/array/array_test.cpp
index b7357512d89..13e46111bfb 100644
--- a/vespalib/src/tests/array/array_test.cpp
+++ b/vespalib/src/tests/array/array_test.cpp
@@ -320,4 +320,33 @@ TEST("test move assignment")
     }
 }
 
+struct UnreserveFixture {
+    Array<int> arr;
+    UnreserveFixture() : arr(1025, 7, alloc::Alloc::allocMMap(0))
+    {
+        EXPECT_EQUAL(1025u, arr.size());
+        EXPECT_EQUAL(2048u, arr.capacity());
+    }
+};
+
+TEST_F("require that try_unreserve() fails if wanted capacity >= current capacity", UnreserveFixture)
+{
+    EXPECT_FALSE(f.arr.try_unreserve(2048));
+}
+
+TEST_F("require that try_unreserve() fails if wanted capacity < current size", UnreserveFixture)
+{
+    EXPECT_FALSE(f.arr.try_unreserve(1024));
+}
+
+TEST_F("require that try_unreserve() succeedes if mmap can be shrinked", UnreserveFixture)
+{
+    int *oldPtr = &f.arr[0];
+    f.arr.resize(512);
+    EXPECT_TRUE(f.arr.try_unreserve(1023));
+    EXPECT_EQUAL(1024u, f.arr.capacity());
+    int *newPtr = &f.arr[0];
+    EXPECT_EQUAL(oldPtr, newPtr);
+}
+
 TEST_MAIN() { TEST_RUN_ALL(); }
diff --git a/vespalib/src/vespa/vespalib/util/array.h b/vespalib/src/vespa/vespalib/util/array.h
index 269bf67bc26..4ca09bd093a 100644
--- a/vespalib/src/vespa/vespalib/util/array.h
+++ b/vespalib/src/vespa/vespalib/util/array.h
@@ -106,6 +106,12 @@ public:
     void resize(size_t n);
     void assign(const_iterator begin_, const_iterator end_);
     void reserve(size_t n);
+    /**
+     * Try to unreserve memory from the underlying memory buffer inplace down to the given limit.
+     * The existing memory buffer is unmodified up to the new size (no copying occurs).
+     * Returns true if it was possible to unreserve memory, false if not.
+     */
+    bool try_unreserve(size_t n);
     void push_back(const T & v)             { std::_Construct(push_back(), v); }
     iterator push_back()                    { extend(size()+1); return array(_sz++); }
     iterator push_back_fast()               { return array(_sz++); }
diff --git a/vespalib/src/vespa/vespalib/util/array.hpp b/vespalib/src/vespa/vespalib/util/array.hpp
index dbdda73ad66..ec418d7d566 100644
--- a/vespalib/src/vespa/vespalib/util/array.hpp
+++ b/vespalib/src/vespa/vespalib/util/array.hpp
@@ -94,6 +94,18 @@ void Array<T>::reserve(size_t n) {
 }
 
 template <typename T>
+bool Array<T>::try_unreserve(size_t n)
+{
+    if (n >= capacity()) {
+        return false;
+    }
+    if (n < size()) {
+        return false;
+    }
+    return _array.resize_inplace(n * sizeof(T));
+}
+
+template <typename T>
 void Array<T>::resize(size_t n)
 {
     if (n > capacity()) {