BLI: Add functions to IndexRange to mirror Span

Adds functions for special cases of slicing and an `is_empty` method.

2 files changed, 125 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_index_range.hh b/source/blender/blenlib/BLI_index_range.hh
index 25edba8c631..832e438fa75 100644
--- a/source/blender/blenlib/BLI_index_range.hh
+++ b/source/blender/blenlib/BLI_index_range.hh
@@ -149,6 +149,14 @@ class IndexRange {
   }
 
   /**
+   * Returns true if the size is zero.
+   */
+  constexpr bool is_empty() const
+  {
+    return size_ == 0;
+  }
+
+  /**
    * Create a new range starting at the end of the current one.
    */
   constexpr IndexRange after(int64_t n) const
@@ -227,6 +235,50 @@ class IndexRange {
   }
 
   /**
+   * Returns a new IndexRange with n elements removed from the beginning. This invokes undefined
+   * behavior when n is negative.
+   */
+  constexpr IndexRange drop_front(int64_t n) const
+  {
+    BLI_assert(n >= 0);
+    const int64_t new_size = std::max<int64_t>(0, size_ - n);
+    return IndexRange(start_ + n, new_size);
+  }
+
+  /**
+   * Returns a new IndexRange with n elements removed from the beginning. This invokes undefined
+   * behavior when n is negative.
+   */
+  constexpr IndexRange drop_back(int64_t n) const
+  {
+    BLI_assert(n >= 0);
+    const int64_t new_size = std::max<int64_t>(0, size_ - n);
+    return IndexRange(start_, new_size);
+  }
+
+  /**
+   * Returns a new IndexRange that only contains the first n elements. This invokes undefined
+   * behavior when n is negative.
+   */
+  constexpr IndexRange take_front(int64_t n) const
+  {
+    BLI_assert(n >= 0);
+    const int64_t new_size = std::min<int64_t>(size_, n);
+    return IndexRange(start_, new_size);
+  }
+
+  /**
+   * Returns a new IndexRange that only contains the last n elements. This invokes undefined
+   * behavior when n is negative.
+   */
+  constexpr IndexRange take_back(int64_t n) const
+  {
+    BLI_assert(n >= 0);
+    const int64_t new_size = std::min<int64_t>(size_, n);
+    return IndexRange(start_ + size_ - new_size, new_size);
+  }
+
+  /**
    * Get read-only access to a memory buffer that contains the range as actual numbers.
    */
   Span<int64_t> as_span() const;
diff --git a/source/blender/blenlib/tests/BLI_index_range_test.cc b/source/blender/blenlib/tests/BLI_index_range_test.cc
index 7f0d2e6c22a..10f6784cd44 100644
--- a/source/blender/blenlib/tests/BLI_index_range_test.cc
+++ b/source/blender/blenlib/tests/BLI_index_range_test.cc
@@ -129,6 +129,79 @@ TEST(index_range, SliceRange)
   EXPECT_EQ(slice.last(), 12);
 }
 
+TEST(index_range, DropBack)
+{
+  IndexRange a(4, 4);
+  auto slice = a.drop_back(2);
+  EXPECT_EQ(slice.size(), 2);
+  EXPECT_EQ(slice.start(), 4);
+  EXPECT_EQ(slice[1], 5);
+}
+
+TEST(index_range, DropBackAll)
+{
+  IndexRange a(4, 4);
+  auto slice = a.drop_back(a.size());
+  EXPECT_TRUE(slice.is_empty());
+}
+
+TEST(index_range, DropFront)
+{
+  IndexRange a(4, 4);
+  auto slice = a.drop_front(1);
+  EXPECT_EQ(slice.size(), 3);
+  EXPECT_EQ(slice[0], 5);
+  EXPECT_EQ(slice[1], 6);
+  EXPECT_EQ(slice.last(), 7);
+}
+
+TEST(index_range, DropFrontLargeN)
+{
+  IndexRange a(1, 5);
+  IndexRange slice = a.drop_front(100);
+  EXPECT_TRUE(slice.is_empty());
+}
+
+TEST(index_range, DropFrontAll)
+{
+  IndexRange a(50);
+  IndexRange slice = a.drop_front(a.size());
+  EXPECT_TRUE(slice.is_empty());
+}
+
+TEST(index_range, TakeFront)
+{
+  IndexRange a(4, 4);
+  IndexRange slice = a.take_front(2);
+  EXPECT_EQ(slice.size(), 2);
+  EXPECT_EQ(slice[0], 4);
+  EXPECT_EQ(slice[1], 5);
+}
+
+TEST(index_range, TakeFrontLargeN)
+{
+  IndexRange a(4, 4);
+  IndexRange slice = a.take_front(100);
+  EXPECT_EQ(slice.size(), 4);
+}
+
+TEST(index_range, TakeBack)
+{
+  IndexRange a(4, 4);
+  auto slice = a.take_back(2);
+  EXPECT_EQ(slice.size(), 2);
+  EXPECT_EQ(slice[0], 6);
+  EXPECT_EQ(slice[1], 7);
+}
+
+TEST(index_range, TakeBackLargeN)
+{
+  IndexRange a(3, 4);
+  IndexRange slice = a.take_back(100);
+  EXPECT_EQ(slice.size(), 4);
+  EXPECT_EQ(slice.size(), 4);
+}
+
 TEST(index_range, AsSpan)
 {
   IndexRange range = IndexRange(4, 6);