IndexRange: Add new `intersect` method

Returns a new range, that contains the intersection of the current one
with the given range.

This is helpful to select a portion of a range without having to deal with
all the asserts of other functions. The resulting range being always a
valid subrange, it can be used to iterate or copy a part of a vector.

2 files changed, 28 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_index_range.hh b/source/blender/blenlib/BLI_index_range.hh
index a0f129c7324..ba49664e91f 100644
--- a/source/blender/blenlib/BLI_index_range.hh
+++ b/source/blender/blenlib/BLI_index_range.hh
@@ -140,6 +140,10 @@ class IndexRange {
   {
     return (a.size_ == b.size_) && (a.start_ == b.start_ || a.size_ == 0);
   }
+  constexpr friend bool operator!=(IndexRange a, IndexRange b)
+  {
+    return !(a == b);
+  }
 
   /**
    * Get the amount of numbers in the range.
@@ -248,6 +252,19 @@ class IndexRange {
   }
 
   /**
+   * Returns a new range, that contains the intersection of the current one with the given range.
+   * Returns empty range is no intersection exists.
+   * Returned range is always a valid slice of this range.
+   */
+  constexpr IndexRange intersect(IndexRange other) const
+  {
+    const int64_t old_end = start_ + size_;
+    const int64_t new_start = std::min(old_end, std::max(start_, other.start_));
+    const int64_t new_end = std::max(new_start, std::min(old_end, other.start_ + other.size_));
+    return IndexRange(new_start, new_end - new_start);
+  }
+
+  /**
    * Returns a new IndexRange with n elements removed from the beginning of the range.
    * This invokes undefined behavior when n is negative.
    */
diff --git a/source/blender/blenlib/tests/BLI_index_range_test.cc b/source/blender/blenlib/tests/BLI_index_range_test.cc
index 955691b3430..8ec7ad85d9c 100644
--- a/source/blender/blenlib/tests/BLI_index_range_test.cc
+++ b/source/blender/blenlib/tests/BLI_index_range_test.cc
@@ -126,6 +126,17 @@ TEST(index_range, Slice)
   EXPECT_EQ(slice.last(), 12);
 }
 
+TEST(index_range, Intersect)
+{
+  IndexRange range = IndexRange(5, 15);
+  EXPECT_EQ(range.intersect(IndexRange(2, 2)), IndexRange(5, 0));
+  EXPECT_EQ(range.intersect(IndexRange(4, 2)), IndexRange(5, 1));
+  EXPECT_EQ(range.intersect(IndexRange(3, 20)), IndexRange(5, 15));
+  EXPECT_EQ(range.intersect(IndexRange(5, 15)), IndexRange(5, 15));
+  EXPECT_EQ(range.intersect(IndexRange(15, 10)), IndexRange(15, 5));
+  EXPECT_EQ(range.intersect(IndexRange(22, 2)), IndexRange(20, 0));
+}
+
 TEST(index_range, SliceRange)
 {
   IndexRange range = IndexRange(5, 15);