feat: add Cell::children()

src/cells.rs

@@ -93,8 +93,33 @@ impl Cell {
         cell_to_parent(self, parent_res)
     }
 
-    // TODO:
-    // Add child and/or children
+    /// Return Cell's children.
+    pub fn children(
+        &self,
+        children_res: u8,
+    ) -> Result<impl Iterator<Item = Result<Self, QuadbinError>>, QuadbinError> {
+        let resolution = self.resolution();
+        if children_res <= resolution || children_res > 26 {
+            return Err(QuadbinError::InvalidResolution(InvalidResolution::new(
+                children_res,
+                "Children resolution should be greater than the current resolution",
+            )));
+        }
+
+        let resolution_diff = children_res - resolution;
+        let block_range = (1 << (resolution_diff << 1)) as u64;
+        let block_shift = (52 - (children_res << 1)) as u64;
+
+        let cell = self.get();
+
+        let child_base = (cell & !(0x1F << 52)) | ((children_res as u64) << 52);
+        let child_base = child_base & !((block_range - 1) << block_shift);
+
+        Ok((0..block_range).map(move |x| {
+            let child = child_base | (x << block_shift);
+            Cell::try_from(child)
+        }))
+    }
 
     /// Find the Cell's neighbor in a specific [Direction].
     ///

src/test/cells.rs

@@ -241,3 +241,69 @@ fn test_cell_neighbors() {
         .neighbors();
     assert_eq!(nn[1], None)
 }
+
+#[test]
+fn test_cell_children_one() {
+    let parent = Cell::new(5192650370358181887);
+    let kids = parent.children(1).unwrap();
+
+    let truth: [u64; 4] = [
+        5193776270265024511,
+        5194902170171867135,
+        5196028070078709759,
+        5197153969985552383,
+    ];
+
+    for (i, cell_result) in kids.enumerate() {
+        let cell = cell_result.expect("cell index");
+        assert_eq!(cell.get(), truth[i]);
+    }
+}
+
+#[test]
+fn test_cell_children_five() {
+    let parent = Cell::new(5209574053332910079);
+    let kids = parent.children(5).unwrap();
+
+    let truth: [u64; 4] = [
+        5214064458820747263,
+        5214068856867258367,
+        5214073254913769471,
+        5214077652960280575,
+    ];
+
+    for (i, cell_result) in kids.enumerate() {
+        let cell = cell_result.expect("cell index");
+        assert_eq!(cell.get(), truth[i]);
+    }
+}
+
+#[test]
+fn test_cell_children_six() {
+    let parent = Cell::new(5209574053332910079);
+    let kids = parent.children(6).unwrap();
+
+    let truth: [u64; 16] = [
+        5218564759913234431,
+        5218565859424862207,
+        5218566958936489983,
+        5218568058448117759,
+        5218569157959745535,
+        5218570257471373311,
+        5218571356983001087,
+        5218572456494628863,
+        5218573556006256639,
+        5218574655517884415,
+        5218575755029512191,
+        5218576854541139967,
+        5218577954052767743,
+        5218579053564395519,
+        5218580153076023295,
+        5218581252587651071,
+    ];
+
+    for (i, cell_result) in kids.enumerate() {
+        let cell = cell_result.expect("cell index");
+        assert_eq!(cell.get(), truth[i]);
+    }
+}

src/test/errors.rs

@@ -56,3 +56,19 @@ fn test_invalid_cell_index() {
         );
     }
 }
+
+#[test]
+fn test_invalid_child_res() {
+    let cell = Cell::try_from(5209574053332910079).expect("cell index");
+    let kids = cell.children(3);
+
+    assert!(kids.is_err());
+
+    assert_eq!(
+        kids.err(),
+        Some(QuadbinError::InvalidResolution(InvalidResolution::new(
+            3,
+            "Children resolution should be greater than the current resolution"
+        )))
+    );
+}

src/test/geo.rs

@@ -36,6 +36,20 @@ fn test_invalid_resolution() {
     );
 }
 
+#[test]
+fn test_invalid_multipoint_resolution() {
+    let orig = multi_point_2d();
+    let res = 27;
+    let cell = Cell::from_multipoint(orig, res);
+    assert_eq!(
+        cell.filter_map(|i| i.err()).next(),
+        Some(QuadbinError::InvalidResolution(InvalidResolution::new(
+            res,
+            "Resolution should be between 0 and 26"
+        )))
+    );
+}
+
 #[test]
 fn test_cell_to_polygon() {
     let bbox = [22.5, -21.943045533438166, 45.0, 0.0];