feat(changes): add initial implementation (#1304)

* feat(changes): add initial implementation

* feat(changes): add docs

src/promql/src/functions/changes.rs

@@ -12,5 +12,107 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//! Implementation of [`reset`](https://prometheus.io/docs/prometheus/latest/querying/functions/#changes) in PromQL. Refer to the [original
+//! Implementation of [`changes`](https://prometheus.io/docs/prometheus/latest/querying/functions/#changes) in PromQL. Refer to the [original
 //! implementation](https://github.com/prometheus/prometheus/blob/main/promql/functions.go#L1023-L1040).
+
+use std::sync::Arc;
+
+use common_function_macro::range_fn;
+use datafusion::arrow::array::{Float64Array, TimestampMillisecondArray};
+use datafusion::arrow::datatypes::TimeUnit;
+use datafusion::common::DataFusionError;
+use datafusion::logical_expr::{ScalarUDF, Signature, TypeSignature, Volatility};
+use datafusion::physical_plan::ColumnarValue;
+use datatypes::arrow::array::Array;
+use datatypes::arrow::datatypes::DataType;
+
+use crate::functions::extract_array;
+use crate::range_array::RangeArray;
+
+/// used to count the number of value changes that occur within a specific time range
+#[range_fn(name = "Changes", ret = "Float64Array", display_name = "prom_changes")]
+pub fn changes(_: &TimestampMillisecondArray, values: &Float64Array) -> Option<f64> {
+    if values.is_empty() {
+        None
+    } else {
+        let (first, rest) = values.values().split_first().unwrap();
+        let mut num_changes = 0;
+        let mut prev_element = first;
+        for cur_element in rest {
+            if cur_element != prev_element && !(cur_element.is_nan() && prev_element.is_nan()) {
+                num_changes += 1;
+            }
+            prev_element = cur_element;
+        }
+        Some(num_changes as f64)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::functions::test_util::simple_range_udf_runner;
+
+    // build timestamp range and value range arrays for test
+    fn build_test_range_arrays(
+        timestamps: Vec<i64>,
+        values: Vec<f64>,
+        ranges: Vec<(u32, u32)>,
+    ) -> (RangeArray, RangeArray) {
+        let ts_array = Arc::new(TimestampMillisecondArray::from_iter(
+            timestamps.into_iter().map(Some),
+        ));
+        let values_array = Arc::new(Float64Array::from_iter(values));
+
+        let ts_range_array = RangeArray::from_ranges(ts_array, ranges.clone()).unwrap();
+        let value_range_array = RangeArray::from_ranges(values_array, ranges).unwrap();
+
+        (ts_range_array, value_range_array)
+    }
+
+    #[test]
+    fn calculate_changes() {
+        let timestamps = vec![
+            1000i64, 3000, 5000, 7000, 9000, 11000, 13000, 15000, 17000, 200000, 500000,
+        ];
+        let ranges = vec![
+            (0, 1),
+            (0, 4),
+            (0, 6),
+            (0, 10),
+            (0, 0), // empty range
+        ];
+
+        // assertion 1
+        let values_1 = vec![1.0, 2.0, 3.0, 0.0, 1.0, 0.0, 0.0, 1.0, 2.0, 0.0];
+        let (ts_array_1, value_array_1) =
+            build_test_range_arrays(timestamps.clone(), values_1, ranges.clone());
+        simple_range_udf_runner(
+            Changes::scalar_udf(),
+            ts_array_1,
+            value_array_1,
+            vec![Some(0.0), Some(3.0), Some(5.0), Some(8.0), None],
+        );
+
+        // assertion 2
+        let values_2 = vec![1.0, 2.0, 3.0, 4.0, 5.0, 1.0, 2.0, 3.0, 4.0, 5.0];
+        let (ts_array_2, value_array_2) =
+            build_test_range_arrays(timestamps.clone(), values_2, ranges.clone());
+        simple_range_udf_runner(
+            Changes::scalar_udf(),
+            ts_array_2,
+            value_array_2,
+            vec![Some(0.0), Some(3.0), Some(5.0), Some(9.0), None],
+        );
+
+        // assertion 3
+        let values_3 = vec![0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 1.0];
+        let (ts_array_3, value_array_3) = build_test_range_arrays(timestamps, values_3, ranges);
+        simple_range_udf_runner(
+            Changes::scalar_udf(),
+            ts_array_3,
+            value_array_3,
+            vec![Some(0.0), Some(0.0), Some(1.0), Some(1.0), None],
+        );
+    }
+}