feat: imple densitysketch

Signed-off-by: Chojan Shang <psiace@apache.org>

Author: PsiACE

datasketches/src/density/mod.rs

@@ -0,0 +1,41 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Density sketch implementation for density estimation from streaming data.
+//!
+//! The sketch maintains a coreset of points using a compaction scheme and
+//! provides density estimates at query points via a kernel function.
+//!
+//! # Usage
+//!
+//! ```rust
+//! # use datasketches::density::DensitySketch;
+//! let mut sketch: DensitySketch<f64> = DensitySketch::new(10, 3);
+//! sketch.update(vec![0.0, 0.0, 0.0]);
+//! sketch.update(vec![1.0, 2.0, 3.0]);
+//! let estimate = sketch.estimate(&[0.0, 0.0, 0.0]);
+//! assert!(estimate > 0.0);
+//! ```
+
+mod serialization;
+mod sketch;
+
+pub use self::sketch::DensityItem;
+pub use self::sketch::DensityKernel;
+pub use self::sketch::DensitySketch;
+pub use self::sketch::DensityValue;
+pub use self::sketch::GaussianKernel;

datasketches/src/density/serialization.rs

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+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+pub(super) const PREAMBLE_INTS_SHORT: u8 = 3;
+pub(super) const PREAMBLE_INTS_LONG: u8 = 6;
+pub(super) const SERIAL_VERSION: u8 = 1;
+pub(super) const DENSITY_FAMILY_ID: u8 = 19;
+pub(super) const FLAGS_IS_EMPTY: u8 = 1 << 2;