value: introduce CqlDecimalBorrowed

Has the same semantics as CqlDecimal, but borrows the bytes.

Author: muzarski

docs/source/data-types/decimal.md

@@ -3,7 +3,7 @@
 
 ## value::CqlDecimal
 
-Without any feature flags, the user can interact with `decimal` type by making use of `value::CqlDecimal` which is a very simple wrapper representing the value as signed binary number in big-endian order with a 32-bit scale.
+Without any feature flags, the user can interact with `decimal` type by making use of `value::CqlDecimal` or `value::CqlDecimalBorrowed` which are very simple wrappers representing the value as signed binary number in big-endian order with a 32-bit scale.
 
 ```rust
 # extern crate scylla;

scylla-cql/src/frame/value.rs

@@ -454,6 +454,9 @@ impl From<CqlVarintBorrowed<'_>> for num_bigint_04::BigInt {
 /// - a [`CqlVarint`] value
 /// - 32-bit integer which determines the position of the decimal point
 ///
+/// This struct holds owned bytes. If you wish to borrow the bytes instead,
+/// see [`CqlDecimalBorrowed`] documentation.
+///
 /// The type is not very useful in most use cases.
 /// However, users can make use of more complex types
 /// such as `bigdecimal::BigDecimal` (v0.4).
@@ -470,6 +473,20 @@ pub struct CqlDecimal {
     scale: i32,
 }
 
+/// Borrowed version of native CQL `decimal` representation.
+///
+/// Represented as a pair:
+/// - a [`CqlVarintBorrowed`] value
+/// - 32-bit integer which determines the position of the decimal point
+///
+/// Refer to the documentation of [`CqlDecimal`].
+/// Especially, see the disclaimer about [non-normalized values](CqlDecimal#db-data-format).
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub struct CqlDecimalBorrowed<'b> {
+    int_val: CqlVarintBorrowed<'b>,
+    scale: i32,
+}
+
 /// Constructors
 impl CqlDecimal {
     /// Creates a [`CqlDecimal`] from an array of bytes
@@ -492,6 +509,20 @@ impl CqlDecimal {
     }
 }
 
+/// Constructors
+impl<'b> CqlDecimalBorrowed<'b> {
+    /// Creates a [`CqlDecimalBorrowed`] from a slice of bytes
+    /// representing [`CqlVarintBorrowed`] and a 32-bit scale.
+    ///
+    /// See: disclaimer about [non-normalized values](CqlVarint#db-data-format).
+    pub fn from_signed_be_bytes_slice_and_exponent(bytes: &'b [u8], scale: i32) -> Self {
+        Self {
+            int_val: CqlVarintBorrowed::from_signed_bytes_be_slice(bytes),
+            scale,
+        }
+    }
+}
+
 /// Conversion to raw bytes
 impl CqlDecimal {
     /// Returns a slice of bytes in two's complement
@@ -507,6 +538,15 @@ impl CqlDecimal {
     }
 }
 
+/// Conversion to raw bytes
+impl CqlDecimalBorrowed<'_> {
+    /// Returns a slice of bytes in two's complement
+    /// binary big-endian representation and a scale.
+    pub fn as_signed_be_bytes_slice_and_exponent(&self) -> (&[u8], i32) {
+        (self.int_val.as_signed_bytes_be_slice(), self.scale)
+    }
+}
+
 #[cfg(feature = "bigdecimal-04")]
 impl From<CqlDecimal> for bigdecimal_04::BigDecimal {
     fn from(value: CqlDecimal) -> Self {
@@ -519,6 +559,18 @@ impl From<CqlDecimal> for bigdecimal_04::BigDecimal {
     }
 }
 
+#[cfg(feature = "bigdecimal-04")]
+impl From<CqlDecimalBorrowed<'_>> for bigdecimal_04::BigDecimal {
+    fn from(value: CqlDecimalBorrowed) -> Self {
+        Self::from((
+            bigdecimal_04::num_bigint::BigInt::from_signed_bytes_be(
+                value.int_val.as_signed_bytes_be_slice(),
+            ),
+            value.scale as i64,
+        ))
+    }
+}
+
 #[cfg(feature = "bigdecimal-04")]
 impl TryFrom<bigdecimal_04::BigDecimal> for CqlDecimal {
     type Error = <i64 as TryInto<i32>>::Error;

scylla-cql/src/types/deserialize/value.rs

@@ -15,12 +15,15 @@ use std::fmt::Display;
 use thiserror::Error;
 
 use super::{make_error_replace_rust_name, DeserializationError, FrameSlice, TypeCheckError};
-use crate::frame::response::result::{deser_cql_value, ColumnType, CqlValue};
 use crate::frame::types;
 use crate::frame::value::{
     Counter, CqlDate, CqlDecimal, CqlDuration, CqlTime, CqlTimestamp, CqlTimeuuid, CqlVarint,
 };
 use crate::frame::{frame_errors::LowLevelDeserializationError, value::CqlVarintBorrowed};
+use crate::frame::{
+    response::result::{deser_cql_value, ColumnType, CqlValue},
+    value::CqlDecimalBorrowed,
+};
 
 /// A type that can be deserialized from a column value inside a row that was
 /// returned from a query.
@@ -269,6 +272,24 @@ impl_emptiable_strict_type!(
     }
 );
 
+impl_emptiable_strict_type!(
+    CqlDecimalBorrowed<'b>,
+    Decimal,
+    |typ: &'metadata ColumnType<'metadata>, v: Option<FrameSlice<'frame>>| {
+        let mut val = ensure_not_null_slice::<Self>(typ, v)?;
+        let scale = types::read_int(&mut val).map_err(|err| {
+            mk_deser_err::<Self>(
+                typ,
+                BuiltinDeserializationErrorKind::BadDecimalScale(err.into()),
+            )
+        })?;
+        Ok(CqlDecimalBorrowed::from_signed_be_bytes_slice_and_exponent(
+            val, scale,
+        ))
+    },
+    'b
+);
+
 #[cfg(feature = "bigdecimal-04")]
 impl_emptiable_strict_type!(
     bigdecimal_04::BigDecimal,

scylla-cql/src/types/deserialize/value_tests.rs

@@ -10,8 +10,8 @@ use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
 
 use crate::frame::response::result::{ColumnType, CqlValue};
 use crate::frame::value::{
-    Counter, CqlDate, CqlDecimal, CqlDuration, CqlTime, CqlTimestamp, CqlTimeuuid, CqlVarint,
-    CqlVarintBorrowed,
+    Counter, CqlDate, CqlDecimal, CqlDecimalBorrowed, CqlDuration, CqlTime, CqlTimestamp,
+    CqlTimeuuid, CqlVarint, CqlVarintBorrowed,
 };
 use crate::types::deserialize::value::{TupleDeserializationErrorKind, TupleTypeCheckErrorKind};
 use crate::types::deserialize::{DeserializationError, FrameSlice, TypeCheckError};
@@ -187,6 +187,12 @@ fn test_varlen_numbers() {
         &mut Bytes::new(),
     );
 
+    assert_ser_de_identity(
+        &ColumnType::Decimal,
+        &CqlDecimalBorrowed::from_signed_be_bytes_slice_and_exponent(b"Ala ma kota", 42),
+        &mut Bytes::new(),
+    );
+
     #[cfg(feature = "bigdecimal-04")]
     assert_ser_de_identity(
         &ColumnType::Decimal,

scylla-cql/src/types/serialize/value.rs

@@ -16,8 +16,8 @@ use uuid::Uuid;
 use crate::frame::response::result::{ColumnType, CqlValue};
 use crate::frame::types::vint_encode;
 use crate::frame::value::{
-    Counter, CqlDate, CqlDecimal, CqlDuration, CqlTime, CqlTimestamp, CqlTimeuuid, CqlVarint,
-    CqlVarintBorrowed, MaybeUnset, Unset, Value,
+    Counter, CqlDate, CqlDecimal, CqlDecimalBorrowed, CqlDuration, CqlTime, CqlTimestamp,
+    CqlTimeuuid, CqlVarint, CqlVarintBorrowed, MaybeUnset, Unset, Value,
 };
 
 #[cfg(feature = "chrono-04")]
@@ -124,6 +124,18 @@ impl SerializeValue for CqlDecimal {
             .map_err(|_| mk_ser_err::<Self>(typ, BuiltinSerializationErrorKind::SizeOverflow))?
     });
 }
+impl SerializeValue for CqlDecimalBorrowed<'_> {
+    impl_serialize_via_writer!(|me, typ, writer| {
+        exact_type_check!(typ, Decimal);
+        let mut builder = writer.into_value_builder();
+        let (bytes, scale) = me.as_signed_be_bytes_slice_and_exponent();
+        builder.append_bytes(&scale.to_be_bytes());
+        builder.append_bytes(bytes);
+        builder
+            .finish()
+            .map_err(|_| mk_ser_err::<Self>(typ, BuiltinSerializationErrorKind::SizeOverflow))?
+    });
+}
 #[cfg(feature = "bigdecimal-04")]
 impl SerializeValue for bigdecimal_04::BigDecimal {
     impl_serialize_via_writer!(|me, typ, writer| {