Add support of converting FixedSizeBinaryArray to DecimalArray (#…

…2041)

* Add support of converting FixedSizeBinaryArray to Decimal

Signed-off-by: remzi <13716567376yh@gmail.com>

* Update arrow/src/array/array_decimal.rs

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

* use better builder API

Signed-off-by: remzi <13716567376yh@gmail.com>

* trigger CI

Signed-off-by: remzi <13716567376yh@gmail.com>

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

arrow/src/array/array_decimal.rs

@@ -20,10 +20,10 @@ use std::convert::From;
 use std::fmt;
 use std::{any::Any, iter::FromIterator};
 
-use super::BooleanBufferBuilder;
 use super::{
     array::print_long_array, raw_pointer::RawPtrBox, Array, ArrayData, FixedSizeListArray,
 };
+use super::{BooleanBufferBuilder, FixedSizeBinaryArray};
 pub use crate::array::DecimalIter;
 use crate::buffer::Buffer;
 use crate::datatypes::DataType;
@@ -148,6 +148,30 @@ pub trait BasicDecimalArray<T: BasicDecimal, U: From<ArrayData>>:
         self.value(row).to_string()
     }
 
+    /// Build a decimal array from [`FixedSizeBinaryArray`].
+    ///
+    /// NB: This function does not validate that each value is in the permissible
+    /// range for a decimal
+    fn from_fixed_size_binary_array(
+        v: FixedSizeBinaryArray,
+        precision: usize,
+        scale: usize,
+    ) -> U {
+        assert!(
+            v.value_length() == Self::VALUE_LENGTH,
+            "Value length of the array ({}) must equal to the byte width of the decimal ({})",
+            v.value_length(),
+            Self::VALUE_LENGTH,
+        );
+        let builder = v
+            .into_data()
+            .into_builder()
+            .data_type(DataType::Decimal(precision, scale));
+
+        let array_data = unsafe { builder.build_unchecked() };
+        U::from(array_data)
+    }
+
     fn from_fixed_size_list_array(
         v: FixedSizeListArray,
         precision: usize,
@@ -646,6 +670,42 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_decimal_array_from_fixed_size_binary() {
+        let value_data = ArrayData::builder(DataType::FixedSizeBinary(16))
+            .offset(1)
+            .len(3)
+            .add_buffer(Buffer::from_slice_ref(&[99999_i128, 2, 34, 560]))
+            .null_bit_buffer(Some(Buffer::from_slice_ref(&[0b1010])))
+            .build()
+            .unwrap();
+
+        let binary_array = FixedSizeBinaryArray::from(value_data);
+        let decimal = DecimalArray::from_fixed_size_binary_array(binary_array, 38, 1);
+
+        assert_eq!(decimal.len(), 3);
+        assert_eq!(decimal.value_as_string(0), "0.2".to_string());
+        assert!(decimal.is_null(1));
+        assert_eq!(decimal.value_as_string(2), "56.0".to_string());
+    }
+
+    #[test]
+    #[should_panic(
+        expected = "Value length of the array (8) must equal to the byte width of the decimal (16)"
+    )]
+    fn test_decimal_array_from_fixed_size_binary_wrong_length() {
+        let value_data = ArrayData::builder(DataType::FixedSizeBinary(8))
+            .offset(1)
+            .len(3)
+            .add_buffer(Buffer::from_slice_ref(&[99999_i64, 2, 34, 560]))
+            .null_bit_buffer(Some(Buffer::from_slice_ref(&[0b1010])))
+            .build()
+            .unwrap();
+
+        let binary_array = FixedSizeBinaryArray::from(value_data);
+        let _ = DecimalArray::from_fixed_size_binary_array(binary_array, 38, 1);
+    }
+
     #[test]
     fn test_decimal_array_from_fixed_size_list() {
         let value_data = ArrayData::builder(DataType::UInt8)