feat: support cast decimal to utf8

Cargo.toml

@@ -46,6 +46,7 @@ hashbrown = { version = "0.14", features = ["rayon", "ahash"] }
 hex = "0.4.3"
 indexmap = { version = "2", features = ["std"] }
 itoa = "1.0.6"
+itoap = { version = "1", features = ["simd"] }
 atoi_simd = "0.15.5"
 fast-float = { version = "0.2" }
 memchr = "2.6"

crates/polars-arrow/Cargo.toml

@@ -36,6 +36,7 @@ ethnum = { workspace = true }
 atoi_simd = { workspace = true, optional = true }
 fast-float = { workspace = true, optional = true }
 itoa = { workspace = true, optional = true }
+itoap = { workspace = true, optional = true }
 ryu = { workspace = true, optional = true }
 
 regex = { workspace = true, optional = true }
@@ -161,7 +162,7 @@ simd = []
 # polars-arrow
 timezones = []
 dtype-array = []
-dtype-decimal = ["atoi"]
+dtype-decimal = ["atoi", "itoap"]
 bigidx = []
 nightly = []
 performant = []

crates/polars-arrow/src/compute/cast/decimal_to.rs

@@ -137,3 +137,28 @@ where
     let from = from.as_any().downcast_ref().unwrap();
     Ok(Box::new(decimal_to_integer::<T>(from)))
 }
+
+/// Returns a [`Utf8Array`] where every element is the utf8 representation of the decimal.
+#[cfg(feature = "dtype-decimal")]
+pub(super) fn decimal_to_binview(from: &PrimitiveArray<i128>) -> BinaryViewArray {
+    let (_, from_scale) = if let ArrowDataType::Decimal(p, s) = from.data_type().to_logical_type() {
+        (*p, *s)
+    } else {
+        panic!("internal error: i128 is always a decimal")
+    };
+
+    let mut mutable = MutableBinaryViewArray::with_capacity(from.len());
+
+    for &x in from.values().iter() {
+        let buf = crate::legacy::compute::decimal::format_decimal(x, from_scale, false);
+        mutable.push_value_ignore_validity(buf.as_str().as_bytes())
+    }
+
+    mutable.freeze().with_validity(from.validity().cloned())
+}
+
+#[cfg(feature = "dtype-decimal")]
+pub(super) fn decimal_to_binview_dyn(from: &dyn Array) -> BinaryViewArray {
+    let from = from.as_any().downcast_ref().unwrap();
+    decimal_to_binview(from)
+}

crates/polars-arrow/src/compute/cast/mod.rs

@@ -778,6 +778,8 @@ fn from_to_binview(
         Binary => binary_to_binview::<i32>(array.as_any().downcast_ref().unwrap()),
         FixedSizeBinary(_) => fixed_size_binary_to_binview(array.as_any().downcast_ref().unwrap()),
         LargeBinary => binary_to_binview::<i64>(array.as_any().downcast_ref().unwrap()),
+        #[cfg(feature = "dtype-decimal")]
+        Decimal(_, _) => decimal_to_binview_dyn(array),
         _ => polars_bail!(InvalidOperation:
             "casting from {from_type:?} to {to_type:?} not supported",
         ),

crates/polars-arrow/src/legacy/compute/decimal.rs

@@ -106,6 +106,109 @@ pub(crate) fn deserialize_decimal(
     }
 }
 
+const BUF_LEN: usize = 48;
+
+#[derive(Clone, Copy)]
+pub struct FormatBuffer {
+    data: [u8; BUF_LEN],
+    len: usize,
+}
+
+impl FormatBuffer {
+    #[inline]
+    pub const fn new() -> Self {
+        Self {
+            data: [0; BUF_LEN],
+            len: 0,
+        }
+    }
+
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        unsafe { std::str::from_utf8_unchecked(&self.data[..self.len]) }
+    }
+}
+
+const POW10: [i128; 38] = [
+    1,
+    10,
+    100,
+    1000,
+    10000,
+    100000,
+    1000000,
+    10000000,
+    100000000,
+    1000000000,
+    10000000000,
+    100000000000,
+    1000000000000,
+    10000000000000,
+    100000000000000,
+    1000000000000000,
+    10000000000000000,
+    100000000000000000,
+    1000000000000000000,
+    10000000000000000000,
+    100000000000000000000,
+    1000000000000000000000,
+    10000000000000000000000,
+    100000000000000000000000,
+    1000000000000000000000000,
+    10000000000000000000000000,
+    100000000000000000000000000,
+    1000000000000000000000000000,
+    10000000000000000000000000000,
+    100000000000000000000000000000,
+    1000000000000000000000000000000,
+    10000000000000000000000000000000,
+    100000000000000000000000000000000,
+    1000000000000000000000000000000000,
+    10000000000000000000000000000000000,
+    100000000000000000000000000000000000,
+    1000000000000000000000000000000000000,
+    10000000000000000000000000000000000000,
+];
+
+pub fn format_decimal(v: i128, scale: usize, trim_zeros: bool) -> FormatBuffer {
+    const ZEROS: [u8; BUF_LEN] = [b'0'; BUF_LEN];
+
+    let mut buf = FormatBuffer::new();
+    let factor = POW10[scale]; //10_i128.pow(scale as _);
+    let (div, rem) = (v / factor, v.abs() % factor);
+
+    unsafe {
+        let mut ptr = buf.data.as_mut_ptr();
+        if div == 0 && v < 0 {
+            *ptr = b'-';
+            ptr = ptr.add(1);
+            buf.len = 1;
+        }
+        let n_whole = itoap::write_to_ptr(ptr, div);
+        buf.len += n_whole;
+        if rem != 0 {
+            ptr = ptr.add(n_whole);
+            *ptr = b'.';
+            ptr = ptr.add(1);
+            let mut frac_buf = [0_u8; BUF_LEN];
+            let n_frac = itoap::write_to_ptr(frac_buf.as_mut_ptr(), rem);
+            std::ptr::copy_nonoverlapping(ZEROS.as_ptr(), ptr, scale - n_frac);
+            ptr = ptr.add(scale - n_frac);
+            std::ptr::copy_nonoverlapping(frac_buf.as_mut_ptr(), ptr, n_frac);
+            buf.len += 1 + scale;
+            if trim_zeros {
+                ptr = ptr.add(n_frac - 1);
+                while *ptr == b'0' {
+                    ptr = ptr.sub(1);
+                    buf.len -= 1;
+                }
+            }
+        }
+    }
+
+    buf
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

crates/polars-core/Cargo.toml

@@ -25,7 +25,6 @@ comfy-table = { version = "7.0.1", default_features = false, optional = true }
 either = { workspace = true }
 hashbrown = { workspace = true }
 indexmap = { workspace = true }
-itoap = { version = "1", optional = true, features = ["simd"] }
 ndarray = { workspace = true, optional = true }
 num-traits = { workspace = true }
 once_cell = { workspace = true }
@@ -109,7 +108,7 @@ dtype-time = ["temporal"]
 dtype-array = ["arrow/dtype-array", "polars-compute/dtype-array"]
 dtype-i8 = []
 dtype-i16 = []
-dtype-decimal = ["dep:itoap", "arrow/dtype-decimal"]
+dtype-decimal = ["arrow/dtype-decimal"]
 dtype-u8 = []
 dtype-u16 = []
 dtype-categorical = []

crates/polars-core/src/fmt.rs

@@ -1153,126 +1153,16 @@ impl Series {
     }
 }
 
+#[inline]
 #[cfg(feature = "dtype-decimal")]
-mod decimal {
-    use std::fmt::Formatter;
-    use std::{fmt, ptr, str};
+pub fn fmt_decimal(f: &mut Formatter<'_>, v: i128, scale: usize) -> fmt::Result {
+    use arrow::legacy::compute::decimal::format_decimal;
 
-    use crate::fmt::{fmt_float_string, get_trim_decimal_zeros};
-
-    const BUF_LEN: usize = 48;
-
-    #[derive(Clone, Copy)]
-    pub struct FormatBuffer {
-        data: [u8; BUF_LEN],
-        len: usize,
-    }
-
-    impl FormatBuffer {
-        #[inline]
-        pub const fn new() -> Self {
-            Self {
-                data: [0; BUF_LEN],
-                len: 0,
-            }
-        }
-
-        #[inline]
-        pub fn as_str(&self) -> &str {
-            unsafe { str::from_utf8_unchecked(&self.data[..self.len]) }
-        }
-    }
-
-    const POW10: [i128; 38] = [
-        1,
-        10,
-        100,
-        1000,
-        10000,
-        100000,
-        1000000,
-        10000000,
-        100000000,
-        1000000000,
-        10000000000,
-        100000000000,
-        1000000000000,
-        10000000000000,
-        100000000000000,
-        1000000000000000,
-        10000000000000000,
-        100000000000000000,
-        1000000000000000000,
-        10000000000000000000,
-        100000000000000000000,
-        1000000000000000000000,
-        10000000000000000000000,
-        100000000000000000000000,
-        1000000000000000000000000,
-        10000000000000000000000000,
-        100000000000000000000000000,
-        1000000000000000000000000000,
-        10000000000000000000000000000,
-        100000000000000000000000000000,
-        1000000000000000000000000000000,
-        10000000000000000000000000000000,
-        100000000000000000000000000000000,
-        1000000000000000000000000000000000,
-        10000000000000000000000000000000000,
-        100000000000000000000000000000000000,
-        1000000000000000000000000000000000000,
-        10000000000000000000000000000000000000,
-    ];
-
-    pub fn format_decimal(v: i128, scale: usize, trim_zeros: bool) -> FormatBuffer {
-        const ZEROS: [u8; BUF_LEN] = [b'0'; BUF_LEN];
-
-        let mut buf = FormatBuffer::new();
-        let factor = POW10[scale]; //10_i128.pow(scale as _);
-        let (div, rem) = (v / factor, v.abs() % factor);
-
-        unsafe {
-            let mut ptr = buf.data.as_mut_ptr();
-            if div == 0 && v < 0 {
-                *ptr = b'-';
-                ptr = ptr.add(1);
-                buf.len = 1;
-            }
-            let n_whole = itoap::write_to_ptr(ptr, div);
-            buf.len += n_whole;
-            if rem != 0 {
-                ptr = ptr.add(n_whole);
-                *ptr = b'.';
-                ptr = ptr.add(1);
-                let mut frac_buf = [0_u8; BUF_LEN];
-                let n_frac = itoap::write_to_ptr(frac_buf.as_mut_ptr(), rem);
-                ptr::copy_nonoverlapping(ZEROS.as_ptr(), ptr, scale - n_frac);
-                ptr = ptr.add(scale - n_frac);
-                ptr::copy_nonoverlapping(frac_buf.as_mut_ptr(), ptr, n_frac);
-                buf.len += 1 + scale;
-                if trim_zeros {
-                    ptr = ptr.add(n_frac - 1);
-                    while *ptr == b'0' {
-                        ptr = ptr.sub(1);
-                        buf.len -= 1;
-                    }
-                }
-            }
-        }
-
-        buf
-    }
-
-    #[inline]
-    pub fn fmt_decimal(f: &mut Formatter<'_>, v: i128, scale: usize) -> fmt::Result {
-        let trim_zeros = get_trim_decimal_zeros();
-        f.write_str(fmt_float_string(format_decimal(v, scale, trim_zeros).as_str()).as_str())
-    }
+    let trim_zeros = get_trim_decimal_zeros();
+    let repr = format_decimal(v, scale, trim_zeros);
+    f.write_str(fmt_float_string(repr.as_str()).as_str())
 }
 
-#[cfg(feature = "dtype-decimal")]
-pub use decimal::fmt_decimal;
-
 #[cfg(all(
     test,
     feature = "temporal",