Fixing BigDecimal conversion for PostgreSQL

Now working properly with numbers, such as `0.01` and `0.012`.

Author: Julius de Bruijn

sqlx-core/src/postgres/types/bigdecimal.rs

@@ -1,7 +1,7 @@
 use std::cmp;
 use std::convert::{TryFrom, TryInto};
 
-use bigdecimal::BigDecimal;
+use bigdecimal::{BigDecimal, ToPrimitive, Zero};
 use num_bigint::{BigInt, Sign};
 
 use crate::decode::Decode;
@@ -77,65 +77,64 @@ impl TryFrom<&'_ BigDecimal> for PgNumeric {
     type Error = BoxDynError;
 
     fn try_from(decimal: &BigDecimal) -> Result<Self, BoxDynError> {
-        let base_10_to_10000 = |chunk: &[u8]| chunk.iter().fold(0i16, |a, &d| a * 10 + d as i16);
+        if decimal.is_zero() {
+            return Ok(PgNumeric::Number {
+                sign: PgNumericSign::Positive,
+                scale: 0,
+                weight: 0,
+                digits: vec![],
+            });
+        }
 
         // NOTE: this unfortunately copies the BigInt internally
         let (integer, exp) = decimal.as_bigint_and_exponent();
 
-        // this routine is specifically optimized for base-10
-        // FIXME: is there a way to iterate over the digits to avoid the Vec allocation
-        let (sign, base_10) = integer.to_radix_be(10);
-
-        // weight is positive power of 10000
-        // exp is the negative power of 10
-        let weight_10 = base_10.len() as i64 - exp;
-
         // scale is only nonzero when we have fractional digits
         // since `exp` is the _negative_ decimal exponent, it tells us
         // exactly what our scale should be
         let scale: i16 = cmp::max(0, exp).try_into()?;
 
-        // there's an implicit +1 offset in the interpretation
-        let weight: i16 = if weight_10 <= 0 {
-            weight_10 / 4 - 1
-        } else {
-            // the `-1` is a fix for an off by 1 error (4 digits should still be 0 weight)
-            (weight_10 - 1) / 4
-        }
-        .try_into()?;
+        let (sign, uint) = integer.into_parts();
+        let mut mantissa = uint.to_u128().unwrap();
 
-        let digits_len = if base_10.len() % 4 != 0 {
-            base_10.len() / 4 + 1
-        } else {
-            base_10.len() / 4
-        };
+        // If our scale is not a multiple of 4, we need to go to the next
+        // multiple.
+        let groups_diff = scale % 4;
+        if groups_diff > 0 {
+            let remainder = 4 - groups_diff as u32;
+            let power = 10u32.pow(remainder as u32) as u128;
 
-        let offset = weight_10.rem_euclid(4) as usize;
+            mantissa = mantissa * power;
+        }
 
-        let mut digits = Vec::with_capacity(digits_len);
+        // Array to store max mantissa of Decimal in Postgres decimal format.
+        let mut digits = Vec::with_capacity(8);
 
-        if let Some(first) = base_10.get(..offset) {
-            if offset != 0 {
-                digits.push(base_10_to_10000(first));
-            }
+        // Convert to base-10000.
+        while mantissa != 0 {
+            digits.push((mantissa % 10_000) as i16);
+            mantissa /= 10_000;
         }
 
-        if let Some(rest) = base_10.get(offset..) {
-            digits.extend(
-                rest.chunks(4)
-                    .map(|chunk| base_10_to_10000(chunk) * 10i16.pow(4 - chunk.len() as u32)),
-            );
-        }
+        // Change the endianness.
+        digits.reverse();
+
+        // Weight is number of digits on the left side of the decimal.
+        let digits_after_decimal = (scale + 3) as u16 / 4;
+        let weight = digits.len() as i16 - digits_after_decimal as i16 - 1;
 
+        // Remove non-significant zeroes.
         while let Some(&0) = digits.last() {
             digits.pop();
         }
 
+        let sign = match sign {
+            Sign::Plus | Sign::NoSign => PgNumericSign::Positive,
+            Sign::Minus => PgNumericSign::Negative,
+        };
+
         Ok(PgNumeric::Number {
-            sign: match sign {
-                Sign::Plus | Sign::NoSign => PgNumericSign::Positive,
-                Sign::Minus => PgNumericSign::Negative,
-            },
+            sign,
             scale,
             weight,
             digits,

tests/postgres/types.rs

@@ -396,7 +396,14 @@ test_type!(bigdecimal<sqlx::types::BigDecimal>(Postgres,
     "10000::numeric" == "10000".parse::<sqlx::types::BigDecimal>().unwrap(),
     "0.1::numeric" == "0.1".parse::<sqlx::types::BigDecimal>().unwrap(),
     "0.01::numeric" == "0.01".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.012::numeric" == "0.012".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.0123::numeric" == "0.0123".parse::<sqlx::types::BigDecimal>().unwrap(),
     "0.01234::numeric" == "0.01234".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.012345::numeric" == "0.012345".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.0123456::numeric" == "0.0123456".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.01234567::numeric" == "0.01234567".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.012345678::numeric" == "0.012345678".parse::<sqlx::types::BigDecimal>().unwrap(),
+    "0.0123456789::numeric" == "0.0123456789".parse::<sqlx::types::BigDecimal>().unwrap(),
     "12.34::numeric" == "12.34".parse::<sqlx::types::BigDecimal>().unwrap(),
     "12345.6789::numeric" == "12345.6789".parse::<sqlx::types::BigDecimal>().unwrap(),
 ));