uucore: format: Use Option for Float precision

The default precision for float actually depends on the format.
It's _usually_ 6, but it's architecture-specific for hexadecimal
floats.

Set the precision as an Option, so that:
 - We don't need to sprinkle `6` in callers
 - We can actually handle unspecified precision correctly in
   float printing (next change).

src/uu/seq/src/seq.rs

@@ -169,7 +169,7 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
                     variant: FloatVariant::Decimal,
                     width: padding,
                     alignment: num_format::NumberAlignment::RightZero,
-                    precision,
+                    precision: Some(precision),
                     ..Default::default()
                 },
                 // format without precision: hexadecimal floats

src/uucore/src/lib/features/format/num_format.rs

@@ -220,7 +220,10 @@ pub struct Float {
     pub width: usize,
     pub positive_sign: PositiveSign,
     pub alignment: NumberAlignment,
-    pub precision: usize,
+    // For float, the default precision depends on the format, usually 6,
+    // but something architecture-specific for %a. Set this to None to
+    // use the default.
+    pub precision: Option<usize>,
 }
 
 impl Default for Float {
@@ -232,7 +235,7 @@ impl Default for Float {
             width: 0,
             positive_sign: PositiveSign::None,
             alignment: NumberAlignment::Left,
-            precision: 6,
+            precision: None,
         }
     }
 }
@@ -315,8 +318,8 @@ impl Formatter<&ExtendedBigDecimal> for Float {
         };
 
         let precision = match precision {
-            Some(CanAsterisk::Fixed(x)) => x,
-            None => 6, // Default float precision (C standard)
+            Some(CanAsterisk::Fixed(x)) => Some(x),
+            None => None,
             Some(CanAsterisk::Asterisk) => return Err(FormatError::WrongSpecType),
         };
 
@@ -360,8 +363,13 @@ fn format_float_non_finite(e: &ExtendedBigDecimal, case: Case) -> String {
     s
 }
 
-fn format_float_decimal(bd: &BigDecimal, precision: usize, force_decimal: ForceDecimal) -> String {
+fn format_float_decimal(
+    bd: &BigDecimal,
+    precision: Option<usize>,
+    force_decimal: ForceDecimal,
+) -> String {
     debug_assert!(!bd.is_negative());
+    let precision = precision.unwrap_or(6); // Default %f precision (C standard)
     if precision == 0 {
         let (bi, scale) = bd.as_bigint_and_scale();
         if scale == 0 && force_decimal != ForceDecimal::Yes {
@@ -376,11 +384,12 @@ fn format_float_decimal(bd: &BigDecimal, precision: usize, force_decimal: ForceD
 
 fn format_float_scientific(
     bd: &BigDecimal,
-    precision: usize,
+    precision: Option<usize>,
     case: Case,
     force_decimal: ForceDecimal,
 ) -> String {
     debug_assert!(!bd.is_negative());
+    let precision = precision.unwrap_or(6); // Default %e precision (C standard)
     let exp_char = match case {
         Case::Lowercase => 'e',
         Case::Uppercase => 'E',
@@ -421,11 +430,12 @@ fn format_float_scientific(
 
 fn format_float_shortest(
     bd: &BigDecimal,
-    precision: usize,
+    precision: Option<usize>,
     case: Case,
     force_decimal: ForceDecimal,
 ) -> String {
     debug_assert!(!bd.is_negative());
+    let precision = precision.unwrap_or(6); // Default %g precision (C standard)
 
     // Note: Precision here is how many digits should be displayed in total,
     // instead of how many digits in the fractional part.
@@ -503,11 +513,23 @@ fn format_float_shortest(
 
 fn format_float_hexadecimal(
     bd: &BigDecimal,
-    precision: usize,
+    precision: Option<usize>,
     case: Case,
     force_decimal: ForceDecimal,
 ) -> String {
     debug_assert!(!bd.is_negative());
+    // Default precision for %a is supposed to be sufficient to represent the
+    // exact value. This is platform specific, GNU coreutils uses a `long double`,
+    // which can be equivalent to a f64, f128, or an x86(-64) specific "f80".
+    // We have arbitrary precision in base 10, so we can't always represent
+    // the value exactly (e.g. 0.1 is c.ccccc...).
+    //
+    // Emulate x86(-64) behavior, where 64 bits are printed in total, that's
+    // 16 hex digits, including 1 before the decimal point (so 15 after).
+    //
+    // TODO: Make this configurable? e.g. arm64 value would be 28 (f128),
+    // arm value 13 (f64).
+    let precision = precision.unwrap_or(15);
 
     let (prefix, exp_char) = match case {
         Case::Lowercase => ("0x", 'p'),
@@ -706,7 +728,8 @@ mod test {
     #[test]
     fn decimal_float() {
         use super::format_float_decimal;
-        let f = |x| format_float_decimal(&BigDecimal::from_f64(x).unwrap(), 6, ForceDecimal::No);
+        let f =
+            |x| format_float_decimal(&BigDecimal::from_f64(x).unwrap(), Some(6), ForceDecimal::No);
         assert_eq!(f(0.0), "0.000000");
         assert_eq!(f(1.0), "1.000000");
         assert_eq!(f(100.0), "100.000000");
@@ -717,11 +740,23 @@ mod test {
         assert_eq!(f(1.999_999_5), "1.999999");
         assert_eq!(f(1.999_999_6), "2.000000");
 
-        let f = |x| format_float_decimal(&BigDecimal::from_f64(x).unwrap(), 0, ForceDecimal::Yes);
+        let f = |x| {
+            format_float_decimal(
+                &BigDecimal::from_f64(x).unwrap(),
+                Some(0),
+                ForceDecimal::Yes,
+            )
+        };
         assert_eq!(f(100.0), "100.");
 
         // Test arbitrary precision: long inputs that would not fit in a f64, print 24 digits after decimal point.
-        let f = |x| format_float_decimal(&BigDecimal::from_str(x).unwrap(), 24, ForceDecimal::No);
+        let f = |x| {
+            format_float_decimal(
+                &BigDecimal::from_str(x).unwrap(),
+                Some(24),
+                ForceDecimal::No,
+            )
+        };
         assert_eq!(f("0.12345678901234567890"), "0.123456789012345678900000");
         assert_eq!(
             f("1234567890.12345678901234567890"),
@@ -737,7 +772,11 @@ mod test {
         // TODO: Enable after https://github.com/akubera/bigdecimal-rs/issues/144 is fixed,
         // as our workaround is in .fmt.
         let f = |digits, scale| {
-            format_float_decimal(&BigDecimal::from_bigint(digits, scale), 6, ForceDecimal::No)
+            format_float_decimal(
+                &BigDecimal::from_bigint(digits, scale),
+                Some(6),
+                ForceDecimal::No,
+            )
         };
         assert_eq!(f(0.into(), 0), "0.000000");
         assert_eq!(f(0.into(), -10), "0.000000");
@@ -750,7 +789,7 @@ mod test {
         let f = |x| {
             format_float_scientific(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                None,
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -766,7 +805,7 @@ mod test {
         let f = |x| {
             format_float_scientific(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                Some(6),
                 Case::Uppercase,
                 ForceDecimal::No,
             )
@@ -778,7 +817,7 @@ mod test {
         let f = |digits, scale| {
             format_float_scientific(
                 &BigDecimal::from_bigint(digits, scale),
-                6,
+                Some(6),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -795,7 +834,7 @@ mod test {
         let f = |x| {
             format_float_scientific(
                 &BigDecimal::from_f64(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -811,7 +850,7 @@ mod test {
         let f = |x| {
             format_float_scientific(
                 &BigDecimal::from_f64(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::Yes,
             )
@@ -831,7 +870,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                None,
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -853,7 +892,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                None,
                 Case::Lowercase,
                 ForceDecimal::Yes,
             )
@@ -875,7 +914,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -894,7 +933,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::Yes,
             )
@@ -920,7 +959,7 @@ mod test {
         let f = |x| {
             format_float_hexadecimal(
                 &BigDecimal::from_str(x).unwrap(),
-                6,
+                Some(6),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -935,7 +974,7 @@ mod test {
         let f = |x| {
             format_float_hexadecimal(
                 &BigDecimal::from_str(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -947,7 +986,7 @@ mod test {
         let f = |x| {
             format_float_hexadecimal(
                 &BigDecimal::from_str(x).unwrap(),
-                0,
+                Some(0),
                 Case::Lowercase,
                 ForceDecimal::Yes,
             )
@@ -959,7 +998,7 @@ mod test {
         let f = |x| {
             format_float_hexadecimal(
                 &BigDecimal::from_str(x).unwrap(),
-                6,
+                Some(6),
                 Case::Uppercase,
                 ForceDecimal::No,
             )
@@ -971,7 +1010,7 @@ mod test {
         let f = |digits, scale| {
             format_float_hexadecimal(
                 &BigDecimal::from_bigint(digits, scale),
-                6,
+                Some(6),
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -1001,7 +1040,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                None,
                 Case::Lowercase,
                 ForceDecimal::No,
             )
@@ -1019,7 +1058,7 @@ mod test {
         let f = |x| {
             format_float_shortest(
                 &BigDecimal::from_f64(x).unwrap(),
-                6,
+                None,
                 Case::Lowercase,
                 ForceDecimal::No,
             )

src/uucore/src/lib/features/format/spec.rs

@@ -432,11 +432,13 @@ impl Spec {
                 precision,
             } => {
                 let width = resolve_asterisk(*width, &mut args).unwrap_or(0);
-                let precision = resolve_asterisk(*precision, &mut args).unwrap_or(6);
+                let precision = resolve_asterisk(*precision, &mut args);
                 let f: ExtendedBigDecimal = args.get_extended_big_decimal();
 
-                if precision as u64 > i32::MAX as u64 {
-                    return Err(FormatError::InvalidPrecision(precision.to_string()));
+                if precision.is_some_and(|p| p as u64 > i32::MAX as u64) {
+                    return Err(FormatError::InvalidPrecision(
+                        precision.unwrap().to_string(),
+                    ));
                 }
 
                 num_format::Float {