implemented ISignedNumber

README.md

@@ -63,6 +63,23 @@ has not been benchmarked, including the allocating exponent-boundary path.
   0–999; other standard and custom formats throw `FormatException`. Default
   `G32` is **not** shortest-round-trip formatting. NaN, infinities, and signed
   zero are supported without converting through decimal.
+- `TryFormat(Span<char>, ...)` implements `ISpanFormattable` with the same formats.
+  It currently allocates via `ToString`; insufficient space returns `false`, writes
+  zero characters, and leaves the destination unchanged.
+
+`DoubleDouble` implements `ISignedNumber<DoubleDouble>`, including the inherited
+`INumberBase` contracts: binary radix, classification, absolute value, magnitude
+selection, increment/decrement, and generic numeric conversions. Integer/parity
+tests and magnitude comparisons retain both components. Magnitude ties prefer
+positive values for maximum and negative values for minimum, including signed zero;
+the `Number` variants prefer a number over NaN.
+
+`CreateChecked`, `CreateSaturating`, and `CreateTruncating` support built-in numeric
+types and `BigInteger`. Floating overflow produces signed infinity in all modes.
+Finite integer output truncates the exact sum, then throws on overflow, clamps,
+or retains the low destination-width bits, respectively. Decimal nonchecked output
+clamps out-of-range values and maps NaN to zero. These policies are distinct from
+the existing casts and `IConvertible` conversions above.
 
 Conversions, parsing, and formatting use allocating `BigInteger` intermediates
 where needed to preserve precision; no additional dependency is required.
@@ -87,16 +104,24 @@ bool success = DoubleDouble.TryParse("1.25e-2".AsSpan(), CultureInfo.InvariantCu
     out DoubleDouble parsed);
 ```
 
-- Supported grammar: optional sign, ASCII decimal digits with an optional decimal
+- Provider-only finite grammar: optional sign, ASCII decimal digits with an optional decimal
   separator, and optional `e`/`E` exponent with sign and digits. At least one
   mantissa digit is required; `.5` and `1.` are accepted with invariant culture.
   Surrounding whitespace is allowed; internal whitespace is not.
 - Signs and the decimal separator come from the supplied culture; a null or
   omitted provider uses the current culture. Culture-specific NaN and infinity
-  symbols are recognized case-insensitively. Signed zero is preserved.
-- Group separators, currency, parentheses, hexadecimal notation, digit separators,
-  and `NumberStyles` overloads are not supported.
-- Input is limited to **4096 characters**, including surrounding whitespace.
+  symbols are recognized case-insensitively. The additional alias `inf` accepts an
+  optional culture-specific sign (`inf`, `+inf`, `-inf` with invariant culture).
+  Exact custom special symbols take precedence over the alias. Special values accept
+  surrounding whitespace and signs even with `NumberStyles.None`; ordinary finite
+  numbers still obey the supplied style flags. Signed zero is preserved.
+- Provider-only overloads reject grouping, currency, and parentheses. Explicit
+  `NumberStyles` overloads support decimal flags through `NumberStyles.Any`, including
+  grouping, currency, parentheses, and trailing signs; group sizes are not validated.
+  Hexadecimal, binary, and undefined style flags throw `ArgumentException`, including
+  in `TryParse`. Hexadecimal notation and programming-language digit separators
+  remain unsupported.
+- Input is limited to **2048 characters**, including surrounding whitespace.
   Huge exponents are bounded before constructing powers of ten. Well-formed
   overflow succeeds with signed infinity; underflow rounds to a subnormal or
   signed zero. A second rounding just below the overflow midpoint stays finite.
@@ -107,8 +132,9 @@ bool success = DoubleDouble.TryParse("1.25e-2".AsSpan(), CultureInfo.InvariantCu
 ## Deferred scope
 
 The planned `PreciseMath` static class and its `Abs`, `Sqrt`, `Pow`, `Exp`, and
-`Log` functions are not implemented. Broader generic-math interfaces, expanded
-parsing/round-trip formatting, and performance benchmarks remain deferred.
+`Log` functions are not implemented. Generic-math interfaces beyond `ISignedNumber`,
+additional text formats/general round-trip formatting, and performance benchmarks
+remain deferred.
 Replacing allocating arithmetic boundary fallbacks is also deferred; the current
 `BigInteger` paths remain in place. That optimization does not require removing
 `BigInteger` from conversions, parsing, formatting, or independent test oracles.