Add std/dec64 and std/dec64math (Douglas Crockford DEC64 decimal floating point)

changelog.md

@@ -68,6 +68,27 @@ errors.
 
 - `std/nre2` is added to replace deprecated NRE.
 
+- `std/dec64` adds Douglas Crockford's
+  [Dec64](https://www.crockford.com/dec64.html) 64-bit decimal floating-point
+  type: a packed `int64` with a 56-bit signed coefficient and an 8-bit signed
+  exponent. Provides arithmetic, comparison, hashing, parsing, and `$`,
+  plus a `'dec` custom literal (`1.25'dec`) and mixed `(Dec64, int)`
+  operator overloads so expressions like `price * 2 + 1` work without
+  wrapping every integer. Decimal-native rounding (`floor`, `ceil`,
+  `trunc`, `round`), `abs`, `sign`, and integer-exponent `pow` stay in
+  pure decimal throughout.
+- `std/dec64math` adds elementary mathematical functions for `Dec64`:
+  `sqrt`, `exp`, `ln`/`log`, `log2`, `log10`, `sin`, `cos`, `tan`,
+  `arcsin`, `arccos`, `arctan`, `arctan2`, `sinh`, `cosh`, `tanh`,
+  `pow(Dec64, Dec64)`, `nthRoot`, and exact-integer `factorial`.
+  Transcendentals are evaluated by round-tripping through `float64` and
+  delegating to `std/math`, which trades ~1 ulp at the 16th decimal
+  digit (Dec64's last) for a pure-Nim implementation that wraps the
+  battle-tested `libm` rather than reimplementing polynomials. For
+  computations that must stay decimal end-to-end (typically money), use
+  the operators in `std/dec64` directly; the transcendentals here are
+  for scientific or measurement-style work where the last ulp is noise.
+
 [//]: # "Changes:"
 
 - `std/math` The `^` symbol now supports floating-point as exponent in addition to the Natural type.

doc/lib.md

@@ -291,6 +291,14 @@ Math libraries
 * [complex](complex.html)
   Complex numbers and relevant mathematical operations.
 
+* [dec64](dec64.html)
+  Crockford's DEC64 64-bit decimal floating-point type: exact decimal
+  arithmetic with a 56-bit coefficient and 8-bit exponent.
+
+* [dec64math](dec64math.html)
+  Elementary mathematical functions (sqrt, exp, log, trig, factorial)
+  for the `Dec64` type.
+
 * [fenv](fenv.html)
   Floating-point environment. Handling of floating-point rounding and
   exceptions (overflow, zero-divide, etc.).