feat: implement MoonBit Ryu number-to-string algorithm

Author: bobzhang

double/internal/ryu/common.mbt

@@ -12,6 +12,39 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Returns e == 0 ? 1 : ceil(log_2(5^e)); requires 0 <= e <= 3528.
+
+///|
+fn pow5bits(e : Int) -> Int {
+  ((e * 1217359).reinterpret_as_uint() >> 19).reinterpret_as_int() + 1
+}
+
+///|
+fn copy_special_str(sign : Bool, exponent : Bool, mantissa : Bool) -> String {
+  if mantissa {
+    return "NaN"
+  }
+  let s = if sign { "-" } else { "" }
+  if exponent {
+    return s + "Infinity"
+  }
+  return s + "0.0"
+}
+
+// Returns floor(log_10(5^e)); requires 0 <= e <= 2620.
+
+///|
+fn log10Pow5(e : Int) -> Int {
+  ((e * 732923).reinterpret_as_uint() >> 20).reinterpret_as_int()
+}
+
+// Returns floor(log_10(2^e)); requires 0 <= e <= 1650.
+
+///|
+fn log10Pow2(e : Int) -> Int {
+  ((e * 78913).reinterpret_as_uint() >> 18).reinterpret_as_int()
+}
+
 // MoonBit implementation of Ryu, https://github.com/ulfjack/ryu
 // This is a fork of the ryu crate adjusted to comply to the ECMAScript number-to-string algorithm,