Change Unit.multiplier type from decimal to float

Remove remnants related to BigDecimal.

lib/core_ext/float.rb

@@ -0,0 +1,54 @@
+# If a decimal string with at most 15 significant digits is converted to the
+# IEEE 754 double-precision format, giving a normal number, and then converted
+# back to a decimal string with the same number of digits, the final result
+# should match the original string.
+# If an IEEE 754 double-precision number is converted to a decimal string with
+# at least 17 significant digits, and then converted back to double-precision
+# representation, the final result must match the original number:
+#   ("%.16e" % self).to_f == self
+class Float
+  def to_scientific
+    sign, significand, exponent = split
+    sign + significand + (exponent != 0 ? "e#{exponent}" : '')
+  end
+
+  # Converts value to HTML formatted scientific notation.
+  def to_html(show_unity: true)
+    result, significand, exponent = split
+    result += significand if significand != '1' || (exponent == 0 && show_unity)
+    if exponent != 0
+      result += "×" if significand != '1'
+      result += "10<sup>% d</sup>" % exponent
+    end
+    result.html_safe
+  end
+
+  # Assume #finite? is true.
+  def limit(precision = DIG)
+    return 0.0 if precision.zero?
+    sign, significand, exponent = split(DIG_MAX)
+    "#{sign}#{significand[..precision]}e#{exponent}".to_f
+  end
+
+  private
+
+  SPLIT_FLOAT = /(-?)(.*?)\.?0*e(.*)/
+
+  # Format `%e` displays starting from significant digit (not 0).
+  def split(digits = DIG)
+    return ['', to_s[..2], 0] unless finite?
+    return ['', '0', 0] if digits.zero?
+    ("%.#{digits - 1}e" % self).match(SPLIT_FLOAT).captures
+      .then { |sign, significand, exponent| [sign, significand, exponent.to_i] }
+  end
+
+  # The maximum number of significant decimal digits in a double-precision
+  # floating point number.
+  DIG_MAX = 17
+  # Smallest and largest double-precision floating point numbers with DIG
+  # precision.
+  # TODO: change MIN_15 to MIN.ceil(MIN_10_EXP - DIG) after #ceil fix in Ruby
+  # v4.0.5: https://bugs.ruby-lang.org/issues/22079
+  MIN_15 = MIN.ceil(-(MIN_10_EXP - 1))
+  MAX_15 = MAX.floor(-(MAX_10_EXP - DIG + 1))
+end