Use symbol ! as factorial

[totosugito]

Hi,
Thank you for creating this great plugin.
I am planned to add factorial function.
Example 5! -> 1 x 2 x 3 x 4 x 5 = 120
Do you have idea how to add this new function (using ! symbol) to compute factorial.
Thank you

[ram6ler]

Thanks for your interest in this library. Sorry for the delay in response!

I'm a little wary of adding support for factorials because they blow up so quickly and don't really fit into the context of double based numeric calculations, which this library aims at. (I have another library, dart-trotter, that uses BigInt representations of factorials and other values that come up in combinatorics so that arbitrarily large values can be handled conveniently — check it out if you're interested.)

However, we could implement this by first adding a function definition to lib/src/defs.dart:

.
.
.
/// A mapping of string representations to functions.
final Map<String, num Function(num)> oneParameterFunctionMap = {
  "abs": (num x) => x.abs(),
  .
  .
  .
  // Add this code: -----------------------------------------------------
  "fact": (num x) =>
      [for (var i = 1; i <= x; i++) i].fold<double>(1.0, (a, b) => a * b),
  // --------------------------------------------------------------- snip!
  .
  .
  .

and then replacing expressions of the form <integer>! with fact(<integer>), which can be done in the cleanExpression function in lib/src/helpers.dart, which is called before the expression is actually used to build a callable tree structure:

/// Returns a sanitized version of `expression`.
String cleanExpression(String expression) {
  final junk = RegExp(r"[^0-9a-zA-Z_.+\-/*%^(),]"),
  // Add this code: --------------------------------------
      factorial = RegExp(r"([0-9]+) *!");
  if (factorial.hasMatch(expression)) {
    expression = expression.replaceAllMapped(
        factorial, (match) => "fact(${match.group(1)!})");
  }
  // ----------------------------------------------- snip!
  .
  .
  .

Now something like

print("2 * 5!".interpret());

should work as expected.

Hope that helps! Feel free to fork and implement if it suits your needs!