Fix power function to return a result

CHANGELOG.txt

@@ -1,3 +1,9 @@
+2.0.0
+-----
+
+* The power function was changed to return a result to prevent calling it with
+  a negative exponent.
+
 1.0.0
 -----
 

README.md

@@ -40,7 +40,7 @@ pub fn main() {
     bigi.from_int(65_535)
   )
 
-  // 1001764965203423232489536175780127875223912737784875709632508486855447029778...
+  // Ok(1001764965203423232489536175780127875223912737784875709632508486855447029778...)
 }
 ```
 

src/bigi.gleam

@@ -109,9 +109,11 @@ pub fn modulo_no_zero(
 ) -> Result(BigInt, Nil)
 
 /// Raise the base to the exponent.
+///
+/// If the exponent is negative, an error is returned.
 @external(erlang, "bigi_ffi", "power")
 @external(javascript, "./bigi_ffi.mjs", "power")
-pub fn power(base a: BigInt, exponent b: BigInt) -> BigInt
+pub fn power(base a: BigInt, exponent b: BigInt) -> Result(BigInt, Nil)
 
 /// Get the digits in a given bigint as a list of integers.
 ///

src/bigi_ffi.erl

@@ -51,12 +51,15 @@ modulo(A, B) -> ((A rem B) + B) rem B.
 modulo_no_zero(_, 0) -> {error, nil};
 modulo_no_zero(A, B) -> {ok, modulo(A, B)}.
 
-power(_, 0) ->
+power(_, Exp) when Exp < 0 -> {error, nil};
+power(Base, Exp) -> {ok, do_power(Base, Exp)}.
+
+do_power(_, 0) ->
     1;
-power(A, 1) ->
+do_power(A, 1) ->
     A;
-power(A, N) ->
-    B = power(A, N div 2),
+do_power(A, N) ->
+    B = do_power(A, N div 2),
     B * B *
         (case N rem 2 of
             0 -> 1;

src/bigi_ffi.mjs

@@ -100,5 +100,9 @@ export function modulo_no_zero(a, b) {
 }
 
 export function power(a, b) {
-  return a ** b;
+  if (b < 0) {
+    return new Error(undefined);
+  }
+
+  return new Ok(a ** b);
 }