big: Add `internal_invmod`.

core/math/big/internal.odin

@@ -1740,6 +1740,29 @@ internal_int_neg :: proc(dest, src: ^Int, allocator := context.allocator) -> (er
 }
 internal_neg :: proc { internal_int_neg, };
 
+/*
+	hac 14.61, pp608.
+*/
+internal_int_inverse_modulo :: proc(dest, a, b: ^Int, allocator := context.allocator) -> (err: Error) {
+	context.allocator = allocator;
+	/*
+		For all n in N and n > 0, n = 0 mod 1.
+	*/
+	if internal_is_positive(a) && internal_cmp(b, 1) == 0 { return internal_zero(dest);	}
+
+	/*
+		`b` cannot be negative and has to be > 1
+	*/
+	if internal_is_negative(b) && internal_cmp(b, 1) != 1 { return .Invalid_Argument; }
+
+	/*
+		If the modulus is odd we can use a faster routine instead.
+	*/
+	if internal_is_odd(b) { return _private_inverse_modulo_odd(dest, a, b); }
+
+	return _private_inverse_modulo(dest, a, b);
+}
+internal_invmod :: proc{ internal_int_inverse_modulo, };
 
 /*
 	Helpers to extract values from the `Int`.
@@ -1991,7 +2014,11 @@ internal_int_get :: proc(a: ^Int, $T: typeid) -> (res: T, err: Error) where intr
 internal_get :: proc { internal_int_get, };
 
 internal_int_get_float :: proc(a: ^Int) -> (res: f64, err: Error) {
-	l   := min(a.used, 17); // log2(max(f64)) is approximately 1020, or 17 legs.
+	/*
+		log2(max(f64)) is approximately 1020, or 17 legs with the 64-bit storage.
+	*/
+	legs :: 1020 / _DIGIT_BITS;
+	l   := min(a.used, legs);
 	fac := f64(1 << _DIGIT_BITS);
 	d   := 0.0;