Fix modular_inverse not handling m == 1 correctly (#16)

Author: vil02

.spec/math/modular_inverse_spec.lua

@@ -13,7 +13,13 @@ describe("Modular inverse", function()
 	it("should handle cases when inputs are not co-prime", function()
 		assert.equal(nil, modular_inverse(2, 2))
 		assert.equal(nil, modular_inverse(5, 15))
+	end)
+
+	it("should handle cases when modulus is 1", function()
+		assert.equal(nil, modular_inverse(-1, 1))
 		assert.equal(nil, modular_inverse(0, 1))
+		assert.equal(nil, modular_inverse(1, 1))
+		assert.equal(nil, modular_inverse(2, 1))
 	end)
 
 	it("should throw error when modulus is zero", function()

src/math/modular_inverse.lua

@@ -8,8 +8,11 @@ return function(
 	m -- modulus
 )
 	assert(m > 0, "modulus must be positive")
+	if m == 1 then
+		return nil
+	end
 	local gcd, x, _ = extended_gcd(a % m, m)
-	if a ~= 0 and gcd == 1 then
+	if gcd == 1 then
 		-- Ensure that result is in (0, m)
 		return x % m
 	end