Add Hamming distance for strings (#15)

.spec/string/distance/hamming_spec.lua

@@ -0,0 +1,39 @@
+describe("Hamming distance", function()
+	local hamming_distance = require("string.distance.hamming")
+
+	local function check_basic(a, b, expected)
+		assert.equal(expected, hamming_distance(a, b))
+		assert.equal(0, hamming_distance(a, a))
+	end
+
+	local function check_with_reversed_inputs(a, b, expected)
+		assert.equal(expected, hamming_distance(a:reverse(), b:reverse()))
+		assert.equal(0, hamming_distance(a:reverse(), a:reverse()))
+	end
+
+	local function check_all(a, b, expected)
+		check_basic(a, b, expected)
+		check_with_reversed_inputs(a, b, expected)
+	end
+
+	local function test(a, b, expected)
+		check_all(a, b, expected)
+		check_all(b, a, expected)
+	end
+
+	it("should handle general cases", function()
+		test("", "", 0)
+		test("a", "a", 0)
+		test("a", "A", 1)
+		test("cąx", "cąy", 1)
+		test("mama", "tata", 2)
+		test("xxx", "Xxx", 1)
+		test("1234", "2345", 4)
+	end)
+
+	it("should throw error for inputs of different length", function()
+		assert.has_error(function()
+			hamming_distance("z", "")
+		end)
+	end)
+end)

src/string/distance/hamming.lua

@@ -0,0 +1,15 @@
+-- Computes the Hamming "edit" distance for two strings of the same length:
+-- The number of indices at which the corresponding bytes are different.
+return function(
+	a, -- some string
+	b -- other string
+)
+	assert(#a == #b, "lengths don't match")
+	local dist = 0
+	for i = 1, #a do
+		if a:byte(i) ~= b:byte(i) then
+			dist = dist + 1
+		end
+	end
+	return dist
+end