diff --git a/c-distance/credit.c b/c-distance/credit.c
index 2bff7f5..7ae5bbc 100644
--- a/c-distance/credit.c
+++ b/c-distance/credit.c
@@ -1,9 +1,43 @@
 #include <lua.h>
 #include <lauxlib.h>
 
-// implement it here
+int distance(lua_State* L) {
+    size_t len1, len2;
+    const char* str1 = luaL_checklstring(L, 1, &len1);
+    const char* str2 = luaL_checklstring(L, 2, &len2);
+    luaL_argcheck(L, len1 == len2, 2,
+            "Expect strings of equal length");
+    char* result = lua_newuserdata(L, len1);
+    int mismatches = 0;
+    int i;
+    for (i = 0; i < len1; i++) {
+        char a = str1[i];
+        char b = str2[i];
+        luaL_argcheck(L, a == '0' || a == '1', 1,
+                "Only characters '0' and '1' are allowed");
+        luaL_argcheck(L, b == '0' || b == '1', 1,
+                "Only characters '0' and '1' are allowed");
+        if (a == b) {
+            result[i] = a;
+        } else {
+            if (mismatches % 2 == 0) {
+                result[i] = a;
+            } else {
+                result[i] = b;
+            }
+            mismatches += 1;
+        }
+    }
+    if (mismatches % 2 == 1) {
+        // result = impossible
+        lua_pushnil(L);
+        return 1;
+    }
+    lua_pushlstring(L, result, len1);
+    return 1;
+}
 
 int luaopen_distance(lua_State* L) {
-    // return it here
-    return 0;
+    lua_pushcfunction(L, distance);
+    return 1;
 }
diff --git a/c-distance/spec.lua b/c-distance/spec.lua
index 6e396b8..b2c476b 100644
--- a/c-distance/spec.lua
+++ b/c-distance/spec.lua
@@ -1,21 +1,21 @@
 describe("distance (implementation)", function()
-    pending("loads module", function()
+    it("loads module", function()
         local f = require 'distance'
     end)
 
-    pending("returns equally distant string", function()
+    it("returns equally distant string", function()
         local f = require 'distance'
         local middle = f('00', '11')
         assert(middle == '01' or middle == '10')
     end)
 
-    pending("returns nil if no equally distant string exists",
+    it("returns nil if no equally distant string exists",
     function()
         local f = require 'distance'
         assert(f('0', '1') == nil)
     end)
 
-    pending("throws error if length if input strings differ",
+    it("throws error if length if input strings differ",
     function()
         local f = require 'distance'
         assert.has_error(function()
@@ -23,7 +23,7 @@ describe("distance (implementation)", function()
         end)
     end)
 
-    pending("throws error if an argument is not string",
+    it("throws error if an argument is not string",
     function()
         local f = require 'distance'
         assert.has_error(function()
@@ -34,7 +34,7 @@ describe("distance (implementation)", function()
         end)
     end)
 
-    pending("throws error if input contains non-0-or-1",
+    it("throws error if input contains non-0-or-1",
     function()
         local f = require 'distance'
         assert.has_error(function()
diff --git a/lua-distance/credit.lua b/lua-distance/credit.lua
index 068643b..e7f4f64 100644
--- a/lua-distance/credit.lua
+++ b/lua-distance/credit.lua
@@ -1,5 +1,42 @@
+local function dist(a, b)
+    local mismatches = 0
+    for i = 1, #a do
+        if a:sub(i, i) ~= b:sub(i, i) then
+            mismatches = mismatches + 1
+        end
+    end
+    return mismatches
+end
+
+local function has_error(f)
+    local status, err = pcall(f)
+    return not status
+end
+
 local function g(f)
-    -- implement it here
+    local p = f('00', '11')
+    assert(p == '01' or p == '10')
+    --
+    local p = f('0', '1')
+    assert(p == nil)
+    --
+    assert(has_error(function()
+        f()
+    end))
+    assert(has_error(function()
+        f('', '0')
+    end))
+    assert(has_error(function()
+        f('a', '0')
+    end))
+    assert(has_error(function()
+        f('00', '0')
+    end))
+    --
+    local p = f('00101', '10100')
+    assert(#p == 5)
+    assert(p:match('^[01]*$'))
+    assert(dist(p, '00101') == dist(p, '10100'))
 end
 
 return g
diff --git a/lua-distance/spec.lua b/lua-distance/spec.lua
index e49acc9..3786872 100644
--- a/lua-distance/spec.lua
+++ b/lua-distance/spec.lua
@@ -1,9 +1,9 @@
 describe("distance (unit test)", function()
-    pending("loads module", function()
+    it("loads module", function()
         local f = require 'distance.credit'
     end)
 
-    pending("accepts true solution", function()
+    it("accepts true solution", function()
         local f = require 'distance.credit'
         assert.has_no_error(function()
             f(function(A, B)
@@ -36,7 +36,7 @@ describe("distance (unit test)", function()
         end)
     end)
 
-    pending("accepts true solution 2", function()
+    it("accepts true solution 2", function()
         local f = require 'distance.credit'
         assert.has_no_error(function()
             f(function(A, B)
@@ -70,7 +70,7 @@ describe("distance (unit test)", function()
         end)
     end)
 
-    pending("throws on bad solutions", function()
+    it("throws on bad solutions", function()
         local f = require 'distance.credit'
         assert.has_error(function()
             f(function(A, B)