diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..6828848
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.5)
+project(md5sum LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 11)
+
+add_library(md5sum STATIC md5.cc md5_impl.cc)
diff --git a/main.cc b/main.cc
deleted file mode 100644
index 88b1ca2..0000000
--- a/main.cc
+++ /dev/null
@@ -1,10 +0,0 @@
-#include <iostream>
-#include "md5sum.h"
-
-int main() {
-    auto ret = md5sum("dnomd343");
-    std::cout << ret << std::endl;
-
-    auto val = md5file("/Users/jucong.lin/klotski/third_party/md5sum/343.txt");
-    std::cout << val << std::endl;
-}
diff --git a/md5.cc b/md5.cc
index 4113085..e03fe08 100644
--- a/md5.cc
+++ b/md5.cc
@@ -1,50 +1,54 @@
-#include "md5sum.h"
-#include "md5def.h"
-
-static char hb2hex(uint8_t hb) {
-    hb &= 0xF;
-    return hb < 10 ? '0' + hb : hb - 10 + 'a';
-}
-
-std::string md5sum(const std::string &dat) {
-    return md5sum(dat.c_str(), dat.length());
-}
-
-std::string md5sum(const void *dat, size_t len) {
-    uint8_t out[16];
-    md5_bin(dat, len, out);
-
-    std::string res;
-    for (auto x : out) {
-        res.push_back(hb2hex(x >> 4));
-        res.push_back(hb2hex(x));
-    }
-    return res;
-}
-
-std::string md5file(const char *filename) {
-	std::FILE *file = std::fopen(filename, "rb");
-	std::string res = md5file(file);
-	std::fclose(file);
-	return res;
-}
-
-std::string md5file(std::FILE *file) {
-	MD5_CTX c;
-    md5_init(&c);
-
-	uint8_t out[16];
-	size_t len = 0;
-    char buff[BUFSIZ];
-	while ((len = std::fread(buff, sizeof(char), BUFSIZ, file)) > 0) {
-		md5_update(&c, buff, len);
-	}
-	md5_final(out, &c);
-
-	std::string res;
-	for (auto x : out) {
-        res.push_back(hb2hex(x >> 4));
-        res.push_back(hb2hex(x));
-    }
-	return res;
-}
+#include "md5sum.h"
+#include "md5def.h"
+
+namespace md5 {
+
+static char hb2hex(uint8_t hb) {
+    hb &= 0xF;
+    return char(hb < 10 ? hb + '0' : hb - 10 + 'a');
+}
+
+std::string md5sum(const std::string &dat) {
+    return md5sum(dat.c_str(), dat.length());
+}
+
+std::string md5sum(const void *dat, size_t len) {
+    uint8_t out[16];
+    md5_bin(dat, len, out);
+
+    std::string res;
+    for (auto x : out) {
+        res.push_back(hb2hex(x >> 4));
+        res.push_back(hb2hex(x));
+    }
+    return res;
+}
+
+std::string md5file(const char *filename) {
+	std::FILE *file = std::fopen(filename, "rb");
+	std::string res = md5file(file);
+	std::fclose(file);
+	return res;
+}
+
+std::string md5file(std::FILE *file) {
+	MD5_CTX c;
+    md5_init(&c);
+
+    size_t len;
+	uint8_t out[16];
+    char buff[BUFSIZ];
+	while ((len = std::fread(buff, sizeof(char), BUFSIZ, file)) > 0) {
+		md5_update(&c, buff, len);
+	}
+	md5_final(out, &c);
+
+	std::string res;
+	for (auto x : out) {
+        res.push_back(hb2hex(x >> 4));
+        res.push_back(hb2hex(x));
+    }
+	return res;
+}
+
+} // namespace md5
diff --git a/md5_impl.cc b/md5_impl.cc
index d408c8e..dc4882e 100644
--- a/md5_impl.cc
+++ b/md5_impl.cc
@@ -23,6 +23,8 @@
 #define GET(n) (ctx->block[(n)])
 #endif
 
+namespace md5 {
+
 static const void* md5_body(MD5_CTX *ctx, const void *data, uint32_t size) {
     const auto *ptr = (uint8_t*)data;
 
@@ -208,3 +210,5 @@ void md5_bin(const void *dat, size_t len, uint8_t out[16]) {
     md5_update(&c, dat, len);
     md5_final(out, &c);
 }
+
+} // namespace md5
diff --git a/md5def.h b/md5def.h
index 2d7e194..838b1f5 100644
--- a/md5def.h
+++ b/md5def.h
@@ -1,5 +1,7 @@
 #pragma once
 
+namespace md5 {
+
 typedef struct {
     uint32_t lo, hi;
     uint32_t a, b, c, d;
@@ -11,3 +13,5 @@ void md5_init(MD5_CTX *ctx);
 void md5_final(uint8_t *result, MD5_CTX *ctx);
 void md5_update(MD5_CTX *ctx, const void *data, uint32_t size);
 void md5_bin(const void *dat, size_t len, uint8_t out[16]);
+
+} // namespace md5
diff --git a/md5sum.h b/md5sum.h
index 3d521df..a6e47e8 100644
--- a/md5sum.h
+++ b/md5sum.h
@@ -1,12 +1,13 @@
-#ifndef MD5_H
-#define MD5_H
-
-#include <string>
-
-std::string md5sum(const std::string &dat);
-std::string md5sum(const void *dat, size_t len);
-
-std::string md5file(std::FILE *file);
-std::string md5file(const char *filename);
-
-#endif // end of MD5_H
+#pragma once
+
+#include <string>
+
+namespace md5 {
+
+std::string md5sum(const std::string &dat);
+std::string md5sum(const void *dat, size_t len);
+
+std::string md5file(std::FILE *file);
+std::string md5file(const char *filename);
+
+} // namespace md5