diff --git a/flake.nix b/flake.nix
index 602f7c4..04a1788 100644
--- a/flake.nix
+++ b/flake.nix
@@ -15,151 +15,190 @@
     };
     pugixml-src = {
       url = "github:zeux/pugixml/latest";
-      flake = false; 
+      flake = false;
     };
   };
 
-  outputs = flakes: let
-    version = "1.3.0";
-    self = flakes.self.packages.x86_64-linux;
-    nixpkgs = flakes.nixpkgs.legacyPackages.x86_64-linux.pkgsStatic;
-    nixpkgs-dyn = flakes.nixpkgs.legacyPackages.x86_64-linux;
-    gourou-src = flakes.gourou-src;
-    updfparser-src = flakes.updfparser-src;
-    base64-src = flakes.base64-src;
-    pugixml-src = flakes.pugixml-src;
-    cxx = "${nixpkgs.stdenv.cc}/bin/x86_64-unknown-linux-musl-g++";
-    ar = "${nixpkgs.stdenv.cc.bintools.bintools_bin}/bin/x86_64-unknown-linux-musl-ar";
-    obj-flags = "-O2 -static";
-  in rec {
-    packages.x86_64-linux.libzip-static = nixpkgs.libzip.overrideAttrs (prev: {
-      cmakeFlags = (prev.cmakeFlags or []) ++ [
-        "-DBUILD_SHARED_LIBS=OFF"
-        "-DBUILD_EXAMPLES=OFF"
-        "-DBUILD_DOC=OFF"
-        "-DBUILD_TOOLS=OFF"
-        "-DBUILD_REGRESS=OFF"
-      ];
-      outputs = ["out"];
-    });
-    packages.x86_64-linux.base64 = derivation {
-      name = "updfparser";
-      system = "x86_64-linux";
-      builder = "${nixpkgs.bash}/bin/bash";
-      PATH = "${nixpkgs.coreutils}/bin";
-      args = ["-c" ''
-        mkdir -p $out/include/base64
-        cp ${base64-src}/Base64.h $out/include/base64/Base64.h
-      ''];
+  outputs = flakes:
+    let
+      version = "1.3.0";
+      self = flakes.self.packages.x86_64-linux;
+      nixpkgs = flakes.nixpkgs.legacyPackages.x86_64-linux.pkgsStatic;
+      nixpkgs-dyn = flakes.nixpkgs.legacyPackages.x86_64-linux;
+      nixpkgs-fmt = flakes.nixpkgs-fmt.defaultPackage.x86_64-linux;
+      gourou-src = flakes.gourou-src;
+      updfparser-src = flakes.updfparser-src;
+      base64-src = flakes.base64-src;
+      pugixml-src = flakes.pugixml-src;
+      cxx = "${nixpkgs.stdenv.cc}/bin/x86_64-unknown-linux-musl-g++";
+      ar = "${nixpkgs.stdenv.cc.bintools.bintools_bin}/bin/x86_64-unknown-linux-musl-ar";
+      obj-flags = "-O2 -static";
+    in
+    rec {
+      packages.x86_64-linux.libzip-static = nixpkgs.libzip.overrideAttrs (prev: {
+        cmakeFlags = (prev.cmakeFlags or [ ]) ++ [
+          "-DBUILD_SHARED_LIBS=OFF"
+          "-DBUILD_EXAMPLES=OFF"
+          "-DBUILD_DOC=OFF"
+          "-DBUILD_TOOLS=OFF"
+          "-DBUILD_REGRESS=OFF"
+        ];
+        outputs = [ "out" ];
+      });
+      packages.x86_64-linux.base64 = derivation {
+        name = "updfparser";
+        system = "x86_64-linux";
+        builder = "${nixpkgs.bash}/bin/bash";
+        PATH = "${nixpkgs.coreutils}/bin";
+        args = [
+          "-c"
+          ''
+            mkdir -p $out/include/base64
+            cp ${base64-src}/Base64.h $out/include/base64/Base64.h
+          ''
+        ];
+      };
+      packages.x86_64-linux.updfparser = derivation {
+        name = "updfparser";
+        system = "x86_64-linux";
+        builder = "${nixpkgs.bash}/bin/bash";
+        PATH = "${nixpkgs.coreutils}/bin";
+        args = [
+          "-c"
+          ''
+            ${cxx} \
+              -c ${updfparser-src}/src/*.cpp \
+              -I ${updfparser-src}/include \
+              ${obj-flags}
+            mkdir -p $out/lib
+            ${ar} crs $out/lib/libupdfparser.a *.o
+          ''
+        ];
+      };
+      packages.x86_64-linux.gourou = derivation {
+        name = "gourou";
+        system = "x86_64-linux";
+        builder = "${nixpkgs.bash}/bin/bash";
+        PATH = "${nixpkgs.coreutils}/bin";
+        args = [
+          "-c"
+          ''
+            ${cxx} \
+              -c \
+              ${gourou-src}/src/*.cpp \
+              ${pugixml-src}/src/pugixml.cpp \
+              -I ${self.base64}/include \
+              -I ${gourou-src}/include \
+              -I ${pugixml-src}/src \
+              -I ${updfparser-src}/include \
+              ${obj-flags}
+            mkdir -p $out/lib $out/debug
+            ${ar} crs $out/lib/libgourou.a *.o
+            cp *.o $out/debug
+          ''
+        ];
+      };
+      packages.x86_64-linux.utils-common = derivation {
+        name = "utils-common";
+        system = "x86_64-linux";
+        builder = "${nixpkgs.bash}/bin/bash";
+        PATH = "${nixpkgs.coreutils}/bin";
+        args = [
+          "-c"
+          ''
+            ${cxx} \
+              -c ${gourou-src}/utils/drmprocessorclientimpl.cpp \
+                 ${gourou-src}/utils/utils_common.cpp \
+              -I ${gourou-src}/utils \
+              -I ${gourou-src}/include \
+              -I ${pugixml-src}/src \
+              -I ${nixpkgs.openssl.dev}/include \
+              -I ${nixpkgs.curl.dev}/include \
+              -I ${nixpkgs.zlib.dev}/include \
+              -I ${self.libzip-static}/include \
+              ${obj-flags}
+            mkdir -p $out/lib
+            ${ar} crs $out/lib/libutils-common.a *.o
+          ''
+        ];
+      };
+      packages.x86_64-linux.knock = derivation {
+        name = "knock";
+        system = "x86_64-linux";
+        builder = "${nixpkgs.bash}/bin/bash";
+        PATH = "${nixpkgs.coreutils}/bin";
+        args = [
+          "-c"
+          ''
+            mkdir -p $out/bin
+            ${cxx} \
+              -o $out/bin/knock \
+              ${./src/knock.cpp} \
+              -D KNOCK_VERSION='"${version}"' \
+              -Wl,--as-needed -static \
+              ${self.utils-common}/lib/libutils-common.a \
+              ${self.gourou}/lib/libgourou.a \
+              ${self.updfparser}/lib/libupdfparser.a \
+              -Wl,--start-group \
+              ${self.libzip-static}/lib/libzip.a \
+              ${nixpkgs.libnghttp2}/lib/libnghttp2.a \
+              ${nixpkgs.libidn2.out}/lib/libidn2.a \
+              ${nixpkgs.libunistring}/lib/libunistring.a \
+              ${nixpkgs.libssh2}/lib/libssh2.a \
+              ${nixpkgs.zstd.out}/lib/libzstd.a \
+              ${nixpkgs.zlib}/lib/libz.a \
+              ${nixpkgs.openssl.out}/lib/libcrypto.a \
+              ${nixpkgs.curl.out}/lib/libcurl.a \
+              ${nixpkgs.openssl.out}/lib/libssl.a \
+              -static-libgcc -static-libstdc++ \
+              -Wl,--end-group \
+              -I ${gourou-src}/utils \
+              -I ${gourou-src}/include \
+              -I ${pugixml-src}/src \
+              -I ${nixpkgs.openssl.dev}/include \
+              -I ${nixpkgs.curl.dev}/include \
+              -I ${nixpkgs.zlib.dev}/include \
+              -I ${self.libzip-static}/include
+          ''
+        ];
+      };
+      packages.x86_64-linux.default = self.knock;
+      packages.x86_64-linux.tests = nixpkgs-dyn.stdenv.mkDerivation {
+        name = "tests";
+        src = ./tests;
+        buildInputs = [
+          (nixpkgs-dyn.python3.withPackages (p: [
+            p.beautifulsoup4
+            p.requests
+          ]))
+        ];
+        patchPhase = ''
+          substituteInPlace tests.py --replace "./result/bin/knock" "${self.knock}/bin/knock"
+        '';
+        installPhase = ''
+          mkdir -p $out/bin
+          cp tests.py $out/bin/tests
+          chmod +x $out/bin/tests
+        '';
+      };
+      devShell.x86_64-linux = nixpkgs.mkShell {
+        packages = [
+          # nix formatter
+          nixpkgs-dyn.nixpkgs-fmt
+          # python formatter
+          nixpkgs-dyn.black
+          # cpp formatter
+          nixpkgs-dyn.clang-tools
+        ];
+        shellHook = ''
+          fmt () {
+            set -ex
+            nixpkgs-fmt .
+            black ./tests
+            clang-format -i --verbose src/*.cpp
+            set +ex
+          }
+        '';
+      };
     };
-    packages.x86_64-linux.updfparser = derivation {
-      name = "updfparser";
-      system = "x86_64-linux";
-      builder = "${nixpkgs.bash}/bin/bash";
-      PATH = "${nixpkgs.coreutils}/bin";
-      args = [ "-c" ''
-        ${cxx} \
-          -c ${updfparser-src}/src/*.cpp \
-          -I ${updfparser-src}/include \
-          ${obj-flags}
-        mkdir -p $out/lib
-        ${ar} crs $out/lib/libupdfparser.a *.o
-      '' ];
-    };
-    packages.x86_64-linux.gourou = derivation {
-      name = "gourou";
-      system = "x86_64-linux";
-      builder = "${nixpkgs.bash}/bin/bash";
-      PATH = "${nixpkgs.coreutils}/bin";
-      args = [ "-c" ''
-        ${cxx} \
-          -c \
-          ${gourou-src}/src/*.cpp \
-          ${pugixml-src}/src/pugixml.cpp \
-          -I ${self.base64}/include \
-          -I ${gourou-src}/include \
-          -I ${pugixml-src}/src \
-          -I ${updfparser-src}/include \
-          ${obj-flags}
-        mkdir -p $out/lib $out/debug
-        ${ar} crs $out/lib/libgourou.a *.o
-        cp *.o $out/debug
-      '' ];
-    };
-    packages.x86_64-linux.utils-common = derivation {
-      name = "utils-common";
-      system = "x86_64-linux";
-      builder = "${nixpkgs.bash}/bin/bash";
-      PATH = "${nixpkgs.coreutils}/bin";
-      args = [ "-c" ''
-        ${cxx} \
-          -c ${gourou-src}/utils/drmprocessorclientimpl.cpp \
-             ${gourou-src}/utils/utils_common.cpp \
-          -I ${gourou-src}/utils \
-          -I ${gourou-src}/include \
-          -I ${pugixml-src}/src \
-          -I ${nixpkgs.openssl.dev}/include \
-          -I ${nixpkgs.curl.dev}/include \
-          -I ${nixpkgs.zlib.dev}/include \
-          -I ${self.libzip-static}/include \
-          ${obj-flags}
-        mkdir -p $out/lib
-        ${ar} crs $out/lib/libutils-common.a *.o
-      '' ];
-    };
-    packages.x86_64-linux.knock = derivation {
-      name = "knock";
-      system = "x86_64-linux";
-      builder = "${nixpkgs.bash}/bin/bash";
-      PATH = "${nixpkgs.coreutils}/bin";
-      args = [ "-c" ''
-        mkdir -p $out/bin
-        ${cxx} \
-          -o $out/bin/knock \
-          ${./src/knock.cpp} \
-          -D KNOCK_VERSION='"${version}"' \
-          -Wl,--as-needed -static \
-          ${self.utils-common}/lib/libutils-common.a \
-          ${self.gourou}/lib/libgourou.a \
-          ${self.updfparser}/lib/libupdfparser.a \
-          -Wl,--start-group \
-          ${self.libzip-static}/lib/libzip.a \
-          ${nixpkgs.libnghttp2}/lib/libnghttp2.a \
-          ${nixpkgs.libidn2.out}/lib/libidn2.a \
-          ${nixpkgs.libunistring}/lib/libunistring.a \
-          ${nixpkgs.libssh2}/lib/libssh2.a \
-          ${nixpkgs.zstd.out}/lib/libzstd.a \
-          ${nixpkgs.zlib}/lib/libz.a \
-          ${nixpkgs.openssl.out}/lib/libcrypto.a \
-          ${nixpkgs.curl.out}/lib/libcurl.a \
-          ${nixpkgs.openssl.out}/lib/libssl.a \
-          -static-libgcc -static-libstdc++ \
-          -Wl,--end-group \
-          -I ${gourou-src}/utils \
-          -I ${gourou-src}/include \
-          -I ${pugixml-src}/src \
-          -I ${nixpkgs.openssl.dev}/include \
-          -I ${nixpkgs.curl.dev}/include \
-          -I ${nixpkgs.zlib.dev}/include \
-          -I ${self.libzip-static}/include
-      '' ];
-    };
-    packages.x86_64-linux.default = self.knock;
-    packages.x86_64-linux.tests = nixpkgs-dyn.stdenv.mkDerivation {
-      name = "tests";
-      src = ./tests;
-      buildInputs = [ (nixpkgs-dyn.python3.withPackages(p: [
-        p.beautifulsoup4
-        p.requests
-      ])) ];
-      patchPhase = ''
-        substituteInPlace tests.py --replace "./result/bin/knock" "${self.knock}/bin/knock"
-      '';
-      installPhase = ''
-        mkdir -p $out/bin
-        cp tests.py $out/bin/tests
-        chmod +x $out/bin/tests
-      '';
-    };
-  };
-}
\ No newline at end of file
+}
diff --git a/readme.md b/readme.md
index 008414a..585a17c 100644
--- a/readme.md
+++ b/readme.md
@@ -44,6 +44,14 @@ nix run .#tests -- ./tests/workspace
 
 Test books can be found [here](https://www.adobe.com/solutions/ebook/digital-editions/sample-ebook-library.html).
 
+### Formatting
+
+```
+nix develop
+fmt
+exit
+```
+
 ## The Name
 
 The name comes from the [D&D 5e spell](https://roll20.net/compendium/dnd5e/Knock#content) for freeing locked items:
diff --git a/src/knock.cpp b/src/knock.cpp
index 014d50d..601fe6a 100644
--- a/src/knock.cpp
+++ b/src/knock.cpp
@@ -1,119 +1,120 @@
-#include <filesystem>
 #include "drmprocessorclientimpl.h"
-#include "libgourou_common.h"
 #include "libgourou.h"
+#include "libgourou_common.h"
+#include <filesystem>
 
 #ifndef KNOCK_VERSION
-    #error KNOCK_VERSION must be defined
+#error KNOCK_VERSION must be defined
 #endif
 
 std::string get_data_dir();
 void verify_absence(std::string file);
 void verify_presence(std::string file);
 
-int main(int argc, char** argv) try {
+int main(int argc, char **argv) try {
 
-    if (argc == 1) {
-        std::cout
-            << "info: knock version " << KNOCK_VERSION << ", libgourou version "
-                << LIBGOUROU_VERSION << "\n"
-            << "usage: " << argv[0] << " [ACSM]" << "\n"
-            << "result: converts file ACSM to a plain EPUB/PDF if present, otherwise prints this"
-            << std::endl;
-        return EXIT_SUCCESS;
-    }
+  if (argc == 1) {
+    std::cout << "info: knock version " << KNOCK_VERSION
+              << ", libgourou version " << LIBGOUROU_VERSION << "\n"
+              << "usage: " << argv[0] << " [ACSM]\n"
+              << "result: converts file ACSM to a plain EPUB/PDF if present, "
+                 "otherwise prints this"
+              << std::endl;
+    return EXIT_SUCCESS;
+  }
 
-    if (argc != 2) {
-        throw std::invalid_argument("the ACSM file must be passed as an argument");
-    }
+  if (argc != 2) {
+    throw std::invalid_argument("the ACSM file must be passed as an argument");
+  }
 
-    const std::string acsm_file = argv[1];
-    verify_presence(acsm_file);
-    const std::string acsm_stem = acsm_file.substr(0, acsm_file.find_last_of("."));
-    const std::string drm_file = acsm_stem + ".drm";
-    const std::string out_file = acsm_stem + ".out";
-    verify_absence(drm_file);
-    verify_absence(out_file);
-    const std::string knock_data = get_data_dir();
+  const std::string acsm_file = argv[1];
+  verify_presence(acsm_file);
+  const std::string acsm_stem =
+      acsm_file.substr(0, acsm_file.find_last_of("."));
+  const std::string drm_file = acsm_stem + ".drm";
+  const std::string out_file = acsm_stem + ".out";
+  verify_absence(drm_file);
+  verify_absence(out_file);
+  const std::string knock_data = get_data_dir();
 
-    DRMProcessorClientImpl client;
-	gourou::DRMProcessor* processor = gourou::DRMProcessor::createDRMProcessor(
-	    &client,
-        false, // don't "always generate a new device" (default)
-        knock_data
-    );
+  DRMProcessorClientImpl client;
+  gourou::DRMProcessor *processor = gourou::DRMProcessor::createDRMProcessor(
+      &client,
+      false, // don't "always generate a new device" (default)
+      knock_data);
 
-    std::cout << "anonymously signing in..." << std::endl;
-    processor->signIn("anonymous", "");
-    processor->activateDevice();
+  std::cout << "anonymously signing in..." << std::endl;
+  processor->signIn("anonymous", "");
+  processor->activateDevice();
 
-    std::cout << "downloading the file from Adobe..." << std::endl;
-    gourou::FulfillmentItem* item = processor->fulfill(acsm_file);
-    gourou::DRMProcessor::ITEM_TYPE type = processor->download(item, drm_file);
-    
-    std::cout << "removing DRM from the file..." << std::endl;
-    std::string ext_file;
-    std::string file_type;
-    switch (type) {
-        case gourou::DRMProcessor::ITEM_TYPE::PDF: {
-            // for pdfs the function moves the pdf while removing drm
-            processor->removeDRM(drm_file, out_file, type, nullptr, 0);
-            std::filesystem::remove(drm_file);
-            ext_file = acsm_stem + ".pdf";
-            file_type = "PDF";
-            break;
-        }
-        case gourou::DRMProcessor::ITEM_TYPE::EPUB: {
-            // for epubs the drm is removed in-place so in == out
-            processor->removeDRM(drm_file, drm_file, type, nullptr, 0);
-            std::filesystem::rename(drm_file, out_file);
-            ext_file = acsm_stem + ".epub";
-            file_type = "EPUB";
-            break;
-        }
-    }
+  std::cout << "downloading the file from Adobe..." << std::endl;
+  gourou::FulfillmentItem *item = processor->fulfill(acsm_file);
+  gourou::DRMProcessor::ITEM_TYPE type = processor->download(item, drm_file);
 
-    if (std::filesystem::exists(ext_file)) {
-        std::cerr
-            << "warning: failed to update file extension; " + ext_file + " already exists"
-            << std::endl;
-        ext_file = out_file;
-    } else {
-        std::filesystem::rename(out_file, ext_file);
-    }
+  std::cout << "removing DRM from the file..." << std::endl;
+  std::string ext_file;
+  std::string file_type;
+  switch (type) {
+  case gourou::DRMProcessor::ITEM_TYPE::PDF: {
+    // for pdfs the function moves the pdf while removing drm
+    processor->removeDRM(drm_file, out_file, type, nullptr, 0);
+    std::filesystem::remove(drm_file);
+    ext_file = acsm_stem + ".pdf";
+    file_type = "PDF";
+    break;
+  }
+  case gourou::DRMProcessor::ITEM_TYPE::EPUB: {
+    // for epubs the drm is removed in-place so in == out
+    processor->removeDRM(drm_file, drm_file, type, nullptr, 0);
+    std::filesystem::rename(drm_file, out_file);
+    ext_file = acsm_stem + ".epub";
+    file_type = "EPUB";
+    break;
+  }
+  }
 
-    std::cout << file_type + " file generated at " + ext_file << std::endl;
+  if (std::filesystem::exists(ext_file)) {
+    std::cerr << "warning: failed to update file extension; " + ext_file +
+                     " already exists"
+              << std::endl;
+    ext_file = out_file;
+  } else {
+    std::filesystem::rename(out_file, ext_file);
+  }
 
-    return 0;
+  std::cout << file_type + " file generated at " + ext_file << std::endl;
 
-} catch (const gourou::Exception& e) {
-    std::cerr << "error:\n" << e.what();
-    return EXIT_FAILURE;
-} catch (const std::exception& e) {
-    std::cerr << "error: " << e.what() << std::endl;
-    return EXIT_FAILURE;
+  return 0;
+
+} catch (const gourou::Exception &e) {
+  std::cerr << "error:\n" << e.what();
+  return EXIT_FAILURE;
+} catch (const std::exception &e) {
+  std::cerr << "error: " << e.what() << std::endl;
+  return EXIT_FAILURE;
 }
 
 std::string get_data_dir() {
-    char* xdg_data_home = std::getenv("XDG_DATA_HOME");
-    std::string knock_data;
-    if (xdg_data_home != nullptr) {
-        knock_data = xdg_data_home;
-    } else {
-        knock_data = std::string(std::getenv("HOME")) + "/.local/share";
-    }
-    knock_data += "/knock/acsm";
-    return knock_data;
+  char *xdg_data_home = std::getenv("XDG_DATA_HOME");
+  std::string knock_data;
+  if (xdg_data_home != nullptr) {
+    knock_data = xdg_data_home;
+  } else {
+    knock_data = std::string(std::getenv("HOME")) + "/.local/share";
+  }
+  knock_data += "/knock/acsm";
+  return knock_data;
 }
 
 void verify_absence(std::string file) {
-    if (std::filesystem::exists(file)) {
-        throw std::runtime_error("file " + file + " must be moved out of the way or deleted");
-    }
+  if (std::filesystem::exists(file)) {
+    throw std::runtime_error("file " + file +
+                             " must be moved out of the way or deleted");
+  }
 }
 
 void verify_presence(std::string file) {
-    if (!std::filesystem::exists(file)) {
-        throw std::runtime_error("file " + file + " does not exist");
-    }
+  if (!std::filesystem::exists(file)) {
+    throw std::runtime_error("file " + file + " does not exist");
+  }
 }
\ No newline at end of file
diff --git a/tests/tests.py b/tests/tests.py
index 3c51ef7..60d25a7 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -14,7 +14,7 @@ if not knock.exists():
 if len(sys.argv) != 2:
     print(
         "error: missing required argument: directory in which to perform the tests",
-        file=sys.stderr
+        file=sys.stderr,
     )
     sys.exit()
 
@@ -31,18 +31,18 @@ if workspace.exists():
     shutil.rmtree(workspace)
 workspace.mkdir()
 
-html = requests \
-    .get("https://www.adobe.com/solutions/ebook/digital-editions/sample-ebook-library.html") \
-    .text
-soup = BeautifulSoup(html, 'html.parser')
+html = requests.get(
+    "https://www.adobe.com/solutions/ebook/digital-editions/sample-ebook-library.html"
+).text
+soup = BeautifulSoup(html, "html.parser")
 
 links = []
-for a_tag in soup.find_all('a'):
+for a_tag in soup.find_all("a"):
     if a_tag.string != "Download eBook":
         continue
     if not urlparse(a_tag.get("href")).path.endswith(".acsm"):
         continue
-    
+
     links.append(a_tag.get("href"))
 
     if len(links) >= 10: