diff --git a/README.md b/README.md
index d50194f2..1b365154 100644
--- a/README.md
+++ b/README.md
@@ -41,6 +41,8 @@ $ make demo
 * [GET alive](docs/api.md#get-alive)  
 * [GET ready](docs/api.md#get-ready)
 * [GET sha](docs/api.md#get-sha)
+* [GET diff_lines_files(was_files,now_files)](docs/api.md#get-diff_lines_fileswas_filesnow_files)
+* [GET diff_summary_files(was_files,now_files)](docs/api.md#get-diff_summary_fileswas_filesnow_files)
 * [GET diff_lines(id,was_index,now_index)](docs/api.md#get-diff_linesidwas_indexnow_index)
 * [GET diff_summary(id,was_index,now_index)](docs/api.md#get-diff_summaryidwas_indexnow_index)
 
diff --git a/bin/echo_env_vars.sh b/bin/echo_env_vars.sh
index 48bc3c02..5ed6e790 100644
--- a/bin/echo_env_vars.sh
+++ b/bin/echo_env_vars.sh
@@ -28,6 +28,7 @@ echo_env_vars()
   #
   echo CYBER_DOJO_DIFFER_CLIENT_CONTAINER_NAME=test_differ_client
   echo CYBER_DOJO_DIFFER_SERVER_CONTAINER_NAME=test_differ_server
+  echo CYBER_DOJO_SAVER_CONTAINER_NAME=test_differ_saver
   #
   local -r AWS_ACCOUNT_ID=244531986313
   local -r AWS_REGION=eu-central-1
diff --git a/bin/lib.sh b/bin/lib.sh
index 7e329427..238f2fde 100644
--- a/bin/lib.sh
+++ b/bin/lib.sh
@@ -41,7 +41,7 @@ exit_non_zero()
 
 copy_in_saver_test_data()
 {
-  local -r SAVER_CID=$(docker ps --filter status=running --format '{{.Names}}' | grep "saver")
+  local -r SAVER_CID="${CYBER_DOJO_SAVER_CONTAINER_NAME}"
   local -r SRC_PATH=${ROOT_DIR}/test/server/data/cyber-dojo
   local -r DEST_PATH=/cyber-dojo
   # You cannot docker cp to a tmpfs, so tar-piping instead...
diff --git a/docker-compose.yml b/docker-compose.yml
index 606f9ab1..9077d3b3 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -49,6 +49,7 @@ services:
 
   saver:
     image: ${CYBER_DOJO_SAVER_IMAGE}:${CYBER_DOJO_SAVER_TAG}
+    container_name: ${CYBER_DOJO_SAVER_CONTAINER_NAME}
     user: saver
     ports: [ "${CYBER_DOJO_SAVER_PORT}:${CYBER_DOJO_SAVER_PORT}" ]
     env_file: [ .env ]
diff --git a/docs/api.md b/docs/api.md
index 2cd850e2..7d7d8eb4 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -3,7 +3,7 @@
 - - - -
 ## GET diff_lines(id,was_index,now_index)
 A diff of two sets of files (designated with **was_index** and **now_index**) from the kata designated with **id**.
-Also includes unchanged files and the content of files renamed but with identical content.
+Also handles unchanged files and files renamed but with identical content.
 - parameters
   * **id:String** the id of the kata.
   * **was_index:Integer** the test-submission index of the first set of files.
@@ -108,6 +108,139 @@ Also includes unchanged files and the content of files renamed but with identica
   ]
   ```
 
+- - - -
+## GET diff_lines_files(was_files,now_files)
+A diff of two sets of files supplied directly as parameters.
+Also handles unchanged files and files renamed but with identical content.
+- parameters
+  * **was_files:Hash** a Hash of filename to content Strings for the first set of files.
+  * **now_files:Hash** a Hash of filename to content Strings for the second set of files.
+  * eg
+  ```json
+  {
+    "was_files": { "hiker.h": "#ifndef HIKER_INCLUDED\n#define WIBBLE\n" },
+    "now_files": { "hiker.h": "#ifndef HIKER_INCLUDED\n#define HIKER_INCLUDED\n" }
+  }
+  ```
+- returns an Array of Hashes with each Hash being the diff of a single file. Each Hash has the following keys:
+  * "type" - one of the Strings [ "created", "deleted", "renamed", "changed", "unchanged" ].
+  * "old_filename" - the String name of the **was_files** file, unless "type" is "created" in which case **null**.
+  * "new_filename" - the String name of the **now_files** file, unless "type" is "deleted" in which case **null**.
+  * "lines" - an Array of Hashes, each Hash detailing an "added", "deleted", or "same" line, or
+    a "section" marker before a diff-chunk.
+  * "line_counts" - a Hash with three entries:
+    - "added" - the number of lines added to **now_files**'s "new_filename" file.
+    - "deleted" - the number of lines deleted from **was_files**'s "old_filename" file.
+    - "same" - the number of lines identical in both **now_files**'s "new_filename" and **was_files**'s "old_filename" file.
+  *
+  * eg a created file, which always has a single "section" marker.
+  ```json
+  [
+    {
+      "type": "created",
+      "old_filename": null,
+      "new_filename": "the.created.filename",
+      "lines":
+      [
+        { "type": "section", "index": 0 },              
+        { "type": "added", "line": "this file", "number": 1 },
+        { "type": "added", "line": "is new",    "number": 2 }
+      ],
+      "line_counts": { "added":2, "deleted":0, "same":0 }
+    }
+    ,
+    ...
+  ]
+  ```
+  * eg a deleted file, which always has a single "section" marker.
+  ```json
+  [
+    {
+      "type": "deleted",
+      "old_filename": "the.deleted.filename",
+      "new_filename": null,
+      "lines":
+      [
+        { "type": "section", "index": 0 },      
+        { "type": "deleted", "line": "this file",   "number": 1 },
+        { "type": "deleted", "line": "had 2 lines", "number": 2 }
+      ],
+      "line_counts": { "added":0, "deleted":2, "same":0 }
+    }
+    ,
+    ...
+  ]
+  ```
+  * eg a renamed file with identical content, which always has zero "section" markers.
+  ```json
+  [
+    {
+      "type": "renamed",
+      "old_filename": "the.old.filename",
+      "new_filename": "the.new.filename",
+      "lines":
+      [
+        { "type": "same", "line": "this file has",   "number": 1 },
+        { "type": "same", "line": "changed its name", "number": 2 }
+        { "type": "same", "line": "but not its contents", "number": 3 }
+      ],
+      "line_counts": { "added":0, "deleted":0, "same":3 }
+    }
+    ,
+    ...
+  ]
+  ```
+  * eg a changed file with two diff-chunks.
+  ```json
+  [
+    {
+      "type": "changed",
+      "old_filename": "hiker.h",
+      "new_filename": "hiker.h",
+      "lines":
+      [
+        { "type": "same", "line": "#ifndef HIKER_INCLUDED",   "number": 1 },
+        { "type": "section", "index": 0 },              
+        { "type": "deleted", "line": "#define WIBBLE",   "number": 2 },
+        { "type": "added",   "line": "#define HIKER_INCLUDED", "number": 2 },
+        { "type": "same", "line": "", "number": 3 },
+        { "type": "section", "index": 1 },              
+        { "type": "deleted", "line": "struct wibble",   "number": 3 },
+        { "type": "added",   "line": "struct hiker", "number": 3 },
+        { "type": "same", "line": "{", "number": 4 },        
+        { "type": "same", "line": "};", "number": 5 },        
+        { "type": "same", "line": "#endif", "number": 6 },        
+      ],
+      "line_counts": { "added":2, "deleted":2, "same":5 }
+    }
+    ,
+    ...
+  ]
+  ```
+
+- - - -
+## GET diff_summary_files(was_files,now_files)
+A diff of two sets of files supplied directly as parameters.
+Also handles unchanged files and files renamed but with identical content.
+- parameters
+  * **was_files:Hash** a Hash of filename to content Strings for the first set of files.
+  * **now_files:Hash** a Hash of filename to content Strings for the second set of files.
+  * eg
+  ```json
+  {
+    "was_files": { "hiker.h": "#ifndef HIKER_INCLUDED\n#define WIBBLE\n" },
+    "now_files": { "hiker.h": "#ifndef HIKER_INCLUDED\n#define HIKER_INCLUDED\n" }
+  }
+  ```
+- returns an Array of Hashes with each Hash being the diff of a single file. Each Hash has the following keys:
+  * "type" - one of the Strings [ "created", "deleted", "renamed", "changed", "unchanged" ].
+  * "old_filename" - the String name of the **was_files** file, unless "type" is "created" in which case **null**.
+  * "new_filename" - the String name of the **now_files** file, unless "type" is "deleted" in which case **null**.
+  * "line_counts" - a Hash with three entries:
+    - "added" - the number of lines added to **now_files**'s "new_filename" file.
+    - "deleted" - the number of lines deleted from **was_files**'s "old_filename" file.
+    - "same" - the number of lines identical in both **now_files**'s "new_filename" and **was_files**'s "old_filename" file.
+
 - - - -
 ## GET diff_summary(id,was_index,now_index)
 The same as `diff_lines` except its Hash entries do *not* include "lines".
diff --git a/source/server/app.rb b/source/server/app.rb
index b373604b..7ff4a278 100644
--- a/source/server/app.rb
+++ b/source/server/app.rb
@@ -7,6 +7,8 @@ class App < AppBase
   get_json(:ready,      Prober)
   get_json(:sha,        Prober)
 
-  get_json(:diff_lines,   Differ)
-  get_json(:diff_summary, Differ)
+  get_json(:diff_lines,         Differ)
+  get_json(:diff_summary,       Differ)
+  get_json(:diff_lines_files,   Differ)
+  get_json(:diff_summary_files, Differ)
 end
diff --git a/test/client/lib/id58_test_base.rb b/test/client/lib/id58_test_base.rb
index 01ba0f08..46bd91ac 100644
--- a/test/client/lib/id58_test_base.rb
+++ b/test/client/lib/id58_test_base.rb
@@ -1,8 +1,11 @@
 require 'English'
+require 'etc'
 require 'minitest/autorun'
 require 'minitest/reporters'
 require_relative 'slim_json_reporter'
 
+Minitest.parallel_executor = Minitest::Parallel::Executor.new(Etc.nprocessors)
+
 reporters = [
   Minitest::Reporters::DefaultReporter.new,
   Minitest::Reporters::SlimJsonReporter.new,
@@ -21,9 +24,12 @@ def initialize(arg)
     super
   end
 
+  parallelize_me!
+
   @@args = (ARGV.sort.uniq - ['--']) # eg 2m4
   @@seen_ids = []
   @@timings = {}
+  TIMINGS_LOCK = Mutex.new
 
   def self.test(id58, *lines, &test_block)
     src = test_block.source_location
@@ -34,7 +40,6 @@ def self.test(id58, *lines, &test_block)
 
     name58 = lines.join(' ')
     execute_around = lambda {
-      ENV['ID58'] = id58
       @_id58 = id58
       @_name58 = name58
       id58_setup
@@ -42,7 +47,7 @@ def self.test(id58, *lines, &test_block)
         t1 = Time.now
         instance_eval(&test_block)
         t2 = Time.now
-        @@timings["#{id58}:#{src_file}:#{src_line}:#{name58}"] = (t2 - t1)
+        TIMINGS_LOCK.synchronize { @@timings["#{id58}:#{src_file}:#{src_line}:#{name58}"] = (t2 - t1) }
       ensure
         puts $ERROR_INFO.message unless $ERROR_INFO.nil?
         id58_teardown
diff --git a/test/client/lib/run.sh b/test/client/lib/run.sh
index 8bb6e878..75d5b065 100755
--- a/test/client/lib/run.sh
+++ b/test/client/lib/run.sh
@@ -10,6 +10,7 @@ readonly TEST_FILES=(${MY_DIR}/../*_test.rb)
 readonly TEST_ARGS=(${@})
 
 readonly SCRIPT="
+\$stdout.sync = true
 require '${MY_DIR}/coverage.rb'
 %w(${TEST_FILES[*]}).shuffle.each{ |file|
   require file
diff --git a/test/server/lib/id58_test_base.rb b/test/server/lib/id58_test_base.rb
index 01ba0f08..46bd91ac 100644
--- a/test/server/lib/id58_test_base.rb
+++ b/test/server/lib/id58_test_base.rb
@@ -1,8 +1,11 @@
 require 'English'
+require 'etc'
 require 'minitest/autorun'
 require 'minitest/reporters'
 require_relative 'slim_json_reporter'
 
+Minitest.parallel_executor = Minitest::Parallel::Executor.new(Etc.nprocessors)
+
 reporters = [
   Minitest::Reporters::DefaultReporter.new,
   Minitest::Reporters::SlimJsonReporter.new,
@@ -21,9 +24,12 @@ def initialize(arg)
     super
   end
 
+  parallelize_me!
+
   @@args = (ARGV.sort.uniq - ['--']) # eg 2m4
   @@seen_ids = []
   @@timings = {}
+  TIMINGS_LOCK = Mutex.new
 
   def self.test(id58, *lines, &test_block)
     src = test_block.source_location
@@ -34,7 +40,6 @@ def self.test(id58, *lines, &test_block)
 
     name58 = lines.join(' ')
     execute_around = lambda {
-      ENV['ID58'] = id58
       @_id58 = id58
       @_name58 = name58
       id58_setup
@@ -42,7 +47,7 @@ def self.test(id58, *lines, &test_block)
         t1 = Time.now
         instance_eval(&test_block)
         t2 = Time.now
-        @@timings["#{id58}:#{src_file}:#{src_line}:#{name58}"] = (t2 - t1)
+        TIMINGS_LOCK.synchronize { @@timings["#{id58}:#{src_file}:#{src_line}:#{name58}"] = (t2 - t1) }
       ensure
         puts $ERROR_INFO.message unless $ERROR_INFO.nil?
         id58_teardown
diff --git a/test/server/lib/run.sh b/test/server/lib/run.sh
index 8bb6e878..75d5b065 100755
--- a/test/server/lib/run.sh
+++ b/test/server/lib/run.sh
@@ -10,6 +10,7 @@ readonly TEST_FILES=(${MY_DIR}/../*_test.rb)
 readonly TEST_ARGS=(${@})
 
 readonly SCRIPT="
+\$stdout.sync = true
 require '${MY_DIR}/coverage.rb'
 %w(${TEST_FILES[*]}).shuffle.each{ |file|
   require file