diff: Make the diff implementation configurable

kittens/diff/diff.conf

@@ -12,6 +12,11 @@ pygments_style default
 # The number of lines of context to show around each change.
 num_context_lines 3
 
+# The diff command to use. Must contain the placeholder _CONTEXT_
+# which will be replaced by the number of lines of context. The default
+# is to search the system for either git or diff and use that, if found.
+diff_cmd auto
+
 # Colors
 foreground            black
 background            white

kittens/diff/main.py

@@ -17,7 +17,7 @@ from ..tui.handler import Handler
 from ..tui.loop import Loop
 from .collect import create_collection, data_for_path, set_highlight_data
 from .config import init_config
-from .patch import Differ
+from .patch import Differ, set_diff_command
 from .render import LineRef, render_diff
 
 try:
@@ -299,6 +299,7 @@ def main(args):
     if os.path.isdir(left) != os.path.isdir(right):
         raise SystemExit('The items to be diffed should both be either directories or files. Comparing a directory to a file is not valid.')
     opts = init_config(args)
+    set_diff_command(opts.diff_cmd)
 
     loop = Loop()
     handler = DiffHandler(args, opts, left, right)

kittens/diff/patch.py

@@ -4,19 +4,40 @@
 
 import concurrent.futures
 import os
+import shlex
+import shutil
 import subprocess
 
 from .collect import lines_for_path
 from .diff_speedup import changed_center
 
 left_lines = right_lines = None
+GIT_DIFF = 'git diff --no-color --no-ext-diff --exit-code -U_CONTEXT_ --no-index --'
+DIFF_DIFF = 'diff -p -U _CONTEXT_ --'
+
+
+def find_differ():
+    if shutil.which('git'):
+        return GIT_DIFF
+    if shutil.which('diff'):
+        return DIFF_DIFF
+
+
+def set_diff_command(opt):
+    if opt == 'auto':
+        cmd = find_differ()
+        if cmd is None:
+            raise SystemExit('Failed to find either the git or diff programs on your system')
+    else:
+        cmd = opt
+    set_diff_command.cmd = cmd
 
 
 def run_diff(file1, file2, context=3):
     # returns: ok, is_different, patch
-    p = subprocess.Popen([
+    cmd = shlex.split(set_diff_command.cmd.replace('_CONTEXT_', str(context)))
-        'git', 'diff', '--no-color', '--no-ext-diff', '--exit-code', '-U' + str(context), '--no-index', '--'
+    p = subprocess.Popen(
-        ] + [file1, file2],
+            cmd + [file1, file2],
             stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.DEVNULL)
     stdout, stderr = p.communicate()
     returncode = p.wait()