Add support for changing the `diff` command via an environment variable

While the file arguments remain fixed the actual `diff` application and its output style can be changed using the `CHANGE_LOG_DIFF_CMD` environment variable.

3 files changed, 18 insertions, 3 deletions

diff --git a/src/change_log.cc b/src/change_log.cc
index c592f20..44a6046 100644
--- a/src/change_log.cc
+++ b/src/change_log.cc
@@ -23,7 +23,13 @@ void init() {
 		logger   = std::make_unique<utility::Logger>(STDERR_FILENO);
 	}
 
-	tracker = std::make_unique<tracking::ChangeTracker>(logger.get());
+	if ( getenv("CHANGE_LOG_DIFF_CMD") != NULL ) {
+		tracker = std::make_unique<tracking::ChangeTracker>(
+			logger.get(), getenv("CHANGE_LOG_DIFF_CMD")
+		);
+	} else {
+		tracker = std::make_unique<tracking::ChangeTracker>(logger.get());
+	}
 }
 
 ssize_t write(int fd, const void* buffer, size_t count) {
diff --git a/src/tracking/change_tracker.cc b/src/tracking/change_tracker.cc
index 913604e..9bbdcb6 100644
--- a/src/tracking/change_tracker.cc
+++ b/src/tracking/change_tracker.cc
@@ -24,10 +24,14 @@ boost::process::context getDefaultContext() {
 
 namespace tracking {
 
-ChangeTracker::ChangeTracker(utility::Logger* logger):
+ChangeTracker::ChangeTracker(utility::Logger* logger, const std::string& diff_cmd):
+	diff_cmd_(diff_cmd),
 	logger_(logger),
 	children_() { }
 
+ChangeTracker::ChangeTracker(utility::Logger* logger):
+	ChangeTracker(logger, "diff -p") { }
+
 ChangeTracker::~ChangeTracker() {
 	for ( auto&& tracked : this->children_ ) {
 		std::get<DIFF_PROCESS>(tracked)->get_stdin().close();
@@ -70,7 +74,10 @@ bool ChangeTracker::track(const std::string& file_path) {
 	auto result = this->children_.emplace(
 		file_path,
 		std::make_unique<boost::process::child>(
-			boost::process::launch_shell("diff -p - " + file_path, getDefaultContext())
+			boost::process::launch_shell(
+				this->diff_cmd_ + " - " + file_path,
+				getDefaultContext()
+			)
 		)
 	);
 
diff --git a/src/tracking/change_tracker.h b/src/tracking/change_tracker.h
index 54af4f7..24251e5 100644
--- a/src/tracking/change_tracker.h
+++ b/src/tracking/change_tracker.h
@@ -11,6 +11,7 @@ namespace tracking {
 
 class ChangeTracker {
 	public:
+		ChangeTracker(utility::Logger*, const std::string&);
 		ChangeTracker(utility::Logger*);
 		~ChangeTracker();
 
@@ -19,6 +20,7 @@ class ChangeTracker {
 		bool track(const std::string&);
 
 	private:
+		const std::string      diff_cmd_;
 		utility::Logger* const logger_;
 
 		std::unordered_map<