From bae777be370c5e1e8fe1a69fad0d7479eff95190 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Wed, 14 Jan 2026 23:57:13 +0100
Subject: -d flag to print deltas

---
 main.cpp | 67 ++++++++++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/main.cpp b/main.cpp
index 047b556..f5e0448 100644
--- a/main.cpp
+++ b/main.cpp
@@ -7,13 +7,23 @@ static void print_usage(const char *argv0) {
 	std::cerr
 		<< "Usage: " << argv0 << " [OPTIONS]\n"
 		<< "  -h    Show this help\n"
-		<< "  -t    Print tags in local time format\n"
+		<< "  -d    Print deltas in seconds, not timestamps\n"
 		<< "  -s    Print tags in timestamp format\n"
+		<< "  -t    Print tags in local time format (default)\n"
 		<< std::flush;
 }
 
+enum Mode {
+	M_STAMP,
+	M_LOCALTIME,
+	M_DELTA,
+};
+
 int main(int argc, char **argv) {
-	bool print_local_time = true;
+	// use std::cin, stdout, std::cerr
+	std::ios::sync_with_stdio(false);
+
+	Mode mode = M_LOCALTIME;
 	for (int i = 1; i < argc; i++) {
 		if (argv[i][0] != '-') {
 			std::cerr << "Unrecognised argument: '" << argv[i] << "'" << std::endl;
@@ -23,33 +33,48 @@ int main(int argc, char **argv) {
 		for (int j = 1; argv[i][j]; j++) {
 			switch (argv[i][j]) {
 				case 'h': print_usage(argv[0]); return 0;
-				case 't': print_local_time = true; break;
-				case 's': print_local_time = false; break;
+				case 'd': mode = M_DELTA; break;
+				case 's': mode = M_STAMP; break;
+				case 't': mode = M_LOCALTIME; break;
+				default: print_usage(argv[0]); return 1;
 			}
 		}
 	}
 
+	struct timeval tv;
+
+	double prevtm;
+	if (mode == M_DELTA) {
+		gettimeofday(&tv, nullptr);
+		prevtm = tv.tv_sec + tv.tv_usec * 1.0e-6;
+	}
+
 	std::string line;
 	while (std::getline(std::cin, line)) {
-		struct timeval tv;
 		gettimeofday(&tv, nullptr);
 
-		char buffer[128];
-		ssize_t nw = 0;
-		if (print_local_time) {
-			const struct tm *tm = localtime(&tv.tv_sec);
-			nw = strftime(buffer, sizeof buffer - 7, "%Y-%m-%d %H:%M:%S.", tm);
-		} else {
-			nw = snprintf(buffer, sizeof buffer - 7, "%ld.", tv.tv_sec);
-		}
-
-		int multiplier = 100000;
-		for (int i = 0; i < 6; i++) {
-			buffer[nw + i] = '0' + tv.tv_usec / multiplier % 10;
-			multiplier /= 10;
+		switch (mode) {
+			case M_STAMP:
+				printf("%.6lf %s\n", tv.tv_sec + tv.tv_usec * 1.0e-6, line.data());
+				break;
+			case M_LOCALTIME: {
+				const struct tm *tm = localtime(&tv.tv_sec);
+				char buffer[128];
+				ssize_t nw = strftime(buffer, sizeof buffer - 7, "%Y-%m-%d %H:%M:%S.", tm);
+				for (int i = 5; i >= 0; i--) {
+					buffer[nw + i] = '0' + tv.tv_usec % 10;
+					tv.tv_usec /= 10;
+				}
+				buffer[nw + 6] = '\0';
+				printf("%s %s\n", buffer, line.data());
+				break;
+			}
+			case M_DELTA: {
+				double tm = tv.tv_sec + tv.tv_usec * 1.0e-6;
+				printf("%.6lf %s\n", tm - prevtm, line.data());
+				prevtm = tm;
+				break;
+			}
 		}
-		buffer[nw + 6] = '\0';
-
-		std::cout << buffer << ' ' << line << std::endl;
 	}
 }
-- 
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