Add the "%{ratio}" formatter (#67).

Makefile.am

@@ -32,6 +32,7 @@ src/pv/format/name.c \
 src/pv/format/previousline.c \
 src/pv/format/progressbar.c \
 src/pv/format/rate.c \
+src/pv/format/ratio.c \
 src/pv/format/sgr.c \
 src/pv/format/timer.c \
 src/pv/loop.c \

src/include/pv-internal.h

@@ -318,6 +318,7 @@ struct pvstate_s {
 
 		bool showing_timer;		 /* set if showing timer */
 		bool showing_bytes;		 /* set if showing byte/line count */
+		bool showing_ratio;		 /* set if showing monitoring in:out ratio */
 		bool showing_rate;		 /* set if showing transfer rate */
 		bool showing_last_written;	 /* set if displaying the last few bytes written */
 		bool showing_previous_line;	 /* set if displaying the previously output line */
@@ -602,6 +603,7 @@ pvdisplay_bytecount_t pv_formatter_fineta(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_rate(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_average_rate(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_bytes(pvformatter_args_t);
+pvdisplay_bytecount_t pv_formatter_ratio(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_buffer_percent(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_last_written(pvformatter_args_t);
 pvdisplay_bytecount_t pv_formatter_previous_line(pvformatter_args_t);

src/pv/display.c

@@ -533,6 +533,7 @@ pvdisplay_bytecount_t pv_formatter_segmentcontent(char *content, pvformatter_arg
 		{ "b", &pv_formatter_bytes, false },
 		{ "{bytes}", &pv_formatter_bytes, false },
 		{ "{transferred}", &pv_formatter_bytes, false },
+		{ "{ratio}", &pv_formatter_ratio, false },
 		{ "T", &pv_formatter_buffer_percent, false },
 		{ "{buffer-percent}", &pv_formatter_buffer_percent, false },
 		{ "A", &pv_formatter_last_written, false },
@@ -544,7 +545,6 @@ pvdisplay_bytecount_t pv_formatter_segmentcontent(char *content, pvformatter_arg
 		{ "{sgr:colour,...}", &pv_formatter_sgr, false },
 		{ NULL, NULL, false }
 	};
-	/* TODO: formatter for ratio, for "-M both" */
 	return format_component_array;
 }
 
@@ -621,6 +621,7 @@ static void pv__format_init(pvprogramstatus_t status, readonly_pvcontrol_t contr
 
 	display->showing_timer = false;
 	display->showing_bytes = false;
+	display->showing_ratio = false;
 	display->showing_rate = false;
 	display->showing_last_written = false;
 	display->showing_previous_line = false;

src/pv/format/ratio.c

@@ -0,0 +1,50 @@
+/*
+ * Formatter function for ratio of bytes transferred by the other monitored
+ * side to bytes transferred by this side.
+ *
+ * Copyright 2026 Andrew Wood
+ *
+ * License GPLv3+: GNU GPL version 3 or later; see `docs/COPYING'.
+ */
+
+#include "config.h"
+#include "pv.h"
+#include "pv-internal.h"
+
+
+/*
+ * Ratio of bytes or lines transferred by the other monitored side ("-M
+ * both") to the bytes or lines transferred by this side.
+ */
+pvdisplay_bytecount_t pv_formatter_ratio(pvformatter_args_t args)
+{
+	char content[128];		 /* flawfinder: ignore - always bounded */
+
+	args->display->showing_ratio = true;
+
+	if (0 == args->buffer_size)
+		return 0;
+
+	content[0] = '\0';
+
+	if (0 == args->transfer->otherside_transferred || 0 == args->transfer->transferred) {
+		(void) pv_snprintf(content, sizeof(content), "%s:%s", "-", "-");
+	} else if (args->transfer->otherside_transferred == args->transfer->transferred) {
+		(void) pv_snprintf(content, sizeof(content), "%d:%d", 1, 1);
+	} else {
+		long double side1, side2, ratio;
+
+		side1 = (long double) (args->transfer->otherside_transferred);
+		side2 = (long double) (args->transfer->transferred);
+
+		if (side1 > side2) {
+			ratio = side1 / side2;
+			(void) pv_snprintf(content, sizeof(content), "%.4Lg:%d", ratio, 1);
+		} else {
+			ratio = side2 / side1;
+			(void) pv_snprintf(content, sizeof(content), "%d:%.4Lg", 1, ratio);
+		}
+	}
+
+	return pv_formatter_segmentcontent(content, args);
+}