From: Earle F. Philhower, III <redacted>
Date: Sat, 2 May 2020 17:45:22 +0000 (-0700)
Subject: output/csv: Avoid accumulated timestamp error for odd samplerates
X-Git-Url: https://sigrok.org/gitaction?a=commitdiff_plain;h=01ac3eedfa23d8fea0804426393404e19e70035b;p=libsigrok.git

output/csv: Avoid accumulated timestamp error for odd samplerates

The conversion of sample rates to sample periods and the repeated
addition of truncated values (integer variables) resulted in the
accumulation of errors in the timestamp column for odd samplerate
values. How to reproduce:

  $ sigrok-cli -d demo:analog_channels=0 \
      -c samplerate=6000000 --samples 1200001 \
      -O csv:time=true | tail

Accept the additional cost to reduce the error. Always get the timestamp
in a new calculation based on the sample number and the sample rate.

This addresses bug #1027.

Signed-off-by: Earle F. Philhower, III <redacted>
[ gsi: rephrased commit message, how to reprodue ]
---

diff --git a/src/output/csv.c b/src/output/csv.c
index ea328c4b..791ddc39 100644
--- a/src/output/csv.c
+++ b/src/output/csv.c
@@ -93,7 +93,8 @@ struct context {
 	uint32_t num_samples;
 	uint32_t channel_count, logic_channel_count;
 	uint32_t channels_seen;
-	uint64_t period;
+	uint64_t sample_rate;
+	uint64_t sample_scale;
 	uint64_t sample_time;
 	uint8_t *previous_sample;
 	float *analog_samples;
@@ -218,31 +219,28 @@ static GString *gen_header(const struct sr_output *o,
 	GString *header;
 	GSList *channels, *l;
 	unsigned int num_channels, i;
-	uint64_t samplerate = 0, sr;
 	char *samplerate_s;
 
 	ctx = o->priv;
 	header = g_string_sized_new(512);
 
-	if (ctx->period == 0) {
+	if (ctx->sample_rate == 0) {
 		if (sr_config_get(o->sdi->driver, o->sdi, NULL,
 				  SR_CONF_SAMPLERATE, &gvar) == SR_OK) {
-			samplerate = g_variant_get_uint64(gvar);
+			ctx->sample_rate = g_variant_get_uint64(gvar);
 			g_variant_unref(gvar);
 		}
 
 		i = 0;
-		sr = 1;
-		while (sr < samplerate) {
+		ctx->sample_scale = 1;
+		while (ctx->sample_scale < ctx->sample_rate) {
 			i++;
-			sr *= 1000;
+			ctx->sample_scale *= 1000;
 		}
-		if (samplerate)
-			ctx->period = sr / samplerate;
 		if (i < ARRAY_SIZE(xlabels))
 			ctx->xlabel = xlabels[i];
-		sr_info("Set sample period to %" PRIu64 " %s",
-			ctx->period, ctx->xlabel);
+		sr_info("Set sample rate, scale to %" PRIu64 ", %" PRIu64 " %s",
+			ctx->sample_rate, ctx->sample_scale, ctx->xlabel);
 	}
 	ctx->title = (o->sdi && o->sdi->driver) ? o->sdi->driver->longname : "unknown";
 
@@ -271,8 +269,8 @@ static GString *gen_header(const struct sr_output *o,
 			g_string_truncate(header, header->len - 1);
 		}
 		g_string_append_printf(header, "\n");
-		if (samplerate != 0) {
-			samplerate_s = sr_samplerate_string(samplerate);
+		if (ctx->sample_rate != 0) {
+			samplerate_s = sr_samplerate_string(ctx->sample_rate);
 			g_string_append_printf(header, "%s Samplerate: %s\n",
 					       ctx->comment, samplerate_s);
 			g_free(samplerate_s);
@@ -442,7 +440,7 @@ static void dump_saved_values(struct context *ctx, GString **out)
 			ctx->previous_sample = g_malloc0(analog_size + ctx->num_logic_channels);
 
 		for (i = 0; i < ctx->num_samples; i++) {
-			ctx->sample_time += ctx->period;
+			ctx->sample_time = (ctx->sample_scale * i) / ctx->sample_rate;
 			analog_sample =
 			    &ctx->analog_samples[i * ctx->num_analog_channels];
 			logic_sample =