metrics/prometheus.v

module metrics

import strings
import io
import arrays

pub struct PrometheusExporter {
	buckets []f64
mut:
	prefix    string
	collector &MetricsCollector = unsafe { nil }
}

pub fn new_prometheus_exporter(buckets []f64) PrometheusExporter {
	return PrometheusExporter{
		buckets: buckets
	}
}

pub fn (mut e PrometheusExporter) load(prefix string, collector &MetricsCollector) {
	unsafe {
		e.collector = collector
	}
	e.prefix = prefix
}

[inline]
fn join_two_array(arr [2]string) string {
	return '${arr[0]}="${arr[1]}"'
}

pub fn (e &PrometheusExporter) serialize_metric(metric Metric) string {
	if metric.labels.len == 0 {
		return '$e.prefix$metric.name'
	}

	return '$e.prefix$metric.name{${metric.labels.map(join_two_array(it)).join(',')}}'
}

pub fn (mut e PrometheusExporter) export_to_string() !string {
	mut builder := strings.new_builder(64)

	e.export_to_writer(mut builder)!

	return builder.str()
}

pub fn (mut e PrometheusExporter) export_to_writer(mut writer io.Writer) ! {
	for counter in e.collector.counters() {
		val := e.collector.counter_get(counter) or { return error("This can't happen.") }
		line := '${e.serialize_metric(counter)} $val\n'

		writer.write(line.bytes())!
	}

	for gauge in e.collector.gauges() {
		val := e.collector.gauge_get(gauge) or { return error("This can't happen.") }
		line := '${e.serialize_metric(gauge)} $val\n'

		writer.write(line.bytes())!
	}

	for hist in e.collector.histograms() {
		data := e.collector.histogram_get(hist) or { return error("This can't happen.") }

		sum := arrays.sum(data) or { 0.0 }
		total_count := data.len

		mut bucket_counts := []u64{len: e.buckets.len}

		mut i := bucket_counts.len - 1

		// For each data point, increment all buckets that the value is
		// contained in. Because the buckets are sorted, we can stop once we
		// encounter one that it doesn't fit in
		for val in data {
			for i >= 0 && val <= e.buckets[i] {
				bucket_counts[i]++

				i -= 1
			}

			i = bucket_counts.len - 1
		}

		mut m := Metric{
			...hist
			name: '${hist.name}_count'
		}
		writer.write('${e.serialize_metric(m)} $total_count\n'.bytes())!

		m = Metric{
			...hist
			name: '${hist.name}_sum'
		}
		writer.write('${e.serialize_metric(m)} $sum\n'.bytes())!

		mut le_labels := [][2]string{}
		le_labels.prepend(hist.labels)
		le_labels << ['le', '']!

		for j, bucket in e.buckets {
			le_labels[le_labels.len - 1][1] = bucket.str()

			m = Metric{
				name: '${hist.name}_bucket'
				labels: le_labels
			}

			writer.write('${e.serialize_metric(m)} ${bucket_counts[j]}\n'.bytes())!
		}

		// Always output the +Inf bucket
		le_labels[le_labels.len - 1][1] = '+Inf'

		m = Metric{
			name: '${hist.name}_bucket'
			labels: le_labels
		}

		writer.write('${e.serialize_metric(m)} $total_count\n'.bytes())!
	}
}