import { Metric } from './metrics'

function escapeString(str: string) {
    return str.replace(/\\/g, '\\\\').replace(/\n/g, '\\n')
}

/**
 * String Attribute values are converted directly to Prometheus attribute values.
 * Non-string values are represented as JSON-encoded strings.
 *
 * `undefined` is converted to an empty string.
 */
function escapeAttributeValue(str: string) {
    if (typeof str !== 'string') {
        str = JSON.stringify(str)
    }
    return escapeString(str).replace(/"/g, '\\"')
}

const invalidCharacterRegex = /[^a-z0-9_]/gi

/**
 * Ensures metric names are valid Prometheus metric names by removing
 * characters allowed by OpenTelemetry but disallowed by Prometheus.
 *
 * https://prometheus.io/docs/concepts/data_model/#metric-names-and-attributes
 *
 * 1. Names must match `[a-zA-Z_:][a-zA-Z0-9_:]*`
 *
 * 2. Colons are reserved for user defined recording rules.
 * They should not be used by exporters or direct instrumentation.
 *
 * OpenTelemetry metric names are already validated in the Meter when they are created,
 * and they match the format `[a-zA-Z][a-zA-Z0-9_.\-]*` which is very close to a valid
 * prometheus metric name, so we only need to strip characters valid in OpenTelemetry
 * but not valid in prometheus and replace them with '_'.
 *
 * @param name name to be sanitized
 */
function sanitizePrometheusMetricName(name: string): string {
    return name.replace(invalidCharacterRegex, '_') // replace all invalid characters with '_'
}

export class MetricsRenderer {
    constructor(private readonly prefix: string) {}

    render(metric: Metric): string {
        const name = this.metricName(metric.name)
        return [
            `# TYPE ${name} ${name.endsWith('_total') ? 'count' : 'gauge'}`,
            `${name}${this.renderLabels(metric.labels)} ${metric.value}${
                metric.timestamp !== null ? ' ' + String(metric.timestamp.getTime()) : ''
            }`,
        ].join('\n')
    }

    private renderLabels(labels: Metric['labels']): string {
        const rendered = Object.entries(labels)
            .map(([key, value]) => `${sanitizePrometheusMetricName(key)}="${escapeAttributeValue(value)}"`)
            .join(',')

        return rendered !== '' ? '{' + rendered + '}' : ''
    }

    private metricName(name: string): string {
        name = name.replace(/^(.*_)?(total)_(.*)$/, '$1$3_$2')

        return sanitizePrometheusMetricName(this.prefix.replace(/_+$/, '') + '_' + name)
    }
}