package metricsutil

import (
	"bytes"
	"encoding/json"
	"fmt"
	"net/http"
	"strings"

	"github.com/armon/go-metrics"
	"github.com/hashicorp/vault/sdk/logical"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/expfmt"
)

const (
	OpenMetricsMIMEType = "application/openmetrics-text"

	PrometheusSchemaMIMEType = "prometheus/telemetry"

	// ErrorContentType is the content type returned by an error response.
	ErrorContentType = "text/plain"
)

const (
	PrometheusMetricFormat = "prometheus"
)

type MetricsHelper struct {
	inMemSink         *metrics.InmemSink
	PrometheusEnabled bool
}

func NewMetricsHelper(inMem *metrics.InmemSink, enablePrometheus bool) *MetricsHelper {
	return &MetricsHelper{inMem, enablePrometheus}
}

func FormatFromRequest(req *logical.Request) string {
	acceptHeaders := req.Headers["Accept"]
	if len(acceptHeaders) > 0 {
		acceptHeader := acceptHeaders[0]
		if strings.HasPrefix(acceptHeader, OpenMetricsMIMEType) {
			return PrometheusMetricFormat
		}

		// Look for prometheus accept header
		for _, header := range acceptHeaders {
			if strings.Contains(header, PrometheusSchemaMIMEType) {
				return PrometheusMetricFormat
			}
		}
	}
	return ""
}

func (m *MetricsHelper) ResponseForFormat(format string) *logical.Response {
	switch format {
	case PrometheusMetricFormat:
		return m.PrometheusResponse()
	case "":
		return m.GenericResponse()
	default:
		return &logical.Response{
			Data: map[string]interface{}{
				logical.HTTPContentType: ErrorContentType,
				logical.HTTPRawBody:     fmt.Sprintf("metric response format \"%s\" unknown", format),
				logical.HTTPStatusCode:  http.StatusBadRequest,
			},
		}
	}
}

func (m *MetricsHelper) PrometheusResponse() *logical.Response {
	resp := &logical.Response{
		Data: map[string]interface{}{
			logical.HTTPContentType: ErrorContentType,
			logical.HTTPStatusCode:  http.StatusBadRequest,
		},
	}

	if !m.PrometheusEnabled {
		resp.Data[logical.HTTPRawBody] = "prometheus is not enabled"
		return resp
	}
	metricsFamilies, err := prometheus.DefaultGatherer.Gather()
	if err != nil && len(metricsFamilies) == 0 {
		resp.Data[logical.HTTPRawBody] = fmt.Sprintf("no prometheus metrics could be decoded: %s", err)
		return resp
	}

	// Initialize a byte buffer.
	buf := &bytes.Buffer{}
	defer buf.Reset()

	e := expfmt.NewEncoder(buf, expfmt.FmtText)
	for _, mf := range metricsFamilies {
		err := e.Encode(mf)
		if err != nil {
			resp.Data[logical.HTTPRawBody] = fmt.Sprintf("error during the encoding of metrics: %s", err)
			return resp
		}
	}
	resp.Data[logical.HTTPContentType] = string(expfmt.FmtText)
	resp.Data[logical.HTTPRawBody] = buf.Bytes()
	resp.Data[logical.HTTPStatusCode] = http.StatusOK
	return resp
}

func (m *MetricsHelper) GenericResponse() *logical.Response {
	resp := &logical.Response{
		Data: map[string]interface{}{
			logical.HTTPContentType: ErrorContentType,
			logical.HTTPStatusCode:  http.StatusBadRequest,
		},
	}

	summary, err := m.inMemSink.DisplayMetrics(nil, nil)
	if err != nil {
		resp.Data[logical.HTTPRawBody] = fmt.Sprintf("error while fetching the in-memory metrics: %s", err)
		return resp
	}
	content, err := json.Marshal(summary)
	if err != nil {
		resp.Data[logical.HTTPRawBody] = fmt.Sprintf("error while marshalling the in-memory metrics: %s", err)
		return resp
	}
	resp.Data[logical.HTTPContentType] = "application/json"
	resp.Data[logical.HTTPRawBody] = content
	resp.Data[logical.HTTPStatusCode] = http.StatusOK
	return resp
}