package log

import (
	"errors"
	"fmt"
	"strings"
)

// An AccessLogField is a field in the access logs.
type AccessLogField string

// known access log fields
const (
	AccessLogFieldAuthority           AccessLogField = "authority"
	AccessLogFieldDuration            AccessLogField = "duration"
	AccessLogFieldForwardedFor        AccessLogField = "forwarded-for"
	AccessLogFieldMethod              AccessLogField = "method"
	AccessLogFieldPath                AccessLogField = "path"
	AccessLogFieldReferer             AccessLogField = "referer"
	AccessLogFieldRequestID           AccessLogField = "request-id"
	AccessLogFieldResponseCode        AccessLogField = "response-code"
	AccessLogFieldResponseCodeDetails AccessLogField = "response-code-details"
	AccessLogFieldSize                AccessLogField = "size"
	AccessLogFieldUpstreamCluster     AccessLogField = "upstream-cluster"
	AccessLogFieldUserAgent           AccessLogField = "user-agent"
)

// DefaultAccessLogFields returns the default access log fields.
func DefaultAccessLogFields() []AccessLogField {
	return []AccessLogField{
		AccessLogFieldUpstreamCluster,
		AccessLogFieldMethod,
		AccessLogFieldAuthority,
		AccessLogFieldPath,
		AccessLogFieldUserAgent,
		AccessLogFieldReferer,
		AccessLogFieldForwardedFor,
		AccessLogFieldRequestID,
		AccessLogFieldDuration,
		AccessLogFieldSize,
		AccessLogFieldResponseCode,
		AccessLogFieldResponseCodeDetails,
	}
}

const accessLogFieldHeaderPrefix = "header."

// AccessLogFieldForHeader returns an access log field for the given header name.
func AccessLogFieldForHeader(header string) AccessLogField {
	return AccessLogField(accessLogFieldHeaderPrefix + header)
}

// IsForHeader returns true if the access log field is for a header.
func (field AccessLogField) IsForHeader() (headerName string, ok bool) {
	if strings.HasPrefix(string(field), accessLogFieldHeaderPrefix) {
		return string(field[len(accessLogFieldHeaderPrefix):]), true
	}
	return "", false
}

// ErrUnknownAccessLogField indicates that an access log field is unknown.
var ErrUnknownAccessLogField = errors.New("unknown access log field")

var accessLogFieldLookup = map[AccessLogField]struct{}{
	AccessLogFieldAuthority:           {},
	AccessLogFieldDuration:            {},
	AccessLogFieldForwardedFor:        {},
	AccessLogFieldMethod:              {},
	AccessLogFieldPath:                {},
	AccessLogFieldReferer:             {},
	AccessLogFieldRequestID:           {},
	AccessLogFieldResponseCode:        {},
	AccessLogFieldResponseCodeDetails: {},
	AccessLogFieldSize:                {},
	AccessLogFieldUpstreamCluster:     {},
	AccessLogFieldUserAgent:           {},
}

// Validate returns an error if the access log field is invalid.
func (field AccessLogField) Validate() error {
	if _, ok := field.IsForHeader(); ok {
		return nil
	}

	_, ok := accessLogFieldLookup[field]
	if !ok {
		return fmt.Errorf("%w: %s", ErrUnknownAccessLogField, field)
	}

	return nil
}