package config

import (
	"errors"
	"fmt"
	"net/http"
	"net/netip"
	"time"

	"github.com/samber/lo"
	"github.com/spf13/viper"

	"github.com/openmeterio/openmeter/pkg/models"
)

// ServerConfig holds HTTP server timeout configuration.
type ServerConfig struct {
	ReadHeaderTimeout time.Duration
	ReadTimeout       time.Duration
	WriteTimeout      time.Duration
	IdleTimeout       time.Duration

	ResponseValidation ResponseValidationConfig

	ClientIPMiddleware ClientIPMiddlewareConfig
}

// ResponseValidationConfig controls optional post-response OpenAPI validation on the v3 API.
type ResponseValidationConfig struct {
	Mode ResponseValidationMode
}

type ResponseValidationMode string

const (
	// ResponseValidationModeOff disables response validation. This is the default.
	ResponseValidationModeOff ResponseValidationMode = "off"
	// ResponseValidationModeUnstable validates only routes marked x-unstable: true in the spec.
	ResponseValidationModeUnstable ResponseValidationMode = "unstable"
	// ResponseValidationModeAll validates every route in the v3 spec.
	ResponseValidationModeAll ResponseValidationMode = "all"
)

func (m ResponseValidationMode) Enabled() bool {
	return m != "" && m != ResponseValidationModeOff
}

func (m ResponseValidationMode) Validate() error {
	switch m {
	case "", ResponseValidationModeOff, ResponseValidationModeUnstable, ResponseValidationModeAll:
		return nil
	default:
		return errors.New("invalid response validation mode (allowed: off, unstable, all)")
	}
}

type ClientIPSource string

const (
	ClientIPSourceRemoteAddr ClientIPSource = "remote-address"
	ClientIPSourceHeader     ClientIPSource = "header"
	ClientIPSourceXFF        ClientIPSource = "x-forwarded-for"
)

var _ models.Validator = (*ClientIPMiddlewareConfig)(nil)

// ClientIPMiddlewareConfig configures the middleware that extracts the client IP address from the HTTP request.
// See: https://adam-p.ca/blog/2022/03/x-forwarded-for/
type ClientIPMiddlewareConfig struct {
	Source ClientIPSource

	// Header defines the header name in the HTTP request containing the real client IP address.
	// Set this only if ClientIPSourceHeader is used as Source.
	// Only use headers your proxy unconditionally overwrites on every request,
	// e.g. "X-Real-IP" (Nginx ngx_http_realip_module), "CF-Connecting-IP" (Cloudflare), or "X-Client-IP" (Apache mod_remoteip).
	// Pass-through headers like "True-Client-IP", "X-Azure-ClientIP", or "Fastly-Client-IP" are client-spoofable
	// unless your edge strips the inbound value.
	Header string

	// TrustedIPPrefixes lists IP prefixes for trusted proxies.
	// Set this only if the ClientIPSourceXFF is used as Source.
	TrustedIPPrefixes []string

	// TrustedProxies defines the number of trusted proxies.
	// Set this only if the ClientIPSourceXFF is used as Source.
	TrustedProxies int
}

func (c ClientIPMiddlewareConfig) Validate() error {
	switch c.Source {
	case ClientIPSourceRemoteAddr:
		return nil
	case ClientIPSourceHeader:
		if c.Header == "" {
			return errors.New("missing client IP header")
		}

		// chi's ClientIPFromHeader takes the LAST header value, which for the append-style
		// X-Forwarded-For header is the nearest proxy hop, not the client.
		if http.CanonicalHeaderKey(c.Header) == "X-Forwarded-For" {
			return fmt.Errorf("X-Forwarded-For cannot be used as client IP header, use the %s source instead", ClientIPSourceXFF)
		}

		return nil
	case ClientIPSourceXFF:
		if len(c.TrustedIPPrefixes) > 0 {
			// Use the same parser as chi's ClientIPFromXFF (netip.MustParsePrefix), which is
			// stricter than net.ParseCIDR; a mismatch would panic at middleware construction.
			invalidPrefixes := lo.Filter(c.TrustedIPPrefixes, func(prefix string, _ int) bool {
				_, err := netip.ParsePrefix(prefix)

				return err != nil
			})

			if len(invalidPrefixes) > 0 {
				return fmt.Errorf("invalid trusted IP prefixes: %+v", invalidPrefixes)
			}

			return nil
		}

		// chi's ClientIPFromXFFTrustedProxies panics if the count is < 1.
		if c.TrustedProxies < 1 {
			return fmt.Errorf("either trusted IP prefixes or a positive number of trusted proxies must be set if real client IP source is set to %s", ClientIPSourceXFF)
		}

		return nil
	default:
		return fmt.Errorf("invalid client IP source: %s", c.Source)
	}
}

func (c ServerConfig) Validate() error {
	var errs []error

	if c.ReadHeaderTimeout < 0 {
		errs = append(errs, errors.New("readHeaderTimeout must be non-negative"))
	}

	if c.ReadTimeout < 0 {
		errs = append(errs, errors.New("readTimeout must be non-negative"))
	}

	if c.WriteTimeout < 0 {
		errs = append(errs, errors.New("writeTimeout must be non-negative"))
	}

	if c.IdleTimeout < 0 {
		errs = append(errs, errors.New("idleTimeout must be non-negative"))
	}

	if err := c.ResponseValidation.Mode.Validate(); err != nil {
		errs = append(errs, err)
	}

	if err := c.ClientIPMiddleware.Validate(); err != nil {
		errs = append(errs, err)
	}

	return errors.Join(errs...)
}

// ConfigureServer sets defaults for HTTP server timeouts.
func ConfigureServer(v *viper.Viper, prefixes ...string) {
	prefixer := NewViperKeyPrefixer(prefixes...)

	v.SetDefault(prefixer("readHeaderTimeout"), 10*time.Second)
	v.SetDefault(prefixer("readTimeout"), 60*time.Second)
	v.SetDefault(prefixer("writeTimeout"), 90*time.Second)
	v.SetDefault(prefixer("idleTimeout"), 120*time.Second)

	v.SetDefault(prefixer("responseValidation.mode"), string(ResponseValidationModeOff))

	v.SetDefault(prefixer("clientIPMiddleware.source"), ClientIPSourceRemoteAddr)
	v.SetDefault(prefixer("clientIPMiddleware.header"), "")
	v.SetDefault(prefixer("clientIPMiddleware.trustedIPPrefixes"), nil)
	v.SetDefault(prefixer("clientIPMiddleware.trustedProxies"), 0)
}