package config

import (
	"errors"
	"fmt"
	"log/slog"
	"math"
	"net/netip"
	"net/url"
	"strconv"
	"strings"
	"time"
)

// RProxy is the settings of `smallwebwaf rproxy`. A size or time limit of zero
// is off.
type RProxy struct {
	// ListenAddr is where rproxy listens (SWRP_LISTEN_ADDR).
	ListenAddr string
	// UpstreamURL is the app (SWRP_UPSTREAM_URL).
	UpstreamURL *url.URL
	// TrustedProxies are the netblocks whose X-Forwarded-For is believed
	// (SWRP_TRUSTED_PROXIES).
	TrustedProxies []netip.Prefix
	// WAFBackend is servewaf's URL (SWRP_WAF_BACKEND).
	WAFBackend *url.URL
	// Timeout is how long rproxy waits for servewaf's answer to a question,
	// and to a report (SWRP_TIMEOUT_MS). It cannot be off.
	Timeout time.Duration
	// DefaultDisposition is what is done with a request servewaf gives no
	// answer about, allow or deny (SWRP_DEFAULT_DISPOSITION).
	DefaultDisposition string
	// MaxRequestSize is the largest request body (SWRP_MAX_REQUEST_SIZE), and
	// MaxResponseSize the largest response body (SWRP_MAX_RESPONSE_SIZE).
	MaxRequestSize  int64
	MaxResponseSize int64
	// MaxRequestTime bounds sending the whole request: the visitor's request
	// line and headers, and then, from their end, its body on its way to the
	// app (SWRP_MAX_REQUEST_TIME_MS). MaxResponseTime bounds the answer, from
	// the end of the request to its last byte reaching the visitor
	// (SWRP_MAX_RESPONSE_TIME_MS).
	MaxRequestTime  time.Duration
	MaxResponseTime time.Duration

	// settings are the values read, as given or by default, for the log line
	// at start.
	settings []slog.Attr
}

var (
	errNeedsWAFBackend = errors.New(
		"is required: it is servewaf's URL, such as http://servewaf:8080")
	errNotDisposition  = errors.New("is not allow or deny")
	errNotMilliseconds = errors.New(
		"is not a whole number of milliseconds above zero, such as 2000")
	errNotMillisecondsOrOff = errors.New(
		"is not a whole number of milliseconds above zero, such as 60000, or off")
)

// RProxyFromEnvironment reads rproxy's settings with lookupEnv, normally
// os.LookupEnv. A setting that is not set takes its default, and
// SWRP_WAF_BACKEND, which has none, must be set. A setting that is set but
// invalid is an error that names it. Since rproxy never touches disk, its
// settings have no _FILE form.
func RProxyFromEnvironment(lookupEnv func(string) (string, bool)) (*RProxy, error) {
	env := &environment{lookupEnv: func(name string) (string, bool) {
		if strings.HasSuffix(name, "_FILE") {
			return "", false
		}

		return lookupEnv(name)
	}}
	cfg := &RProxy{
		ListenAddr:         env.address("SWRP_LISTEN_ADDR", defaultListenAddr),
		UpstreamURL:        env.appURL("SWRP_UPSTREAM_URL", defaultUpstreamURL),
		TrustedProxies:     env.netblocks("SWRP_TRUSTED_PROXIES", privateRanges),
		WAFBackend:         env.wafBackend("SWRP_WAF_BACKEND"),
		Timeout:            env.milliseconds("SWRP_TIMEOUT_MS", "2000"),
		DefaultDisposition: env.disposition("SWRP_DEFAULT_DISPOSITION", "allow"),
		MaxRequestSize:     env.size("SWRP_MAX_REQUEST_SIZE", "100M"),
		MaxResponseSize:    env.size("SWRP_MAX_RESPONSE_SIZE", "1G"),
		MaxRequestTime:     env.millisecondsOrOff("SWRP_MAX_REQUEST_TIME_MS", "60000"),
		MaxResponseTime:    env.millisecondsOrOff("SWRP_MAX_RESPONSE_TIME_MS", "1800000"),
	}

	if env.err != nil {
		return nil, env.err
	}

	cfg.settings = env.settings

	return cfg, nil
}

// LogValue makes an RProxy log as each setting's name with the value it was
// given, or its default.
func (c *RProxy) LogValue() slog.Value {
	return slog.GroupValue(c.settings...)
}

// wafBackend reads the setting that is servewaf's URL, which must be set.
func (e *environment) wafBackend(name string) *url.URL {
	value := e.value(name, "")
	if value == "" {
		e.check(name, errNeedsWAFBackend)

		return nil
	}

	backend, err := parseUpstreamURL(value)
	e.check(name, err)

	return backend
}

// disposition reads the setting that is what is done with a request servewaf
// gives no answer about: allow or deny.
func (e *environment) disposition(name, defaultValue string) string {
	value := e.value(name, defaultValue)
	if value != "allow" && value != "deny" {
		e.check(name, fmt.Errorf("%q %w", value, errNotDisposition))
	}

	return value
}

// milliseconds reads a setting that is a whole number of milliseconds above
// zero.
func (e *environment) milliseconds(name, defaultValue string) time.Duration {
	value := e.value(name, defaultValue)

	duration, valid := parseMilliseconds(value)
	if !valid {
		e.check(name, fmt.Errorf("%q %w", value, errNotMilliseconds))
	}

	return duration
}

// millisecondsOrOff reads a setting that is a whole number of milliseconds
// above zero, or off, which is 0.
func (e *environment) millisecondsOrOff(name, defaultValue string) time.Duration {
	value := e.value(name, defaultValue)
	if value == off {
		return 0
	}

	duration, valid := parseMilliseconds(value)
	if !valid {
		e.check(name, fmt.Errorf("%q %w", value, errNotMillisecondsOrOff))
	}

	return duration
}

// parseMilliseconds reads a whole number of milliseconds above zero, and
// reports whether it is one.
func parseMilliseconds(value string) (time.Duration, bool) {
	n, err := strconv.ParseInt(value, 10, 64)
	if err != nil || n <= 0 || n > math.MaxInt64/int64(time.Millisecond) {
		return 0, false
	}

	return time.Duration(n) * time.Millisecond, true
}