package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"net/url"
	"os"
	"regexp"
	"strings"
)

// Config is {datadir}/config/forge.json. It has no field for a secret: every
// secret is environment only. SiteName, SiteDesc, Logo, and Icon are read once
// at startup, into Site.
type Config struct {
	Listen   string         `json:"listen"`
	SiteName string         `json:"site_name"`
	SiteDesc string         `json:"site_desc"`
	Logo     string         `json:"logo"`
	Icon     string         `json:"icon"`
	BaseURL  string         `json:"base_url"`
	OIDC     []OIDCProvider `json:"oidc"`
	Mail     MailConfig     `json:"mail"`

	// ClientIPHeader names the header the client address arrives in, behind a
	// proxy. Empty means the connection is the address.
	ClientIPHeader string `json:"client_ip_header"`

	host string // BaseURL's host without the port, for generated addresses
}

type OIDCProvider struct {
	Key      string   `json:"key"`
	Label    string   `json:"label"`
	Issuer   string   `json:"issuer"`
	ClientID string   `json:"client_id"`
	Allow    []string `json:"allow,omitempty"`

	ClientSecret string `json:"-"` // GIT_FORGE_OIDC_<key>_CLIENT_SECRET
}

// An entry beginning with @ is a whole domain, anything else is one address,
// and an empty list admits anyone. Checked at every login, so removing an
// entry revokes.
func (p OIDCProvider) allows(email string) bool {
	if len(p.Allow) == 0 {
		return true
	}
	email = strings.ToLower(email)
	_, domain, split := strings.Cut(email, "@")
	for _, entry := range p.Allow {
		entry = strings.ToLower(entry)
		if want, isDomain := strings.CutPrefix(entry, "@"); isDomain {
			// Whole label, not suffix.
			if split && domain == want {
				return true
			}
			continue
		}
		if email == entry {
			return true
		}
	}
	return false
}

type MailConfig struct {
	Driver  string        `json:"driver"`
	From    string        `json:"from"`
	Mailgun MailgunConfig `json:"mailgun"`
}

type MailgunConfig struct {
	Domain  string `json:"domain"`
	BaseURL string `json:"base_url"`

	APIKey string `json:"-"` // GIT_FORGE_MAIL_MAILGUN_API_KEY
}

// An OIDC key prefixes the accounts it mints, as in sso:carol, and names the
// client secret's environment variable.
var oidcKeyRE = regexp.MustCompile(`^[a-z][a-z0-9]{0,15}$`)

// A header name. One with a colon in it never matches, which looks like the
// proxy failing to set it.
var headerNameRE = regexp.MustCompile(`^[A-Za-z][A-Za-z0-9_-]*$`)

// The name reaches the realm git is prompted with, where a quote or backslash
// ends it early.
func validSiteName(name string) bool {
	return name != "" && !strings.ContainsAny(name, `"\`) && !hasControlByte(name)
}

var errSiteName = userError("the site needs a name, without a quote, a backslash, or a control byte in it")

func defaultConfig() *Config {
	return &Config{
		Listen:   ":8080",
		SiteName: "git-forge",
		SiteDesc: "a small self-hosted git forge",
		BaseURL:  "http://localhost:8080",
		OIDC:     []OIDCProvider{},
		Mail: MailConfig{
			Driver:  "none",
			From:    "git-forge <git-forge@localhost>",
			Mailgun: MailgunConfig{BaseURL: "https://api.mailgun.net/v3"},
		},
	}
}

// readConfig is the file alone. An unknown field is an error, so a typo cannot
// silently do nothing.
func readConfig(path string) (*Config, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	cfg := &Config{}
	dec := json.NewDecoder(f)
	dec.DisallowUnknownFields()
	if err := dec.Decode(cfg); err != nil {
		return nil, err
	}
	return cfg, nil
}

// loadConfig is the file with the environment on top, validated.
func loadConfig(path string) (*Config, error) {
	cfg, err := readConfig(path)
	if err != nil {
		return nil, err
	}
	cfg.applyEnv()
	if err := cfg.validate(); err != nil {
		return nil, err
	}
	return cfg, nil
}

func (c *Config) applyEnv() {
	fromEnv(&c.Listen, "GIT_FORGE_LISTEN")
	fromEnv(&c.SiteName, "GIT_FORGE_SITE_NAME")
	fromEnv(&c.SiteDesc, "GIT_FORGE_SITE_DESC")
	fromEnv(&c.BaseURL, "GIT_FORGE_BASE_URL")
	fromEnv(&c.ClientIPHeader, "GIT_FORGE_CLIENT_IP_HEADER")

	c.Mail.Mailgun.APIKey = os.Getenv("GIT_FORGE_MAIL_MAILGUN_API_KEY")
	for i := range c.OIDC {
		key := strings.ToUpper(c.OIDC[i].Key)
		c.OIDC[i].ClientSecret = os.Getenv("GIT_FORGE_OIDC_" + key + "_CLIENT_SECRET")
	}
}

func fromEnv(dst *string, name string) {
	if v, ok := os.LookupEnv(name); ok {
		*dst = v
	}
}

// validate reports everything wrong at once.
func (c *Config) validate() error {
	var errs []error
	fail := func(format string, a ...any) {
		errs = append(errs, fmt.Errorf(format, a...))
	}

	if c.Listen == "" {
		fail("listen: empty")
	}
	if !validSiteName(c.SiteName) {
		fail("site_name: empty, or holds a quote, a backslash, or a control byte")
	}
	if c.ClientIPHeader != "" && !headerNameRE.MatchString(c.ClientIPHeader) {
		fail("client_ip_header: %q is not a header name", c.ClientIPHeader)
	}

	c.BaseURL = strings.TrimSuffix(c.BaseURL, "/")
	u, err := url.Parse(c.BaseURL)
	switch {
	case err != nil:
		fail("base_url: %v", err)
	case u.Scheme != "http" && u.Scheme != "https":
		fail("base_url: scheme must be http or https, not %q", u.Scheme)
	case u.Host == "":
		fail("base_url: no host")
	case u.Path != "" || u.RawQuery != "" || u.Fragment != "":
		fail("base_url: scheme and host only, git-forge serves the root")
	default:
		c.host = u.Hostname() // no port: this ends up in mail addresses
	}

	seen := map[string]bool{}
	for i, p := range c.OIDC {
		at := fmt.Sprintf("oidc[%d]", i)
		switch {
		case !oidcKeyRE.MatchString(p.Key):
			fail("%s: key %q must match %s, it prefixes account names", at, p.Key, oidcKeyRE)
		case seen[p.Key]:
			fail("%s: duplicate key %q", at, p.Key)
		default:
			seen[p.Key] = true
		}
		if p.Label == "" {
			fail("%s: label: empty", at)
		}
		if p.ClientID == "" {
			fail("%s: client_id: empty", at)
		}
		iss, err := url.Parse(p.Issuer)
		switch {
		case err != nil:
			fail("%s: issuer: %v", at, err)
		case iss.Scheme != "https":
			fail("%s: issuer: must be https", at)
		case iss.Host == "":
			fail("%s: issuer: no host", at)
		case strings.HasSuffix(p.Issuer, "/"):
			fail("%s: issuer: no trailing slash, it is compared to the iss claim", at)
		}
		// An entry that can never match is a lockout, refused here rather than
		// at login.
		for j, entry := range p.Allow {
			switch {
			case entry == "" || strings.TrimSpace(entry) != entry:
				fail("%s: allow[%d]: empty or padded with spaces", at, j)
			case entry == "@":
				fail("%s: allow[%d]: a domain entry needs a domain after the @", at, j)
			case !strings.Contains(entry, "@"):
				fail("%s: allow[%d]: %q is neither an address nor an @domain", at, j, entry)
			}
		}
	}

	if c.Mail.From == "" {
		fail("mail.from: empty")
	}
	switch c.Mail.Driver {
	case "none", "log":
	case "mailgun":
		if c.Mail.Mailgun.Domain == "" {
			fail("mail.mailgun.domain: empty")
		}
		if c.Mail.Mailgun.APIKey == "" {
			fail("mail.mailgun: GIT_FORGE_MAIL_MAILGUN_API_KEY is not set")
		}
		base, err := url.Parse(c.Mail.Mailgun.BaseURL)
		switch {
		case err != nil:
			fail("mail.mailgun.base_url: %v", err)
		case base.Scheme != "https" || base.Host == "":
			fail("mail.mailgun.base_url: must be an https url")
		case strings.HasSuffix(c.Mail.Mailgun.BaseURL, "/"):
			fail("mail.mailgun.base_url: no trailing slash")
		}
	default:
		fail("mail.driver: %q is not none, log, or mailgun", c.Mail.Driver)
	}

	return errors.Join(errs...)
}