// Package config loads, validates and resolves syncbot's TOML configuration.
//
// The on-disk document is deliberately forgiving: every knob has a default,
// [global] supplies fallbacks for all repositories, and each [[repo]] overrides
// only what it needs. Load flattens all of that into a []Job in which every
// value is already resolved, so the rest of the daemon never has to reason
// about defaults or inheritance — and so the hot-reload logic can decide
// whether a job changed with a plain reflect.DeepEqual.
package config

import (
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"runtime"
	"sort"
	"strings"
	"time"

	"github.com/BurntSushi/toml"
)

// Duration is a time.Duration that decodes from a TOML string such as "5m".
type Duration time.Duration

// UnmarshalText implements encoding.TextUnmarshaler.
func (d *Duration) UnmarshalText(b []byte) error {
	v, err := time.ParseDuration(string(b))
	if err != nil {
		return fmt.Errorf("invalid duration %q (want e.g. \"30s\", \"5m\", \"2h\")", b)
	}
	*d = Duration(v)
	return nil
}

// D returns the underlying time.Duration.
func (d Duration) D() time.Duration { return time.Duration(d) }

// Endpoint is one side of a mirror. In TOML it may be written either as a bare
// URL string or as a table when it needs its own credentials:
//
//	dst = "git@github.com:me/repo.git"
//
//	[repo.dst]
//	url     = "git@github.com:me/repo.git"
//	ssh_key = "/etc/syncbot/keys/repo"
type Endpoint struct {
	URL           string `json:"url"`
	SSHKey        string `json:"-"`
	KnownHosts    string `json:"-"`
	StrictHostKey string `json:"-"`
}

// UnmarshalTOML accepts both the string and the table spelling of an endpoint.
func (e *Endpoint) UnmarshalTOML(v any) error {
	switch t := v.(type) {
	case string:
		e.URL = t
		return nil
	case map[string]any:
		for _, k := range sortedKeys(t) {
			s, ok := t[k].(string)
			if !ok {
				return fmt.Errorf("key %q must be a string", k)
			}
			switch k {
			case "url":
				e.URL = s
			case "ssh_key":
				e.SSHKey = s
			case "known_hosts":
				e.KnownHosts = s
			case "strict_host_key":
				e.StrictHostKey = s
			default:
				return fmt.Errorf("unknown key %q (want url, ssh_key, known_hosts or strict_host_key)", k)
			}
		}
		if e.URL == "" {
			return fmt.Errorf("missing required key \"url\"")
		}
		return nil
	default:
		return fmt.Errorf("must be a URL string or a table, got %T", v)
	}
}

// Global holds process-wide settings plus the defaults inherited by every repo.
type Global struct {
	WorkDir        string   `toml:"work_dir"`
	Listen         string   `toml:"listen"`
	LogLevel       string   `toml:"log_level"`
	LogFormat      string   `toml:"log_format"`
	Concurrency    int      `toml:"concurrency"`
	ReloadInterval Duration `toml:"reload_interval"`

	// Inherited by every [[repo]] unless overridden there.
	Interval      Duration `toml:"interval"`
	Timeout       Duration `toml:"timeout"`
	MaxBackoff    Duration `toml:"max_backoff"`
	Refs          []string `toml:"refs"`
	Prune         *bool    `toml:"prune"`
	Force         *bool    `toml:"force"`
	Atomic        *bool    `toml:"atomic"`
	AllowEmpty    *bool    `toml:"allow_empty"`
	SSHKey        string   `toml:"ssh_key"`
	KnownHosts    string   `toml:"known_hosts"`
	StrictHostKey string   `toml:"strict_host_key"`
	GitConfig     []string `toml:"git_config"`
}

// Repo is one [[repo]] block as written by the user.
type Repo struct {
	Name    string   `toml:"name"`
	Src     Endpoint `toml:"src"`
	Dst     Endpoint `toml:"dst"`
	Enabled *bool    `toml:"enabled"`

	Interval      Duration `toml:"interval"`
	Timeout       Duration `toml:"timeout"`
	MaxBackoff    Duration `toml:"max_backoff"`
	Refs          []string `toml:"refs"`
	Prune         *bool    `toml:"prune"`
	Force         *bool    `toml:"force"`
	Atomic        *bool    `toml:"atomic"`
	AllowEmpty    *bool    `toml:"allow_empty"`
	SSHKey        string   `toml:"ssh_key"`
	KnownHosts    string   `toml:"known_hosts"`
	StrictHostKey string   `toml:"strict_host_key"`
	GitConfig     []string `toml:"git_config"`
}

// file mirrors the TOML document itself.
type file struct {
	Global Global `toml:"global"`
	Repos  []Repo `toml:"repo"`
}

// Job is a fully resolved sync unit: all defaults folded in, ready to run.
// Every field is comparable with reflect.DeepEqual, which is how the manager
// detects that a reloaded config actually changed something for this repo.
type Job struct {
	Name string
	Src  Endpoint
	Dst  Endpoint
	Dir  string // local bare mirror

	Interval   time.Duration
	Timeout    time.Duration
	MaxBackoff time.Duration

	Refs       []string
	Prune      bool
	Force      bool
	Atomic     bool
	AllowEmpty bool
	GitConfig  []string
}

// Config is the resolved configuration the daemon runs on.
type Config struct {
	WorkDir        string
	Listen         string
	LogLevel       string
	LogFormat      string
	Concurrency    int
	ReloadInterval time.Duration
	Jobs           []Job
}

// Defaults applied when the document leaves a value out.
const (
	DefaultWorkDir        = "/var/lib/syncbot"
	DefaultInterval       = 5 * time.Minute
	DefaultTimeout        = 30 * time.Minute
	DefaultMaxBackoff     = time.Hour
	DefaultReloadInterval = 5 * time.Second
	DefaultLogLevel       = "info"
	DefaultLogFormat      = "text"
)

// DefaultRefs mirrors branches and tags, which is what almost everyone wants.
var DefaultRefs = []string{"refs/heads/*", "refs/tags/*"}

// nameRe keeps repo names usable as directory names and Prometheus labels.
var nameRe = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._-]{0,63}$`)

// Load reads, validates and resolves the config file at path.
func Load(path string) (*Config, error) {
	b, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	return Parse(b)
}

// Parse resolves an in-memory TOML document. Load is the usual entry point;
// Parse exists so tests (and -check) can work without touching disk.
func Parse(b []byte) (*Config, error) {
	var f file
	md, err := toml.Decode(string(b), &f)
	if err != nil {
		return nil, err
	}
	if undec := unknownKeys(md); len(undec) > 0 {
		return nil, fmt.Errorf("unknown config key(s): %s", strings.Join(undec, ", "))
	}
	return resolve(&f)
}

// unknownKeys reports keys the decoder did not recognise, so a typo like
// "intervall" fails loudly at startup instead of silently doing nothing.
func unknownKeys(md toml.MetaData) []string {
	var out []string
	for _, k := range md.Undecoded() {
		s := k.String()
		// Endpoint consumes its own subtree via UnmarshalTOML and validates the
		// keys itself; the decoder cannot see into it, so ignore those paths.
		if strings.Contains(s, ".src.") || strings.Contains(s, ".dst.") {
			continue
		}
		out = append(out, s)
	}
	sort.Strings(out)
	return out
}

func resolve(f *file) (*Config, error) {
	g := f.Global

	workDir, err := expandEnv(orString(g.WorkDir, DefaultWorkDir))
	if err != nil {
		return nil, fmt.Errorf("global.work_dir: %w", err)
	}
	if !filepath.IsAbs(workDir) {
		if workDir, err = filepath.Abs(workDir); err != nil {
			return nil, fmt.Errorf("global.work_dir: %w", err)
		}
	}

	cfg := &Config{
		WorkDir:        workDir,
		Listen:         g.Listen,
		LogLevel:       strings.ToLower(orString(g.LogLevel, DefaultLogLevel)),
		LogFormat:      strings.ToLower(orString(g.LogFormat, DefaultLogFormat)),
		Concurrency:    g.Concurrency,
		ReloadInterval: orDuration(g.ReloadInterval, DefaultReloadInterval),
	}
	if cfg.Concurrency <= 0 {
		cfg.Concurrency = min(4, runtime.NumCPU())
	}
	switch cfg.LogLevel {
	case "debug", "info", "warn", "error":
	default:
		return nil, fmt.Errorf("global.log_level: %q is not one of debug, info, warn, error", cfg.LogLevel)
	}
	switch cfg.LogFormat {
	case "text", "json":
	default:
		return nil, fmt.Errorf("global.log_format: %q is not one of text, json", cfg.LogFormat)
	}
	if cfg.ReloadInterval < time.Second {
		return nil, fmt.Errorf("global.reload_interval: must be at least 1s")
	}
	if len(f.Repos) == 0 {
		return nil, fmt.Errorf("no [[repo]] blocks defined: nothing to sync")
	}

	seen := make(map[string]bool, len(f.Repos))
	for i := range f.Repos {
		r := &f.Repos[i]
		where := fmt.Sprintf("repo[%d]", i)
		if r.Name != "" {
			where = fmt.Sprintf("repo %q", r.Name)
		}
		if !nameRe.MatchString(r.Name) {
			return nil, fmt.Errorf("%s: name must match %s", where, nameRe)
		}
		if seen[r.Name] {
			return nil, fmt.Errorf("%s: duplicate name", where)
		}
		seen[r.Name] = true

		if enabled := r.Enabled; enabled != nil && !*enabled {
			continue
		}

		job, err := resolveJob(r, &g, workDir)
		if err != nil {
			return nil, fmt.Errorf("%s: %w", where, err)
		}
		cfg.Jobs = append(cfg.Jobs, *job)
	}
	if len(cfg.Jobs) == 0 {
		return nil, fmt.Errorf("every [[repo]] is disabled: nothing to sync")
	}
	sort.Slice(cfg.Jobs, func(i, j int) bool { return cfg.Jobs[i].Name < cfg.Jobs[j].Name })
	return cfg, nil
}

func resolveJob(r *Repo, g *Global, workDir string) (*Job, error) {
	j := &Job{
		Name:       r.Name,
		Dir:        filepath.Join(workDir, "mirrors", r.Name+".git"),
		Interval:   pick(r.Interval, g.Interval, DefaultInterval),
		Timeout:    pick(r.Timeout, g.Timeout, DefaultTimeout),
		MaxBackoff: pick(r.MaxBackoff, g.MaxBackoff, DefaultMaxBackoff),
		Refs:       orSlice(r.Refs, g.Refs, DefaultRefs),
		Prune:      orBool(true, r.Prune, g.Prune),
		Force:      orBool(true, r.Force, g.Force),
		Atomic:     orBool(false, r.Atomic, g.Atomic),
		AllowEmpty: orBool(false, r.AllowEmpty, g.AllowEmpty),
		GitConfig:  orSlice(r.GitConfig, g.GitConfig, nil),
	}

	var err error
	// SSH settings cascade: endpoint table -> [[repo]] -> [global].
	if j.Src, err = resolveEndpoint(r.Src, r, g); err != nil {
		return nil, fmt.Errorf("src: %w", err)
	}
	if j.Dst, err = resolveEndpoint(r.Dst, r, g); err != nil {
		return nil, fmt.Errorf("dst: %w", err)
	}
	if j.Src.URL == "" {
		return nil, fmt.Errorf("src is required")
	}
	if j.Dst.URL == "" {
		return nil, fmt.Errorf("dst is required")
	}
	if j.Src.URL == j.Dst.URL {
		return nil, fmt.Errorf("src and dst are the same repository")
	}
	if j.Interval <= 0 {
		return nil, fmt.Errorf("interval must be positive")
	}
	if j.Timeout <= 0 {
		return nil, fmt.Errorf("timeout must be positive")
	}
	if j.MaxBackoff < j.Interval {
		j.MaxBackoff = j.Interval
	}
	if len(j.Refs) == 0 {
		return nil, fmt.Errorf("refs must not be empty")
	}
	for _, p := range j.Refs {
		if !strings.HasPrefix(p, "refs/") {
			return nil, fmt.Errorf("refs: %q must start with \"refs/\"", p)
		}
		if strings.Count(p, "*") > 1 {
			return nil, fmt.Errorf("refs: %q may contain at most one \"*\"", p)
		}
	}
	for _, kv := range j.GitConfig {
		if !strings.Contains(kv, "=") {
			return nil, fmt.Errorf("git_config: %q must be in key=value form", kv)
		}
	}
	return j, nil
}

func resolveEndpoint(e Endpoint, r *Repo, g *Global) (Endpoint, error) {
	out := Endpoint{
		URL:           e.URL,
		SSHKey:        orString(e.SSHKey, r.SSHKey, g.SSHKey),
		KnownHosts:    orString(e.KnownHosts, r.KnownHosts, g.KnownHosts),
		StrictHostKey: orString(e.StrictHostKey, r.StrictHostKey, g.StrictHostKey),
	}

	var err error
	for _, p := range []*string{&out.URL, &out.SSHKey, &out.KnownHosts} {
		if *p, err = expandEnv(*p); err != nil {
			return out, err
		}
	}

	if out.StrictHostKey == "" {
		// With a pinned known_hosts file we can afford to be strict; without
		// one, trust-on-first-use is the only thing that can work unattended.
		if out.KnownHosts != "" {
			out.StrictHostKey = "yes"
		} else {
			out.StrictHostKey = "accept-new"
		}
	}
	switch out.StrictHostKey {
	case "yes", "no", "accept-new":
	default:
		return out, fmt.Errorf("strict_host_key: %q is not one of yes, no, accept-new", out.StrictHostKey)
	}
	if out.SSHKey != "" && !filepath.IsAbs(out.SSHKey) {
		return out, fmt.Errorf("ssh_key: %q must be an absolute path", out.SSHKey)
	}
	return out, nil
}

// envRe matches ${VAR}. Bare $VAR is deliberately not expanded so that secrets
// containing a literal '$' survive unharmed.
var envRe = regexp.MustCompile(`\$\{([A-Za-z_][A-Za-z0-9_]*)\}`)

// expandEnv substitutes ${VAR} references, failing loudly on undefined names —
// silently expanding to "" would produce a subtly broken URL instead.
func expandEnv(s string) (string, error) {
	if !strings.Contains(s, "${") {
		return s, nil
	}
	var missing []string
	out := envRe.ReplaceAllStringFunc(s, func(m string) string {
		name := m[2 : len(m)-1]
		v, ok := os.LookupEnv(name)
		if !ok {
			missing = append(missing, name)
			return ""
		}
		return v
	})
	if len(missing) > 0 {
		return "", fmt.Errorf("undefined environment variable(s): %s", strings.Join(missing, ", "))
	}
	return out, nil
}

func orString(vs ...string) string {
	for _, v := range vs {
		if v != "" {
			return v
		}
	}
	return ""
}

func orBool(def bool, vs ...*bool) bool {
	for _, v := range vs {
		if v != nil {
			return *v
		}
	}
	return def
}

func orSlice(vs ...[]string) []string {
	for _, v := range vs {
		if len(v) > 0 {
			return append([]string(nil), v...)
		}
	}
	return nil
}

func orDuration(v Duration, def time.Duration) time.Duration {
	if v != 0 {
		return v.D()
	}
	return def
}

func pick(repo, global Duration, def time.Duration) time.Duration {
	if repo != 0 {
		return repo.D()
	}
	if global != 0 {
		return global.D()
	}
	return def
}

func sortedKeys(m map[string]any) []string {
	out := make([]string, 0, len(m))
	for k := range m {
		out = append(out, k)
	}
	sort.Strings(out)
	return out
}

// orBool with a literal default needs a *bool; these make the call sites read
// naturally without sprinkling helper variables around.
func boolPtr(b bool) *bool { return &b }