// Package spec loads and validates a declared Telegram presence.
//
// The normative schema is presence/telegram.schema.yaml. The validation here
// mirrors it deliberately rather than interpreting it at runtime: the rules are
// few, and a spec that fails should say why in the tool's own words. The
// mirroring is a real drift risk, so schema_test.go asserts that the schema file
// and this package still agree on every rule.
package spec

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"os"
	"regexp"
	"strings"

	"gopkg.in/yaml.v3"
)

// PostMessages is the only administrator right this system will ever hold.
//
// InterfaceEvolutionIntent.md 7 forbids the Daimon from using a wider right, and
// a right that is never exercised is still a right that was granted. Enforcing
// it where the grant is declared is cheaper than auditing that it stayed unused.
const PostMessages = "post_messages"

type File struct {
	Presence Presence `yaml:"presence"`
}

type Presence struct {
	SchemaVersion         string             `yaml:"schema_version"`
	Campaign              string             `yaml:"campaign"`
	Interface             string             `yaml:"interface"`
	Bot                   Bot                `yaml:"bot"`
	Channels              map[string]Channel `yaml:"channels"`
	LinkedDiscussionGroup bool               `yaml:"linked_discussion_group"`
}

type Bot struct {
	Name               string   `yaml:"name"`
	UsernamePreference []string `yaml:"username_preference"`
	About              string   `yaml:"about"`
	Description        string   `yaml:"description"`
	Avatar             string   `yaml:"avatar"`
}

type Channel struct {
	Title              string   `yaml:"title"`
	Description        string   `yaml:"description"`
	Visibility         string   `yaml:"visibility"`
	UsernamePreference []string `yaml:"username_preference"`
	AdminRights        []string `yaml:"admin_rights"`
}

const (
	Private = "private"
	Public  = "public"

	// The two channels every presence declares. Test is created first and is
	// where renderings are checked; nothing reaches Public until a person has
	// looked at one.
	Test = "test"
	Live = "public"
)

var (
	campaignRe     = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
	botUsernameRe  = regexp.MustCompile(`^[A-Za-z][A-Za-z0-9_]{3,30}([Bb]ot|_bot)$`)
	chanUsernameRe = regexp.MustCompile(`^[A-Za-z][A-Za-z0-9_]{4,31}$`)
)

// Load reads a spec and validates it. The digest is over the file bytes, so any
// edit changes it -- that is what makes drift detectable in the resolved state.
func Load(path string) (*Presence, string, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return nil, "", fmt.Errorf("read spec: %w", err)
	}
	var f File
	dec := yaml.NewDecoder(strings.NewReader(string(raw)))
	dec.KnownFields(true) // additionalProperties: false
	if err := dec.Decode(&f); err != nil {
		return nil, "", fmt.Errorf("parse spec: %w", err)
	}
	if err := f.Presence.Validate(); err != nil {
		return nil, "", err
	}
	sum := sha256.Sum256(raw)
	return &f.Presence, "sha256:" + hex.EncodeToString(sum[:]), nil
}

func (p *Presence) Validate() error {
	var errs []string
	add := func(format string, a ...any) { errs = append(errs, fmt.Sprintf(format, a...)) }

	if p.SchemaVersion != "0.1" {
		add("presence.schema_version must be %q, got %q", "0.1", p.SchemaVersion)
	}
	if !campaignRe.MatchString(p.Campaign) {
		add("presence.campaign %q must be a lowercase slug", p.Campaign)
	}
	if p.Interface == "" {
		add("presence.interface is required")
	}

	if p.Bot.Name == "" {
		add("bot.name is required")
	} else if n := len([]rune(p.Bot.Name)); n > 64 {
		add("bot.name is %d characters, limit is 64", n)
	}
	if len(p.Bot.UsernamePreference) == 0 {
		add("bot.username_preference needs at least one candidate; BotFather usernames are globally unique and a first choice is often taken")
	}
	for _, u := range p.Bot.UsernamePreference {
		if !botUsernameRe.MatchString(u) {
			add("bot username %q must be 4-31 characters and end in \"bot\" or \"_bot\"", u)
		}
	}
	if n := len([]rune(p.Bot.About)); n > 120 {
		add("bot.about is %d characters, limit is 120", n)
	}
	if n := len([]rune(p.Bot.Description)); n > 512 {
		add("bot.description is %d characters, limit is 512", n)
	}

	for _, name := range []string{Test, Live} {
		if _, ok := p.Channels[name]; !ok {
			add("channels.%s is required", name)
		}
	}
	for name, c := range p.Channels {
		if name != Test && name != Live {
			add("channels.%s is not a known channel; expected %q and %q", name, Test, Live)
			continue
		}
		errs = append(errs, c.validate(name)...)
	}

	if len(errs) > 0 {
		return fmt.Errorf("spec is invalid:\n  - %s", strings.Join(errs, "\n  - "))
	}
	return nil
}

func (c Channel) validate(name string) []string {
	var errs []string
	add := func(format string, a ...any) { errs = append(errs, fmt.Sprintf(format, a...)) }

	if c.Title == "" {
		add("channels.%s.title is required", name)
	} else if n := len([]rune(c.Title)); n > 128 {
		add("channels.%s.title is %d characters, limit is 128", name, n)
	}
	if n := len([]rune(c.Description)); n > 255 {
		add("channels.%s.description is %d characters, limit is 255", name, n)
	}

	switch c.Visibility {
	case Private:
		if len(c.UsernamePreference) > 0 {
			add("channels.%s is private and must not declare a username; a dropped field is how a spec stops describing what exists", name)
		}
	case Public:
		if len(c.UsernamePreference) == 0 {
			add("channels.%s is public and must declare at least one username candidate", name)
		}
	default:
		add("channels.%s.visibility must be %q or %q, got %q", name, Private, Public, c.Visibility)
	}
	for _, u := range c.UsernamePreference {
		if !chanUsernameRe.MatchString(u) {
			add("channels.%s username %q must be 5-32 characters, starting with a letter", name, u)
		}
	}

	// The clamp. A spec asking for a wider right fails here rather than being
	// quietly trimmed: someone asked for it, and they should find out.
	for _, r := range c.AdminRights {
		if r != PostMessages {
			add("channels.%s.admin_rights may only contain %q; %q is refused", name, PostMessages, r)
		}
	}
	if len(c.AdminRights) > 1 {
		add("channels.%s.admin_rights has %d entries; only %q is permitted", name, len(c.AdminRights), PostMessages)
	}
	return errs
}

// Rights returns the rights to grant. Always exactly post_messages: the spec may
// state it for readability, but it is not the spec's decision.
func (c Channel) Rights() []string { return []string{PostMessages} }