// Package plan computes what provisioning would do, without doing any of it.
//
// A plan is read by a person before it is applied, so it is written to be read:
// it says what it will attempt, what it cannot know in advance, and what it
// refuses. The refusals are the important part -- a reconciler that quietly
// works around a situation it does not understand is worse than one that stops.
package plan

import (
	"errors"
	"fmt"
	"strings"

	"github.com/tegwick/fluid-telegram/internal/avatar"
	"github.com/tegwick/fluid-telegram/internal/spec"
	"github.com/tegwick/fluid-telegram/internal/state"
)

type Kind int

const (
	// Create makes something that does not exist.
	Create Kind = iota
	// Update changes something that does, in a way that is safe to repeat.
	Update
	// Attempt is a Create whose outcome cannot be known in advance -- claiming a
	// username, for instance. The plan promises the attempt, not the result.
	Attempt
	// Warn is something the operator should know that the tool will not act on.
	Warn
	// Defer is an action held back until a precondition the design expects to be
	// met later. It is not an error: the rest of the plan still applies. The
	// public channel waiting on a checked rendering is the case this exists for,
	// and it is normal on every first run.
	Defer
	// Block is a refusal caused by the world disagreeing with the state file.
	// A plan containing one applies nothing, because the tool no longer knows
	// what it is looking at.
	Block
)

func (k Kind) String() string {
	switch k {
	case Create:
		return "create"
	case Update:
		return "update"
	case Attempt:
		return "attempt"
	case Warn:
		return "warn"
	case Defer:
		return "defer"
	case Block:
		return "BLOCK"
	}
	return "?"
}

type Action struct {
	Kind   Kind
	Target string
	Detail string
	// Why is present on Warn and Block, where the reason matters more than the
	// action. An operator who is told only "blocked" will look for a way around.
	Why string
}

type Plan struct {
	Campaign   string
	SpecDigest string
	SpecPath   string
	Actions    []Action

	// Avatar is the validated image, when the spec declares one and it loaded.
	// apply uses it rather than re-reading the file, so the thing that was
	// checked is the thing that is sent.
	Avatar *avatar.Image
}

func (p *Plan) add(k Kind, target, detail string) {
	p.Actions = append(p.Actions, Action{k, target, detail, ""})
}
func (p *Plan) addWhy(k Kind, target, detail, why string) {
	p.Actions = append(p.Actions, Action{k, target, detail, why})
}

// Blocked reports whether the plan may not be applied.
func (p *Plan) Blocked() bool {
	for _, a := range p.Actions {
		if a.Kind == Block {
			return true
		}
	}
	return false
}

// Empty reports whether applying would change nothing. Warnings and deferrals do
// not count as changes: a converged presence with a standing warning, or with the
// public channel still waiting on its first checked rendering, is still converged.
func (p *Plan) Empty() bool {
	for _, a := range p.Actions {
		if a.Kind != Warn && a.Kind != Defer {
			return false
		}
	}
	return true
}

// Live is what the provisioner can observe about the presence right now. It is
// an interface so that plan computation stays pure and testable: the MTProto
// client is one implementation, a fake is another.
type Live interface {
	// BotExists reports whether a bot with this username is ours and reachable.
	BotExists(username string) (bool, error)
	// ChannelAdminRights returns the rights our bot actually holds on a chat.
	ChannelAdminRights(chatID int64) ([]string, error)
	// ChannelUsername returns the username a chat currently carries.
	ChannelUsername(chatID int64) (string, error)
}

// Compute diffs the spec against the resolved state and live observation.
// specPath locates the spec so that relative asset paths resolve against the
// repository that declares them.
func Compute(specPath string, sp *spec.Presence, digest string, rs *state.Resolved, live Live) (*Plan, error) {
	p := &Plan{Campaign: sp.Campaign, SpecDigest: digest, SpecPath: specPath}

	if rs.Campaign != "" && rs.Campaign != sp.Campaign {
		p.addWhy(Block, "campaign", fmt.Sprintf("state is for %q, spec is for %q", rs.Campaign, sp.Campaign),
			"The campaign slug names the state file and the OpenBao subtree. Changing it "+
				"does not rename a presence, it points at a different one, so the tool will "+
				"not guess which was meant.")
		return p, nil
	}

	if err := planBot(p, sp, rs, live); err != nil {
		return nil, err
	}
	if err := planChannels(p, sp, rs, live); err != nil {
		return nil, err
	}

	// Nothing here deletes. Removing a channel from the spec destroys its
	// subscribers and its post history irreversibly, and no spec is trusted
	// with that.
	for name := range rs.Channels {
		if _, ok := sp.Channels[name]; !ok {
			p.addWhy(Warn, "channel."+name, "present in state, absent from spec",
				"Not deleted. Deleting a channel destroys its subscribers and history "+
					"irreversibly; remove it by hand if that is genuinely what you want.")
		}
	}
	return p, nil
}

func planBot(p *Plan, sp *spec.Presence, rs *state.Resolved, live Live) error {
	if rs.Bot.ID == 0 {
		p.add(Attempt, "bot", fmt.Sprintf("register %q via BotFather, username from %d candidate(s): %s",
			sp.Bot.Name, len(sp.Bot.UsernamePreference), strings.Join(sp.Bot.UsernamePreference, ", ")))
		p.add(Create, "bot.profile", "set name, about text and description")
		planAvatar(p, sp, rs)
		return nil
	}

	ok, err := live.BotExists(rs.Bot.Username)
	if err != nil {
		return fmt.Errorf("check bot: %w", err)
	}
	if !ok {
		p.addWhy(Block, "bot", fmt.Sprintf("@%s is in the resolved state but is not reachable", rs.Bot.Username),
			"Either the bot was deleted or the operator session no longer has access to it. "+
				"Both mean the state file is describing something that is not there, and "+
				"re-creating the bot would silently orphan the token in OpenBao.")
		return nil
	}
	// Profile fields are safe to reassert: BotFather takes them idempotently and
	// the tool does not know what a person may have changed by hand.
	p.add(Update, "bot.profile", "reassert name, about text and description from the spec")
	planAvatar(p, sp, rs)
	return nil
}

// planAvatar decides whether the picture needs sending. It is content
// addressed: replacing the file is what triggers an update, because a timestamp
// says when something was touched and a digest says whether it differs.
func planAvatar(p *Plan, sp *spec.Presence, rs *state.Resolved) {
	if sp.Bot.Avatar == "" {
		return
	}
	img, err := avatar.Load(p.SpecPath, sp.Bot.Avatar)
	if errors.Is(err, avatar.ErrMissing) {
		p.addWhy(Defer, "bot.avatar", sp.Bot.Avatar,
			"The spec declares an avatar but the file is not there. Everything else "+
				"applies; add the file and run again to set the picture.")
		return
	}
	if err != nil {
		p.addWhy(Block, "bot.avatar", sp.Bot.Avatar, err.Error()+
			". The picture is checked before the conversation starts, so that a "+
			"rejected image cannot leave the bot registered without one.")
		return
	}
	p.Avatar = &img

	if rs.Bot.AvatarDigest == img.Digest {
		return // unchanged
	}
	kind := Create
	detail := fmt.Sprintf("%s (%dx%d, %d KiB)", sp.Bot.Avatar, img.Width, img.Height, img.Bytes/1024)
	if rs.Bot.AvatarDigest != "" {
		kind = Update
		detail = "replace picture with " + detail
	}
	if note := img.CropNote(); note != "" {
		p.addWhy(kind, "bot.avatar", detail, note)
		return
	}
	p.add(kind, "bot.avatar", detail)
}

func planChannels(p *Plan, sp *spec.Presence, rs *state.Resolved, live Live) error {
	// Test first, always. The ordering is the guarantee, not a convention.
	for _, name := range []string{spec.Test, spec.Live} {
		c, ok := sp.Channels[name]
		if !ok {
			continue
		}
		rc, provisioned := rs.Channels[name]

		if name == spec.Live && !rs.TestChannelVerified(spec.Test) {
			p.addWhy(Defer, "channel.public", "held until the test channel has a checked rendering",
				"Nothing reaches the public channel until a person has seen a rendering in "+
					"the test channel. This is expected on a first run and does not stop the "+
					"rest of the plan; publish once to the test channel, then plan again.")
			continue
		}

		if !provisioned {
			p.add(Create, "channel."+name, fmt.Sprintf("%s channel %q", c.Visibility, c.Title))
			if c.Visibility == spec.Public {
				p.add(Attempt, "channel."+name+".username",
					"claim from: "+strings.Join(c.UsernamePreference, ", "))
			}
			p.add(Create, "channel."+name+".admin",
				"add bot as administrator with post_messages only")
			continue
		}

		rights, err := live.ChannelAdminRights(rc.ChatID)
		if err != nil {
			return fmt.Errorf("check rights on %s: %w", name, err)
		}
		if !hasOnly(rights, spec.PostMessages) {
			p.addWhy(Block, "channel."+name+".admin",
				fmt.Sprintf("bot holds %v, expected [%s]", rights, spec.PostMessages),
				"Rights drifted. Widening is a permission this system is not allowed to "+
					"exercise; losing post_messages means it cannot publish. Either way a "+
					"person decides what happened before a tool changes it back.")
		}

		if c.Visibility == spec.Public {
			cur, err := live.ChannelUsername(rc.ChatID)
			if err != nil {
				return fmt.Errorf("check username on %s: %w", name, err)
			}
			if cur != rc.Username {
				p.addWhy(Block, "channel."+name+".username",
					fmt.Sprintf("is @%s, state says @%s", cur, rc.Username),
					"A channel's public username changing under us is not something to "+
						"reconcile. It may have been taken over, or the state may be stale.")
			}
		}
	}
	return nil
}

func hasOnly(got []string, want string) bool {
	return len(got) == 1 && got[0] == want
}

// Render writes the plan the way an operator reads it: refusals first, because
// they decide whether the rest matters.
func (p *Plan) Render() string {
	var b strings.Builder
	fmt.Fprintf(&b, "presence plan for %q\n", p.Campaign)
	fmt.Fprintf(&b, "spec %s\n\n", p.SpecDigest)

	if p.Empty() && !p.Blocked() {
		b.WriteString("  no changes -- the declared presence matches what exists\n")
	}
	for _, k := range []Kind{Block, Warn, Defer, Attempt, Create, Update} {
		for _, a := range p.Actions {
			if a.Kind != k {
				continue
			}
			fmt.Fprintf(&b, "  %-8s %-26s %s\n", a.Kind, a.Target, a.Detail)
			if a.Why != "" {
				for _, line := range wrap(a.Why, 72) {
					fmt.Fprintf(&b, "           %s\n", line)
				}
			}
		}
	}
	if p.Blocked() {
		b.WriteString("\nnothing will be applied while a BLOCK stands: the presence does not\n" +
			"match the state file, and a person decides what happened before a tool acts\n")
	}
	return b.String()
}

func wrap(s string, width int) []string {
	var out []string
	line := ""
	for _, w := range strings.Fields(s) {
		if line != "" && len(line)+1+len(w) > width {
			out = append(out, line)
			line = ""
		}
		if line == "" {
			line = w
		} else {
			line += " " + w
		}
	}
	if line != "" {
		out = append(out, line)
	}
	return out
}