package apply

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"github.com/novox/mesh-host/internal/declaration"
	"github.com/novox/mesh-host/internal/store"
)

// Running the mesh's own code, without the module choosing how.
//
// **A daemon is an intent and this is one answer to it.** A module says what to run and the
// machine's own supervisor is how — which means the module is not writing a unit file, and not
// choosing between a container and a service before it can declare anything
// (novox/hq 03-DESIGN/01-to-be/18-building-a-module.md).
//
// What this does, in order: fetch the bundle, refuse it unless it hashes to what was declared,
// unpack it where the mesh keeps such things, write the unit, and put it in the state asked for.
// The unit is the mesh's — an operator editing it loses the edit at the next declaration, which is
// the same rule every managed file on a machine follows (ADR 0011).

// daemonRoot is where unpacked daemons live.
//
// Under the mesh's own directory rather than somewhere a distribution owns: these are files the
// mesh puts there and replaces, and putting them where a package manager also writes is how two
// owners end up disagreeing about one path.
const daemonRoot = "/var/lib/mesh/daemons"

// unitDir is where the mesh writes the units it owns.
const unitDir = "/etc/systemd/system"

func applyDaemon(ctx context.Context, r *declaration.Daemon, run Runner,
	changed map[string]bool, previous store.Applied) (Outcome, error) {
	out := begin(r)
	out.Action = "unchanged"

	body, err := fetch(ctx, r.Source)
	if err != nil {
		return out, err
	}
	sum := sha256.Sum256(body)
	got := "sha256:" + hex.EncodeToString(sum[:])
	if got != r.Digest {
		// Refused before anything is written or started. What is at that address is not what was
		// declared, and running it would be running something nobody reviewed.
		return out, fmt.Errorf(
			"%s was declared as %s and what arrived is %s; nothing was unpacked or started",
			r.Source, r.Digest, got)
	}

	// **The identity of a daemon is its bytes AND how it is run.** Two daemons from one bundle
	// differing only in their command are different daemons, and a record that tracked the digest
	// alone would call the second one unchanged.
	want := got + " " + unitFor(r)
	at := filepath.Join(daemonRoot, r.Name)

	// **Something it reads changed, so it must be restarted even though it is unchanged.** A
	// running process does not re-read its configuration: replace the file, find the daemon
	// already up, do nothing, and the machine keeps behaving the way it did before while every
	// check passes. The same rule a service follows, for the same reason.
	var because string
	for _, id := range r.RestartOn {
		if changed[id] {
			because = id
			break
		}
	}

	if previous.Wrote == want && because == "" {
		// Everything about it is as declared. Still asked whether it is RUNNING, because a
		// declaration that is satisfied by a record rather than by the machine is how a stopped
		// service reports success.
		if active, err := run(ctx, "systemctl", "is-active", "--quiet", r.Name+".service"); err == nil {
			_ = active
			return out, nil
		}
		if _, err := run(ctx, "systemctl", "start", r.Name+".service"); err != nil {
			return out, fmt.Errorf("%s is installed and would not start: %w", r.Name, err)
		}
		out.Action = "updated"
		out.Detail = "restarted a daemon that had stopped"
		out.wrote = want
		return out, nil
	}

	// Replaced rather than merged: the bundle is the whole of what it runs, and files left from a
	// previous version would be loaded by a runtime that walks a directory.
	if err := os.RemoveAll(at); err != nil {
		return out, err
	}
	if err := os.MkdirAll(at, 0o755); err != nil {
		return out, err
	}
	written, err := unpack(body, at)
	if err != nil {
		return out, err
	}
	if err := ownAll(at, r.User); err != nil {
		return out, err
	}

	unit := filepath.Join(unitDir, r.Name+".service")
	if err := os.WriteFile(unit, []byte(unitFor(r)), 0o644); err != nil {
		return out, err
	}
	if _, err := run(ctx, "systemctl", "daemon-reload"); err != nil {
		return out, err
	}
	// Enabled and restarted, in that order: enabled so it survives a reboot, restarted rather than
	// started because this path is also how a new version arrives and the old one is still running.
	if _, err := run(ctx, "systemctl", "enable", r.Name+".service"); err != nil {
		return out, err
	}
	if _, err := run(ctx, "systemctl", "restart", r.Name+".service"); err != nil {
		return out, fmt.Errorf("%s was installed and would not start: %w", r.Name, err)
	}

	out.Action = "updated"
	if previous.Wrote == "" {
		out.Action = "created"
	}
	out.Detail = fmt.Sprintf("%d file(s), running as %s.service", written, r.Name)
	if because != "" {
		out.Detail += ", restarted because " + because + " changed"
	}
	out.wrote = want
	return out, nil
}

// unitFor is the unit the mesh writes for a daemon.
//
// **Generated whole and never edited in place**, the same rule as every other managed file: an
// edit survives until the next declaration and then vanishes, which is worse than not being
// allowed at all, so the file says so.
//
// Deterministic — environment sorted — because this string is half the daemon's identity, and a
// map iterated in Go's order would make every apply look like a change.
func unitFor(r *declaration.Daemon) string {
	var b strings.Builder
	b.WriteString("# Generated by the mesh. Do not edit — this file is replaced whenever the\n")
	b.WriteString("# declaration changes, and an edit would survive until then and vanish.\n")
	b.WriteString("[Unit]\n")
	fmt.Fprintf(&b, "Description=%s, a mesh daemon\n", r.Name)
	b.WriteString("After=network-online.target\n")
	b.WriteString("Wants=network-online.target\n\n")

	b.WriteString("[Service]\n")
	b.WriteString("Type=simple\n")
	fmt.Fprintf(&b, "WorkingDirectory=%s\n", filepath.Join(daemonRoot, r.Name))
	for _, file := range r.EnvFile {
		fmt.Fprintf(&b, "EnvironmentFile=%s\n", file)
	}
	for _, key := range sortedKeys(r.Env) {
		fmt.Fprintf(&b, "Environment=%s=%s\n", key, r.Env[key])
	}
	if r.User != "" {
		fmt.Fprintf(&b, "User=%s\n", r.User)
	}
	fmt.Fprintf(&b, "ExecStart=%s\n", strings.Join(r.Run, " "))
	// Restarted when it exits, because a daemon that stops is not a daemon. Delayed, so a process
	// that fails at once does not spin the machine.
	b.WriteString("Restart=always\nRestartSec=5\n\n")

	b.WriteString("[Install]\nWantedBy=multi-user.target\n")
	return b.String()
}