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.
var daemonRoot = "/var/lib/mesh/daemons"

// unitDir is where the mesh writes the units it owns. A variable only so a test can point it at a
// directory of its own.
var unitDir = "/etc/systemd/system"

func applyProcess(ctx context.Context, r *declaration.Process, 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)
	}

	// **Its identity is its bytes AND how it is run.** Two processes from one bundle differing
	// only in their command are different, and a record tracking 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
	}

	// **A step is run to completion, not installed.** What follows it is gated on it finishing,
	// so the machine is not asked to start something that needed a migration that did not happen.
	// Nothing is left behind to ask afterwards: the record that it ran is the digest, which is why
	// the identity above includes the command.
	if r.RunOnce {
		if _, err := run(ctx, r.Run[0], r.Run[1:]...); err != nil {
			return out, fmt.Errorf("the %s step did not complete: %w", r.Name, err)
		}
		out.Action = "created"
		if previous.Wrote != "" {
			out.Action = "updated"
		}
		out.Detail = fmt.Sprintf("%d file(s), step completed", written)
		out.wrote = want
		return out, nil
	}

	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.
	// **Scheduled means a timer, not a service that stays up.** The unit above is written either
	// way and describes what to run; what differs is whether the machine is asked to keep it
	// running or to start it when the timer says so.
	if r.Schedule != "" {
		timer := filepath.Join(unitDir, r.Name+".timer")
		if err := os.WriteFile(timer, []byte(timerFor(r)), 0o644); err != nil {
			return out, err
		}
		if _, err := run(ctx, "systemctl", "daemon-reload"); err != nil {
			return out, err
		}
		// The timer is enabled and started; the service is neither. Enabling the service too
		// would have it run at boot as well as on its cadence, which is a second schedule nobody
		// asked for.
		if _, err := run(ctx, "systemctl", "enable", r.Name+".timer"); err != nil {
			return out, err
		}
		if _, err := run(ctx, "systemctl", "restart", r.Name+".timer"); err != nil {
			return out, fmt.Errorf("%s was installed and its timer would not start: %w", r.Name, err)
		}
		out.Action = "updated"
		if previous.Wrote == "" {
			out.Action = "created"
		}
		out.Detail = fmt.Sprintf("%d file(s), scheduled as %s.timer", written, r.Name)
		out.wrote = want
		return out, nil
	}

	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.Process) 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\n", unitValue(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, " "))
	if r.Schedule != "" {
		// Started by its timer and expected to finish. Restarting it would have it run
		// continuously between fires, which is the opposite of a schedule.
		b.WriteString("Type=oneshot\n")
		b.WriteString("\n")
		return strings.Replace(b.String(), "Type=simple\n", "", 1)
	}
	// Restarted when it exits, because something that stops is not something that stays up.
	// 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()
}

// timerFor is the cadence a scheduled process runs on.
//
// **The mesh's cron expression, handed to the machine's own timer.** The host already parses and
// evaluates five-field cron (ADR 0053) for a scheduled container; a machine with a supervisor can
// be told the cadence directly rather than have the host wake up and decide.
func timerFor(r *declaration.Process) 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, on a schedule the mesh set\n\n", r.Name)
	b.WriteString("[Timer]\n")
	fmt.Fprintf(&b, "OnCalendar=%s\n", calendarFor(r.Schedule))
	// A fire missed because the machine was off happens when it comes back, rather than being
	// skipped silently — which is the difference between a machine that was down and a schedule
	// that quietly stopped.
	b.WriteString("Persistent=true\n\n")
	b.WriteString("[Install]\nWantedBy=timers.target\n")
	return b.String()
}

// calendarFor turns five-field cron into what a systemd timer reads.
//
// minute hour day-of-month month day-of-week  ->  DayOfWeek Year-Month-Day Hour:Minute:Second
func calendarFor(cron string) string {
	fields := strings.Fields(cron)
	if len(fields) != 5 {
		// Refused at validation, so this is unreachable — and returning something that would fire
		// constantly is worse than returning something that never does.
		return "*-*-* 00:00:00"
	}
	minute, hour, dom, month, dow := fields[0], fields[1], fields[2], fields[3], fields[4]
	day := dow
	if dow == "*" {
		day = ""
	} else {
		day += " "
	}
	return fmt.Sprintf("%s*-%s-%s %s:%s:00", day, month, dom, hour, minute)
}

// unitValue renders one environment assignment so a unit file means what the declaration said.
//
// **Three things a unit file does to a value that nothing else does**, and a module's environment
// routinely contains all three — a generated password is arbitrary bytes.
//
//   - `%` begins a specifier. `%H` is the hostname, `%i` the instance. A password containing one
//     is silently replaced by something else, and the failure is an authentication error nobody
//     can explain by looking at the declaration.
//   - whitespace separates assignments. `Environment=K=a b` sets K to "a" and then tries to read
//     "b" as another assignment.
//   - a newline ends the line. What follows it is read as a unit DIRECTIVE, so a value carrying
//     one could write ExecStart= and have the machine run something nobody declared.
//
// Quoted, with quotes and backslashes escaped and percent doubled. A container needs none of this
// because `--env` is passed through literally, which is why this had to be found here rather than
// noticed in both.
func unitValue(key, value string) string {
	escaped := strings.NewReplacer(
		`\`, `\\`,
		`"`, `\"`,
		"%", "%%",
	).Replace(value)
	return `"` + key + "=" + escaped + `"`
}

// removeProcess takes away a process the mesh no longer declares: its timer and unit stopped and
// disabled, their files removed, the supervisor told, and the unpacked bundle deleted.
//
// **All of it is the host's**, which is why all of it goes (novox/hq ADR 0118). Before this there
// was no way to remove a process at all, and one left undeclared failed every apply on its node
// until someone edited the host's state by hand — unassigning any module that ran its own code
// stranded the machine.
//
// Idempotent, like every removal: a unit already gone is not an error, and a record whose files
// have all vanished is forgotten rather than reported as removed.
func removeProcess(ctx context.Context, a store.Applied, run Runner) (string, string, error) {
	name := a.Target
	if problem := declaration.ProcessNameProblem(name); problem != "" {
		// The name is a unit name and a directory under the mesh's own, and what goes is that
		// directory, whole. One that could climb out of either — ".." is the mesh's own directory's
		// parent — is refused rather than acted on, whatever wrote it into the record.
		return "", "", fmt.Errorf("a process recorded under %q cannot be removed by name: %s", name, problem)
	}
	found := false
	for _, unit := range []string{name + ".timer", name + ".service"} {
		path := filepath.Join(unitDir, unit)
		if _, err := os.Stat(path); err != nil {
			continue
		}
		found = true
		// The timer first, so a scheduled run cannot start the service between the two.
		if _, err := run(ctx, "systemctl", "disable", "--now", unit); err != nil {
			return "", "", fmt.Errorf("stopping %s: %w", unit, err)
		}
		if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
			return "", "", err
		}
	}
	if found {
		if _, err := run(ctx, "systemctl", "daemon-reload"); err != nil {
			return "", "", err
		}
	}
	bundle := filepath.Join(daemonRoot, name)
	if _, err := os.Stat(bundle); err == nil {
		found = true
		if err := os.RemoveAll(bundle); err != nil {
			return "", "", err
		}
	}
	if !found {
		return "forgotten", "no longer there", nil
	}
	return "removed", "stopped; its unit and its bundle removed — the mesh's own code", nil
}