package upgrade

import (
	"bufio"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"time"
)

// The host's own successor, witnessed by its launcher (novox/hq to-be 45 §8, ADR 0227 rule 8).
//
// The launcher runs a delivered host that is not the known-good one **on trial**, and rolls back
// from one that crashes repeatedly, stops for nothing, or does not report within its bound. Two
// files are the whole conversation between them, both plain enough for a shell:
//
//   - known-good — written by this host when the mesh has taken a report it made about its
//     declaration under its own build. That is what ends a trial.
//   - rolled-back — written by the launcher: one line per verdict, tab-separated — from, to, when
//     (seconds since the epoch), outcome, why. Read here and said on every report while it stands.

// RolledBackName is the launcher's record of its verdicts, beside the state.
const RolledBackName = "rolled-back"

// RolledBackPath is where it lives, given where the state lives.
func RolledBackPath(statePath string) string {
	return filepath.Join(filepath.Dir(statePath), RolledBackName)
}

// Verdict is one line of the launcher's record.
type Verdict struct {
	From, To, Outcome, Why string
	At                     time.Time
}

// ReadRolledBack is the launcher's record, oldest first. A line it cannot read is skipped and named
// in the error, never guessed at; absence is no verdicts.
func ReadRolledBack(path string) ([]Verdict, error) {
	file, err := os.Open(path)
	if errors.Is(err, os.ErrNotExist) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	defer file.Close()
	var out []Verdict
	var bad []string
	lines := bufio.NewScanner(file)
	for n := 1; lines.Scan(); n++ {
		line := strings.TrimRight(lines.Text(), "\r")
		if strings.TrimSpace(line) == "" {
			continue
		}
		fields := strings.SplitN(line, "\t", 5)
		if len(fields) != 5 || fields[0] == "" {
			bad = append(bad, strconv.Itoa(n))
			continue
		}
		seconds, err := strconv.ParseInt(fields[2], 10, 64)
		if err != nil {
			bad = append(bad, strconv.Itoa(n))
			continue
		}
		out = append(out, Verdict{From: fields[0], To: fields[1], At: time.Unix(seconds, 0).UTC(),
			Outcome: fields[3], Why: fields[4]})
	}
	if err := lines.Err(); err != nil {
		return out, err
	}
	if len(bad) > 0 {
		return out, fmt.Errorf("the launcher's record %s has line(s) %s that are not from, to, when, outcome, why",
			path, strings.Join(bad, ", "))
	}
	return out, nil
}

// RolledBackVersions are the versions the launcher refuses to start again.
func RolledBackVersions(verdicts []Verdict) []string {
	var out []string
	for _, v := range verdicts {
		out = append(out, v.From)
	}
	return out
}

// Proved is this host's version seen to report its declaration under its own build: the evidence a
// trial waits for, and the only evidence known-good has ever claimed (novox/hq ADR 0005).
//
//   - known-good becomes this version, which ends the launcher's trial;
//   - the start counter is cleared, so months of ordinary restarts never add up to a rollback;
//   - versions older than this one's predecessor are retired — never one newer, never this one;
//   - and when this version is not the one a rollback went back to, the launcher's verdicts end: a
//     newer host has proved itself, and what was concluded about an older one no longer stands.
//
// Returns the versions retired. Every step is attempted; the errors are joined.
func Proved(statePath, versionsDir, version string) ([]string, error) {
	if strings.TrimSpace(version) == "" {
		return nil, errors.New("a host that does not know its own version cannot prove it")
	}
	var errs []error
	if err := RecordKnownGood(KnownGoodPath(statePath), version); err != nil {
		errs = append(errs, fmt.Errorf("recording %s as known-good: %w", version, err))
	}
	if err := ClearAttempts(AttemptsPath(statePath)); err != nil {
		errs = append(errs, fmt.Errorf("clearing the start counter: %w", err))
	}
	retired, err := Retire(versionsDir, version)
	if err != nil {
		errs = append(errs, err)
	}
	verdicts, err := ReadRolledBack(RolledBackPath(statePath))
	if err != nil {
		errs = append(errs, err)
	}
	wentBackTo := false
	for _, v := range verdicts {
		if v.To == version {
			wentBackTo = true
		}
	}
	if len(verdicts) > 0 && !wentBackTo {
		if err := os.Remove(RolledBackPath(statePath)); err != nil && !errors.Is(err, os.ErrNotExist) {
			errs = append(errs, err)
		}
	}
	return retired, errors.Join(errs...)
}