package session

import (
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"time"

	"github.com/warrenronsiek/ctask/internal/lockfile"
	"github.com/warrenronsiek/ctask/internal/shell"
)

// LaunchOpts configures a session launch.
type LaunchOpts struct {
	WsDir   string
	EnvVars map[string]string
	Agent   string
	Mode    string
	Slug    string
	Shell   bool // true = interactive shell, false = agent

	// Force suppresses both the active-session warning (Layer 1) and the
	// stale-workspace warning (Layer 3). It does NOT disable the metadata
	// write lock or the session summary. Used for scripted/automated runs.
	Force bool

	// NewlyCreated signals that this invocation just created WsDir (via
	// `ctask new`). When true and the session's manifest diff is empty, the
	// workspace is treated as provisional and removed on exit — see
	// handleProvisional. Set to false by resume/open/last.
	NewlyCreated bool
}

// manifestStartPath returns the path to the start manifest file.
func manifestStartPath(wsDir string) string {
	return filepath.Join(wsDir, ".ctask", "manifest-start.json")
}

// ctaskWriteLockPath returns the path to the workspace write lock file.
func ctaskWriteLockPath(wsDir string) string {
	return filepath.Join(wsDir, ".ctask", "write.lock")
}

const (
	sessionWriteLockTimeout    = 2 * time.Second
	sessionWriteLockStaleAfter = 10 * time.Second
)

// Run launches an agent or shell session. It sequences the full v0.4
// concurrency-protection lifecycle:
//
//  1. Layer 3: stale-workspace detection (warn if files changed outside a
//     ctask session; skipped if Force is true).
//  2. Layer 1: active-session lease check (warn or auto-clean stale;
//     skipped if Force is true).
//  3. Layer 2: write lock taken to install the new lease (unless coexisting
//     with an already-active lease, in which case no lease is written).
//  4. Start manifest capture + write.
//  5. Launch context banner (Layer 4 — prints the last-session summary).
//  6. Heartbeat goroutine starts (Layer 1) when we own a lease.
//  7. Agent/shell runs to completion.
//  8. Heartbeat stops.
//  9. End manifest + diff.
//  10. Write lock taken to append log, write summary (Layer 4), remove
//     lease (Layer 1), remove start manifest.
//
// Returns the child process's error (for exit-code propagation).
func Run(opts LaunchOpts) error {
	// ---- Preflight (Layers 3 + 1) ----
	preflight, err := PreflightFull(PreflightOpts{
		WsDir: opts.WsDir,
		Force: opts.Force,
		In:    os.Stdin,
		Out:   os.Stderr,
	})
	if err != nil {
		fmt.Fprintf(os.Stderr, "[ctask] Warning: preflight check failed: %v\n", err)
		// Continue anyway — preflight errors must never block the session.
	}
	if !preflight.Proceed {
		return nil
	}

	startTime := time.Now().UTC().Truncate(time.Second)

	// ---- Install a lease unless we're coexisting with an active one ----
	// Lease creation is "important but non-fatal": if the lock times out,
	// the session runs without concurrency protection; make that visible.
	leasePath := LeasePath(opts.WsDir)
	ownLease := !preflight.ActiveLeaseFound
	if ownLease {
		lease := NewLease(startTime, opts.Agent, opts.Mode)
		skipped, lockErr := lockfile.WithLock(
			ctaskWriteLockPath(opts.WsDir),
			sessionWriteLockTimeout, sessionWriteLockStaleAfter,
			func() error { return WriteLease(leasePath, lease) },
		)
		if lockErr != nil {
			fmt.Fprintf(os.Stderr, "[ctask] Warning: could not write session lease: %v; this session will not be visible to concurrent ctask processes\n", lockErr)
			ownLease = false
		}
		if skipped {
			fmt.Fprintf(os.Stderr, "[ctask] Warning: could not acquire metadata lock, skipping write to %s; this session will not be visible to concurrent ctask processes\n", leasePath)
			ownLease = false
		}
	}

	// ---- Start manifest (important but non-fatal) ----
	startManifest, err := CaptureManifest(opts.WsDir)
	if err != nil {
		fmt.Fprintf(os.Stderr, "[ctask] warning: failed to capture start manifest: %v; end-of-session diff will be skipped\n", err)
	}
	if startManifest != nil {
		mPath := manifestStartPath(opts.WsDir)
		skipped, lockErr := lockfile.WithLock(
			ctaskWriteLockPath(opts.WsDir),
			sessionWriteLockTimeout, sessionWriteLockStaleAfter,
			func() error { return WriteManifest(mPath, startManifest) },
		)
		if skipped {
			fmt.Fprintf(os.Stderr, "[ctask] Warning: could not acquire metadata lock, skipping write to %s; end-of-session diff will be skipped\n", mPath)
			startManifest = nil
		} else if lockErr != nil {
			fmt.Fprintf(os.Stderr, "[ctask] Warning: could not write start manifest: %v\n", lockErr)
			startManifest = nil
		}
	}

	// ---- Launch context banner (Layer 4 — best-effort read) ----
	if summary, err := ReadSummary(SummaryPath(opts.WsDir)); err == nil && summary != nil {
		fmt.Fprint(os.Stderr, FormatLaunchContext(summary))
	}

	// ---- Heartbeat (Layer 1) ----
	var hb *Heartbeat
	if ownLease {
		hb = StartHeartbeat(leasePath, HeartbeatInterval)
	}

	// ---- Run the child ----
	var childErr error
	if opts.Shell {
		childErr = shell.ExecShell(opts.WsDir, opts.EnvVars, opts.Slug, opts.Mode)
	} else {
		for _, line := range shell.BannerLines(opts.Mode, opts.Slug, opts.WsDir) {
			fmt.Println(line)
		}
		childErr = shell.ExecAgent(opts.Agent, opts.WsDir, opts.EnvVars)
	}

	if hb != nil {
		hb.Stop()
	}

	// ---- Provisional-workspace cleanup ----
	// If `ctask new` launched this session, the child exited non-zero
	// (canceled before real work), and no files changed, remove the workspace
	// entirely and skip finalize. A zero child exit means the user entered
	// the agent and exited normally — those workspaces are preserved even
	// with an empty diff.
	if handleProvisional(opts, startManifest, childExitCode(childErr)) {
		return childErr
	}

	// ---- Post-session: manifest + summary + cleanup ----
	endTime := time.Now().UTC().Truncate(time.Second)
	if startManifest != nil {
		if err := finalize(opts, startManifest, startTime, endTime, ownLease); err != nil {
			fmt.Fprintf(os.Stderr, "[ctask] warning: session finalize failed: %v\n", err)
			// Leave manifest-start.json for debugging on failure.
		}
	}

	return childErr
}

// childExitCode extracts the exit code from the error returned by cmd.Run().
// Returns 0 when err is nil (clean exit). Returns the reported code for
// *exec.ExitError. Returns -1 for any other error (agent not found, OS-level
// failure) so provisional cleanup still kicks in — the child never ran.
func childExitCode(err error) int {
	if err == nil {
		return 0
	}
	if ee, ok := err.(*exec.ExitError); ok {
		return ee.ExitCode()
	}
	return -1
}

// finalize runs all end-of-session metadata writes under one write-lock
// acquisition: session log append (best-effort), last-session-summary.json
// (best-effort), lease removal (best-effort, if we owned it), and
// manifest-start.json cleanup.
func finalize(opts LaunchOpts, startManifest *Manifest, startTime, endTime time.Time, ownLease bool) error {
	endManifest, err := CaptureManifest(opts.WsDir)
	if err != nil {
		return fmt.Errorf("capturing end manifest: %w", err)
	}
	diff := DiffManifests(startManifest, endManifest)

	agent := opts.Agent
	if opts.Shell {
		agent = "shell"
	}
	info := &SessionInfo{
		Agent:     agent,
		Mode:      opts.Mode,
		StartTime: startTime,
		EndTime:   endTime,
		Diff:      diff,
	}

	// Pull the session ID from the lease (if we own one) so the summary
	// references the same ID used during the session. Fall back to a
	// constructed ID to keep the summary self-describing when leaseless.
	sessionID := NewSessionID(currentHostname(), os.Getpid(), startTime)
	if ownLease {
		if l, err := ReadLease(LeasePath(opts.WsDir)); err == nil && l != nil {
			sessionID = l.SessionID
		}
	}

	summary := SummarizeFromDiff(
		sessionID, currentHostname(), agent, opts.Mode,
		startTime, endTime, diff, endManifest,
	)

	skipped, lockErr := lockfile.WithLock(
		ctaskWriteLockPath(opts.WsDir),
		sessionWriteLockTimeout, sessionWriteLockStaleAfter,
		func() error {
			if err := AppendSessionLog(opts.WsDir, info); err != nil {
				fmt.Fprintf(os.Stderr, "[ctask] warning: append session log failed: %v\n", err)
			}
			if err := WriteSummary(SummaryPath(opts.WsDir), summary); err != nil {
				return fmt.Errorf("write summary: %w", err)
			}
			if ownLease {
				if rmErr := os.Remove(LeasePath(opts.WsDir)); rmErr != nil && !os.IsNotExist(rmErr) {
					fmt.Fprintf(os.Stderr, "[ctask] warning: could not remove lease: %v\n", rmErr)
				}
			}
			if rmErr := os.Remove(manifestStartPath(opts.WsDir)); rmErr != nil && !os.IsNotExist(rmErr) {
				fmt.Fprintf(os.Stderr, "[ctask] warning: could not remove start manifest: %v\n", rmErr)
			}
			return nil
		},
	)
	if skipped {
		fmt.Fprintf(os.Stderr, "[ctask] Warning: could not acquire metadata lock, skipping session-end writes (summary and lease cleanup)\n")
		return nil
	}
	return lockErr
}