// Package symlinks manages the shared-state symlinks that every Claude
// account home must expose, so that account failover does not create state
// divergence (duplicated JSONL transcripts, broken undo history, drifted
// session env).
//
// Rationale
//
// Claude Code stores three directories whose content MUST be identical
// across the two configured accounts for failover to be a no-op:
//
//   - projects/      — session JSONL transcripts (used by `claude --resume`)
//   - session-env/   — per-session environment and working-dir metadata
//   - file-history/  — undo/redo history persistence
//
// If account A writes under `~/.claude-compte1/projects/...` while account
// B later runs under `~/.claude-compte2/projects/...`, resume fails
// silently with "session not found" and the operator loses every in-flight
// conversation.
//
// Historically we fixed this by creating symlinks manually on the
// operator's VM. Any fresh deployment that forgets those links silently
// reintroduces the bug. This package encodes the convention in code:
// EnsureForAccount creates missing links, ValidateAll fails fast at
// startup when an account home is misconfigured.
package symlinks

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

// DefaultSharedRoot is the directory under which the three shared targets
// live. All SharedSymlink.Target values default to a subdirectory of this
// root so tests can override the root without rewriting the shared list.
const DefaultSharedRoot = "/home/ubuntu"

// SharedSymlink describes one required link inside a Claude account home.
//
// Target is the absolute path on disk that holds the real shared
// directory. Name is the basename of the link that must exist inside each
// account home (e.g. `session-env`, `file-history`, `projects`).
type SharedSymlink struct {
	Target string
	Name   string
}

// RequiredShared lists the three symlinks every Claude account home must
// expose. The list is package-level so integration tests can read it, but
// callers SHOULD prefer the EnsureForAccount / ValidateAll entry points
// that accept an override list for isolation.
var RequiredShared = []SharedSymlink{
	{Target: "/home/ubuntu/.claude-session-env-shared", Name: "session-env"},
	{Target: "/home/ubuntu/.claude-file-history-shared", Name: "file-history"},
	{Target: "/home/ubuntu/.claude-projects-shared", Name: "projects"},
}

// EnsureForAccount verifies (and creates if missing) every required shared
// symlink for a single account home. Behaviour:
//
//   - If accountHome does not exist, it is created (mode 0700).
//   - If Target (the shared destination) does not exist, it is created
//     (mode 0700). Both accounts pointing at a non-existent target would
//     produce two separate state trees on first write.
//   - If the link is absent, it is created.
//   - If the link is present and points at Target, nothing happens.
//   - If the link is present but points elsewhere, an error is returned.
//     We REFUSE to auto-correct a divergent link because fixing it blindly
//     could delete user data: the "wrong" target may contain the only copy
//     of the session transcripts.
//   - If a regular file or directory exists where the link should be,
//     an error is returned for the same reason.
func EnsureForAccount(accountHome string, required []SharedSymlink) error {
	if accountHome == "" {
		return errors.New("symlinks: accountHome is empty")
	}

	if err := os.MkdirAll(accountHome, 0700); err != nil {
		return fmt.Errorf("symlinks: create account home %q: %w", accountHome, err)
	}

	for _, sl := range required {
		if err := ensureTarget(sl.Target); err != nil {
			return err
		}
		if err := ensureLink(accountHome, sl); err != nil {
			return err
		}
	}
	return nil
}

// ValidateAll runs EnsureForAccount on every account home. It aggregates
// all errors and returns a single error with every failure inlined, so the
// operator sees the full picture at startup rather than fixing one link,
// restarting, hitting the next one, repeat.
func ValidateAll(accountHomes []string, required []SharedSymlink) error {
	if len(accountHomes) == 0 {
		return errors.New("symlinks: no account homes provided")
	}
	var errs []string
	for _, home := range accountHomes {
		if err := EnsureForAccount(home, required); err != nil {
			errs = append(errs, err.Error())
		}
	}
	if len(errs) > 0 {
		return fmt.Errorf("symlinks: validation failed for %d account home(s): %s",
			len(errs), strings.Join(errs, "; "))
	}
	return nil
}

// ensureTarget creates Target as an empty directory when absent.
// An existing file (non-directory, non-symlink) at Target is an operator
// error we cannot resolve automatically.
func ensureTarget(target string) error {
	info, err := os.Stat(target)
	if err != nil {
		if !os.IsNotExist(err) {
			return fmt.Errorf("symlinks: stat shared target %q: %w", target, err)
		}
		if mkErr := os.MkdirAll(target, 0700); mkErr != nil {
			return fmt.Errorf("symlinks: create shared target %q: %w", target, mkErr)
		}
		return nil
	}
	if !info.IsDir() {
		return fmt.Errorf("symlinks: shared target %q is not a directory", target)
	}
	return nil
}

// ensureLink reconciles one link entry inside accountHome.
func ensureLink(accountHome string, sl SharedSymlink) error {
	linkPath := filepath.Join(accountHome, sl.Name)

	info, err := os.Lstat(linkPath)
	if err != nil {
		if os.IsNotExist(err) {
			if linkErr := os.Symlink(sl.Target, linkPath); linkErr != nil {
				return fmt.Errorf("symlinks: create %q → %q: %w", linkPath, sl.Target, linkErr)
			}
			return nil
		}
		return fmt.Errorf("symlinks: lstat %q: %w", linkPath, err)
	}

	// Path exists — must be a symlink pointing at Target.
	if info.Mode()&os.ModeSymlink == 0 {
		return fmt.Errorf("symlinks: %q exists but is not a symlink (expected → %q)",
			linkPath, sl.Target)
	}
	currentTarget, err := os.Readlink(linkPath)
	if err != nil {
		return fmt.Errorf("symlinks: readlink %q: %w", linkPath, err)
	}
	if currentTarget != sl.Target {
		return fmt.Errorf("symlinks: divergent link at %q: points to %q, expected %q (refusing to auto-correct to avoid data loss)",
			linkPath, currentTarget, sl.Target)
	}
	return nil
}