package apply

import (
	"fmt"
	"os"
	"strconv"
	"strings"
	"syscall"
)

// Applier makes this machine match one kind of resource, and reads back to prove it took.
//
// Read-back is not optional and it is not this package's habit alone: setting a value is not
// evidence the value took (novox/hq how-we-build §5, and 05-the-node-host.md as a component
// requirement). So Apply writes AND confirms, and returns an error if the machine does not then
// match — a firewall is asked whether the rule loaded, and a file is read back byte for byte.
type Applier interface {
	// Type is the resource type this handles — the key in the shape table.
	Type() string
	// Apply makes the machine match r and reads back to confirm. created reports whether the
	// host brought the resource into being (as opposed to adopting one already present), which
	// is what the store needs so removal never deletes what the host did not create.
	Apply(r Resource) (created bool, err error)
	// Remove undoes a resource the host created. Only ever called for a store Record whose
	// Created is true, and written to never destroy data it did not put there.
	Remove(rec Record) error
}

// Appliers is the set of types this host can apply, keyed by type name.
func Appliers() map[string]Applier {
	return map[string]Applier{
		"directory": directoryApplier{},
		"file":      fileApplier{},
	}
}

// --- directory ---

type directoryApplier struct{}

func (directoryApplier) Type() string { return "directory" }

func (directoryApplier) Apply(r Resource) (bool, error) {
	path := r.Path()
	mode, err := parseMode(r.stringField("mode"), 0o755)
	if err != nil {
		return false, err
	}

	created := false
	info, statErr := os.Lstat(path)
	switch {
	case statErr == nil:
		if !info.IsDir() {
			return false, fmt.Errorf("%s exists and is not a directory", path)
		}
	case os.IsNotExist(statErr):
		if err := os.Mkdir(path, mode); err != nil {
			return false, fmt.Errorf("creating %s: %w", path, err)
		}
		created = true
	default:
		return false, fmt.Errorf("inspecting %s: %w", path, statErr)
	}

	if err := os.Chmod(path, mode); err != nil {
		return created, fmt.Errorf("setting mode on %s: %w", path, err)
	}
	if err := applyOwner(path, r.stringField("owner")); err != nil {
		return created, err
	}

	// Read back: the directory must now exist, be a directory, and hold the mode and owner
	// asked for. Anything else is a value that did not take.
	if err := verifyPathState(path, true, mode, r.stringField("owner")); err != nil {
		return created, fmt.Errorf("%s did not take: %w", path, err)
	}
	return created, nil
}

func (directoryApplier) Remove(rec Record) error {
	// os.Remove, never RemoveAll: it fails on a non-empty directory, and that failure is the
	// point. A directory the host created but that now holds something is not the host's to
	// delete — data outlives the mesh that declared it (ADR 0030).
	err := os.Remove(rec.Path)
	if os.IsNotExist(err) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("removing directory %s (left in place): %w", rec.Path, err)
	}
	return nil
}

// --- file ---

type fileApplier struct{}

func (fileApplier) Type() string { return "file" }

func (fileApplier) Apply(r Resource) (bool, error) {
	path := r.Path()
	mode, err := parseMode(r.stringField("mode"), 0o644)
	if err != nil {
		return false, err
	}
	content := []byte(r.stringField("content"))

	_, statErr := os.Lstat(path)
	created := os.IsNotExist(statErr)
	if statErr != nil && !created {
		return false, fmt.Errorf("inspecting %s: %w", path, statErr)
	}

	// Idempotent: the file is rewritten only when the bytes differ, so applying the same
	// declaration twice changes nothing the second time. Mode and owner are still reconciled
	// below, because those can drift without the content doing so.
	needsWrite := created
	if !created {
		existing, readErr := os.ReadFile(path)
		needsWrite = readErr != nil || string(existing) != string(content)
	}
	if needsWrite {
		if err := os.WriteFile(path, content, mode); err != nil {
			return created, fmt.Errorf("writing %s: %w", path, err)
		}
	}
	if err := os.Chmod(path, mode); err != nil {
		return created, fmt.Errorf("setting mode on %s: %w", path, err)
	}
	if err := applyOwner(path, r.stringField("owner")); err != nil {
		return created, err
	}

	// Read back: the file must now hold exactly these bytes and this mode. A file whose content
	// was composed on the machine, or whose write was short, is a value that did not take.
	got, err := os.ReadFile(path)
	if err != nil {
		return created, fmt.Errorf("%s did not take: reading it back: %w", path, err)
	}
	if string(got) != string(content) {
		return created, fmt.Errorf("%s did not take: content read back does not match", path)
	}
	if err := verifyPathState(path, false, mode, r.stringField("owner")); err != nil {
		return created, fmt.Errorf("%s did not take: %w", path, err)
	}
	return created, nil
}

func (fileApplier) Remove(rec Record) error {
	err := os.Remove(rec.Path)
	if os.IsNotExist(err) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("removing file %s: %w", rec.Path, err)
	}
	return nil
}

// --- shared ---

func parseMode(s string, fallback os.FileMode) (os.FileMode, error) {
	if s == "" {
		return fallback, nil
	}
	n, err := strconv.ParseUint(s, 8, 32)
	if err != nil {
		return 0, fmt.Errorf("mode %q is not an octal number like \"0700\": %w", s, err)
	}
	return os.FileMode(n), nil
}

// applyOwner sets uid:gid when an owner is named. Owners are numeric because a container's user
// has no name on the machine (novox/mesh-control: "an owner may be numeric"). An empty owner is
// left untouched — not every resource asserts one.
func applyOwner(path, owner string) error {
	if owner == "" {
		return nil
	}
	uid, gid, err := parseOwner(owner)
	if err != nil {
		return err
	}
	if err := os.Chown(path, uid, gid); err != nil {
		return fmt.Errorf("setting owner %s on %s: %w", owner, path, err)
	}
	return nil
}

func parseOwner(owner string) (int, int, error) {
	parts := strings.SplitN(owner, ":", 2)
	if len(parts) != 2 {
		return 0, 0, fmt.Errorf("owner %q is not \"uid:gid\"", owner)
	}
	uid, err := strconv.Atoi(parts[0])
	if err != nil {
		return 0, 0, fmt.Errorf("owner %q: uid is not a number", owner)
	}
	gid, err := strconv.Atoi(parts[1])
	if err != nil {
		return 0, 0, fmt.Errorf("owner %q: gid is not a number", owner)
	}
	return uid, gid, nil
}

// verifyPathState reads back mode and (when asserted) owner, and reports the first mismatch.
func verifyPathState(path string, wantDir bool, mode os.FileMode, owner string) error {
	info, err := os.Lstat(path)
	if err != nil {
		return err
	}
	if info.IsDir() != wantDir {
		return fmt.Errorf("expected directory=%v, found directory=%v", wantDir, info.IsDir())
	}
	if info.Mode().Perm() != mode.Perm() {
		return fmt.Errorf("expected mode %04o, found %04o", mode.Perm(), info.Mode().Perm())
	}
	if owner != "" {
		wantUID, wantGID, err := parseOwner(owner)
		if err != nil {
			return err
		}
		st, ok := info.Sys().(*syscall.Stat_t)
		if !ok {
			return fmt.Errorf("cannot read owner back on this platform")
		}
		if int(st.Uid) != wantUID || int(st.Gid) != wantGID {
			return fmt.Errorf("expected owner %d:%d, found %d:%d", wantUID, wantGID, st.Uid, st.Gid)
		}
	}
	return nil
}