package apply

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"syscall"
	"time"

	"github.com/novox/mesh-host/internal/declaration"
	"github.com/novox/mesh-host/internal/store"
)

// Keep records the original of a file found on an adopted node, before anything else happens to
// it, and says where (novox/hq ADR 0100). It never overwrites an original it already kept: the
// first copy is the one that was there before the mesh.
type Keep func(path string, content []byte, mode os.FileMode) (string, error)

// KeepIn keeps originals under dir/kept, each named for the path it came from, readable by root
// alone — a predecessor's configuration may carry its credentials.
func KeepIn(dir string) Keep {
	return func(path string, content []byte, _ os.FileMode) (string, error) {
		sum := sha256.Sum256([]byte(path))
		kept := filepath.Join(dir, "kept",
			hex.EncodeToString(sum[:])[:16]+"-"+filepath.Base(path))
		if _, err := os.Lstat(kept); err == nil {
			return kept, nil
		}
		if err := os.MkdirAll(filepath.Dir(kept), 0o700); err != nil {
			return "", err
		}
		if err := writeAtomically(kept, content, 0o600); err != nil {
			return "", err
		}
		back, err := os.ReadFile(kept)
		if err != nil || string(back) != string(content) {
			return "", fmt.Errorf("kept the original of %s at %s and cannot read it back", path, kept)
		}
		return kept, nil
	}
}

// holdable is whether a resource is one a predecessor can already have on the machine: a file at
// a path, or a container under a name. A file written into is not: it replaces nothing that was
// found, only adds the mesh's keys beside it (novox/hq ADR 0102).
func holdable(r declaration.Resource) bool {
	if f, ok := r.(*declaration.File); ok {
		return f.Into == ""
	}
	return r.Kind() == declaration.TypeContainer
}

// found is whether a declared file or container is present on the machine with no record of this
// host making it (novox/hq ADR 0100). A container carrying the host's own spec label was made by
// a host, whatever this store says, so it is never found.
func found(ctx context.Context, r declaration.Resource, run Runner, known store.State) (bool, error) {
	if known.Recorded(string(r.Kind()), r.Target()) {
		return false, nil
	}
	switch res := r.(type) {
	case *declaration.File:
		_, err := os.Lstat(res.Path)
		if errors.Is(err, os.ErrNotExist) {
			return false, nil
		}
		return err == nil, err
	case *declaration.Container:
		seen, exists, err := inspectFound(ctx, res.Name, run)
		if err != nil || !exists {
			return false, err
		}
		return seen.spec == "", nil
	}
	return false, nil
}

type foundContainer struct {
	id      string
	running bool
	spec    string
}

// inspectFound reads a container by name the way a hold needs it: its id, whether it runs, and
// whether a host made it.
func inspectFound(ctx context.Context, name string, run Runner) (foundContainer, bool, error) {
	cri, err := containerRuntime(ctx, run)
	if err != nil {
		return foundContainer{}, false, fmt.Errorf("%w, so nothing can be said about %q", err, name)
	}
	out, err := run(ctx, cri, "inspect", "--format",
		"{{.Id}}\t{{.State.Running}}\t{{index .Config.Labels \""+specLabel+"\"}}", name)
	if err != nil {
		return foundContainer{}, false, nil
	}
	parts := strings.Split(strings.TrimSpace(out), "\t")
	for len(parts) < 3 {
		parts = append(parts, "")
	}
	spec := strings.TrimSpace(parts[2])
	if spec == "<no value>" {
		spec = ""
	}
	return foundContainer{id: strings.TrimSpace(parts[0]), running: parts[1] == "true", spec: spec}, true, nil
}

// hold keeps a found file or container as it is, and reports it — the first time by recording
// what was found, every time after by comparing against that. Nothing is reverted, restarted or
// created: a held target that disappears stays held and gone until its module is taken.
func hold(ctx context.Context, r declaration.Resource, module string, was store.Held, already bool,
	run Runner, keep Keep, now time.Time) (Outcome, store.Held, error) {
	out := begin(r)
	h := was
	if !already {
		h = store.Held{ID: r.Identity(), Module: module, Kind: string(r.Kind()),
			Target: r.Target(), Since: now}
	}
	h.Module = module

	var changed string
	switch res := r.(type) {
	case *declaration.File:
		info, err := os.Lstat(res.Path)
		switch {
		case errors.Is(err, os.ErrNotExist):
			if !already {
				return out, h, fmt.Errorf("%s was found and is gone before it could be kept", res.Path)
			}
			changed = "gone"
		case err != nil:
			return out, h, err
		default:
			content, err := os.ReadFile(res.Path)
			if err != nil {
				return out, h, fmt.Errorf("%s was found and cannot be read to keep it: %w", res.Path, err)
			}
			if !already {
				// The original first, before anything is recorded: a hold with no kept copy
				// would be a promise the host cannot keep.
				if keep == nil {
					return out, h, fmt.Errorf(
						"%s was found on this adopted node and this host has nowhere to keep its original", res.Path)
				}
				kept, err := keep(res.Path, content, info.Mode().Perm())
				if err != nil {
					return out, h, fmt.Errorf("keeping the original of %s: %w", res.Path, err)
				}
				h.Kept = kept
				h.Digest = digestOf(string(content))
				h.Mode = fmt.Sprintf("%04o", info.Mode().Perm())
				if st, ok := info.Sys().(*syscall.Stat_t); ok {
					h.Owner = fmt.Sprintf("%d:%d", st.Uid, st.Gid)
				}
			} else if digestOf(string(content)) != h.Digest {
				changed = "rewritten"
			}
		}
	case *declaration.Container:
		seen, exists, err := inspectFound(ctx, res.Name, run)
		if err != nil {
			return out, h, err
		}
		switch {
		case !exists && !already:
			return out, h, fmt.Errorf("container %s was found and is gone before it could be held", res.Name)
		case !already:
			h.Container, h.Running = seen.id, seen.running
		case !exists:
			changed = "gone"
		case seen.id != h.Container:
			changed = "replaced"
		case h.Running && !seen.running:
			changed = "stopped"
		}
	default:
		return out, h, fmt.Errorf("a %s cannot be held", r.Kind())
	}

	if changed != h.Changed {
		h.Changed = changed
		h.ChangedAt = now
		if changed == "" {
			h.ChangedAt = time.Time{}
		}
	}
	out.Action = "held"
	out.Detail = "found on the machine; kept until " + module + " is taken"
	if h.Changed != "" {
		out.Detail += "; " + h.Changed + " by something other than the mesh since it was found, and not reverted"
	}
	return out, h, nil
}

// takenDetail is what an outcome says when a module's cutover replaced what was held for it.
func takenDetail(h store.Held) string {
	if h.Kept != "" {
		return "taken: replaced what was found; original kept at " + h.Kept
	}
	return "taken: replaced what was found"
}