package apply

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"slices"
	"sort"

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

// A file written into, never over (novox/hq ADR 0102).
//
// **The file is the machine's; the mesh owns keys in it.** The container runtime's configuration
// is the case that needed it: the mesh states one fact there — its registry is trusted over the
// private network — and writing the file whole replaced everything the machine had set, down to
// where the runtime keeps its data. So the host reads what is there, sets only the declared keys,
// keeps every other key as it found it, and records what each of its keys held before. Undeclared,
// each key goes back, and a file the mesh created goes only if nothing but its keys is left.

// applyInto writes a file's declared keys into the object already at its path.
func applyInto(r *declaration.File, previous store.Applied) (Outcome, error) {
	out := begin(r)
	if r.Into != declaration.IntoJSON {
		return out, fmt.Errorf("%s: into %q is not a format this host writes into", r.Path, r.Into)
	}
	var declared map[string]json.RawMessage
	if err := json.Unmarshal([]byte(r.Content), &declared); err != nil {
		return out, fmt.Errorf("%s: the keys to write are not a JSON object: %w", r.Path, err)
	}

	existing, err := os.ReadFile(r.Path)
	existed := err == nil
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return out, err
	}
	object := map[string]json.RawMessage{}
	if existed && len(bytes.TrimSpace(existing)) > 0 {
		if err := json.Unmarshal(existing, &object); err != nil || object == nil {
			// Refused, never replaced: a file the host cannot read as an object is a file it
			// cannot write into without losing whatever it is.
			return out, fmt.Errorf("%s is not a JSON object, so the mesh cannot write its keys into it "+
				"without replacing what is there; it was left as it is", r.Path)
		}
	}

	rec := store.Into{Format: declaration.IntoJSON, Before: map[string]json.RawMessage{}}
	if previous.Into != nil {
		rec.Created = previous.Into.Created
		for k, v := range previous.Into.Before {
			rec.Before[k] = v
		}
		rec.Absent = slices.Clone(previous.Into.Absent)
	} else {
		rec.Created = !existed
	}
	tracked := func(k string) bool { _, ok := rec.Before[k]; return ok || slices.Contains(rec.Absent, k) }

	// Drift: the machine no longer holds what this host last set in its keys.
	drifted := previous.Wrote != "" && existed && digestOf(keysOf(object, keysTracked(rec))) != previous.Wrote

	// Keys the mesh set before and no longer declares go back to what they held.
	for _, k := range keysTracked(rec) {
		if _, still := declared[k]; still {
			continue
		}
		giveBack(object, &rec, k)
	}
	// Declared keys: remember what each held the first time, then set it.
	for _, k := range keysIn(declared) {
		if !tracked(k) {
			if v, had := object[k]; had {
				rec.Before[k] = v
			} else {
				rec.Absent = append(rec.Absent, k)
			}
		}
		object[k] = declared[k]
	}

	want, err := render(object)
	if err != nil {
		return out, err
	}
	same := existed && canonical(existing) == canonical(want)
	if !same {
		mode := os.FileMode(0o644)
		if info, err := os.Stat(r.Path); err == nil {
			mode = info.Mode().Perm() // the machine's file keeps the machine's mode
		} else if r.Mode != "" {
			if m, err := modeOf(r.Mode, mode); err == nil {
				mode = m
			}
		}
		if err := os.MkdirAll(filepath.Dir(r.Path), 0o755); err != nil {
			return out, err
		}
		if err := writeAtomically(r.Path, want, mode); err != nil {
			return out, err
		}
	}
	// Read back: every declared key holds what was declared.
	written, err := os.ReadFile(r.Path)
	if err != nil {
		return out, fmt.Errorf("wrote into %s and cannot read it back: %w", r.Path, err)
	}
	var check map[string]json.RawMessage
	if err := json.Unmarshal(written, &check); err != nil {
		return out, fmt.Errorf("%s is not a JSON object after writing into it: %w", r.Path, err)
	}
	for k, v := range declared {
		if canonical(check[k]) != canonical(v) {
			return out, fmt.Errorf("%s does not hold the declared %q after writing into it", r.Path, k)
		}
	}

	if len(rec.Before) == 0 {
		rec.Before = nil
	}
	out.into = &rec
	out.wrote = digestOf(keysOf(check, keysIn(declared)))
	switch {
	case !existed:
		out.Action = "created"
		out.Detail = "written into; the file was not there"
	case same:
		out.Action = "unchanged"
	case drifted:
		out.Action = "corrected"
		out.Detail = "the mesh's keys had been changed on the machine; the rest of the file was kept"
	default:
		out.Action = "updated"
		out.Detail = "the mesh's keys written in; every other key kept as it was"
	}
	return out, nil
}

// removeInto gives back what a file written into held before the mesh's keys.
func removeInto(a store.Applied) (string, string, error) {
	existing, err := os.ReadFile(a.Target)
	if errors.Is(err, os.ErrNotExist) {
		return "forgotten", "no longer there", nil
	}
	if err != nil {
		return "", "", err
	}
	object := map[string]json.RawMessage{}
	if len(bytes.TrimSpace(existing)) > 0 {
		if err := json.Unmarshal(existing, &object); err != nil || object == nil {
			return "kept", "no longer a JSON object, so the mesh's keys were left in it; " +
				"remove them by hand", nil
		}
	}
	rec := *a.Into
	for _, k := range keysTracked(rec) {
		giveBack(object, &rec, k)
	}
	if a.Into.Created && len(object) == 0 {
		if err := os.Remove(a.Target); err != nil {
			return "", "", err
		}
		return "removed", "no longer declared; the mesh had created it and nothing else was in it", nil
	}
	want, err := render(object)
	if err != nil {
		return "", "", err
	}
	info, err := os.Stat(a.Target)
	if err != nil {
		return "", "", err
	}
	if err := writeAtomically(a.Target, want, info.Mode().Perm()); err != nil {
		return "", "", err
	}
	return "restored", "no longer declared; the mesh's keys were given back what they held", nil
}

func giveBack(object map[string]json.RawMessage, rec *store.Into, k string) {
	if v, had := rec.Before[k]; had {
		object[k] = v
		delete(rec.Before, k)
		return
	}
	delete(object, k)
	rec.Absent = slices.DeleteFunc(rec.Absent, func(a string) bool { return a == k })
}

func keysTracked(rec store.Into) []string {
	var keys []string
	for k := range rec.Before {
		keys = append(keys, k)
	}
	keys = append(keys, rec.Absent...)
	sort.Strings(keys)
	return slices.Compact(keys)
}

func keysOf(object map[string]json.RawMessage, keys []string) string {
	var b bytes.Buffer
	for _, k := range keys {
		b.WriteString(k + "=" + canonical(object[k]) + "\n")
	}
	return b.String()
}

func keysIn(m map[string]json.RawMessage) []string {
	keys := make([]string, 0, len(m))
	for k := range m {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	return keys
}

// canonical is a JSON value compacted, so formatting is not mistaken for a change.
func canonical(raw []byte) string {
	var b bytes.Buffer
	if err := json.Compact(&b, raw); err != nil {
		return string(raw)
	}
	return b.String()
}

func render(object map[string]json.RawMessage) ([]byte, error) {
	b, err := json.MarshalIndent(object, "", "  ")
	if err != nil {
		return nil, err
	}
	return append(b, '\n'), nil
}