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{},
		Added: 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)
		for k, v := range previous.Into.Added {
			rec.Added[k] = slices.Clone(v)
		}
	} else {
		rec.Created = !existed
	}
	tracked := func(k string) bool {
		_, before := rec.Before[k]
		_, added := rec.Added[k]
		return before || added || slices.Contains(rec.Absent, k)
	}

	// Drift: the machine no longer holds what this host last set in its keys.
	drifted := previous.Wrote != "" && existed && digestOf(viewOf(object, rec, 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. A list is the
	// machine's too — a predecessor's own trusted registries, say — so the mesh adds its members
	// to it rather than replacing it, and remembers exactly which it added.
	for _, k := range keysIn(declared) {
		_, scalar := rec.Before[k]
		if isList(declared[k]) && !scalar {
			current, had := object[k]
			if had && !isList(current) {
				return out, fmt.Errorf("%s: the mesh adds to the list %q, and the machine holds something "+
					"other than a list there; it was left as it is", r.Path, k)
			}
			if !tracked(k) && !had {
				rec.Absent = append(rec.Absent, k)
			}
			merged, added, err := addMembers(current, declared[k], rec.Added[k])
			if err != nil {
				return out, fmt.Errorf("%s: %q: %w", r.Path, k, err)
			}
			rec.Added[k] = added
			object[k] = merged
			continue
		}
		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 _, list := rec.Added[k]; list {
			members, _ := membersOf(v)
			have, err := membersOf(check[k])
			if err != nil {
				return out, fmt.Errorf("%s does not hold a list at %q after writing into it", r.Path, k)
			}
			for _, m := range members {
				if !hasMember(have, m) {
					return out, fmt.Errorf("%s does not hold the declared %s in %q after writing into it", r.Path, m, k)
				}
			}
			continue
		}
		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
	}
	if len(rec.Added) == 0 {
		rec.Added = nil
	}
	out.into = &rec
	out.wrote = digestOf(viewOf(check, rec, 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 added, list := rec.Added[k]; list {
		// Only the members the mesh added go; the list and everything else in it stay, unless
		// the mesh made the key and nothing is left in it.
		wasAbsent := slices.Contains(rec.Absent, k)
		if current, had := object[k]; had && isList(current) {
			have, _ := membersOf(current)
			have = slices.DeleteFunc(have, func(m json.RawMessage) bool { return hasMember(added, m) })
			if len(have) == 0 && wasAbsent {
				delete(object, k)
			} else {
				object[k] = listOf(have)
			}
		}
		delete(rec.Added, k)
		rec.Absent = slices.DeleteFunc(rec.Absent, func(a string) bool { return a == k })
		return
	}
	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)
	}
	for k := range rec.Added {
		keys = append(keys, k)
	}
	keys = append(keys, rec.Absent...)
	sort.Strings(keys)
	return slices.Compact(keys)
}

// viewOf is what the mesh holds itself to in a file written into: each scalar key's value, and for
// a list only whether each member the mesh added is still there — what the machine keeps beside
// them is not the mesh's to judge.
func viewOf(object map[string]json.RawMessage, rec store.Into, keys []string) string {
	var b bytes.Buffer
	for _, k := range keys {
		if added, list := rec.Added[k]; list {
			have, _ := membersOf(object[k])
			b.WriteString(k + " holds")
			for _, m := range added {
				fmt.Fprintf(&b, " %s=%v", canonical(m), hasMember(have, m))
			}
			b.WriteString("\n")
			continue
		}
		b.WriteString(k + "=" + canonical(object[k]) + "\n")
	}
	return b.String()
}

func isList(raw json.RawMessage) bool {
	t := bytes.TrimSpace(raw)
	return len(t) > 0 && t[0] == '['
}

func membersOf(raw json.RawMessage) ([]json.RawMessage, error) {
	if len(bytes.TrimSpace(raw)) == 0 {
		return nil, nil
	}
	var members []json.RawMessage
	if err := json.Unmarshal(raw, &members); err != nil {
		return nil, err
	}
	return members, nil
}

func hasMember(list []json.RawMessage, m json.RawMessage) bool {
	for _, have := range list {
		if canonical(have) == canonical(m) {
			return true
		}
	}
	return false
}

func listOf(members []json.RawMessage) json.RawMessage {
	if members == nil {
		members = []json.RawMessage{}
	}
	raw, _ := json.Marshal(members)
	return raw
}

// addMembers adds the declared members to the machine's list, dropping only members the mesh
// added before and no longer declares. It returns the list and exactly which members the mesh
// added — a declared member the machine already had is the machine's, and is never recorded.
func addMembers(current, declared json.RawMessage, addedBefore []json.RawMessage) (json.RawMessage,
	[]json.RawMessage, error) {
	have, err := membersOf(current)
	if err != nil {
		return nil, nil, err
	}
	want, err := membersOf(declared)
	if err != nil {
		return nil, nil, err
	}
	added := []json.RawMessage{}
	for _, a := range addedBefore {
		if hasMember(want, a) {
			added = append(added, a)
			continue
		}
		have = slices.DeleteFunc(have, func(m json.RawMessage) bool { return canonical(m) == canonical(a) })
	}
	for _, m := range want {
		if !hasMember(have, m) {
			have = append(have, m)
			if !hasMember(added, m) {
				added = append(added, m)
			}
		}
	}
	return listOf(have), added, nil
}

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
}