package meshmoduleassignerclient

import (
	"encoding/json"
	"errors"
	"fmt"
	"sort"
	"strings"
	"sync"

	seat "git.novox.be/novox/mesh-contracts/placement/go/seats/meshmoduleassigner"
)

// **One key per module and node** (research 072 row 37; issue 774). The seat's state `placement` keeps a node's placement
// under the node's own keys, so a node watches `<node>.>` alone and a change of one module writes that module's key:
// `<node>.node`, the node key (what a person decided of the node, its pins and holdings, and the modules placed on it,
// each with the digest of its module key's value), and `<node>.<module>`, a module key. The node key is the commit of a
// write; a reader takes for each module the node key names the value of its module key whose digest the node key names,
// the current one once the node key is written, the previous one while the write is under way, so a reader never
// composes a part of a write. The form before, one value `placement.<node>` per node, is still read and mirrored until
// a later change deletes it.

// KeyKind is the kind of a key of the state `placement`.
type KeyKind int

// The kinds of key: a node key, a module key, and the form before (one value per node).
const (
	NodeKind KeyKind = iota + 1
	ModuleKind
	LegacyKind
)

// NodeKey is a node's node key, `<node>.node`. A node named `placement` is refused: its keys would read as the
// legacy form's.
func NodeKey(node string) (string, error) {
	if err := checkPlacementNode(node); err != nil {
		return "", err
	}
	return node + "." + NodeWord, nil
}

// ModuleKey is a module's key on a node, `<node>.<module>`; a module named `node` is refused.
func ModuleKey(node, module string) (string, error) {
	if err := checkPlacementNode(node); err != nil {
		return "", err
	}
	if err := checkPlacementModule(module); err != nil {
		return "", err
	}
	return node + "." + module, nil
}

// NodeKeys is the pattern of every key of one node, `<node>.>`: what a node watches of its own placement.
func NodeKeys(node string) (string, error) {
	if err := checkPlacementNode(node); err != nil {
		return "", err
	}
	return node + ".>", nil
}

// LegacyKey is a node's key in the form before, `placement.<node>`, read while this change is delivered.
func LegacyKey(node string) (string, error) {
	if err := checkName("node", node); err != nil {
		return "", err
	}
	return legacyWord + "." + node, nil
}

// ParseKey reads a key of the state `placement` in either form: its node, its module (empty for a node key and
// a legacy key) and its kind. Any other key is refused, naming it.
func ParseKey(key string) (node, module string, kind KeyKind, err error) {
	first, rest, ok := strings.Cut(key, ".")
	if !ok || strings.Contains(rest, ".") || checkName("node", first) != nil || checkName("module", rest) != nil {
		return "", "", 0, fmt.Errorf("meshmoduleassignerclient: %q is not a placement key, <node>.node or <node>.<module>", key)
	}
	switch {
	case first == legacyWord:
		return rest, "", LegacyKind, nil
	case rest == NodeWord:
		return first, "", NodeKind, nil
	default:
		return first, rest, ModuleKind, nil
	}
}

func checkPlacementNode(node string) error {
	if err := checkName("node", node); err != nil {
		return err
	}
	if node == legacyWord {
		return fmt.Errorf("meshmoduleassignerclient: no node is named %q: its placement keys would read as the form before", legacyWord)
	}
	return nil
}

func checkPlacementModule(module string) error {
	if err := checkName("module", module); err != nil {
		return err
	}
	if module == NodeWord {
		return fmt.Errorf("meshmoduleassignerclient: no module is named %q: it is the word of a node's own key", NodeWord)
	}
	return nil
}

// EncodeNode is n as JSON at Version.
func EncodeNode(n seat.PlacementByNodeNodeValue) ([]byte, error) {
	if err := checkPlacementNode(n.Node); err != nil {
		return nil, err
	}
	n.V = Version
	return json.Marshal(n)
}

// DecodeNode reads a node key's value; it refuses an unknown version, a node or module that is not a name, and a module
// named twice or without a digest.
func DecodeNode(data []byte) (seat.PlacementByNodeNodeValue, error) {
	if err := checkVersion(data); err != nil {
		return seat.PlacementByNodeNodeValue{}, err
	}
	n, err := seat.DecodePlacementByNodeNodeValue(data)
	if err != nil {
		return n, fmt.Errorf("meshmoduleassignerclient: value does not fit its shape: %w", err)
	}
	if err := checkPlacementNode(n.Node); err != nil {
		return seat.PlacementByNodeNodeValue{}, err
	}
	seen := map[string]bool{}
	for _, r := range n.Modules {
		if err := checkPlacementModule(r.Module); err != nil {
			return seat.PlacementByNodeNodeValue{}, err
		}
		if seen[r.Module] || r.Digest == "" {
			return seat.PlacementByNodeNodeValue{}, fmt.Errorf("meshmoduleassignerclient: the node key of %s names %s twice or without a digest", n.Node, r.Module)
		}
		seen[r.Module] = true
	}
	return n, nil
}

// EncodeModule is m as JSON at Version.
func EncodeModule(m seat.PlacementByNodeModuleValue) ([]byte, error) {
	if err := checkPlacementNode(m.Node); err != nil {
		return nil, err
	}
	if err := checkPlacementModule(m.Placed.Module); err != nil {
		return nil, err
	}
	if m.Previous != nil && m.Previous.Module != m.Placed.Module {
		return nil, fmt.Errorf("meshmoduleassignerclient: the key of %s on %s carries %s as its previous value", m.Placed.Module, m.Node, m.Previous.Module)
	}
	m.V = Version
	return json.Marshal(m)
}

// DecodeModule reads a module key's value.
func DecodeModule(data []byte) (seat.PlacementByNodeModuleValue, error) {
	if err := checkVersion(data); err != nil {
		return seat.PlacementByNodeModuleValue{}, err
	}
	m, err := seat.DecodePlacementByNodeModuleValue(data)
	if err != nil {
		return m, fmt.Errorf("meshmoduleassignerclient: value does not fit its shape: %w", err)
	}
	if err := checkPlacementNode(m.Node); err != nil {
		return seat.PlacementByNodeModuleValue{}, err
	}
	if err := checkPlacementModule(m.Placed.Module); err != nil {
		return seat.PlacementByNodeModuleValue{}, err
	}
	return m, nil
}

// Keys is the state `placement` as a reader holds it, key by key in either form, and every node's placement
// assembled from it: the one reader of the state's keys for every part (issue 774), so no part parses them itself. A
// write of the holder's under way is never assembled half: a module key's value is taken only as far as the node key
// names its digest (see the package's comment above NodeWord), and a node whose keys do not agree keeps the
// placement last assembled for it, Put saying why.
//
// Safe for use from a watch and from a reader at once.
type Keys struct {
	mu    sync.Mutex
	nodes map[string]*keysOf
}

// keysOf is what is held of one node's keys.
type keysOf struct {
	node       *seat.PlacementByNodeNodeValue
	nodeRev    uint64
	modules    map[string]seat.PlacementByNodeModuleValue
	moduleRevs map[string]uint64
	legacy     *seat.Placement
	legacyRev  uint64
	last       *seat.Placement // the placement last assembled whole, nil when none
}

// NewKeys holds no key yet.
func NewKeys() *Keys { return &Keys{nodes: map[string]*keysOf{}} }

// PlacementsOf is every node's placement assembled from the state's keys as a reader holds them (key to value; a
// deleted key absent), and an error naming every key that did not read and every node whose keys do not agree (that
// node is then assembled from its module keys' values as they are).
func PlacementsOf(keys map[string][]byte) (map[string]seat.Placement, error) {
	k := NewKeys()
	var errs []error
	names := make([]string, 0, len(keys))
	for key := range keys {
		names = append(names, key)
	}
	sort.Strings(names)
	for _, key := range names {
		// A node key read before its module keys does not agree yet; whether the keys agree is asked once all are in.
		if _, readErr, _ := k.put(key, keys[key], 0, false); readErr != nil {
			errs = append(errs, readErr)
		}
	}
	k.mu.Lock()
	for _, node := range sortedKeys(k.nodes) {
		if _, _, err := k.nodes[node].assemble(node); err != nil {
			errs = append(errs, err)
		}
	}
	k.mu.Unlock()
	return k.All(), errors.Join(errs...)
}

// Put takes one key's value in (revision: the bus's revision of it, 0 when the reader has none; deleted: a delete or a
// purge). It answers the node the key is of and an error for a key or a value that does not read, or for a node whose
// keys do not agree; the node then keeps what it had.
func (k *Keys) Put(key string, value []byte, revision uint64, deleted bool) (string, error) {
	node, readErr, err := k.put(key, value, revision, deleted)
	if readErr != nil {
		return node, readErr
	}
	return node, err
}

// put is Put telling an error of the key or value it took (readErr) from the node's keys not agreeing after it (err).
func (k *Keys) put(key string, value []byte, revision uint64, deleted bool) (node string, readErr, err error) {
	node, module, kind, err := ParseKey(key)
	if err != nil {
		return "", err, nil
	}
	k.mu.Lock()
	defer k.mu.Unlock()
	of := k.nodes[node]
	if of == nil {
		of = &keysOf{modules: map[string]seat.PlacementByNodeModuleValue{}, moduleRevs: map[string]uint64{}}
		k.nodes[node] = of
	}
	switch kind {
	case NodeKind:
		of.node, of.nodeRev = nil, 0
		if !deleted {
			n, derr := DecodeNode(value)
			switch {
			case derr != nil:
				readErr = fmt.Errorf("the node key %s does not read, so it places nothing there: %w", key, derr)
			case n.Node != node:
				readErr = fmt.Errorf("the node key %s names the node %q, so it places nothing there", key, n.Node)
			default:
				of.node, of.nodeRev = &n, revision
			}
		}
	case ModuleKind:
		delete(of.modules, module)
		delete(of.moduleRevs, module)
		if !deleted {
			m, derr := DecodeModule(value)
			switch {
			case derr != nil:
				readErr = fmt.Errorf("the module key %s does not read: %w", key, derr)
			case m.Node != node || m.Placed.Module != module:
				readErr = fmt.Errorf("the module key %s holds %s on %s", key, m.Placed.Module, m.Node)
			default:
				of.modules[module] = m
			}
			// Its revision is kept even when its value does not read, so the holder can write over it.
			of.moduleRevs[module] = revision
		}
	case LegacyKind:
		of.legacy, of.legacyRev = nil, 0
		if !deleted {
			p, derr := DecodePlacement(value)
			switch {
			case derr != nil:
				readErr = fmt.Errorf("the placement of %s does not read, so it places nothing there: %w", node, derr)
			case p.Node != node:
				readErr = fmt.Errorf("the placement under %s names the node %q, so it places nothing there", key, p.Node)
			default:
				of.legacy, of.legacyRev = &p, revision
			}
		}
	}
	p, whole, err := of.assemble(node)
	switch {
	case p == nil:
		of.last = nil
	case whole || of.last == nil:
		of.last = p
	}
	return node, readErr, err
}

// assemble is the node's placement from its keys: nil when none places it, whole false (and an error) when the node key
// names a module digest no module key's value has.
func (of *keysOf) assemble(node string) (*seat.Placement, bool, error) {
	if of.node == nil {
		if of.legacy == nil {
			return nil, true, nil
		}
		p := clonePlacement(*of.legacy)
		return &p, true, nil
	}
	n := of.node
	p := seat.Placement{V: Version, Node: node, Decided: n.Decided, Pins: n.Pins, Holds: n.Holds, Modules: make([]seat.PlacedModule, 0, len(n.Modules))}
	var astray []string
	for _, r := range n.Modules {
		m, ok := of.modules[r.Module]
		switch {
		case ok && Digest(m.Placed) == r.Digest:
			p.Modules = append(p.Modules, m.Placed)
		case ok && m.Previous != nil && Digest(*m.Previous) == r.Digest:
			p.Modules = append(p.Modules, *m.Previous)
		case ok:
			astray = append(astray, r.Module)
			p.Modules = append(p.Modules, m.Placed)
		default:
			astray = append(astray, r.Module)
		}
	}
	if len(astray) > 0 {
		return &p, false, fmt.Errorf("the node key of %s names %s at a value no module key holds: %s keeps the placement it had",
			node, strings.Join(astray, ", "), node)
	}
	return &p, true, nil
}

// Of is node's placement and the revision of the key it was read from (its node key, or its legacy key), and whether
// any key places it.
func (k *Keys) Of(node string) (seat.Placement, uint64, bool) {
	k.mu.Lock()
	defer k.mu.Unlock()
	of := k.nodes[node]
	if of == nil || of.last == nil {
		return seat.Placement{}, 0, false
	}
	rev := of.nodeRev
	if of.node == nil {
		rev = of.legacyRev
	}
	return clonePlacement(*of.last), rev, true
}

// All is every node's placement, by node, and the revision of each one's key (Of).
func (k *Keys) All() map[string]seat.Placement {
	out, _ := k.AllRevisions()
	return out
}

// AllRevisions is All with the revision of each node's key.
func (k *Keys) AllRevisions() (map[string]seat.Placement, map[string]uint64) {
	k.mu.Lock()
	nodes := make([]string, 0, len(k.nodes))
	for n := range k.nodes {
		nodes = append(nodes, n)
	}
	k.mu.Unlock()
	out, revs := map[string]seat.Placement{}, map[string]uint64{}
	for _, n := range nodes {
		if p, rev, ok := k.Of(n); ok {
			out[n], revs[n] = p, rev
		}
	}
	return out, revs
}

// KeyWrite is one write of a key of the state `placement`, by compare-and-set: Value nil deletes the key at
// Base, Base 0 creates it, any other Base updates it at that revision.
type KeyWrite struct {
	Node  string
	Key   string
	Value []byte
	Base  uint64
	// Commit says this write is the node key's: once it is written, the node's placement is the new one.
	Commit bool
	// Mirror says this write is the legacy key's, `placement.<node>`, kept in step after the commit for the readers not
	// yet delivered: a reader of both forms never reads it once the node key is there, so a mirror write that fails
	// harms no such reader, and is made again at the node's next write.
	Mirror bool
}

// Writes are the writes that take node from the placement its keys hold to after (nil: no placement), in the order
// they are made: the module keys that change, each carrying the value it replaces; then the node key, the commit; then
// the legacy key `placement.<node>` with the whole placement, a mirror for the readers not yet delivered, unless it
// holds it already; then the deletes of the module keys after no longer names. For no placement the legacy key goes
// first, then the node key, then the module keys, so a crash between them never leaves the legacy value to be read
// again. A module key whose value stays is not written.
func (k *Keys) Writes(node string, after *seat.Placement) ([]KeyWrite, error) {
	nodeKey, err := NodeKey(node)
	if err != nil {
		return nil, err
	}
	k.mu.Lock()
	defer k.mu.Unlock()
	of := k.nodes[node]
	if of == nil {
		of = &keysOf{modules: map[string]seat.PlacementByNodeModuleValue{}, moduleRevs: map[string]uint64{}}
	}
	var out []KeyWrite
	deleteModules := func(keep map[string]bool) {
		for _, module := range sortedKeys(of.moduleRevs) {
			if keep[module] {
				continue
			}
			key, _ := ModuleKey(node, module)
			out = append(out, KeyWrite{Node: node, Key: key, Base: of.moduleRevs[module]})
		}
	}
	if after == nil {
		if of.legacy != nil {
			key, _ := LegacyKey(node)
			out = append(out, KeyWrite{Node: node, Key: key, Base: of.legacyRev})
		}
		if of.node != nil {
			out = append(out, KeyWrite{Node: node, Key: nodeKey, Base: of.nodeRev, Commit: true})
		}
		deleteModules(nil)
		return out, nil
	}
	if after.Node != node {
		return nil, fmt.Errorf("meshmoduleassignerclient: the placement written under %s names the node %q", node, after.Node)
	}
	if err := CheckPlacement(*after); err != nil {
		return nil, err
	}
	committed := map[string]seat.PlacedModule{}
	if of.last != nil {
		for _, m := range of.last.Modules {
			committed[m.Module] = m
		}
	}
	n := seat.PlacementByNodeNodeValue{Node: node, Decided: after.Decided, Pins: after.Pins, Holds: after.Holds, Modules: make([]seat.PlacementByNodeNodeValueModulesItem, 0, len(after.Modules))}
	keep := map[string]bool{}
	for _, m := range after.Modules {
		if err := checkPlacementModule(m.Module); err != nil {
			return nil, err
		}
		digest := Digest(m)
		n.Modules = append(n.Modules, seat.PlacementByNodeNodeValueModulesItem{Module: m.Module, Digest: digest})
		keep[m.Module] = true
		was, wasPlaced := committed[m.Module]
		if held, ok := of.modules[m.Module]; ok && Digest(held.Placed) == digest &&
			(held.Previous == nil || wasPlaced && Digest(was) == digest) {
			continue // the key holds it, and no previous value a reader could take in its place
		}
		v := seat.PlacementByNodeModuleValue{Node: node, Placed: m}
		if wasPlaced && Digest(was) != digest {
			v.Previous = &was
		}
		raw, err := EncodeModule(v)
		if err != nil {
			return nil, err
		}
		key, _ := ModuleKey(node, m.Module)
		out = append(out, KeyWrite{Node: node, Key: key, Value: raw, Base: of.moduleRevs[m.Module]})
	}
	raw, err := EncodeNode(n)
	if err != nil {
		return nil, err
	}
	out = append(out, KeyWrite{Node: node, Key: nodeKey, Value: raw, Base: of.nodeRev, Commit: true})
	// The mirror of the form before, while readers of it alone may run (the delivery of this change); a later change
	// stops writing it and deletes it.
	if of.legacy == nil || !SamePlacement(*of.legacy, *after) {
		legacy, err := EncodePlacement(*after)
		if err != nil {
			return nil, err
		}
		key, _ := LegacyKey(node)
		out = append(out, KeyWrite{Node: node, Key: key, Value: legacy, Base: of.legacyRev, Mirror: true})
	}
	deleteModules(keep)
	return out, nil
}

// Migrations are, by node, the writes that convert every node held only in the form before (its legacy key, no node
// key) into its node and module keys, its legacy key kept as the mirror for the readers not yet delivered (Writes); empty
// once every node is converted, so the holder's conversion at each start is idempotent.
func (k *Keys) Migrations() (map[string][]KeyWrite, error) {
	k.mu.Lock()
	var legacy []string
	for node, of := range k.nodes {
		if of.node == nil && of.legacy != nil && checkPlacementNode(node) == nil {
			legacy = append(legacy, node)
		}
	}
	k.mu.Unlock()
	sort.Strings(legacy)
	out := map[string][]KeyWrite{}
	var errs []error
	for _, node := range legacy {
		p, _, _ := k.Of(node)
		w, err := k.Writes(node, &p)
		if err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", node, err))
			continue
		}
		out[node] = w
	}
	return out, errors.Join(errs...)
}

// Legacy says node is held only in the form before.
func (k *Keys) Legacy(node string) bool {
	k.mu.Lock()
	defer k.mu.Unlock()
	of := k.nodes[node]
	return of != nil && of.node == nil && of.legacy != nil
}

// PlacementValues is every key and value of p written whole onto no keys before: its module keys, its node key and the
// legacy mirror, as a writer that holds no key of the node (a test's bus, a fresh mesh) writes them.
func PlacementValues(p seat.Placement) (map[string][]byte, error) {
	writes, err := NewKeys().Writes(p.Node, &p)
	if err != nil {
		return nil, err
	}
	out := make(map[string][]byte, len(writes))
	for _, w := range writes {
		out[w.Key] = w.Value
	}
	return out, nil
}

// clonePlacement is p with its own maps and slices, so a reader's copy is not a held one.
func clonePlacement(p seat.Placement) seat.Placement {
	raw, _ := json.Marshal(p)
	var out seat.Placement
	_ = json.Unmarshal(raw, &out)
	if out.Modules == nil {
		out.Modules = []seat.PlacedModule{}
	}
	return out
}