// Package views is the owners' side and the readers' side of a first view (novox/hq ADR 0615, ADR 0621, ADR 0622;
// design 25 § 12.2): the indexer and the assigner answer a reader's first view of one of their states as a tool of
// their own module (Answer, FirstView), say each change of that state as one event on their views family (Event,
// Emitter), and a reader keeps its picture from the answer and the events, asking the tool again only on a gap in
// revisions (Follower). The tools' read side (Read) answers the ordinary read tools of § 12.2 too.
package views

import (
	"encoding/json"
	"errors"
	"fmt"
	"slices"
	"strings"
	"time"
)

// Version is the version of every value this package writes and the only one it reads.
const Version = 1

// The owners' views families (design 25 § 12.2; ADR 0615 §5, ADR 0619 §2): one stream each on its owner's old `streams`
// section, kept by limits 7 days and at most 256 MiB, dropping its oldest when full, so a reader that missed a dropped
// event sees the gap in revisions and asks once more. A stream's name is the mesh's, so each carries its owner's name.
const (
	IndexerStream  = "indexer-views"
	AssignerStream = "assigner-views"
	StreamDays     = 7
	StreamMiB      = 256
	// EventPrefix starts each event's local name: `views.<state>`, one event per changed key of that state.
	EventPrefix = "views."
	// EventPattern is every event of a views family, as a stream's events and a reader's consumes name it.
	EventPattern = EventPrefix + "**"
)

// MarksState is an owner's own state of the revision each followed state was said up to (Mark), one key per state:
// what a first view answers its revision from, so a reader that takes the answer misses no event after it.
const MarksState = "view-marks"

// MaxValueBytes is the largest value an event carries whole: under the bus's largest message (1 MiB, the server's
// max_payload) with the event's own fields. A larger one is said as its revision alone (Event.Gap), and the owner
// writes a finding naming the key (design 25 § 12.2).
const MaxValueBytes = 960 * 1024

// PageBytes bounds the values one answer carries, so the answer fits one bus message; Answer.Next continues it.
const PageBytes = 768 * 1024

// Bound is how long a reader waits for its first view before it raises the condition naming the owner and the verb
// (ADR 0615 §6): longer than an owner's start, so a normal start, readers and owners coming up together, tells nobody.
const Bound = 2 * time.Minute

// EventName is the local name of the event saying a change of state: `views.<state>`.
func EventName(state string) string { return EventPrefix + state }

// StateOfEvent is the state an event's local name (or the last tokens of its subject) says, or "".
func StateOfEvent(name string) string {
	if i := strings.LastIndex(name, ".event."+EventPrefix); i >= 0 {
		name = name[i+len(".event."):]
	}
	s, ok := strings.CutPrefix(name, EventPrefix)
	if !ok || s == "" || strings.ContainsAny(s, ".*> ") {
		return ""
	}
	return s
}

// Event is one change of one key of an owner's state, said on its views family: the key's value whole, or a delete
// mark, at the revision of the state the change was written at. Created is when the owner's state was made on the bus:
// a state made again (the bus module's recreate) starts its revisions again, and a reader takes another Created as a gap.
type Event struct {
	V        int             `json:"v"`
	Owner    string          `json:"owner"`
	State    string          `json:"state"`
	Key      string          `json:"key"`
	Revision uint64          `json:"revision"`
	Created  time.Time       `json:"created"`
	Deleted  bool            `json:"deleted,omitempty"`
	Value    json.RawMessage `json:"value,omitempty"`
	// Gap says the value was too large to carry (MaxValueBytes): the reader asks its first view once.
	Gap bool `json:"gap,omitempty"`
}

// ID is the event's id on the bus, so the same change said twice (an owner's restart) is kept once.
func (e Event) ID() string {
	return fmt.Sprintf("%s-%s-%d-%d", e.Owner, e.State, e.Created.UnixNano(), e.Revision)
}

// Encode writes an event.
func (e Event) Encode() ([]byte, error) {
	e.V = Version
	if e.Owner == "" || e.State == "" || e.Key == "" || e.Revision == 0 {
		return nil, errors.New("views: an event names its owner, state, key and revision")
	}
	if e.Deleted || e.Gap {
		e.Value = nil
	}
	return json.Marshal(e)
}

// DecodeEvent reads an event of this version.
func DecodeEvent(data []byte) (Event, error) {
	var e Event
	if err := decode(data, &e); err != nil {
		return Event{}, err
	}
	if e.State == "" || e.Key == "" || e.Revision == 0 {
		return Event{}, errors.New("views: an event names no state, key or revision")
	}
	return e, nil
}

// Mark is the revision an owner said its state up to, and when that state was made (MarksState).
type Mark struct {
	V        int       `json:"v"`
	State    string    `json:"state"`
	Revision uint64    `json:"revision"`
	Created  time.Time `json:"created"`
	At       time.Time `json:"at"`
	// Fill is the state's bucket as the bus held it when the mark was written: what a read tool says of its fill.
	Fill Fill `json:"fill"`
}

// Fill is a state's bucket's fill against its cap, as the bus says it.
type Fill struct {
	Keys     uint64 `json:"keys"`
	Bytes    uint64 `json:"bytes"`
	MaxBytes int64  `json:"max-bytes"`
}

// DecodeMark reads a mark of this version.
func DecodeMark(data []byte) (Mark, error) {
	var m Mark
	return m, decode(data, &m)
}

// Answer is a first view, or one page of an ordinary read: the state's values by key, from After on, at the revision
// of the state the owner said up to (Mark) before it read them; Next is the key to ask after for the rest, empty when
// the answer is whole. A reader keeps the first page's revision: a value on a later page may be newer, and the events
// after that revision bring every key to its newest in order.
type Answer struct {
	V        int                        `json:"v"`
	Owner    string                     `json:"owner"`
	State    string                     `json:"state"`
	Revision uint64                     `json:"revision"`
	Created  time.Time                  `json:"created"`
	Values   map[string]json.RawMessage `json:"values"`
	Next     string                     `json:"next,omitempty"`
	At       time.Time                  `json:"at"`
}

// DecodeAnswer reads an answer of this version.
func DecodeAnswer(data []byte) (Answer, error) {
	var a Answer
	if err := decode(data, &a); err != nil {
		return Answer{}, err
	}
	if a.Values == nil {
		a.Values = map[string]json.RawMessage{}
	}
	return a, nil
}

// Entry is one key's value and revision, as a state read gives it.
type Entry struct {
	Value    json.RawMessage
	Revision uint64
}

// Source is the read side of one state, as a tools bundle reaches its own state (stdio.KeptState's Keys and Get).
type Source interface {
	Keys() ([]string, error)
	Get(key string) (*Entry, bool, error)
}

// FirstView answers one page of state from its owner's source: the mark first, so no change after the revision it
// answers at is missed, then the values in key order after after, until PageBytes. A state not said yet (no mark)
// answers revision 0: a first view of whatever it holds, which the reader takes as a gap at the first event.
func FirstView(owner, state string, values, marks Source, after string, now time.Time) (Answer, error) {
	a := Answer{V: Version, Owner: owner, State: state, Values: map[string]json.RawMessage{}, At: now.UTC()}
	if marks != nil {
		e, ok, err := marks.Get(state)
		if err != nil {
			return Answer{}, fmt.Errorf("the mark of %s cannot be read: %w", state, err)
		}
		if ok {
			m, err := DecodeMark(e.Value)
			if err != nil {
				return Answer{}, fmt.Errorf("the mark of %s: %w", state, err)
			}
			a.Revision, a.Created = m.Revision, m.Created
		}
	}
	keys, err := values.Keys()
	if err != nil {
		return Answer{}, fmt.Errorf("the state %s cannot be listed: %w", state, err)
	}
	slices.Sort(keys)
	size := 0
	for _, k := range keys {
		if after != "" && k <= after {
			continue
		}
		e, ok, err := values.Get(k)
		if err != nil {
			return Answer{}, fmt.Errorf("%s.%s cannot be read: %w", state, k, err)
		}
		if !ok {
			continue
		}
		if size > 0 && size+len(k)+len(e.Value) > PageBytes {
			a.Next = lastKey(a.Values)
			break
		}
		a.Values[k] = e.Value
		size += len(k) + len(e.Value)
	}
	return a, nil
}

func lastKey(m map[string]json.RawMessage) string {
	last := ""
	for k := range m {
		if k > last {
			last = k
		}
	}
	return last
}

func decode(data []byte, into any) error {
	var probe struct {
		V *int `json:"v"`
	}
	if err := json.Unmarshal(data, &probe); err != nil {
		return fmt.Errorf("views: value does not fit its shape: %w", err)
	}
	if probe.V == nil || *probe.V != Version {
		return fmt.Errorf("views: unknown value version: this reader reads %d", Version)
	}
	if err := json.Unmarshal(data, into); err != nil {
		return fmt.Errorf("views: value does not fit its shape: %w", err)
	}
	return nil
}