package bus

import (
	"encoding/json"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/nats-io/nats.go"
)

// A module reading its own stream back (novox/hq ADR 0363; the operator, 2026-10-30: a build step's whole failure output
// is said on its build module's stream, and the module's tools read it there). As a log's read: one batch direct get,
// no consumer, and only a stream the module's own membership names (`streams`); the bus grants its owner that stream's
// info and direct get and no other's (mesh-sdk buscompose ownStreamGrants).

// StreamEntry is one event of a module's own stream: its sequence, its event (the subject after the stream's
// `…event.`), its body and when the bus took it.
type StreamEntry struct {
	Sequence uint64          `json:"sequence"`
	Event    string          `json:"event"`
	Body     json.RawMessage `json:"body"`
	At       time.Time       `json:"at"`
}

// StreamPage is one page of a stream's read: Next is the sequence to continue from, 0 when nothing follows; Last the
// stream's last sequence.
type StreamPage struct {
	Entries []StreamEntry `json:"entries"`
	Next    uint64        `json:"next"`
	Last    uint64        `json:"last"`
}

// streamBusName is a stream's local name as the bus holds it (mesh-sdk catalogue.StreamBusName): capitals, a hyphen an
// underscore; a bus name is itself.
func streamBusName(name string) string { return strings.ToUpper(strings.ReplaceAll(name, "-", "_")) }

// issuedStream is the stream of its own a module may read by a name, local or the bus's, or the reason it may not.
func (c *Conn) issuedStream(module, name string) (StreamIssued, error) {
	m := c.Membership(module)
	if m == nil {
		return StreamIssued{}, fmt.Errorf("%s has no membership issued on %s yet, so no stream of it is readable "+
			"until the mesh issues one (novox/hq ADR 0363)", module, c.node)
	}
	var names []string
	for _, s := range m.Streams {
		if s.Stream == streamBusName(name) && strings.Contains(s.Subject, "{event}") {
			return s, nil
		}
		names = append(names, s.Stream)
	}
	sort.Strings(names)
	issued := "none"
	if len(names) > 0 {
		issued = strings.Join(names, ", ")
	}
	return StreamIssued{}, fmt.Errorf("%s keeps no stream called %q: a module reads the streams it declares under `streams`, "+
		"and no other's; it was issued: %s (novox/hq ADR 0363)", module, name, issued)
}

// StreamRead is a module's own stream's events from a sequence on, whose event names match a pattern (`*` one name, `>`
// the rest, last; empty is every event), at most limit (LogReadDefault when 0, at most LogReadMax) and about LogPageBytes.
func (c *Conn) StreamRead(module, name, events string, from uint64, limit int) (*StreamPage, error) {
	s, err := c.issuedStream(module, name)
	if err != nil {
		return nil, err
	}
	switch {
	case limit == 0:
		limit = LogReadDefault
	case limit < 0 || limit > LogReadMax:
		return nil, fmt.Errorf("a read's limit is 1 to %d events, %d when none is given, not %d", LogReadMax, LogReadDefault, limit)
	}
	if events == "" {
		events = ">"
	}
	parts := strings.Split(events, ".")
	for i, p := range parts {
		if !(p == ">" && i == len(parts)-1) && p != "*" && !logToken.MatchString(p) {
			return nil, fmt.Errorf("%q is not an event pattern: names, `*` for one and `>` for the rest, last", events)
		}
	}
	prefix, _, _ := strings.Cut(s.Subject, "{event}")
	missing := func() error {
		return fmt.Errorf("the stream %s is issued and not on the bus yet: the controller creates it from the catalogue "+
			"on its next raise, and reads it directly once it allows that (novox/hq ADR 0363)", s.Stream)
	}
	msgs, next, err := c.directBatch(s.Stream, prefix+events, from, limit, s.Stream, missing)
	if err != nil {
		return nil, err
	}
	page := &StreamPage{Entries: []StreamEntry{}, Next: next}
	for _, msg := range msgs {
		e, err := streamEntryOf(s.Stream, prefix, msg)
		if err != nil {
			return nil, err
		}
		page.Entries = append(page.Entries, *e)
	}
	info, err := c.logInfo(LogIssued{Name: s.Stream, Stream: s.Stream})
	if err != nil {
		return nil, err
	}
	page.Last = info.Last
	return page, nil
}

// streamEntryOf is an event a direct get answered: its event is its subject after prefix.
func streamEntryOf(stream, prefix string, msg *nats.Msg) (*StreamEntry, error) {
	subject := msg.Header.Get("Nats-Subject")
	event, ok := strings.CutPrefix(subject, prefix)
	if !ok {
		return nil, fmt.Errorf("the bus answered an event of %s on %q, which is not under %s", stream, subject, prefix)
	}
	seq, err := strconv.ParseUint(msg.Header.Get("Nats-Sequence"), 10, 64)
	if err != nil {
		return nil, fmt.Errorf("the bus answered an event of %s with no sequence", stream)
	}
	at, err := time.Parse(time.RFC3339Nano, msg.Header.Get("Nats-Time-Stamp"))
	if err != nil {
		return nil, fmt.Errorf("the bus answered event %d of %s with no time: %v", seq, stream, err)
	}
	return &StreamEntry{Sequence: seq, Event: event, Body: valueOf(msg.Data), At: at.UTC()}, nil
}