// Package announce is how a module says on the bus which states it keeps and which of them others may read (novox/hq
// ADR 0495 clauses 2 and 3; issue 1790): the subject it says it on, the value it says, and where the controller keeps
// the last one said by each module. What a module announced is the truth of what it keeps; a reader is granted a read
// from it, never from a list or from the owner's manifest.
//
// A module announces from its code (process.Conn.Keeps, stdio.Keeps, the TypeScript SDK's keeps), never from its
// manifest, which only the core reads (clause 1). A module that keeps no state on the bus announces nothing.
package announce

import (
	"encoding/json"
	"fmt"
	"regexp"
	"sort"
)

// Version is the version of the value this package writes.
const Version = 1

// Event is the event type of an announcement, under the module's own namespace.
const Event = "states"

// Subject is where module announces what it keeps: its own event subject, mesh.mod.<module>.event.states, which its
// account may already publish to and the events stream keeps.
func Subject(module string) string { return "mesh.mod." + module + ".event." + Event }

// SubjectAll is every module's announcement, what the controller consumes.
const SubjectAll = "mesh.mod.*.event." + Event

// KeptBucket is the controller's bucket of the last announcement of each module, by Key.
const KeptBucket = "mesh-controller_states-announced"

var name = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)

// Key is module's key in KeptBucket: the module itself, which must be one plain name, since a module's name is part of
// its buckets' names (catalogue.StateProblems).
func Key(module string) (string, error) {
	if !name.MatchString(module) {
		return "", fmt.Errorf("announce: %q is not a module that keeps state: lower-case letters, digits and hyphens", module)
	}
	return module, nil
}

// State is one state a module keeps, by its local name, and whether others may read it. Others never write it: a
// state another module writes is the owner's own code's business.
type State struct {
	Name     string `json:"name"`
	Readable bool   `json:"readable,omitempty"`
	// Readers, when named, are the only modules that may read the state (novox/hq ADR 0547 clause 9, issue 1945): a
	// state readable by some and not all, such as the vault's bus account hashes. Only ReadableBy grants them; Readable,
	// which a composer that knows no readers asks, stays false for it, so such a composer grants nobody.
	Readers []string `json:"readers,omitempty"`
}

// States is an announcement: the module, the machine whose instance said it, and every state the module keeps, sorted
// by name. An empty list says the module keeps nothing.
type States struct {
	V      int     `json:"v"`
	Module string  `json:"module"`
	Node   string  `json:"node,omitempty"`
	States []State `json:"states"`
}

// Keeps says whether the module announced keeping the state.
func (s States) Keeps(state string) bool {
	for _, k := range s.States {
		if k.Name == state {
			return true
		}
	}
	return false
}

// ReadableBy says whether the module announced the state readable by reader: readable, and reader among the readers
// when it names any.
func (s States) ReadableBy(state, reader string) bool {
	for _, k := range s.States {
		if k.Name != state {
			continue
		}
		if len(k.Readers) > 0 {
			for _, r := range k.Readers {
				if r == reader {
					return true
				}
			}
			return false
		}
		return k.Readable
	}
	return false
}

// Readable says whether the module announced the state readable by others.
func (s States) Readable(state string) bool {
	for _, k := range s.States {
		if k.Name == state {
			return k.Readable
		}
	}
	return false
}

func (s *States) check() error {
	if s.Module == "" {
		return fmt.Errorf("announce: an announcement names no module")
	}
	if _, err := Key(s.Module); err != nil {
		return err
	}
	seen := map[string]bool{}
	for _, k := range s.States {
		if !name.MatchString(k.Name) {
			return fmt.Errorf("announce: %s announces the state %q, and a state is named locally: lower-case letters, digits and hyphens", s.Module, k.Name)
		}
		if seen[k.Name] {
			return fmt.Errorf("announce: %s announces the state %q twice", s.Module, k.Name)
		}
		seen[k.Name] = true
		for _, r := range k.Readers {
			if !name.MatchString(r) {
				return fmt.Errorf("announce: %s names %q as a reader of %s, and a reader is a module: lower-case letters, digits and hyphens", s.Module, r, k.Name)
			}
		}
	}
	return nil
}

// Encode is s as JSON at Version, its states sorted; it refuses a module or a state no bucket could be named by and a
// state announced twice, naming it.
func Encode(s States) ([]byte, error) {
	if err := s.check(); err != nil {
		return nil, err
	}
	s.V = Version
	s.States = append([]State{}, s.States...)
	sort.Slice(s.States, func(i, j int) bool { return s.States[i].Name < s.States[j].Name })
	return json.Marshal(s)
}

// Decode reads an announcement; it refuses another version and what Encode refuses.
func Decode(data []byte) (States, error) {
	var s States
	if err := json.Unmarshal(data, &s); err != nil {
		return States{}, fmt.Errorf("announce: the announcement is not JSON: %w", err)
	}
	if s.V != Version {
		return States{}, fmt.Errorf("announce: the announcement is version %d, and this reads version %d", s.V, Version)
	}
	if err := s.check(); err != nil {
		return States{}, err
	}
	sort.Slice(s.States, func(i, j int) bool { return s.States[i].Name < s.States[j].Name })
	return s, nil
}