package catalogue

import (
	"bytes"
	"encoding/json"
	"fmt"
	"regexp"
	"strings"
)

// What a module may call its state, and whose state it may ask to read (novox/hq ADR 0201).
//
// A module names its state **locally** — `servers`, never a bucket or a subject — and another
// module's as `<module>.<name>`, the way a consumed event names its emitter (design 32 §1). The
// mesh derives the bucket from the two names, so the module and the local name must each be one
// token: the bucket joins them with an underscore, which neither may contain, so two modules can
// never derive one bucket.

// stateName is one local name of a module's state: lower-case, no dot, no underscore.
var stateName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)

// The mesh's caps on what a module may ask of a bucket's history.
const (
	// StateMostHistory is the most past values a key may keep. The server's own limit.
	StateMostHistory = 64
)

// StateDeclaration is one bucket a module owns: its local name, and the options that are the
// owner's to choose, as a seat chooses how long its backlog survives (design 32 §3).
type StateDeclaration struct {
	Name string `json:"name"`
	// History is how many values a key keeps, the current one included; zero is one.
	History int `json:"history,omitempty"`
	// TTLSeconds is how long a value lives once written; zero is until it is replaced or deleted.
	TTLSeconds int `json:"ttl-seconds,omitempty"`
}

// UnmarshalJSON reads a bucket as its bare name, or as {name, history, ttl-seconds}.
func (s *StateDeclaration) UnmarshalJSON(raw []byte) error {
	trimmed := bytes.TrimSpace(raw)
	if len(trimmed) > 0 && trimmed[0] == '"' {
		return json.Unmarshal(trimmed, &s.Name)
	}
	type plain StateDeclaration
	var full plain
	dec := json.NewDecoder(bytes.NewReader(trimmed))
	dec.DisallowUnknownFields()
	if err := dec.Decode(&full); err != nil {
		return fmt.Errorf("a state is either a name or {name, history, ttl-seconds}: %w", err)
	}
	*s = StateDeclaration(full)
	return nil
}

// MarshalJSON writes back the short form when there is nothing else to say.
func (s StateDeclaration) MarshalJSON() ([]byte, error) {
	if s.History == 0 && s.TTLSeconds == 0 {
		return json.Marshal(s.Name)
	}
	type plain StateDeclaration
	return json.Marshal(plain(s))
}

// ReadState splits a read into the owning module and the local name, or says why it is not one.
func ReadState(read string) (module, local string, err error) {
	at := strings.LastIndex(read, ".")
	if at <= 0 || at == len(read)-1 {
		return "", "", fmt.Errorf("%q does not name a module and its state: a read is <module>.<name>", read)
	}
	module, local = read[:at], read[at+1:]
	if !stateName.MatchString(module) {
		return "", "", fmt.Errorf("%q cannot own state: a module whose state is read is one plain name", module)
	}
	if !stateName.MatchString(local) {
		return "", "", fmt.Errorf("%q is not a state name: lower-case letters, digits and hyphens", local)
	}
	return module, local, nil
}

// StateProblems is what is wrong with a manifest's state and reads.
//
// Refused at registration, because a bucket name the bus cannot hold is a module that installs,
// starts, and is refused on its first write with a reason about a bucket nobody named.
func StateProblems(m Manifest) []string {
	var problems []string
	if len(m.State) > 0 && !stateName.MatchString(m.Module) {
		problems = append(problems, fmt.Sprintf(
			"%s keeps state, and a module's name is part of its buckets' names, which take one plain "+
				"name — no dot (novox/hq ADR 0201)", m.Module))
	}
	seen := map[string]bool{}
	for _, s := range m.State {
		switch {
		case !stateName.MatchString(s.Name):
			problems = append(problems, fmt.Sprintf(
				"%s keeps state %q: a state is named locally — lower-case letters, digits and hyphens, "+
					"no dot and no underscore; the mesh derives the bucket (novox/hq ADR 0201)", m.Module, s.Name))
		case seen[s.Name]:
			problems = append(problems, fmt.Sprintf("%s keeps state %q twice", m.Module, s.Name))
		}
		seen[s.Name] = true
		if s.History < 0 || s.History > StateMostHistory {
			problems = append(problems, fmt.Sprintf(
				"%s keeps %d values of %q; a key keeps between 1 and %d", m.Module, s.History, s.Name, StateMostHistory))
		}
		if s.TTLSeconds < 0 {
			problems = append(problems, fmt.Sprintf("%s gives %q a negative lifetime", m.Module, s.Name))
		}
	}
	for _, r := range m.Reads {
		module, _, err := ReadState(r)
		if err != nil {
			problems = append(problems, fmt.Sprintf("%s reads %v", m.Module, err))
			continue
		}
		if module == m.Module {
			problems = append(problems, fmt.Sprintf(
				"%s reads %q, which is its own state: a module reads and writes what it keeps already", m.Module, r))
		}
	}
	return problems
}

// StateReadsNothingDeclares is every read across a catalogue whose owner is present and declares no
// such state. An absent owner says nothing — a module may be installed long before the one whose
// state it reads, as a consumer may before its emitter (design 32 §1).
func StateReadsNothingDeclares(manifests []Manifest) []string {
	declared := map[string]map[string]bool{}
	for _, m := range manifests {
		own := map[string]bool{}
		for _, s := range m.State {
			own[s.Name] = true
		}
		declared[m.Module] = own
	}
	var problems []string
	for _, m := range manifests {
		for _, r := range m.Reads {
			module, local, err := ReadState(r)
			if err != nil {
				continue
			}
			if own, present := declared[module]; present && !own[local] {
				problems = append(problems, fmt.Sprintf(
					"%s reads %q, and %s keeps no state called %q", m.Module, r, module, local))
			}
		}
	}
	return problems
}