// Package catalogue is what modules are, and what a node gets when it is assigned some.
//
// novox/hq ADR 0009: everything is a module, a module declares what it provides and requires,
// and a module declares what it claims. This turns a set of assignments into the one declaration
// a node is sent — which is the first thing the control plane decides rather than relays.
package catalogue

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

// Scopes a claim can have.
//
// Not everything singular is singular per machine: a seat is one per node, a DHCP server is one
// per segment, and the hub is one per mesh. Scope says which, and it is the same idea the mesh
// already enforces by hand for the hub.
const (
	ScopeNode = "node"
	ScopeSite = "site"
	ScopeMesh = "mesh"
)

// name is what a module, a provision or a claim may be called.
//
// Constrained because these become resource identities, permission patterns and error messages,
// and a name that is valid in one and not the others is a fault found late.
var name = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*(\.[a-z0-9][a-z0-9-]*)*$`)

// Claim is a singular resource a module takes over.
type Claim struct {
	Name string `json:"name"`
	// Scope defaults to the node, which is where nearly everything singular is singular.
	Scope string `json:"scope,omitempty"`
}

// At is this claim's scope, with the default applied.
func (c Claim) At() string {
	if c.Scope == "" {
		return ScopeNode
	}
	return c.Scope
}

// Manifest is everything a module says about itself.
type Manifest struct {
	Module  string `json:"module"`
	Version string `json:"version,omitempty"`

	// Provides are the names other modules may require. A module always provides its own name;
	// this is for the rest — `zsh` provides `shell`, `xorg` provides `display-server`.
	Provides []string `json:"provides,omitempty"`

	// Requires are names that must be provided by something assigned to the same node.
	Requires []string `json:"requires,omitempty"`

	// Claims are singular resources. Two modules claiming one thing within a scope cannot both
	// be assigned there — which is how exclusivity is expressed, rather than as a list of rivals
	// that every new module would force its predecessors to update.
	Claims []Claim `json:"claims,omitempty"`

	// Capabilities the machine must have. A different field from Requires because the remedy
	// differs: a missing module can be assigned, and a missing capability means the wrong
	// machine.
	Capabilities []string `json:"capabilities,omitempty"`

	// Resources are what this module puts on a node, in the host's own vocabulary.
	Resources []map[string]any `json:"resources,omitempty"`

	// Computed names something in the control plane that works this module's resources out per
	// node, instead of them being fixed here.
	//
	// Because some files cannot be written in advance. A machine's peer list on the private
	// network is derived from every other machine, so it differs on each one and changes when any
	// of them changes — there is nothing to put in a manifest.
	//
	// Being a module anyway is the point: it is assigned like anything else, so a machine that
	// should not be on the private network simply is not given it, and the network is worked out
	// over the machines that have it. Before this, connectivity was code beside the module system
	// doing the same job, and every machine with an address was on the network whether or not
	// anybody wanted it there.
	Computed string `json:"computed,omitempty"`

	// Contributes is what this module tells whatever answers a requirement.
	//
	// The other half of an edge. `requires` says a thing must be there; this says what to do with
	// it — a web application requiring a reverse proxy has to say *which name, which port*, and
	// until now there was nowhere to put that. Every module that needed it was reduced to
	// reaching into the control plane's database directly, which is how two of them came to hold
	// a credential to it permanently.
	//
	// Keyed by the requirement, because that is what the contribution is *about*. Contributing to
	// something is requiring it: asking to be published means a publisher must exist, and a
	// module that had to say both would eventually say one.
	Contributes map[string]map[string]any `json:"contributes,omitempty"`

	// Receives is where this module wants its consumers' contributions written, per requirement
	// it provides.
	//
	// A file, in the mesh's own shape, replaced whenever the set changes. **The control plane
	// does not know what a reverse proxy is** and does not write one's configuration — it
	// delivers the facts, and the module turns them into whatever it runs. That boundary is why
	// swapping the proxy does not touch a single module that publishes through it.
	Receives map[string]string `json:"receives,omitempty"`
}

// Wants is everything that must be provided on the same node: what this module requires, and what
// it contributes to.
func (m Manifest) Wants() []string {
	out := append([]string{}, m.Requires...)
	for to := range m.Contributes {
		var already bool
		for _, r := range m.Requires {
			if r == to {
				already = true
			}
		}
		if !already {
			out = append(out, to)
		}
	}
	sort.Strings(out)
	return out
}

// ReceivedID is the resource identity of the file a provider is given its contributions in.
//
// Named rather than positional so a module can point `restart-on` at it: a proxy that got a new
// route and did not reload is a route that silently does not work, which is the same fault the
// overlay had when a peer list changed under a running interface.
func ReceivedID(requirement string) string { return "received-" + requirement }

// ParseManifest reads a module manifest, refusing anything it cannot act on.
//
// Every problem is reported rather than the first, because somebody writing a manifest fixes
// them in one pass or in four.
func ParseManifest(raw []byte) (Manifest, error) {
	var m Manifest
	if err := json.Unmarshal(raw, &m); err != nil {
		return Manifest{}, fmt.Errorf("this is not a module manifest: %w", err)
	}

	var problems []string
	if !name.MatchString(m.Module) {
		problems = append(problems, fmt.Sprintf(
			"%q is not a usable module name: lower-case letters, digits, dashes and dots", m.Module))
	}
	for _, p := range m.Provides {
		if !name.MatchString(p) {
			problems = append(problems, fmt.Sprintf("%q is not a usable name to provide", p))
		}
		if p == m.Module {
			// Harmless and worth saying: a module always provides its own name, so writing it
			// suggests the author expected it not to.
			problems = append(problems, fmt.Sprintf(
				"%s provides its own name already; listing it says nothing", m.Module))
		}
	}
	for _, r := range m.Requires {
		if !name.MatchString(r) {
			problems = append(problems, fmt.Sprintf("%q is not a usable name to require", r))
		}
		if r == m.Module {
			problems = append(problems, fmt.Sprintf("%s requires itself", m.Module))
		}
	}
	for _, c := range m.Claims {
		if !name.MatchString(c.Name) {
			problems = append(problems, fmt.Sprintf("%q is not a usable claim name", c.Name))
		}
		switch c.At() {
		case ScopeNode, ScopeSite, ScopeMesh:
		default:
			problems = append(problems, fmt.Sprintf(
				"%s claims %s at scope %q; a claim is held per node, per site or per mesh",
				m.Module, c.Name, c.Scope))
		}
	}
	if m.Computed != "" && len(m.Resources) > 0 {
		// One or the other. A module that both ships files and has them computed would leave
		// nobody able to say where a given file came from.
		problems = append(problems, fmt.Sprintf(
			"%s has resources of its own and says they are computed by %q; it is one or the other",
			m.Module, m.Computed))
	}
	for to, values := range m.Contributes {
		if !name.MatchString(to) {
			problems = append(problems, fmt.Sprintf("%q is not a usable name to contribute to", to))
		}
		if len(values) == 0 {
			// An empty contribution is either a mistake or a requirement written the long way
			// round, and both are better said plainly.
			problems = append(problems, fmt.Sprintf(
				"%s contributes nothing to %q; if it only needs one, require it", m.Module, to))
		}
	}
	for to, where := range m.Receives {
		if !name.MatchString(to) {
			problems = append(problems, fmt.Sprintf("%q is not a usable name to receive", to))
		}
		if !strings.HasPrefix(where, "/") {
			problems = append(problems, fmt.Sprintf(
				"%s receives %q at %q, which is not an absolute path", m.Module, to, where))
		}
		var offered bool
		for _, o := range m.Offers() {
			if o == to {
				offered = true
			}
		}
		if !offered {
			// Receiving contributions to something you do not provide would create a file nobody
			// ever writes to, on a machine where nothing asked for it.
			problems = append(problems, fmt.Sprintf(
				"%s receives contributions to %q and does not provide it", m.Module, to))
		}
	}
	for i, r := range m.Resources {
		id, _ := r["id"].(string)
		if id == "" {
			problems = append(problems, fmt.Sprintf("resource %d has no id", i))
		}
		if _, ok := r["type"].(string); !ok {
			problems = append(problems, fmt.Sprintf("resource %q has no type", id))
		}
	}

	if len(problems) > 0 {
		sort.Strings(problems)
		return Manifest{}, fmt.Errorf("this manifest cannot be used:\n  - %s",
			strings.Join(problems, "\n  - "))
	}
	return m, nil
}

// Offers is everything this module can satisfy: its own name, and what it provides.
func (m Manifest) Offers() []string {
	out := append([]string{m.Module}, m.Provides...)
	sort.Strings(out)
	return out
}