// Package apply is tier 0's one job: take a declaration and make this machine match it.
//
// A declaration is data, not instructions — an ordered list of typed resources the host owns,
// settled by novox/hq ADR 0043. This package is the *consumer* side of that record: what the
// host accepts, and what it does with it. What produces a declaration (the control plane, or a
// hand-authored substrate.lock) is deliberately not here.
//
// The cardinal rule of the whole project appears twice in this file, because a declaration is
// exactly where it bites: an unknown version, an unknown resource type, or an unknown field is
// a refusal of the WHOLE declaration — never a skip, never best-effort. A host that applied the
// parts it understood would leave a machine that looks configured and is not, which is
// novox/hq 04-ISSUES/003 with the declaration on the other side of the wire.
package apply

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

// Version is the one declaration vocabulary this host understands. A declaration naming any
// other version is refused whole — an older host cannot be handed a newer vocabulary and
// quietly do half of it (ADR 0043).
const Version = 1

// Declaration is what crosses the link, or what substrate.lock carries: an ordered list of
// resources, addressed to one node.
type Declaration struct {
	// Version of the vocabulary. Refused whole if it is not exactly Version.
	Version int `json:"version"`
	// For names the node this is meant for. A host with an identity refuses a declaration
	// addressed elsewhere; a host with no identity yet — the first node — has nothing to check
	// against and applies it (ADR 0043). Empty means unaddressed, which any host applies.
	For string `json:"for"`
	// Resources, in the order they are to be applied. The host does not sort them and does not
	// resolve dependencies: ordering is the control plane's decision, stated rather than derived
	// (ADR 0037).
	Resources []Resource `json:"resources"`
}

// Resource is one thing the host owns on this machine. Its identity is a name the control plane
// keeps stable across declarations — not a position, not a hash of its content — because that
// stable name is what lets the store say "this is the same resource I applied last time", which
// is what makes convergence and removal possible at all (ADR 0043).
//
// Beyond id and type, a resource's fields are type-specific and validated against the shape the
// type declares. They are kept as raw JSON so an unknown field can be refused rather than
// silently dropped by struct decoding.
type Resource struct {
	ID     string
	Type   string
	Fields map[string]json.RawMessage
}

// Path is the host-owned filesystem path this resource lives at. Every type this host applies
// so far is addressed by a path, and the store needs it to remove the resource later.
func (r Resource) Path() string {
	return r.stringField("path")
}

func (r Resource) stringField(key string) string {
	raw, ok := r.Fields[key]
	if !ok {
		return ""
	}
	var s string
	if err := json.Unmarshal(raw, &s); err != nil {
		return ""
	}
	return s
}

// shape is the set of field keys a resource type may carry, beyond the common id and type.
// This is the host's half of a wire contract whose other half is the control plane's catalogue
// (novox/mesh-control examples/modules/modules_test.go). Duplicated deliberately, because the
// host shares no code with any other tier (ADR 0041) — and checked on both sides, because a
// contract with two copies and no check is a contract only until someone edits one.
//
// A type absent from this table is unknown TO THIS HOST, and unknown is refused. That is not a
// gap to apologise for: the network-free types come first (ADR 0043), and a host refusing a
// container resource it cannot yet apply is the same protection as refusing an unknown one —
// it never does half a declaration.
var shapes = map[string][]string{
	"directory": {"path", "mode", "owner"},
	"file":      {"path", "content", "mode", "owner"},
}

// Parse reads a declaration and refuses anything it does not fully understand.
//
// The refusal is whole and it is specific: the error names what it could not accept, because a
// boundary that refuses without saying why is worse than the thing it guards (ADR 0039).
func Parse(raw []byte) (Declaration, error) {
	// The envelope is decoded strictly: an unknown top-level field is refused like any other.
	var envelope struct {
		Version   int               `json:"version"`
		For       string            `json:"for"`
		Resources []json.RawMessage `json:"resources"`
	}
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.DisallowUnknownFields()
	if err := dec.Decode(&envelope); err != nil {
		return Declaration{}, fmt.Errorf("not a declaration this host accepts: %w", err)
	}

	if envelope.Version != Version {
		return Declaration{}, fmt.Errorf(
			"declaration version %d, and this host speaks version %d — refused whole rather than "+
				"applying a vocabulary it does not know", envelope.Version, Version)
	}

	d := Declaration{Version: envelope.Version, For: envelope.For}
	seen := map[string]bool{}
	for i, rawRes := range envelope.Resources {
		r, err := parseResource(rawRes)
		if err != nil {
			return Declaration{}, fmt.Errorf("resource %d: %w", i, err)
		}
		if seen[r.ID] {
			return Declaration{}, fmt.Errorf(
				"resource %d: id %q appears twice — an id is how the store tells one resource "+
					"from another, so two cannot share one", i, r.ID)
		}
		seen[r.ID] = true
		d.Resources = append(d.Resources, r)
	}
	return d, nil
}

func parseResource(raw json.RawMessage) (Resource, error) {
	var fields map[string]json.RawMessage
	if err := json.Unmarshal(raw, &fields); err != nil {
		return Resource{}, fmt.Errorf("not an object: %w", err)
	}

	id := decodeString(fields["id"])
	typ := decodeString(fields["type"])
	if id == "" {
		return Resource{}, fmt.Errorf("has no id, and every resource must have one")
	}
	if typ == "" {
		return Resource{}, fmt.Errorf("%q has no type", id)
	}

	allowed, known := shapes[typ]
	if !known {
		return Resource{}, fmt.Errorf(
			"%q is a %q, which this host cannot apply — refused whole, because applying the rest "+
				"would leave a machine that looks configured and is not", id, typ)
	}

	// Every field beyond the common two must belong to the type's shape. An unknown one is
	// refused: a firewall-scope key read by nothing is exactly the fault this prevents
	// (04-ISSUES/003).
	ok := map[string]bool{"id": true, "type": true}
	for _, k := range allowed {
		ok[k] = true
	}
	extra := make(map[string]json.RawMessage, len(fields))
	for k, v := range fields {
		if !ok[k] {
			return Resource{}, fmt.Errorf(
				"%q is a %s and carries %q, which that shape does not have", id, typ, k)
		}
		if k != "id" && k != "type" {
			extra[k] = v
		}
	}

	if _, hasPath := extra["path"]; !hasPath {
		return Resource{}, fmt.Errorf("%q is a %s and names no path", id, typ)
	}

	return Resource{ID: id, Type: typ, Fields: extra}, nil
}

func decodeString(raw json.RawMessage) string {
	if raw == nil {
		return ""
	}
	var s string
	_ = json.Unmarshal(raw, &s)
	return s
}

// KnownTypes lists the resource types this host can apply, in stable order. Exists so the CLI
// and tests can state the host's reach rather than restating the shape table.
func KnownTypes() []string {
	out := make([]string, 0, len(shapes))
	for t := range shapes {
		out = append(out, t)
	}
	sort.Strings(out)
	return out
}