// Package roomversion is the registry of Matrix room versions and of what
// each one changes.
//
// A room version fixes the rules every server in that room must follow:
// how event IDs are derived, which keys survive a redaction, and which
// authorisation rules apply. Rooms never change version in place, so a
// server that wants to take part in old rooms has to keep every old rule
// set working — the differences are small but they are not optional, and a
// server that gets one wrong corrupts the room's history for everybody.
//
// Rather than branch on the version string in a dozen places, each version
// is described once here as a Rules value, and the event, auth and stateres
// packages ask it what to do.
//
// # Supported versions
//
// Versions 3 to 12 are implemented. Versions 1 and 2 are listed as known
// but unsupported: they carry the event ID inside the event, write
// prev_events and auth_events as [id, hashes] tuples rather than plain
// lists, and use state resolution v1, which is enough of a separate event
// format to be its own piece of work. It is only needed to join rooms
// created before 2018 on other servers, which federation (PLAN.md phase 5)
// does not yet do; the registry rejects those rooms clearly until it does.
package roomversion

import (
	"errors"
	"fmt"
	"slices"
	"strings"
)

// Version is a room version identifier, such as "11". It is an opaque
// string by spec: do not compare it with < or parse it as a number.
type Version string

// The versions this package knows about.
const (
	V1  Version = "1"
	V2  Version = "2"
	V3  Version = "3"
	V4  Version = "4"
	V5  Version = "5"
	V6  Version = "6"
	V7  Version = "7"
	V8  Version = "8"
	V9  Version = "9"
	V10 Version = "10"
	V11 Version = "11"
	V12 Version = "12"
)

// Default is the version `createRoom` uses when the client does not ask for
// one.
//
// The spec says servers SHOULD default to 12, but also asks them to stay on
// 11 for a while after the 1.16 release so that clients and bridges catch
// up (MSC4304). We follow that advice: 12 is implemented and a client can
// ask for it, but a plain createRoom still makes an 11 room.
const Default = V11

// Errors from Get. They are distinct because the answers differ: an
// unknown version is a client typo or a room from the future, while an
// unsupported one is a gap in this server.
var (
	// ErrUnknownVersion means no such room version exists, as far as this
	// build of Warp knows.
	ErrUnknownVersion = errors.New("unknown room version")

	// ErrUnsupportedVersion means the version exists but Warp cannot take
	// part in rooms that use it.
	ErrUnsupportedVersion = errors.New("unsupported room version")
)

// EventIDFormat says how the event ID of an event is arrived at.
type EventIDFormat int

const (
	// EventIDFormatExplicit carries the ID in the event's event_id
	// property, as "$random:server" (room versions 1 and 2).
	EventIDFormatExplicit EventIDFormat = iota

	// EventIDFormatSHA256 computes the ID from the reference hash, in
	// unpadded standard base64 (room version 3).
	EventIDFormatSHA256

	// EventIDFormatSHA256URLSafe computes the ID from the reference hash,
	// in URL-safe unpadded base64 (room version 4 and later). The change
	// was made because an ID containing '/' cannot be put in a path.
	EventIDFormatSHA256URLSafe
)

// StateResVersion says which state resolution algorithm a room uses.
type StateResVersion int

const (
	// StateResV1 is the original algorithm, used only by room version 1.
	StateResV1 StateResVersion = iota

	// StateResV2 is the algorithm from room version 2 onwards.
	StateResV2

	// StateResV2Subgraph is state resolution v2 as amended by room version
	// 12: iterative auth checks start from an empty state map, and the full
	// conflicted set also contains the conflicted state subgraph. Both
	// changes exist to make state resets rarer.
	StateResV2Subgraph
)

// Rules is everything that varies between room versions. A zero Rules is
// not usable; get one from Get or MustGet.
type Rules struct {
	// Version is the identifier these rules belong to.
	Version Version

	// Supported reports whether Warp can take part in rooms of this
	// version. Get refuses to return rules that are not supported, so a
	// Rules in hand is always supported; the field exists so that Known
	// can report the full picture.
	Supported bool

	// EventIDFormat says where an event's ID comes from.
	EventIDFormat EventIDFormat

	// StateRes says which state resolution algorithm applies.
	StateRes StateResVersion

	// StrictCanonicalJSON makes a malformed number or a non-UTF-8 string a
	// hard rejection rather than something to tolerate (version 6+). In
	// older rooms such events exist and still have to be processed.
	StrictCanonicalJSON bool

	// EnforceSigningKeyValidity requires a signing key's valid_until_ts to
	// be at least the event's origin_server_ts (version 5+).
	EnforceSigningKeyValidity bool

	// --- Authorisation ---

	// AliasesEventAuth gives m.room.aliases its own authorisation rule,
	// under which any server could edit its own alias list (versions 1 to
	// 5). Version 6 dropped it, and with it the ability to put another
	// server's aliases in a room.
	AliasesEventAuth bool

	// CreateEventRequiresCreator rejects an m.room.create event whose
	// content has no creator property (versions 1 to 10). Version 11
	// removed the property, since it only ever repeated sender.
	CreateEventRequiresCreator bool

	// KnockJoinRule enables the knock membership and join rule (version 7+).
	KnockJoinRule bool

	// RestrictedJoinRule enables the restricted join rule, where
	// membership of another room grants entry, authorised by a server that
	// can see that membership (version 8+).
	RestrictedJoinRule bool

	// KnockRestrictedJoinRule enables knock_restricted, the two combined
	// (version 10+).
	KnockRestrictedJoinRule bool

	// RequireIntegerPowerLevels rejects a power level given as a string
	// (version 10+). Earlier versions accepted "50" and parsed it, which
	// meant two servers could disagree about what a value meant.
	RequireIntegerPowerLevels bool

	// RoomIDIsCreateEventID makes the room ID the create event's own ID
	// with a '!' sigil instead of '$', so it carries no domain. The create
	// event has no room_id of its own (version 12+).
	RoomIDIsCreateEventID bool

	// CreateEventExcludedFromAuthEvents keeps m.room.create out of every
	// event's auth_events, because the room ID now identifies it (version
	// 12+).
	CreateEventExcludedFromAuthEvents bool

	// CreatorsHaveInfinitePower gives the room's creators a power level
	// above any that can be written down, and forbids naming them in
	// m.room.power_levels (version 12+).
	CreatorsHaveInfinitePower bool

	// AdditionalCreators allows m.room.create content to name further
	// creators alongside the sender (version 12+).
	AdditionalCreators bool

	// --- Redaction ---

	// RedactionProtectsLegacyKeys keeps the top-level prev_state, origin
	// and membership properties through a redaction (versions 1 to 10).
	// Version 11 stopped protecting them: none was load-bearing, and
	// membership in particular let a redaction preserve a claim the
	// content no longer backed up.
	RedactionProtectsLegacyKeys bool

	// RedactionKeepsWholeCreateContent keeps all of an m.room.create
	// event's content (version 11+). Before that only creator survived,
	// which threw away the room version — the one thing a server needs to
	// read the rest of the room.
	RedactionKeepsWholeCreateContent bool

	// RedactsInContent puts the target of an m.room.redaction event in
	// content.redacts rather than at the top level (version 11+). The
	// property moved so that the redaction algorithm, and therefore the
	// event's hashes and signatures, would cover it at all.
	RedactsInContent bool

	// RedactionKeepsRedacts keeps content.redacts on an m.room.redaction
	// event (version 11+), where the property moved so that it would be
	// covered by hashes and signatures at all.
	RedactionKeepsRedacts bool

	// RedactionKeepsPowerLevelsInvite keeps content.invite on an
	// m.room.power_levels event (version 11+).
	RedactionKeepsPowerLevelsInvite bool

	// RedactionKeepsThirdPartyInviteSigned keeps
	// content.third_party_invite.signed on an m.room.member event
	// (version 11+), which is what proves a third-party invite was real.
	RedactionKeepsThirdPartyInviteSigned bool

	// RedactionKeepsJoinAuthorisedViaUsersServer keeps
	// content.join_authorised_via_users_server on an m.room.member event
	// (version 9+). Version 8 redacted it, which made a redacted join in a
	// restricted room fail to authorise for ever after.
	RedactionKeepsJoinAuthorisedViaUsersServer bool

	// RedactionKeepsJoinRulesAllow keeps content.allow on an
	// m.room.join_rules event (version 8+).
	RedactionKeepsJoinRulesAllow bool

	// RedactionKeepsAliases keeps content.aliases on an m.room.aliases
	// event (versions 1 to 5).
	RedactionKeepsAliases bool
}

// String returns the version identifier.
func (r Rules) String() string { return string(r.Version) }

// versions is the registry, in the order Known reports them.
//
// Each entry is written out in full rather than derived from the one before
// it. It is more lines, but a reader can see what a single version does
// without replaying the whole history, and a new version cannot change an
// old one by accident.
var versions = []Rules{
	{
		Version:                    V1,
		Supported:                  false,
		EventIDFormat:              EventIDFormatExplicit,
		StateRes:                   StateResV1,
		AliasesEventAuth:           true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
		RedactionKeepsAliases:       true,
	},
	{
		Version:                    V2,
		Supported:                  false,
		EventIDFormat:              EventIDFormatExplicit,
		StateRes:                   StateResV2,
		AliasesEventAuth:           true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
		RedactionKeepsAliases:       true,
	},
	{
		Version:                    V3,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256,
		StateRes:                   StateResV2,
		AliasesEventAuth:           true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
		RedactionKeepsAliases:       true,
	},
	{
		Version:                    V4,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		AliasesEventAuth:           true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
		RedactionKeepsAliases:       true,
	},
	{
		Version:                    V5,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		EnforceSigningKeyValidity:  true,
		AliasesEventAuth:           true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
		RedactionKeepsAliases:       true,
	},
	{
		Version:                    V6,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		StrictCanonicalJSON:        true,
		EnforceSigningKeyValidity:  true,
		CreateEventRequiresCreator: true,

		RedactionProtectsLegacyKeys: true,
	},
	{
		Version:                    V7,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		StrictCanonicalJSON:        true,
		EnforceSigningKeyValidity:  true,
		CreateEventRequiresCreator: true,
		KnockJoinRule:              true,

		RedactionProtectsLegacyKeys: true,
	},
	{
		Version:                    V8,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		StrictCanonicalJSON:        true,
		EnforceSigningKeyValidity:  true,
		CreateEventRequiresCreator: true,
		KnockJoinRule:              true,
		RestrictedJoinRule:         true,

		RedactionProtectsLegacyKeys:  true,
		RedactionKeepsJoinRulesAllow: true,
	},
	{
		Version:                    V9,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		StrictCanonicalJSON:        true,
		EnforceSigningKeyValidity:  true,
		CreateEventRequiresCreator: true,
		KnockJoinRule:              true,
		RestrictedJoinRule:         true,

		RedactionProtectsLegacyKeys:                true,
		RedactionKeepsJoinRulesAllow:               true,
		RedactionKeepsJoinAuthorisedViaUsersServer: true,
	},
	{
		Version:                    V10,
		Supported:                  true,
		EventIDFormat:              EventIDFormatSHA256URLSafe,
		StateRes:                   StateResV2,
		StrictCanonicalJSON:        true,
		EnforceSigningKeyValidity:  true,
		CreateEventRequiresCreator: true,
		KnockJoinRule:              true,
		RestrictedJoinRule:         true,
		KnockRestrictedJoinRule:    true,
		RequireIntegerPowerLevels:  true,

		RedactionProtectsLegacyKeys:                true,
		RedactionKeepsJoinRulesAllow:               true,
		RedactionKeepsJoinAuthorisedViaUsersServer: true,
	},
	{
		Version:                   V11,
		Supported:                 true,
		EventIDFormat:             EventIDFormatSHA256URLSafe,
		StateRes:                  StateResV2,
		StrictCanonicalJSON:       true,
		EnforceSigningKeyValidity: true,
		KnockJoinRule:             true,
		RestrictedJoinRule:        true,
		KnockRestrictedJoinRule:   true,
		RequireIntegerPowerLevels: true,
		RedactsInContent:          true,

		RedactionKeepsWholeCreateContent:           true,
		RedactionKeepsRedacts:                      true,
		RedactionKeepsPowerLevelsInvite:            true,
		RedactionKeepsThirdPartyInviteSigned:       true,
		RedactionKeepsJoinRulesAllow:               true,
		RedactionKeepsJoinAuthorisedViaUsersServer: true,
	},
	{
		Version:                   V12,
		Supported:                 true,
		EventIDFormat:             EventIDFormatSHA256URLSafe,
		StateRes:                  StateResV2Subgraph,
		StrictCanonicalJSON:       true,
		EnforceSigningKeyValidity: true,
		KnockJoinRule:             true,
		RestrictedJoinRule:        true,
		KnockRestrictedJoinRule:   true,
		RequireIntegerPowerLevels: true,

		RedactsInContent:                  true,
		RoomIDIsCreateEventID:             true,
		CreateEventExcludedFromAuthEvents: true,
		CreatorsHaveInfinitePower:         true,
		AdditionalCreators:                true,

		RedactionKeepsWholeCreateContent:           true,
		RedactionKeepsRedacts:                      true,
		RedactionKeepsPowerLevelsInvite:            true,
		RedactionKeepsThirdPartyInviteSigned:       true,
		RedactionKeepsJoinRulesAllow:               true,
		RedactionKeepsJoinAuthorisedViaUsersServer: true,
	},
}

// index is versions keyed by identifier, built once at startup.
var index = func() map[Version]Rules {
	m := make(map[Version]Rules, len(versions))
	for _, v := range versions {
		m[v.Version] = v
	}
	return m
}()

// Get returns the rules for a room version.
//
// It fails for a version Warp does not support, so every Rules in
// circulation is one we can actually act on.
func Get(v Version) (Rules, error) {
	rules, ok := index[v]
	if !ok {
		return Rules{}, fmt.Errorf("%w: %q", ErrUnknownVersion, v)
	}
	if !rules.Supported {
		return Rules{}, fmt.Errorf("%w: %q", ErrUnsupportedVersion, v)
	}
	return rules, nil
}

// MustGet is Get for a version fixed in the source, such as a test or
// Default. It panics rather than returning an error, because a typo in a
// constant is a bug to fix, not a condition to handle.
func MustGet(v Version) Rules {
	rules, err := Get(v)
	if err != nil {
		panic(err)
	}
	return rules
}

// IsSupported reports whether Warp can take part in rooms of this version.
// It is the test behind the authorisation rule that rejects an
// m.room.create event naming a version we do not recognise.
func IsSupported(v Version) bool {
	return index[v].Supported
}

// Supported lists the versions Warp can take part in, oldest first. It
// feeds the room_versions capability.
func Supported() []Version {
	out := make([]Version, 0, len(versions))
	for _, v := range versions {
		if v.Supported {
			out = append(out, v.Version)
		}
	}
	return out
}

// Known lists every version in the registry, supported or not.
func Known() []Rules {
	return slices.Clone(versions)
}

// MaxVersionLength is the spec's cap on a room version identifier.
const MaxVersionLength = 32

// Validate checks a string against the room version grammar: 1 to 32
// characters of lowercase letters, digits, '.' and '-'.
//
// It is for strings arriving from a client or another server, where the
// answer "that is not a room version at all" is more useful than "unknown
// room version". Note that a valid identifier need not be one we know.
func Validate(s string) error {
	if s == "" {
		return errors.New("room version must not be empty")
	}
	if n := len([]rune(s)); n > MaxVersionLength {
		return fmt.Errorf("room version is %d characters, the limit is %d", n, MaxVersionLength)
	}
	for _, r := range s {
		switch {
		case r >= 'a' && r <= 'z', r >= '0' && r <= '9', r == '.', r == '-':
		default:
			return fmt.Errorf("room version %q contains %q; only lowercase letters, digits, '.' and '-' are allowed", s, r)
		}
	}
	return nil
}

// SupportedString renders Supported for a log line or an error message.
func SupportedString() string {
	out := make([]string, 0, len(versions))
	for _, v := range Supported() {
		out = append(out, string(v))
	}
	return strings.Join(out, ", ")
}