// Package pushrules is the push rule language of the Client-Server API:
// the rule model, the server-default ruleset, and the evaluation that
// decides whether an event notifies a user.
//
// It is pure library code, like internal/matrix: no storage, no HTTP, no
// rooms. The caller supplies the event and the handful of facts about the
// room an evaluation needs — the user's display name in it, how many
// members it has, how much power the sender holds — and gets back the rule
// that matched. Where those facts come from, and what is done with the
// answer, is internal/pushserver's business.
package pushrules

import (
	"encoding/json"
	"fmt"
	"slices"
	"strings"
)

// Kind is one of the five kinds of push rule, which are also the order
// they are evaluated in.
type Kind string

// The kinds, as the spec spells them in paths and in a ruleset.
const (
	KindOverride  Kind = "override"
	KindContent   Kind = "content"
	KindRoom      Kind = "room"
	KindSender    Kind = "sender"
	KindUnderride Kind = "underride"
)

// Kinds lists the kinds in evaluation order.
var Kinds = []Kind{KindOverride, KindContent, KindRoom, KindSender, KindUnderride}

// ParseKind reads a kind from a path segment.
func ParseKind(s string) (Kind, bool) {
	k := Kind(s)
	return k, slices.Contains(Kinds, k)
}

// Rule is one push rule, in the shape the Client-Server API reads and
// writes it.
type Rule struct {
	RuleID  string `json:"rule_id"`
	Default bool   `json:"default"`
	Enabled bool   `json:"enabled"`

	// Actions are kept as the JSON they arrived as. The spec has the
	// server pass unknown tweaks through to the push gateway untouched,
	// and a typed model would quietly drop them.
	Actions []json.RawMessage `json:"actions"`

	// Conditions are only on override and underride rules. A room or
	// sender rule's condition is its ID, and a content rule's is its
	// pattern.
	Conditions []Condition `json:"conditions,omitempty"`

	// Pattern is only on a content rule.
	Pattern *string `json:"pattern,omitempty"`
}

// MarshalJSON writes conditions whenever the rule has a list of them, an
// empty one included: .m.rule.master's "conditions": [] is how a client
// tells an override rule that always matches from a malformed one.
func (r Rule) MarshalJSON() ([]byte, error) {
	type plain Rule
	if r.Conditions == nil || len(r.Conditions) > 0 {
		return json.Marshal(plain(r))
	}
	return json.Marshal(struct {
		plain
		Conditions []Condition `json:"conditions"`
	}{plain: plain(r), Conditions: []Condition{}})
}

// Condition is one condition of an override or underride rule.
type Condition struct {
	Kind string `json:"kind"`

	// Key is the dotted path into the event for event_match,
	// event_property_is and event_property_contains, and the notification
	// key for sender_notification_permission.
	Key string `json:"key,omitempty"`

	// Pattern is event_match's glob.
	Pattern *string `json:"pattern,omitempty"`

	// Is is room_member_count's comparison, e.g. "2" or ">=10".
	Is string `json:"is,omitempty"`

	// Value is what event_property_is and event_property_contains compare
	// against. It is raw so that `null` is a value rather than an absence.
	Value json.RawMessage `json:"value,omitempty"`
}

// The condition kinds this server evaluates. A rule with any other kind of
// condition never matches, which is what the spec asks of a server that
// meets a condition it does not understand.
const (
	CondEventMatch                   = "event_match"
	CondEventPropertyIs              = "event_property_is"
	CondEventPropertyContains        = "event_property_contains"
	CondContainsDisplayName          = "contains_display_name"
	CondRoomMemberCount              = "room_member_count"
	CondSenderNotificationPermission = "sender_notification_permission"
)

// Ruleset is a user's rules, one list per kind, each in priority order.
type Ruleset struct {
	Override  []Rule `json:"override"`
	Content   []Rule `json:"content"`
	Room      []Rule `json:"room"`
	Sender    []Rule `json:"sender"`
	Underride []Rule `json:"underride"`
}

// Rules returns the list of one kind.
func (rs *Ruleset) Rules(kind Kind) []Rule {
	switch kind {
	case KindOverride:
		return rs.Override
	case KindContent:
		return rs.Content
	case KindRoom:
		return rs.Room
	case KindSender:
		return rs.Sender
	case KindUnderride:
		return rs.Underride
	}
	return nil
}

// SetRules replaces the list of one kind.
func (rs *Ruleset) SetRules(kind Kind, rules []Rule) {
	switch kind {
	case KindOverride:
		rs.Override = rules
	case KindContent:
		rs.Content = rules
	case KindRoom:
		rs.Room = rules
	case KindSender:
		rs.Sender = rules
	case KindUnderride:
		rs.Underride = rules
	}
}

// Find returns a rule by kind and ID.
func (rs *Ruleset) Find(kind Kind, ruleID string) (Rule, bool) {
	for _, r := range rs.Rules(kind) {
		if r.RuleID == ruleID {
			return r, true
		}
	}
	return Rule{}, false
}

// normalise replaces nil lists with empty ones, so that a ruleset always
// marshals with all five keys as arrays: a client reading `null` where it
// expects a list of rules is a client that crashes.
func (rs *Ruleset) normalise() {
	for _, kind := range Kinds {
		rules := rs.Rules(kind)
		if rules == nil {
			rules = []Rule{}
		}
		for i := range rules {
			if rules[i].Actions == nil {
				rules[i].Actions = []json.RawMessage{}
			}
		}
		rs.SetRules(kind, rules)
	}
}

// MarshalJSON writes the ruleset with every kind present.
func (rs Ruleset) MarshalJSON() ([]byte, error) {
	rs.normalise()
	type plain Ruleset
	return json.Marshal(plain(rs))
}

// Clone returns a deep enough copy that editing one list of it leaves the
// original alone.
func (rs Ruleset) Clone() Ruleset {
	var out Ruleset
	for _, kind := range Kinds {
		out.SetRules(kind, slices.Clone(rs.Rules(kind)))
	}
	return out
}

// IsReserved reports whether a rule ID is in the namespace the spec keeps
// for server-default rules. A client may change the actions of such a rule
// and switch it on and off, but may not create, replace or delete one.
func IsReserved(ruleID string) bool { return strings.HasPrefix(ruleID, ".") }

// Merge lays a user's stored rules over the server defaults.
//
// What a user owns is the rules they made, and the enabled flag and
// actions of each default rule; the default rules themselves — their
// conditions, and which of them exist — are the server's, and come from
// defaults. Storing the whole ruleset and re-reading only those parts is
// what lets a server upgrade bring in a default rule a newer spec version
// added, without a migration rewriting every user's rules.
//
// The order within a kind is the spec's: user rules outrank the defaults
// of their kind, except that .m.rule.master stays ahead of everything so
// that switching it on still silences all.
func Merge(stored, defaults Ruleset) Ruleset {
	var out Ruleset
	for _, kind := range Kinds {
		overrides := map[string]Rule{}
		var user []Rule
		for _, r := range stored.Rules(kind) {
			if r.Default {
				overrides[r.RuleID] = r
				continue
			}
			if IsReserved(r.RuleID) {
				// A user rule cannot have a reserved ID; one that does was
				// not written by this server and is ignored rather than
				// allowed to shadow a default.
				continue
			}
			user = append(user, r)
		}

		var prepend, appended []Rule
		for _, d := range defaults.Rules(kind) {
			if o, ok := overrides[d.RuleID]; ok {
				d.Enabled = o.Enabled
				d.Actions = o.Actions
			}
			if d.RuleID == RuleMaster {
				prepend = append(prepend, d)
				continue
			}
			appended = append(appended, d)
		}

		rules := make([]Rule, 0, len(prepend)+len(user)+len(appended))
		rules = append(rules, prepend...)
		rules = append(rules, user...)
		rules = append(rules, appended...)
		out.SetRules(kind, rules)
	}
	out.normalise()
	return out
}

// Document is the content of the m.push_rules account data: the ruleset
// under the one scope Matrix still has.
type Document struct {
	Global Ruleset `json:"global"`
}

// ParseDocument reads stored m.push_rules content.
func ParseDocument(raw []byte) (Ruleset, error) {
	var doc Document
	if err := json.Unmarshal(raw, &doc); err != nil {
		return Ruleset{}, fmt.Errorf("pushrules: stored ruleset is not readable: %w", err)
	}
	return doc.Global, nil
}

// MarshalDocument renders a ruleset as m.push_rules content.
func MarshalDocument(rs Ruleset) ([]byte, error) {
	return json.Marshal(Document{Global: rs})
}