1 files changed, 165 insertions, 0 deletions
diff --git a/internal/pushrules/evaluate.go b/internal/pushrules/evaluate.go
new file mode 100644
index 00000000..df22cb04
--- /dev/null
+++ b/internal/pushrules/evaluate.go
@@ -0,0 +1,165 @@
+package pushrules
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// A RuleSetEvaluator encapsulates context to evaluate an event
+// against a rule set.
+type RuleSetEvaluator struct {
+	ec      EvaluationContext
+	ruleSet []kindAndRules
+}
+
+// An EvaluationContext gives a RuleSetEvaluator access to the
+// environment, for rules that require that.
+type EvaluationContext interface {
+	// UserDisplayName returns the current user's display name.
+	UserDisplayName() string
+
+	// RoomMemberCount returns the number of members in the room of
+	// the current event.
+	RoomMemberCount() (int, error)
+
+	// HasPowerLevel returns whether the user has at least the given
+	// power in the room of the current event.
+	HasPowerLevel(userID, levelKey string) (bool, error)
+}
+
+// A kindAndRules is just here to simplify iteration of the (ordered)
+// kinds of rules.
+type kindAndRules struct {
+	Kind  Kind
+	Rules []*Rule
+}
+
+// NewRuleSetEvaluator creates a new evaluator for the given rule set.
+func NewRuleSetEvaluator(ec EvaluationContext, ruleSet *RuleSet) *RuleSetEvaluator {
+	return &RuleSetEvaluator{
+		ec: ec,
+		ruleSet: []kindAndRules{
+			{OverrideKind, ruleSet.Override},
+			{ContentKind, ruleSet.Content},
+			{RoomKind, ruleSet.Room},
+			{SenderKind, ruleSet.Sender},
+			{UnderrideKind, ruleSet.Underride},
+		},
+	}
+}
+
+// MatchEvent returns the first matching rule. Returns nil if there
+// was no match rule.
+func (rse *RuleSetEvaluator) MatchEvent(event *gomatrixserverlib.Event) (*Rule, error) {
+	// TODO: server-default rules have lower priority than user rules,
+	// but they are stored together with the user rules. It's a bit
+	// unclear what the specification (11.14.1.4 Predefined rules)
+	// means the ordering should be.
+	//
+	// The most reasonable interpretation is that default overrides
+	// still have lower priority than user content rules, so we
+	// iterate twice.
+	for _, rsat := range rse.ruleSet {
+		for _, defRules := range []bool{false, true} {
+			for _, rule := range rsat.Rules {
+				if rule.Default != defRules {
+					continue
+				}
+				ok, err := ruleMatches(rule, rsat.Kind, event, rse.ec)
+				if err != nil {
+					return nil, err
+				}
+				if ok {
+					return rule, nil
+				}
+			}
+		}
+	}
+
+	// No matching rule.
+	return nil, nil
+}
+
+func ruleMatches(rule *Rule, kind Kind, event *gomatrixserverlib.Event, ec EvaluationContext) (bool, error) {
+	if !rule.Enabled {
+		return false, nil
+	}
+
+	switch kind {
+	case OverrideKind, UnderrideKind:
+		for _, cond := range rule.Conditions {
+			ok, err := conditionMatches(cond, event, ec)
+			if err != nil {
+				return false, err
+			}
+			if !ok {
+				return false, nil
+			}
+		}
+		return true, nil
+
+	case ContentKind:
+		// TODO: "These configure behaviour for (unencrypted) messages
+		// that match certain patterns." - Does that mean "content.body"?
+		return patternMatches("content.body", rule.Pattern, event)
+
+	case RoomKind:
+		return rule.RuleID == event.RoomID(), nil
+
+	case SenderKind:
+		return rule.RuleID == event.Sender(), nil
+
+	default:
+		return false, nil
+	}
+}
+
+func conditionMatches(cond *Condition, event *gomatrixserverlib.Event, ec EvaluationContext) (bool, error) {
+	switch cond.Kind {
+	case EventMatchCondition:
+		return patternMatches(cond.Key, cond.Pattern, event)
+
+	case ContainsDisplayNameCondition:
+		return patternMatches("content.body", ec.UserDisplayName(), event)
+
+	case RoomMemberCountCondition:
+		cmp, err := parseRoomMemberCountCondition(cond.Is)
+		if err != nil {
+			return false, fmt.Errorf("parsing room_member_count condition: %w", err)
+		}
+		n, err := ec.RoomMemberCount()
+		if err != nil {
+			return false, fmt.Errorf("RoomMemberCount failed: %w", err)
+		}
+		return cmp(n), nil
+
+	case SenderNotificationPermissionCondition:
+		return ec.HasPowerLevel(event.Sender(), cond.Key)
+
+	default:
+		return false, nil
+	}
+}
+
+func patternMatches(key, pattern string, event *gomatrixserverlib.Event) (bool, error) {
+	re, err := globToRegexp(pattern)
+	if err != nil {
+		return false, err
+	}
+
+	var eventMap map[string]interface{}
+	if err = json.Unmarshal(event.JSON(), &eventMap); err != nil {
+		return false, fmt.Errorf("parsing event: %w", err)
+	}
+	v, err := lookupMapPath(strings.Split(key, "."), eventMap)
+	if err != nil {
+		// An unknown path is a benign error that shouldn't stop rule
+		// processing. It's just a non-match.
+		return false, nil
+	}
+
+	return re.MatchString(fmt.Sprint(v)), nil
+}