package pushrules

import (
	"bytes"
	"encoding/json"
	"fmt"
	"regexp"
	"strconv"
	"strings"
	"sync"
)

// Event is an event as the push rules see it: a tree of JSON values that
// dotted keys are looked up in.
type Event struct {
	tree map[string]any

	// hasMentions is whether content carries m.mentions at all, which
	// switches the legacy mention rules off.
	hasMentions bool
}

// NewEvent parses an event. Any shape of event works — the federation
// form or the client one — because the keys rules look at (type, sender,
// state_key, room_id, content) are the same in both.
func NewEvent(raw []byte) (*Event, error) {
	dec := json.NewDecoder(bytes.NewReader(raw))
	// Numbers stay as written, so that an integer compares as one: a float
	// round trip would make 9007199254740993 equal to its neighbour.
	dec.UseNumber()
	var tree map[string]any
	if err := dec.Decode(&tree); err != nil {
		return nil, fmt.Errorf("pushrules: event is not a JSON object: %w", err)
	}
	ev := &Event{tree: tree}
	if content, ok := tree["content"].(map[string]any); ok {
		_, ev.hasMentions = content["m.mentions"]
	}
	return ev, nil
}

// lookup follows a dotted key into the event.
func (e *Event) lookup(key string) (any, bool) {
	var cur any = e.tree
	for _, part := range SplitKey(key) {
		obj, ok := cur.(map[string]any)
		if !ok {
			return nil, false
		}
		if cur, ok = obj[part]; !ok {
			return nil, false
		}
	}
	return cur, true
}

// str is the string at a key, if there is one.
func (e *Event) str(key string) (string, bool) {
	v, ok := e.lookup(key)
	if !ok {
		return "", false
	}
	s, ok := v.(string)
	return s, ok
}

// SplitKey splits a dotted key into its parts. A dot preceded by a
// backslash is part of a name rather than a separator, which is how a key
// reaches into `m.mentions`; a backslash preceded by one is a literal
// backslash. Any other backslash is kept as it is.
func SplitKey(key string) []string {
	var parts []string
	var cur strings.Builder
	for i := 0; i < len(key); i++ {
		c := key[i]
		switch {
		case c == '\\' && i+1 < len(key) && (key[i+1] == '.' || key[i+1] == '\\'):
			cur.WriteByte(key[i+1])
			i++
		case c == '.':
			parts = append(parts, cur.String())
			cur.Reset()
		default:
			cur.WriteByte(c)
		}
	}
	return append(parts, cur.String())
}

// Context is what an evaluation needs to know beyond the event itself.
type Context struct {
	// UserID is the user the rules belong to.
	UserID string

	// DisplayName is the user's display name in the room, or empty.
	DisplayName string

	// MemberCount is how many users are joined to the room.
	MemberCount int

	// SenderMayNotify reports whether the event's sender has the power a
	// notification key — "room", for @room — requires. Nil means nobody
	// has.
	SenderMayNotify func(key string) bool
}

// Evaluate runs the ruleset against an event and returns the first
// enabled rule that matches it, kind by kind in priority order.
func (rs *Ruleset) Evaluate(ev *Event, ctx Context) (Rule, Kind, bool) {
	for _, kind := range Kinds {
		for _, rule := range rs.Rules(kind) {
			if !rule.Enabled {
				continue
			}
			if rule.Default && ev.hasMentions && legacyMentionRules[rule.RuleID] {
				continue
			}
			if ruleMatches(kind, rule, ev, ctx) {
				return rule, kind, true
			}
		}
	}
	return Rule{}, "", false
}

func ruleMatches(kind Kind, rule Rule, ev *Event, ctx Context) bool {
	switch kind {
	case KindOverride, KindUnderride:
		for _, cond := range rule.Conditions {
			if !conditionMatches(cond, ev, ctx) {
				return false
			}
		}
		return true
	case KindContent:
		if rule.Pattern == nil {
			return false
		}
		body, ok := ev.str("content.body")
		return ok && globMatches(*rule.Pattern, body, true)
	case KindRoom:
		roomID, ok := ev.str("room_id")
		return ok && roomID == rule.RuleID
	case KindSender:
		sender, ok := ev.str("sender")
		return ok && sender == rule.RuleID
	}
	return false
}

func conditionMatches(cond Condition, ev *Event, ctx Context) bool {
	switch cond.Kind {
	case CondEventMatch:
		if cond.Pattern == nil {
			return false
		}
		// event_match is about strings: a number or a boolean at the key
		// is not a match for any pattern, "*" included.
		value, ok := ev.str(cond.Key)
		return ok && globMatches(*cond.Pattern, value, cond.Key == "content.body")

	case CondEventPropertyIs:
		want, ok := scalar(cond.Value)
		if !ok {
			return false
		}
		got, ok := ev.lookup(cond.Key)
		return ok && scalarEqual(got, want)

	case CondEventPropertyContains:
		want, ok := scalar(cond.Value)
		if !ok {
			return false
		}
		got, ok := ev.lookup(cond.Key)
		if !ok {
			return false
		}
		list, ok := got.([]any)
		if !ok {
			return false
		}
		for _, item := range list {
			if scalarEqual(item, want) {
				return true
			}
		}
		return false

	case CondContainsDisplayName:
		if ctx.DisplayName == "" {
			return false
		}
		body, ok := ev.str("content.body")
		return ok && literalMatches(ctx.DisplayName, body)

	case CondRoomMemberCount:
		return memberCountMatches(cond.Is, ctx.MemberCount)

	case CondSenderNotificationPermission:
		return cond.Key != "" && ctx.SenderMayNotify != nil && ctx.SenderMayNotify(cond.Key)
	}
	return false
}

// scalar decodes a condition's value. Only the canonical JSON scalars may
// be compared: a string, an integer, a boolean or null.
func scalar(raw json.RawMessage) (any, bool) {
	if len(raw) == 0 {
		return nil, false
	}
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.UseNumber()
	var v any
	if err := dec.Decode(&v); err != nil {
		return nil, false
	}
	switch v.(type) {
	case string, bool, nil, json.Number:
		return v, true
	}
	return nil, false
}

// scalarEqual compares an event value with a condition value. Numbers are
// compared as integers, which is all canonical JSON allows.
func scalarEqual(got, want any) bool {
	switch w := want.(type) {
	case nil:
		return got == nil
	case string:
		g, ok := got.(string)
		return ok && g == w
	case bool:
		g, ok := got.(bool)
		return ok && g == w
	case json.Number:
		g, ok := got.(json.Number)
		if !ok {
			return false
		}
		gi, err1 := g.Int64()
		wi, err2 := w.Int64()
		return err1 == nil && err2 == nil && gi == wi
	}
	return false
}

// memberCountMatches evaluates room_member_count's "is": an optional
// comparison operator, then an integer. No operator means equality.
func memberCountMatches(is string, count int) bool {
	op := "=="
	for _, candidate := range []string{"==", "<=", ">=", "<", ">"} {
		if rest, ok := strings.CutPrefix(is, candidate); ok {
			op, is = candidate, rest
			break
		}
	}
	n, err := strconv.Atoi(is)
	if err != nil {
		return false
	}
	switch op {
	case "==":
		return count == n
	case "<":
		return count < n
	case ">":
		return count > n
	case "<=":
		return count <= n
	case ">=":
		return count >= n
	}
	return false
}

// Glob matching.
//
// A pattern matches case-insensitively, with `*` for any run of characters
// and `?` for one. It normally has to match the whole value. Against
// content.body it instead has to match a run of whole words somewhere in
// it: a body is prose, and the rule that looks for "alice" should fire on
// "hi alice!" and not on "malice".

// wordChar is the class a word boundary is the edge of: letters and digits
// in any script, and the underscore, as Python's \w — and so Synapse —
// has it.
const wordChar = `\p{L}\p{N}_`

var (
	globCacheMu sync.Mutex
	globCache   = map[string]*regexp.Regexp{}
)

// maxGlobCache bounds the compiled pattern cache. Patterns come from
// users, so an unbounded cache would be a way to fill the server's memory;
// past the bound it starts again, which costs a compile per pattern.
const maxGlobCache = 4096

func globMatches(pattern, value string, words bool) bool {
	re := compileGlob(pattern, words)
	return re != nil && re.MatchString(value)
}

func compileGlob(pattern string, words bool) *regexp.Regexp {
	key := "e\x00" + pattern
	if words {
		key = "w\x00" + pattern
	}

	globCacheMu.Lock()
	re, ok := globCache[key]
	globCacheMu.Unlock()
	if ok {
		return re
	}

	var b strings.Builder
	for _, r := range pattern {
		switch r {
		case '*':
			b.WriteString(".*?")
		case '?':
			b.WriteString(".")
		default:
			b.WriteString(regexp.QuoteMeta(string(r)))
		}
	}
	re = boundaryRegexp(b.String(), words)

	globCacheMu.Lock()
	if len(globCache) >= maxGlobCache {
		globCache = map[string]*regexp.Regexp{}
	}
	globCache[key] = re
	globCacheMu.Unlock()
	return re
}

// literalMatches is a word match of a literal string, which is what
// contains_display_name asks: a display name is not a pattern, and one
// called "*" must not match every message.
func literalMatches(literal, value string) bool {
	re := boundaryRegexp(regexp.QuoteMeta(literal), true)
	return re != nil && re.MatchString(value)
}

func boundaryRegexp(expr string, words bool) *regexp.Regexp {
	if words {
		expr = `(?:^|[^` + wordChar + `])(?:` + expr + `)(?:[^` + wordChar + `]|$)`
	} else {
		expr = `^(?:` + expr + `)$`
	}
	re, err := regexp.Compile(`(?is)` + expr)
	if err != nil {
		return nil
	}
	return re
}