perf(db): keyset pagination for revision prefixes

Keyset по id/ord вместо OFFSET; anti-join для RevisionDiff.

Co-authored-by: Cursor <[email protected]>

internal/store/memory_crud.go

@@ -1,8 +1,6 @@
 package store
 
 import (
-	"fmt"
-	"strconv"
 	"strings"
 	"time"
 
@@ -795,31 +793,37 @@ func (m *Memory) ListRevisionPrefixes(tenantID, revisionID string, cursor string
 	if limit <= 0 {
 		limit = 50
 	}
+	afterID, off, useOffset := ParsePrefixPageCursor(cursor)
 	m.mu.RLock()
 	defer m.mu.RUnlock()
 	if _, err := m.getRevisionLocked(tenantID, revisionID); err != nil {
 		return nil, "", false
 	}
-	all := m.revPrefixes[revisionID]
-	off := 0
-	if cursor != "" {
-		if n, err := strconv.Atoi(cursor); err == nil && n >= 0 {
-			off = n
-		}
+	allRows := m.revPrefixes[revisionID]
+	start := 0
+	if useOffset {
+		start = off
+	} else if afterID != nil {
+		start = int(*afterID) + 1
 	}
-	end := off + limit
-	next := ""
-	more := false
-	if end > len(all) {
-		end = len(all)
-	} else {
-		more = true
-		next = fmt.Sprintf("%d", end)
-	}
-	if off >= len(all) {
+	if start > len(allRows) {
 		return nil, "", false
 	}
-	return all[off:end], next, more
+	end := start + limit
+	next := ""
+	more := false
+	if end > len(allRows) {
+		end = len(allRows)
+	} else {
+		more = true
+		next = FormatPrefixPageCursor(int64(end - 1))
+	}
+	if start >= end {
+		return nil, "", false
+	}
+	out := make([]PrefixRow, end-start)
+	copy(out, allRows[start:end])
+	return out, next, more
 }
 
 func (m *Memory) ListGlobalSettings(tenantID string) (map[string]any, error) {

internal/store/pagination.go

@@ -0,0 +1,30 @@
+package store
+
+import (
+	"strconv"
+	"strings"
+)
+
+// ParsePrefixPageCursor decodes opaque cursors for revision prefix pagination.
+func ParsePrefixPageCursor(cursor string) (afterID *int64, offset int, useOffset bool) {
+	cursor = strings.TrimSpace(cursor)
+	if cursor == "" {
+		return nil, 0, false
+	}
+	if strings.HasPrefix(cursor, "o:") {
+		n, err := strconv.Atoi(strings.TrimPrefix(cursor, "o:"))
+		if err != nil || n < 0 {
+			return nil, 0, false
+		}
+		return nil, n, true
+	}
+	if n, err := strconv.ParseInt(cursor, 10, 64); err == nil && n >= 0 {
+		return &n, 0, false
+	}
+	return nil, 0, false
+}
+
+// FormatPrefixPageCursor encodes the keyset cursor (last row id or slice index).
+func FormatPrefixPageCursor(lastID int64) string {
+	return strconv.FormatInt(lastID, 10)
+}