channeldb/invoices: extend invoice queries to allow backwards pagination

channeldb/invoices.go

@@ -402,15 +402,19 @@ func (d *DB) FetchAllInvoices(pendingOnly bool) ([]Invoice, error) {
 type InvoiceQuery struct {
 	// IndexOffset is the offset within the add indices to start at. This
 	// can be used to start the response at a particular invoice.
-	IndexOffset uint32
+	IndexOffset uint64
 
 	// NumMaxInvoices is the maximum number of invoices that should be
 	// starting from the add index.
-	NumMaxInvoices uint32
+	NumMaxInvoices uint64
 
 	// PendingOnly, if set, returns unsettled invoices starting from the
 	// add index.
 	PendingOnly bool
+
+	// Reversed, if set, indicates that the invoices returned should start
+	// from the IndexOffset and go backwards.
+	Reversed bool
 }
 
 // InvoiceSlice is the response to a invoice query. It includes the original
@@ -423,13 +427,19 @@ type InvoiceSlice struct {
 	InvoiceQuery
 
 	// Invoices is the set of invoices that matched the query above.
-	Invoices []*Invoice
+	Invoices []Invoice
+
+	// FirstIndexOffset is the index of the first element in the set of
+	// returned Invoices above. Callers can use this to resume their query
+	// in the event that the slice has too many events to fit into a single
+	// response.
+	FirstIndexOffset uint64
 
 	// LastIndexOffset is the index of the last element in the set of
 	// returned Invoices above. Callers can use this to resume their query
-	// in the event that the time slice has too many events to fit into a
-	// single response.
-	LastIndexOffset uint32
+	// in the event that the slice has too many events to fit into a single
+	// response.
+	LastIndexOffset uint64
 }
 
 // QueryInvoices allows a caller to query the invoice database for invoices
@@ -439,12 +449,6 @@ func (d *DB) QueryInvoices(q InvoiceQuery) (InvoiceSlice, error) {
 		InvoiceQuery: q,
 	}
 
-	// If the caller provided an index offset, then we'll not know how many
-	// records we need to skip. We'll also keep track of the record offset
-	// as that's part of the final return value.
-	invoicesToSkip := q.IndexOffset
-	invoiceOffset := q.IndexOffset
-
 	err := d.View(func(tx *bolt.Tx) error {
 		// If the bucket wasn't found, then there aren't any invoices
 		// within the database yet, so we can simply exit.
@@ -452,38 +456,50 @@ func (d *DB) QueryInvoices(q InvoiceQuery) (InvoiceSlice, error) {
 		if invoices == nil {
 			return ErrNoInvoicesCreated
 		}
-		invoiceAddedIndex := invoices.Bucket(addIndexBucket)
-		if invoiceAddedIndex == nil {
+		invoiceAddIndex := invoices.Bucket(addIndexBucket)
+		if invoiceAddIndex == nil {
 			return ErrNoInvoicesCreated
 		}
 
-		// We'll be using a cursor to seek into the database, so we'll
-		// populate byte slices that represent the start of the key
-		// space we're interested in.
-		var startIndex [8]byte
-		switch q.PendingOnly {
-		case true:
-			// We have to start from the beginning so we know
-			// how many pending invoices we're skipping.
-			byteOrder.PutUint64(startIndex[:], uint64(1))
-		default:
-			// We can seek right to the invoice offset we want
-			// to start with.
-			invoicesToSkip = 0
-			byteOrder.PutUint64(startIndex[:], uint64(invoiceOffset+1))
+		// keyForIndex is a helper closure that retrieves the invoice
+		// key for the given add index of an invoice.
+		keyForIndex := func(c *bolt.Cursor, index uint64) []byte {
+			var keyIndex [8]byte
+			byteOrder.PutUint64(keyIndex[:], index)
+			_, invoiceKey := c.Seek(keyIndex[:])
+			return invoiceKey
+		}
+
+		// nextKey is a helper closure to determine what the next
+		// invoice key is when iterating over the invoice add index.
+		nextKey := func(c *bolt.Cursor) ([]byte, []byte) {
+			if q.Reversed {
+				return c.Prev()
+			}
+			return c.Next()
+		}
+
+		// We'll be using a cursor to seek into the database and return
+		// a slice of invoices. We'll need to determine where to start
+		// our cursor depending on the parameters set within the query.
+		c := invoiceAddIndex.Cursor()
+		invoiceKey := keyForIndex(c, q.IndexOffset+1)
+		if q.Reversed {
+			_, invoiceKey = c.Last()
+			if q.IndexOffset != 0 {
+				invoiceKey = keyForIndex(c, q.IndexOffset-1)
+			}
 		}
 
 		// If we know that a set of invoices exists, then we'll begin
 		// our seek through the bucket in order to satisfy the query.
-		// We'll continue until either we reach the end of the range,
-		// or reach our max number of events.
-		cursor := invoiceAddedIndex.Cursor()
-		_, invoiceKey := cursor.Seek(startIndex[:])
-		for ; invoiceKey != nil; _, invoiceKey = cursor.Next() {
+		// We'll continue until either we reach the end of the range, or
+		// reach our max number of invoices.
+		for ; invoiceKey != nil; _, invoiceKey = nextKey(c) {
 			// If our current return payload exceeds the max number
 			// of invoices, then we'll exit now.
-			if uint32(len(resp.Invoices)) >= q.NumMaxInvoices {
-				return nil
+			if uint64(len(resp.Invoices)) >= q.NumMaxInvoices {
+				break
 			}
 
 			invoice, err := fetchInvoice(invoiceKey, invoices)
@@ -496,17 +512,22 @@ func (d *DB) QueryInvoices(q InvoiceQuery) (InvoiceSlice, error) {
 			if q.PendingOnly && invoice.Terms.Settled {
 				continue
 			}
-			// If we're not yet past the user defined offset, then
-			// we'll continue to seek forward.
-			if invoicesToSkip > 0 {
-				invoicesToSkip--
-				continue
-			}
 
 			// At this point, we've exhausted the offset, so we'll
 			// begin collecting invoices found within the range.
-			resp.Invoices = append(resp.Invoices, &invoice)
-			invoiceOffset++
+			resp.Invoices = append(resp.Invoices, invoice)
+		}
+
+		// If we iterated through the add index in reverse order, then
+		// we'll need to reverse the slice of invoices to return them in
+		// forward order.
+		if q.Reversed {
+			numInvoices := len(resp.Invoices)
+			for i := 0; i < numInvoices/2; i++ {
+				opposite := numInvoices - i - 1
+				resp.Invoices[i], resp.Invoices[opposite] =
+					resp.Invoices[opposite], resp.Invoices[i]
+			}
 		}
 
 		return nil
@@ -515,9 +536,12 @@ func (d *DB) QueryInvoices(q InvoiceQuery) (InvoiceSlice, error) {
 		return resp, err
 	}
 
-	// Finally, record the index of the last invoice added so that the
-	// caller can resume from this point later on.
-	resp.LastIndexOffset = invoiceOffset
+	// Finally, record the indexes of the first and last invoices returned
+	// so that the caller can resume from this point later on.
+	if len(resp.Invoices) > 0 {
+		resp.FirstIndexOffset = resp.Invoices[0].AddIndex
+		resp.LastIndexOffset = resp.Invoices[len(resp.Invoices)-1].AddIndex
+	}
 
 	return resp, nil
 }