Add RequestLimit (#234)

* support for RequestLimit in Count

* more docs

encoding.go

@@ -16,15 +16,13 @@ var typeCache sync.Map // unmarshalKey → *typedef
 type typedef struct {
 	decoders map[unmarshalKey]decodeFunc
 	fields   []structField
-	root     reflect.Type
 	info     *structInfo
 }
 
 func newTypedef(rt reflect.Type) (*typedef, error) {
 	def := &typedef{
 		decoders: make(map[unmarshalKey]decodeFunc),
 		// encoders: make(map[encodeKey]encodeFunc),
-		root: rt,
 	}
 	err := def.init(rt)
 	return def, err

query.go

@@ -185,10 +185,11 @@ func (q *Query) SearchLimit(limit int64) *Query {
 }
 
 // RequestLimit specifies the maximum amount of requests to make against DynamoDB's API.
-// func (q *Query) RequestLimit(limit int) *Query {
-// 	q.reqLimit = limit
-// 	return q
-// }
+// A limit of zero or less means unlimited requests.
+func (q *Query) RequestLimit(limit int) *Query {
+	q.reqLimit = limit
+	return q
+}
 
 // Order specifies the desired result order.
 // Requires a range key (a.k.a. partition key) to be specified.
@@ -286,22 +287,29 @@ func (q *Query) CountWithContext(ctx context.Context) (int64, error) {
 		return 0, q.err
 	}
 
-	var count int64
+	var count, scanned int64
+	var reqs int
 	var res *dynamodb.QueryOutput
 	for {
-		req := q.queryInput()
-		req.Select = selectCount
+		input := q.queryInput()
+		input.Select = selectCount
 
 		err := q.table.db.retry(ctx, func() error {
 			var err error
-			res, err = q.table.db.client.QueryWithContext(ctx, req)
+			res, err = q.table.db.client.QueryWithContext(ctx, input)
 			if err != nil {
 				return err
 			}
+			reqs++
+
 			if res.Count == nil {
-				return errors.New("nil count")
+				return errors.New("malformed DynamoDB response: count is nil")
 			}
 			count += *res.Count
+			if res.ScannedCount != nil {
+				scanned += *res.ScannedCount
+			}
+
 			return nil
 		})
 		if err != nil {
@@ -312,7 +320,10 @@ func (q *Query) CountWithContext(ctx context.Context) (int64, error) {
 		}
 
 		q.startKey = res.LastEvaluatedKey
-		if res.LastEvaluatedKey == nil || q.searchLimit > 0 {
+		if res.LastEvaluatedKey == nil ||
+			(q.limit > 0 && count >= q.limit) ||
+			(q.searchLimit > 0 && scanned >= q.searchLimit) ||
+			(q.reqLimit > 0 && reqs >= q.reqLimit) {
 			break
 		}
 	}

reflect.go

@@ -193,11 +193,11 @@ type encodeKey struct {
 
 type structInfo struct {
 	root   reflect.Type
+	parent *structInfo
 	fields map[string]*structField // by name
 	refs   map[encodeKey][]*structField
 	types  map[encodeKey]encodeFunc
 	zeros  map[reflect.Type]func(reflect.Value) bool
-	parent *structInfo
 
 	seen  map[encodeKey]struct{}
 	queue []encodeKey

scan.go

@@ -2,6 +2,7 @@ package dynamo
 
 import (
 	"context"
+	"errors"
 	"strings"
 	"sync"
 
@@ -131,10 +132,11 @@ func (s *Scan) SearchLimit(limit int64) *Scan {
 }
 
 // RequestLimit specifies the maximum amount of requests to make against DynamoDB's API.
-// func (s *Scan) RequestLimit(limit int) *Scan {
-// 	s.reqLimit = limit
-// 	return s
-// }
+// A limit of zero or less means unlimited requests.
+func (s *Scan) RequestLimit(limit int) *Scan {
+	s.reqLimit = limit
+	return s
+}
 
 // ConsumedCapacity will measure the throughput capacity consumed by this operation and add it to cc.
 func (s *Scan) ConsumedCapacity(cc *ConsumedCapacity) *Scan {
@@ -260,6 +262,7 @@ func (s *Scan) CountWithContext(ctx context.Context) (int64, error) {
 	var count, scanned int64
 	input := s.scanInput()
 	input.Select = aws.String(dynamodb.SelectCount)
+	var reqs int
 	for {
 		var out *dynamodb.ScanOutput
 		err := s.table.db.retry(ctx, func() error {
@@ -268,23 +271,26 @@ func (s *Scan) CountWithContext(ctx context.Context) (int64, error) {
 			return err
 		})
 		if err != nil {
-			return count, err
+			return 0, err
 		}
+		reqs++
 
+		if out.Count == nil {
+			return count, errors.New("malformed DynamoDB outponse: count is nil")
+		}
 		count += *out.Count
-		scanned += *out.ScannedCount
+		if out.ScannedCount != nil {
+			scanned += *out.ScannedCount
+		}
 
 		if s.cc != nil {
 			addConsumedCapacity(s.cc, out.ConsumedCapacity)
 		}
 
-		if s.limit > 0 && count >= s.limit {
-			break
-		}
-		if s.searchLimit > 0 && scanned >= s.searchLimit {
-			break
-		}
-		if out.LastEvaluatedKey == nil {
+		if out.LastEvaluatedKey == nil ||
+			(s.limit > 0 && count >= s.limit) ||
+			(s.searchLimit > 0 && scanned >= s.searchLimit) ||
+			(s.reqLimit > 0 && reqs >= s.reqLimit) {
 			break
 		}