feat(Query): Add random keyword in DQL (#7693)

This change implements the `random` function argument.
If an argument `random: k` is provided to a dql function then 
random k results will be returned.
For example:
`func(has(name), random:5)` will choose 5 random nodes having
the name predicate.

gql/parser.go

@@ -2595,7 +2595,7 @@ func parseLanguageList(it *lex.ItemIterator) ([]string, error) {
 
 func validKeyAtRoot(k string) bool {
 	switch k {
-	case "func", "orderasc", "orderdesc", "first", "offset", "after":
+	case "func", "orderasc", "orderdesc", "first", "offset", "after", "random":
 		return true
 	case "from", "to", "numpaths", "minweight", "maxweight":
 		// Specific to shortest path
@@ -2609,7 +2609,7 @@ func validKeyAtRoot(k string) bool {
 // Check for validity of key at non-root nodes.
 func validKey(k string) bool {
 	switch k {
-	case "orderasc", "orderdesc", "first", "offset", "after":
+	case "orderasc", "orderdesc", "first", "offset", "after", "random":
 		return true
 	}
 	return false

query/query.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"fmt"
 	"math"
+	"math/rand"
 	"sort"
 	"strconv"
 	"strings"
@@ -114,6 +115,8 @@ type params struct {
 	Count int
 	// Offset is the value of the "offset" parameter.
 	Offset int
+	// Random is the value of the "random" parameter
+	Random int
 	// AfterUID is the value of the "after" parameter.
 	AfterUID uint64
 	// DoCount is true if the count of the predicate is requested instead of its value.
@@ -745,6 +748,15 @@ func (args *params) fill(gq *gql.GraphQuery) error {
 		}
 		args.Count = int(first)
 	}
+
+	if v, ok := gq.Args["random"]; ok {
+		random, err := strconv.ParseInt(v, 0, 32)
+		if err != nil {
+			return err
+		}
+		args.Random = int(random)
+	}
+
 	return nil
 }
 
@@ -2307,6 +2319,13 @@ func ProcessGraph(ctx context.Context, sg, parent *SubGraph, rch chan error) {
 		}
 	}
 
+	if sg.Params.Random > 0 {
+		if err = sg.applyRandom(ctx); err != nil {
+			rch <- err
+			return
+		}
+	}
+
 	// Here we consider handling count with filtering. We do this after
 	// pagination because otherwise, we need to do the count with pagination
 	// taken into account. For example, a PL might have only 50 entries but the
@@ -2404,6 +2423,43 @@ func ProcessGraph(ctx context.Context, sg, parent *SubGraph, rch chan error) {
 	rch <- childErr
 }
 
+// stores index of a uid as the index in the uidMatrix (x)
+// and index in the corresponding list of the uidMatrix (y)
+type UidKey struct {
+	x int
+	y int
+}
+
+// applies "random" to lists inside uidMatrix
+// sg.Params.Random number of nodes are selected in each uid list
+// duplicates are avoided (random selection without replacement)
+// if sg.Params.Random is more than the number of available nodes
+// all nodes are returned
+func (sg *SubGraph) applyRandom(ctx context.Context) error {
+	sg.updateUidMatrix()
+
+	for i := 0; i < len(sg.uidMatrix); i++ {
+		// shuffle the uid list and select the
+		// first sg.Params.Random uids
+
+		uidList := sg.uidMatrix[i].Uids
+
+		rand.Shuffle(len(uidList), func(i, j int) {
+			uidList[i], uidList[j] = uidList[j], uidList[i]
+		})
+
+		numRandom := sg.Params.Random
+		if sg.Params.Random > len(uidList) {
+			numRandom = len(uidList)
+		}
+
+		sg.uidMatrix[i].Uids = uidList[:numRandom]
+	}
+
+	sg.DestMap = codec.Merge(sg.uidMatrix)
+	return nil
+}
+
 // applyPagination applies count and offset to lists inside uidMatrix.
 func (sg *SubGraph) applyPagination(ctx context.Context) error {
 	if sg.Params.Count == 0 && sg.Params.Offset == 0 { // No pagination.
@@ -2647,7 +2703,7 @@ func (sg *SubGraph) sortAndPaginateUsingVar(ctx context.Context) error {
 func isValidArg(a string) bool {
 	switch a {
 	case "numpaths", "from", "to", "orderasc", "orderdesc", "first", "offset", "after", "depth",
-		"minweight", "maxweight":
+		"minweight", "maxweight", "random":
 		return true
 	}
 	return false