Add random function (#514)

cmd/genji/doc/functions.go

@@ -18,6 +18,7 @@ var builtinDocs = functionDocs{
 	"typeof":   "The typeof function returns the type of arg1.",
 	"len":      "The len function returns length of the arg1 expression if arg1 evals to string, array or document, either returns NULL.",
 	"coalesce": "The coalesce function returns the first non-null argument. NULL is returned if all arguments are null.",
+	"random":   "The random function returns a random number between math.MinInt64 and math.MaxInt64.",
 }
 
 var mathDocs = functionDocs{

internal/expr/functions/math.go

@@ -3,6 +3,7 @@ package functions
 import (
 	"fmt"
 	"math"
+	"math/rand"
 
 	"github.com/genjidb/genji/document"
 	"github.com/genjidb/genji/types"
@@ -22,6 +23,7 @@ var mathFunctions = Definitions{
 	"asinh":  asinh,
 	"atan":   atan,
 	"atan2":  atan2,
+	"random": random,
 }
 
 var floor = &ScalarDefinition{
@@ -164,3 +166,12 @@ var atan2 = &ScalarDefinition{
 		return types.NewDoubleValue(res), nil
 	},
 }
+
+var random = &ScalarDefinition{
+	name:  "random",
+	arity: 0,
+	callFn: func(args ...types.Value) (types.Value, error) {
+		randomNum := rand.Int63()
+		return types.NewIntegerValue(randomNum), nil
+	},
+}