add push operation

README.md

@@ -41,6 +41,7 @@ Available operations:
 | del(path)  | delete a key  |
 | merge(path, value)  | merge json value into the path. Only JSON values are allowed  |
 | pop(path)  | remove last element from an array  |
+| push(path, value)  | add new element to an array  |
 
 ## Examples:
 

pkg/operations/parse.go

@@ -35,6 +35,7 @@ var operations = map[string]Operation{
 	"del":   Delete,
 	"merge": Merge,
 	"pop":   Pop,
+	"push":  Push,
 }
 
 //nolint:govet

pkg/operations/push.go

@@ -0,0 +1,34 @@
+package operations
+
+import (
+	"github.com/can3p/sackmesser/pkg/traverse/types"
+	"github.com/pkg/errors"
+)
+
+func Push(root types.Node, path []types.PathElement, args ...any) error {
+	if len(args) != 1 {
+		return errors.Errorf("set operation expects one argument")
+	}
+
+	value := args[0]
+	node, lastChunk, err := traverseButOne(root, path)
+
+	if err != nil {
+		return err
+	}
+
+	val, err := node.GetField(lastChunk)
+
+	if err != nil {
+		return err
+	}
+
+	typed, ok := val.([]any)
+	if !ok {
+		return types.ErrWrongVisit
+	}
+
+	typed = append(typed, value)
+
+	return node.SetField(lastChunk, typed)
+}

pkg/operations/push_test.go

@@ -0,0 +1,52 @@
+package operations
+
+import (
+	"testing"
+
+	"github.com/alecthomas/assert/v2"
+	"github.com/can3p/sackmesser/pkg/traverse/simplejson"
+	"github.com/can3p/sackmesser/pkg/traverse/types"
+)
+
+func TestPushOperation(t *testing.T) {
+	jstr := `{ "abc": [ 1, 2, 3 ] }`
+
+	examples := []struct {
+		description string
+		path        []types.PathElement
+		expected    string
+		arg         any
+		isErr       bool
+	}{
+		{
+			description: "push array item",
+			path:        testPath("abc"),
+			arg:         true,
+			expected:    `{ "abc": [ 1, 2, 3, true ]}`,
+		},
+		{
+			description: "push into scalar",
+			path:        testPath("abc", 0),
+			arg:         true,
+			isErr:       true,
+		},
+	}
+
+	for idx, ex := range examples {
+		node := simplejson.MustParse([]byte(jstr))
+
+		err := Push(node, ex.path, ex.arg)
+
+		if ex.isErr {
+			assert.Error(t, err, "[Ex %d - %s]", idx+1, ex.description)
+			continue
+		} else {
+			assert.NoError(t, err, "[Ex %d - %s]", idx+1, ex.description)
+		}
+
+		expected := simplejson.MustParse([]byte(ex.expected))
+
+		assert.Equal(t, expected.Value(), node.Value(), "[Ex %d - %s]", idx+1, ex.description)
+	}
+
+}