Add unix epoch timestamp transform

Author: Kim Ngo

README.md

@@ -227,6 +227,8 @@ syntax. **Note**: this operation is done in-place. If you want to preserve the
 original string(s), pair the transform with `shift`. This transform also 
 supports the `$now` operator for `inputFormat`, which will set the current 
 timestamp at the specified path, formatted according to the `outputFormat`.
+`$unix` is supported for both input and output formats as a Unix time, the
+number of seconds elapsed since January 1, 1970 UTC as an integer string.
 ```javascript
 {
   "operation": "timestamp",
@@ -237,7 +239,11 @@ timestamp at the specified path, formatted according to the `outputFormat`.
   "nowTimestamp": {
     "inputFormat": "$now",
     "outputFormat": "2006-01-02T15:04:05-0700"
-    } 
+  },
+  "epochTimestamp": {
+    "inputFormat": "2006-01-02T15:04:05-0700",
+    "outputFormat": "$unix"
+  }
 }
 
 ```

transform/timestamp.go

@@ -13,6 +13,8 @@ import (
 // This is necessary for testing purposes
 var now = time.Now
 
+const unixFormat = "$unix"
+
 // Timestamp parses and formats timestamp strings using the golang syntax
 func Timestamp(spec *Config, data []byte) ([]byte, error) {
 	for k, v := range *spec.Spec {
@@ -96,10 +98,29 @@ func Timestamp(spec *Config, data []byte) ([]byte, error) {
 
 // parseAndFormatValue generates a properly formatted timestamp
 func parseAndFormatValue(inputFormat, outputFormat, unformattedItem string) (string, error) {
-	parsedItem, err := time.Parse(inputFormat, unformattedItem)
-	if err != nil {
-		return "", err
+	var (
+		parsedItem    time.Time
+		formattedItem string
+		err           error
+	)
+
+	if inputFormat == unixFormat {
+		i, err := strconv.ParseInt(unformattedItem, 10, 64)
+		if err != nil {
+			return "", err
+		}
+		parsedItem = time.Unix(i, 0)
+	} else {
+		parsedItem, err = time.Parse(inputFormat, unformattedItem)
+		if err != nil {
+			return "", err
+		}
+	}
+
+	if outputFormat == unixFormat {
+		formattedItem = strconv.FormatInt(parsedItem.Unix(), 10)
+	} else {
+		formattedItem = parsedItem.Format(outputFormat)
 	}
-	formattedItem := strings.Join([]string{"\"", parsedItem.Format(outputFormat), "\""}, "")
-	return formattedItem, nil
+	return strings.Join([]string{"\"", formattedItem, "\""}, ""), nil
 }

transform/timestamp_test.go

@@ -169,6 +169,7 @@ func TestParseAndFormatValue(t *testing.T) {
 		{time.UnixDate, "\"Fri Jul 21 08:15:27 +0100 2017\""},
 		{time.RFC3339, "\"2017-07-21T08:15:27+01:00\""},
 		{time.StampNano, "\"Jul 21 08:15:27.000000000\""},
+		{"$unix", "\"1500621327\""},
 	}
 	for _, testItem := range parseAndFormatTests {
 		actual, _ := parseAndFormatValue(inputFormat, testItem.outputFormat, inputTimestamp)
@@ -179,3 +180,27 @@ func TestParseAndFormatValue(t *testing.T) {
 		}
 	}
 }
+
+func TestParseAndFormatValueOutputUnix(t *testing.T) {
+	parseAndFormatTests := []struct {
+		inputFormat    string
+		inputTimestamp string
+		expectedOutput string
+	}{
+		// test against a sampling of common formats
+		{"2006-01-02T15:04:05-0700", "2017-07-21T08:15:27+0100", "\"1500621327\""},
+		{"January _2, 2006", "July 21, 2017", "\"1500595200\""},
+		{time.ANSIC, "Fri Jul 21 08:15:27 2017", "\"1500624927\""},
+		{time.UnixDate, "Fri Jul 21 08:15:27 GMT 2017", "\"1500624927\""},
+		{time.RFC3339, "2017-07-21T08:15:27+01:00", "\"1500621327\""},
+		{"$unix", "1500621327", "\"1500621327\""},
+	}
+	for _, testItem := range parseAndFormatTests {
+		actual, _ := parseAndFormatValue(testItem.inputFormat, "$unix", testItem.inputTimestamp)
+		if actual != testItem.expectedOutput {
+			t.Error("Error data does not match expectation.", testItem.inputFormat)
+			t.Log("Expected:   ", testItem.expectedOutput)
+			t.Log("Actual:     ", actual)
+		}
+	}
+}