如何在pyspark中从与上一年相同的列中减去行值？

from pyspark.sql import SparkSession
from pyspark.sql.window import Window
from pyspark.sql.functions import lag, col

spark = SparkSession.builder.getOrCreate()

data = [(2019, 100), (2019, 200), (2020, 300), (2020, 400), (2021, 500)]
df = spark.createDataFrame(data, ["year", "value"])
df.show()

+----+-----+
|year|value|
+----+-----+
|2019|  100|
|2019|  200|
|2020|  300|
|2020|  400|
|2021|  500|
+----+-----+

windowSpec = Window.partitionBy("year").orderBy(col("year").desc())

df = df.withColumn("previous_value", lag("value").over(windowSpec))
df.show()

+----+-----+--------------+
|year|value|previous_value|
+----+-----+--------------+
|2021|  500|          null|
|2020|  400|          null|
|2020|  300|           400|
|2019|  200|          null|
|2019|  100|           200|
+----+-----+--------------+

df = df.withColumn("difference", col("value") - col("previous_value"))
df.show()

+----+-----+--------------+----------+
|year|value|previous_value|difference|
+----+-----+--------------+----------+
|2021|  500|          null|      null|
|2020|  400|          null|      null|
|2020|  300|           400|      -100|
|2019|  200|          null|      null|
|2019|  100|           200|      -100|
+----+-----+--------------+----------+