Improve performance by caching java's output

.gitignore

@@ -11,3 +11,4 @@ build/
 mjtest.egg-info
 .preprocessed
 .DS_Store
+.java_output
\ No newline at end of file

mjtest/test/exec_tests.py

@@ -2,7 +2,6 @@ import logging
 import os
 import shutil
 from os import path
-import sys
 from mjtest.environment import TestMode, Environment
 from mjtest.test.syntax_tests import BasicSyntaxTest
 from mjtest.test.tests import TestCase, BasicDiffTestResult, BasicTestResult
@@ -22,7 +21,16 @@ class JavaExecTest(BasicSyntaxTest):
     def __init__(self, env: Environment, type: str, file: str, preprocessed_file: str):
         super().__init__(env, type, file, preprocessed_file)
         self._expected_output_file = file + self.OUTPUT_FILE_ENDING
+        prev_out_dir = path.join(path.dirname(file), ".java_output")
+        if not path.exists(prev_out_dir):
+            os.mkdir(prev_out_dir)
+        self._prev_out_file = path.join(prev_out_dir, path.basename(self._expected_output_file))
         self._has_expected_output_file = path.exists(self._expected_output_file)
+        if not self._has_expected_output_file:
+            if path.exists(self._prev_out_file) and self.compare_files_by_age(file, self._prev_out_file) == -1:
+                self._has_expected_output_file = True
+                self._expected_output_file = self._prev_out_file
+                _LOG.info("Reuse old java output file \"{}\"".format(path.relpath(self._prev_out_file)))
         self._should_succeed = True
 
     def run(self) -> BasicDiffTestResult:
@@ -43,6 +51,9 @@ class JavaExecTest(BasicSyntaxTest):
             exp_out, _, _ = \
                 self.env.run_command("java", base_filename)
             exp_out = exp_out.decode().strip()
+            with open(self._prev_out_file, "w") as f:
+                f.write(exp_out)
+                f.flush()
         if self._has_expected_output_file and self.type == self.MODE and self.env.mode == self.MODE:
             with open(self._expected_output_file, "r") as f:
                 exp_out = f.read()
@@ -62,4 +73,18 @@ class JavaExecTest(BasicSyntaxTest):
             os.chdir(cwd)
             raise
 
+    def get_file_time(self, file: str) -> float:
+        return max(os.path.getmtime(file), os.path.getatime(file))
+
+    def compare_files_by_age(self, file: str, other_file: str) -> int:
+        """
+        :return: -1 == file is older, 0 == almost equal ages, 1 == other file is older
+        """
+        diff = self.get_file_time(file) - self.get_file_time(other_file)
+        if diff < 60:
+            return -1
+        if diff > -60:
+            return 1
+        return 0
+
 TestCase.TEST_CASE_CLASSES[TestMode.comile_firm].append(JavaExecTest)
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