Initial project import

tools/v/vlib/flag/flag_from_test.v

@@ -0,0 +1,55 @@
+import flag
+
+const some_args_1 = ['--mix', '-m', 'ok', '-d', 'one', '--test=abc', '-d', 'two', '/path/to/a',
+	'path/to/b']
+
+struct Config {
+	am       string   @[only: m]
+	def_test string = 'def'   @[long: test; short: t]
+	device   []string @[short: d]
+	paths    []string @[tail]
+mut:
+	amount   int = 1
+	mix      bool
+	mix_hard bool = true
+}
+
+fn test_using() {
+	mut config := Config{
+		mix_hard: false
+		amount:   8
+	}
+
+	config, _ = flag.using[Config](config, some_args_1)!
+	assert config.mix
+	assert config.mix_hard == false
+	assert config.am == 'ok'
+	assert config.def_test == 'abc'
+	assert config.device[0] == 'one'
+	assert config.device[1] == 'two'
+	assert config.amount == 8
+	assert config.paths.len == 2
+	assert config.paths[0] == '/path/to/a'
+	assert config.paths[1] == 'path/to/b'
+
+	config.mix = false // is changed to true via `--mix`
+	config.mix_hard = true // should be kept as `true`, since no flags changed it
+	config.amount = 888
+
+	config2, _ := flag.using[Config](config, some_args_1)!
+	assert config2.mix
+	assert config2.mix_hard
+	assert config2.am == 'ok'
+	assert config2.def_test == 'abc'
+	assert config2.device[0] == 'one'
+	assert config2.device[1] == 'two'
+	assert config2.device[2] == 'one' // `config` already had items pushed from `some_args_1` so this grows when using `using[T](struct,...)`
+	assert config2.device[3] == 'two'
+	assert config2.device.len == 4
+	assert config2.amount == 888
+	assert config2.paths.len == 4
+	assert config2.paths[0] == '/path/to/a'
+	assert config2.paths[1] == 'path/to/b'
+	assert config2.paths[2] == '/path/to/a'
+	assert config2.paths[3] == 'path/to/b'
+}