🐐 String indexing and slicing

tests.py

@@ -639,6 +639,10 @@ error_tests = [
   ("I * \"hello\"", CentvrionError),                                   # multiplication with string
   ("\"hello\" MINVS \"world\"", CentvrionError),                       # comparison with strings
   ("I[I]", CentvrionError),                                           # indexing a non-array
+  ('"SALVTE"[VII]', CentvrionError),                                   # string index out of range
+  ('"SALVTE"[NVLLVS]', CentvrionError),                                # string index with non-integer
+  ('"SALVTE"[II VSQVE VII]', CentvrionError),                          # string slice out of range
+  ('"SALVTE"[III VSQVE II]', CentvrionError),                          # string slice from > to
   ("DESIGNA x VT I\nDESIGNA x[I] VT II", CentvrionError),             # index-assign to non-array
   ("SEMEN(I)", CentvrionError),                                        # requires FORS module
   ('CVM FORS\nSEMEN("abc")', CentvrionError),                          # SEMEN requires integer seed
@@ -1583,6 +1587,88 @@ class TestArraySlice(unittest.TestCase):
     run_test(self, source, nodes, value)
 
 
+# --- String indexing ---
+
+string_index_tests = [
+  # first character
+  ('"SALVTE"[I]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("I")))]),
+    ValStr("S")),
+  # last character
+  ('"SALVTE"[VI]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("VI")))]),
+    ValStr("E")),
+  # middle character
+  ('"SALVTE"[III]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("III")))]),
+    ValStr("L")),
+  # string index via variable
+  ('DESIGNA s VT "SALVTE"\ns[II]',
+    Program([], [
+      Designa(ID("s"), String("SALVTE")),
+      ExpressionStatement(ArrayIndex(ID("s"), Numeral("II"))),
+    ]),
+    ValStr("A")),
+  # expression as index
+  ('"SALVTE"[I + II]',
+    Program([], [ExpressionStatement(ArrayIndex(
+      String("SALVTE"),
+      BinOp(Numeral("I"), Numeral("II"), "SYMBOL_PLUS")))]),
+    ValStr("L")),
+]
+
+class TestStringIndex(unittest.TestCase):
+  @parameterized.expand(string_index_tests)
+  def test_string_index(self, source, nodes, value):
+    run_test(self, source, nodes, value)
+
+
+# --- String slicing ---
+
+string_slice_tests = [
+  # substring from middle
+  ('"SALVTE"[II VSQVE IV]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("II"), Numeral("IV")))]),
+    ValStr("ALV")),
+  # full string slice
+  ('"SALVTE"[I VSQVE VI]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("I"), Numeral("VI")))]),
+    ValStr("SALVTE")),
+  # single-char slice
+  ('"SALVTE"[III VSQVE III]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("III"), Numeral("III")))]),
+    ValStr("L")),
+  # slice on variable
+  ('DESIGNA s VT "SALVTE"\ns[II VSQVE IV]',
+    Program([], [
+      Designa(ID("s"), String("SALVTE")),
+      ExpressionStatement(ArraySlice(ID("s"), Numeral("II"), Numeral("IV"))),
+    ]),
+    ValStr("ALV")),
+  # chaining: slice then index
+  ('"SALVTE"[I VSQVE III][II]',
+    Program([], [ExpressionStatement(ArrayIndex(
+      ArraySlice(String("SALVTE"), Numeral("I"), Numeral("III")),
+      Numeral("II")))]),
+    ValStr("A")),
+  # expression as slice bounds
+  ('"SALVTE"[I + I VSQVE II + II]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"),
+      BinOp(Numeral("I"), Numeral("I"), "SYMBOL_PLUS"),
+      BinOp(Numeral("II"), Numeral("II"), "SYMBOL_PLUS")))]),
+    ValStr("ALV")),
+]
+
+class TestStringSlice(unittest.TestCase):
+  @parameterized.expand(string_slice_tests)
+  def test_string_slice(self, source, nodes, value):
+    run_test(self, source, nodes, value)
+
+
 # --- Comments ---
 
 comment_tests = [