fix: adjust checks in ForthMachine to prevent segfault when num_items is negative

awkward-cpp/src/libawkward/forth/ForthMachine.cpp

@@ -3075,7 +3075,7 @@ namespace awkward {
             uint64_t tmp;
             uint8_t tmpbyte;
 
-            if (items_remaining != 0) {
+            if (items_remaining > 0) {
               tmpbyte = input->read_byte(current_error_);
               if (current_error_ != util::ForthError::none) {
                 return;
@@ -3087,7 +3087,7 @@ namespace awkward {
               }
               data = tmp;
             }
-            while (items_remaining != 0) {
+            while (items_remaining > 0) {
               if (bits_wnd_r >= 8) {
                 bits_wnd_r -= 8;
                 bits_wnd_l -= 8;
@@ -3230,6 +3230,7 @@ namespace awkward {
                 break;                                                         \
               }
 
+            if (num_items < 0) num_items = 0;
             switch (format) {
               case READ_BOOL:    WRITE_DIRECTLY(bool, bool)
               case READ_INT8:    WRITE_DIRECTLY(int8_t, int8)
@@ -3311,6 +3312,7 @@ namespace awkward {
                 break;                                                         \
               }
 
+            if (num_items < 0) num_items = 0;
             switch (format) {
               case READ_BOOL:    WRITE_TO_STACK(bool)
               case READ_INT8:    WRITE_TO_STACK(int8_t)

docs/reference/awkwardforth.rst

@@ -1219,6 +1219,20 @@ and
 
 but the second is faster because it involves two Forth instructions and one ``memcpy``.
 
+If the number of items to read is negative then it is interpreted as zero.
+
+.. code-block:: python
+
+    >>> vm = ForthMachine32("""
+    ... input x
+    ... output y float32
+    ... 
+    ... -1000000 x #d-> y
+    ... """)
+    >>> vm.run({"x": np.arange(1000000) * 1.1})
+    >>> np.asarray(vm["y"])
+    array([], dtype=float32)
+
 Type codes
 """"""""""
 

tests/test_3209_awkwardforth_read_negative_number_of_items.py

@@ -0,0 +1,45 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+
+from __future__ import annotations
+
+import numpy as np
+
+import awkward as ak
+
+
+def test_read_negative_number_of_items():
+    vm = ak.forth.ForthMachine32("input source -5 source #q-> stack")
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.stack == []
+
+    vm = ak.forth.ForthMachine32("input source output sink float64 -5 source #q-> sink")
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.output("sink").tolist() == []
+
+
+def test_read_negative_and_positive_number_of_items():
+    vm = ak.forth.ForthMachine32(
+        "input source -5 source #q-> stack 5 source #q-> stack"
+    )
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.stack == [1, 2, 3, 4, 5]
+
+    vm = ak.forth.ForthMachine32(
+        "input source output sink float64 -5 source #q-> sink 5 source #q-> sink"
+    )
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.output("sink").tolist() == [1, 2, 3, 4, 5]
+
+
+def test_read_positive_and_negative_number_of_items():
+    vm = ak.forth.ForthMachine32(
+        "input source 5 source #q-> stack -5 source #q-> stack"
+    )
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.stack == [1, 2, 3, 4, 5]
+
+    vm = ak.forth.ForthMachine32(
+        "input source output sink float64 5 source #q-> sink -5 source #q-> sink"
+    )
+    vm.run({"source": np.array([1, 2, 3, 4, 5], dtype=np.int64)})
+    assert vm.output("sink").tolist() == [1, 2, 3, 4, 5]