core/irrt: add Slice and Range

Needed for implementing general ndarray indexing.

Currently the IRRT slice and range have nothing to do with NAC3's slice
and range.

nac3core/irrt/irrt.cpp

@@ -5,3 +5,5 @@
 #include <irrt/ndarray/def.hpp>
 #include <irrt/ndarray/iter.hpp>
 #include <irrt/original.hpp>
+#include <irrt/range.hpp>
+#include <irrt/slice.hpp>

nac3core/irrt/irrt/range.hpp

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <irrt/debug.hpp>
+#include <irrt/int_types.hpp>
+
+namespace
+{
+namespace range
+{
+template <typename T> T len(T start, T stop, T step)
+{
+    // Reference:
+    // https://github.com/python/cpython/blob/9dbd12375561a393eaec4b21ee4ac568a407cdb0/Objects/rangeobject.c#L933
+    if (step > 0 && start < stop)
+        return 1 + (stop - 1 - start) / step;
+    else if (step < 0 && start > stop)
+        return 1 + (start - 1 - stop) / (-step);
+    else
+        return 0;
+}
+} // namespace range
+
+/**
+ * @brief A Python range.
+ */
+template <typename T> struct Range
+{
+    T start;
+    T stop;
+    T step;
+
+    /**
+     * @brief Calculate the `len()` of this range.
+     */
+    template <typename SizeT> T len()
+    {
+        debug_assert(SizeT, step != 0);
+        return range::len(start, stop, step);
+    }
+};
+} // namespace

nac3core/irrt/irrt/slice.hpp

@@ -0,0 +1,158 @@
+#pragma once
+
+#include <irrt/debug.hpp>
+#include <irrt/exception.hpp>
+#include <irrt/int_types.hpp>
+#include <irrt/math_util.hpp>
+#include <irrt/range.hpp>
+
+namespace
+{
+namespace slice
+{
+/**
+ * @brief Resolve a possibly negative index in a list of a known length.
+ *
+ * Returns -1 if the resolved index is out of the list's bounds.
+ */
+template <typename T> T resolve_index_in_length(T length, T index)
+{
+    T resolved = index < 0 ? length + index : index;
+    if (0 <= resolved && resolved < length)
+    {
+        return resolved;
+    }
+    else
+    {
+        return -1;
+    }
+}
+
+/**
+ * @brief Resolve a slice as a range.
+ *
+ * This is equivalent to `range(*slice(start, stop, step).indices(length))` in Python.
+ */
+template <typename T>
+void indices(bool start_defined, T start, bool stop_defined, T stop, bool step_defined, T step, T length,
+             T *range_start, T *range_stop, T *range_step)
+{
+    // Reference: https://github.com/python/cpython/blob/main/Objects/sliceobject.c#L388
+    *range_step = step_defined ? step : 1;
+    bool step_is_negative = *range_step < 0;
+
+    T lower, upper;
+    if (step_is_negative)
+    {
+        lower = -1;
+        upper = length - 1;
+    }
+    else
+    {
+        lower = 0;
+        upper = length;
+    }
+
+    if (start_defined)
+    {
+        *range_start = start < 0 ? max(lower, start + length) : min(upper, start);
+    }
+    else
+    {
+        *range_start = step_is_negative ? upper : lower;
+    }
+
+    if (stop_defined)
+    {
+        *range_stop = stop < 0 ? max(lower, stop + length) : min(upper, stop);
+    }
+    else
+    {
+        *range_stop = step_is_negative ? lower : upper;
+    }
+}
+} // namespace slice
+
+/**
+ * @brief A Python-like slice with **unresolved** indices.
+ */
+template <typename T> struct Slice
+{
+    bool start_defined;
+    T start;
+
+    bool stop_defined;
+    T stop;
+
+    bool step_defined;
+    T step;
+
+    Slice()
+    {
+        this->reset();
+    }
+
+    void reset()
+    {
+        this->start_defined = false;
+        this->stop_defined = false;
+        this->step_defined = false;
+    }
+
+    void set_start(T start)
+    {
+        this->start_defined = true;
+        this->start = start;
+    }
+
+    void set_stop(T stop)
+    {
+        this->stop_defined = true;
+        this->stop = stop;
+    }
+
+    void set_step(T step)
+    {
+        this->step_defined = true;
+        this->step = step;
+    }
+
+    /**
+     * @brief Resolve this slice as a range.
+     *
+     * In Python, this would be `range(*slice(start, stop, step).indices(length))`.
+     */
+    template <typename SizeT> Range<T> indices(T length)
+    {
+        // Reference:
+        // https://github.com/python/cpython/blob/main/Objects/sliceobject.c#L388
+        debug_assert(SizeT, length >= 0);
+
+        Range<T> result;
+        slice::indices(start_defined, start, stop_defined, stop, step_defined, step, length, &result.start,
+                       &result.stop, &result.step);
+        return result;
+    }
+
+    /**
+     * @brief Like `.indices()` but with assertions.
+     */
+    template <typename SizeT> Range<T> indices_checked(T length)
+    {
+        // TODO: Switch to `SizeT length`
+
+        if (length < 0)
+        {
+            raise_exception(SizeT, EXN_VALUE_ERROR, "length should not be negative, got {0}", length, NO_PARAM,
+                            NO_PARAM);
+        }
+
+        if (this->step_defined && this->step == 0)
+        {
+            raise_exception(SizeT, EXN_VALUE_ERROR, "slice step cannot be zero", NO_PARAM, NO_PARAM, NO_PARAM);
+        }
+
+        return this->indices<SizeT>(length);
+    }
+};
+} // namespace