core/irrt: add Slice and Range

Needed for implementing general ndarray indexing

nac3core/irrt/irrt.cpp

@@ -5,3 +5,4 @@
 #include <irrt/ndarray/def.hpp>
 #include <irrt/ndarray/iter.hpp>
 #include <irrt/original.hpp>
+#include <irrt/slice.hpp>

nac3core/irrt/irrt/slice.hpp

@@ -0,0 +1,200 @@
+#pragma once
+
+#include <irrt/debug.hpp>
+#include <irrt/exception.hpp>
+#include <irrt/int_types.hpp>
+#include <irrt/math_util.hpp>
+
+// The type of an index or a value describing the length of a
+// range/slice is always `int32_t`.
+using SliceIndex = int32_t;
+
+namespace
+{
+
+/**
+ * @brief A Python range.
+ */
+struct Range
+{
+    SliceIndex start;
+    SliceIndex stop;
+    SliceIndex step;
+
+    /**
+     * @brief Calculate the `len()` of this range.
+     */
+    template <typename SizeT> SliceIndex len()
+    {
+        // Reference: https://github.com/python/cpython/blob/9dbd12375561a393eaec4b21ee4ac568a407cdb0/Objects/rangeobject.c#L933
+        debug_assert(SizeT, step != 0);
+
+        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 slice
+{
+/**
+ * @brief Resolve a slice index under a given length like Python indexing.
+ *
+ * In Python, if you have a `list` of length 100, `list[-1]` resolves to
+ * `list[99]`, so `resolve_index_in_length_clamped(100, -1)` returns `99`.
+ *
+ * If `length` is 0, 0 is returned for any value of `index`.
+ *
+ * If `index` is out of bounds, clamps the returned value between `0` and
+ * `length - 1` (inclusive).
+ *
+ */
+SliceIndex resolve_index_in_length_clamped(SliceIndex length, SliceIndex index)
+{
+    if (index < 0)
+    {
+        return max<SliceIndex>(length + index, 0);
+    }
+    else
+    {
+        return min<SliceIndex>(length, index);
+    }
+}
+
+const SliceIndex OUT_OF_BOUNDS = -1;
+
+/**
+ * @brief Like `resolve_index_in_length_clamped`, but returns `OUT_OF_BOUNDS`
+ * if `index` is out of bounds.
+ */
+SliceIndex resolve_index_in_length(SliceIndex length, SliceIndex index)
+{
+    SliceIndex resolved = index < 0 ? length + index : index;
+    if (0 <= resolved && resolved < length)
+    {
+        return resolved;
+    }
+    else
+    {
+        return OUT_OF_BOUNDS;
+    }
+}
+} // namespace slice
+
+/**
+ * @brief A Python-like slice with **unresolved** indices.
+ */
+struct Slice
+{
+    bool start_defined;
+    SliceIndex start;
+
+    bool stop_defined;
+    SliceIndex stop;
+
+    bool step_defined;
+    SliceIndex step;
+
+    Slice()
+    {
+        this->reset();
+    }
+
+    void reset()
+    {
+        this->start_defined = false;
+        this->stop_defined = false;
+        this->step_defined = false;
+    }
+
+    void set_start(SliceIndex start)
+    {
+        this->start_defined = true;
+        this->start = start;
+    }
+
+    void set_stop(SliceIndex stop)
+    {
+        this->stop_defined = true;
+        this->stop = stop;
+    }
+
+    void set_step(SliceIndex 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 indices(SliceIndex length)
+    {
+        // Reference: https://github.com/python/cpython/blob/main/Objects/sliceobject.c#L388
+        debug_assert(SizeT, length >= 0);
+
+        Range result;
+
+        result.step = step_defined ? step : 1;
+        bool step_is_negative = result.step < 0;
+
+        SliceIndex lower, upper;
+        if (step_is_negative)
+        {
+            lower = -1;
+            upper = length - 1;
+        }
+        else
+        {
+            lower = 0;
+            upper = length;
+        }
+
+        if (start_defined)
+        {
+            result.start = start < 0 ? max(lower, start + length) : min(upper, start);
+        }
+        else
+        {
+            result.start = step_is_negative ? upper : lower;
+        }
+
+        if (stop_defined)
+        {
+            result.stop = stop < 0 ? max(lower, stop + length) : min(upper, stop);
+        }
+        else
+        {
+            result.stop = step_is_negative ? lower : upper;
+        }
+
+        return result;
+    }
+
+    /**
+     * @brief Like `.indices()` but with assertions.
+     */
+    template <typename SizeT> Range indices_checked(SliceIndex 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