py/objslice: Add support for indices() method on slice objects.

Instances of the slice class are passed to __getitem__() on objects when
the user indexes them with a slice.  In practice the majority of the time
(other than passing it on untouched) is to work out what the slice means in
the context of an array dimension of a particular length.  Since Python 2.3
there has been a method on the slice class, indices(), that takes a
dimension length and returns the real start, stop and step, accounting for
missing or negative values in the slice spec.  This commit implements such
a indices() method on the slice class.

It is configurable at compile-time via MICROPY_PY_BUILTINS_SLICE_INDICES,
disabled by default, enabled on unix, stm32 and esp32 ports.

This commit also adds new tests for slice indices and for slicing unicode
strings.

9 files changed, 160 insertions, 74 deletions

diff --git a/ports/esp32/mpconfigport.h b/ports/esp32/mpconfigport.h
index 1924cf2186..983c882ae8 100644
--- a/ports/esp32/mpconfigport.h
+++ b/ports/esp32/mpconfigport.h
@@ -73,6 +73,7 @@
 #define MICROPY_PY_BUILTINS_SET             (1)
 #define MICROPY_PY_BUILTINS_SLICE           (1)
 #define MICROPY_PY_BUILTINS_SLICE_ATTRS     (1)
+#define MICROPY_PY_BUILTINS_SLICE_INDICES   (1)
 #define MICROPY_PY_BUILTINS_FROZENSET       (1)
 #define MICROPY_PY_BUILTINS_PROPERTY        (1)
 #define MICROPY_PY_BUILTINS_RANGE_ATTRS     (1)
diff --git a/ports/stm32/mpconfigport.h b/ports/stm32/mpconfigport.h
index 7ae8ee7d72..55f09f81f0 100644
--- a/ports/stm32/mpconfigport.h
+++ b/ports/stm32/mpconfigport.h
@@ -97,6 +97,7 @@
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_BUILTINS_SLICE_ATTRS (1)
+#define MICROPY_PY_BUILTINS_SLICE_INDICES (1)
 #define MICROPY_PY_BUILTINS_ROUND_INT (1)
 #define MICROPY_PY_ALL_SPECIAL_METHODS (1)
 #define MICROPY_PY_BUILTINS_COMPILE (MICROPY_ENABLE_COMPILER)
diff --git a/ports/unix/mpconfigport.h b/ports/unix/mpconfigport.h
index 40cd1f5702..d633726a16 100644
--- a/ports/unix/mpconfigport.h
+++ b/ports/unix/mpconfigport.h
@@ -90,6 +90,7 @@
 #define MICROPY_PY_REVERSE_SPECIAL_METHODS (1)
 #define MICROPY_PY_ARRAY_SLICE_ASSIGN (1)
 #define MICROPY_PY_BUILTINS_SLICE_ATTRS (1)
+#define MICROPY_PY_BUILTINS_SLICE_INDICES (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_ATEXIT       (1)
 #if MICROPY_PY_SYS_SETTRACE
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 93d67accdb..d6f4d92328 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -894,6 +894,11 @@ typedef double mp_float_t;
 #define MICROPY_PY_BUILTINS_SLICE_ATTRS (0)
 #endif
 
+// Whether to support the .indices(len) method on slice objects
+#ifndef MICROPY_PY_BUILTINS_SLICE_INDICES
+#define MICROPY_PY_BUILTINS_SLICE_INDICES (0)
+#endif
+
 // Whether to support frozenset object
 #ifndef MICROPY_PY_BUILTINS_FROZENSET
 #define MICROPY_PY_BUILTINS_FROZENSET (0)
diff --git a/py/obj.h b/py/obj.h
index efeb14b433..ab5b1e6ec0 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -778,8 +778,16 @@ static inline mp_map_t *mp_obj_dict_get_map(mp_obj_t dict) {
 // set
 void mp_obj_set_store(mp_obj_t self_in, mp_obj_t item);
 
+// slice indexes resolved to particular sequence
+typedef struct {
+    mp_int_t start;
+    mp_int_t stop;
+    mp_int_t step;
+} mp_bound_slice_t;
+
 // slice
 void mp_obj_slice_get(mp_obj_t self_in, mp_obj_t *start, mp_obj_t *stop, mp_obj_t *step);
+void mp_obj_slice_indices(mp_obj_t self_in, mp_int_t length, mp_bound_slice_t *result);
 
 // functions
 
@@ -836,13 +844,6 @@ const mp_obj_t *mp_obj_property_get(mp_obj_t self_in);
 
 // sequence helpers
 
-// slice indexes resolved to particular sequence
-typedef struct {
-    mp_uint_t start;
-    mp_uint_t stop;
-    mp_int_t step;
-} mp_bound_slice_t;
-
 void mp_seq_multiply(const void *items, size_t item_sz, size_t len, size_t times, void *dest);
 #if MICROPY_PY_BUILTINS_SLICE
 bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice_t *indexes);
diff --git a/py/objslice.c b/py/objslice.c
index cfc819edcc..d17dbf6057 100644
--- a/py/objslice.c
+++ b/py/objslice.c
@@ -27,7 +27,7 @@
 #include <stdlib.h>
 #include <assert.h>
 
-#include "py/obj.h"
+#include "py/runtime.h"
 
 /******************************************************************************/
 /* slice object                                                               */
@@ -53,6 +53,22 @@ STATIC void slice_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kind_t
     mp_print_str(print, ")");
 }
 
+#if MICROPY_PY_BUILTINS_SLICE_INDICES
+STATIC mp_obj_t slice_indices(mp_obj_t self_in, mp_obj_t length_obj) {
+    mp_int_t length = mp_obj_int_get_checked(length_obj);
+    mp_bound_slice_t bound_indices;
+    mp_obj_slice_indices(self_in, length, &bound_indices);
+
+    mp_obj_t results[3] = {
+        MP_OBJ_NEW_SMALL_INT(bound_indices.start),
+        MP_OBJ_NEW_SMALL_INT(bound_indices.stop),
+        MP_OBJ_NEW_SMALL_INT(bound_indices.step),
+    };
+    return mp_obj_new_tuple(3, results);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(slice_indices_obj, slice_indices);
+#endif
+
 #if MICROPY_PY_BUILTINS_SLICE_ATTRS
 STATIC void slice_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     if (dest[0] != MP_OBJ_NULL) {
@@ -60,22 +76,37 @@ STATIC void slice_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         return;
     }
     mp_obj_slice_t *self = MP_OBJ_TO_PTR(self_in);
+
     if (attr == MP_QSTR_start) {
         dest[0] = self->start;
     } else if (attr == MP_QSTR_stop) {
         dest[0] = self->stop;
     } else if (attr == MP_QSTR_step) {
         dest[0] = self->step;
+    #if MICROPY_PY_BUILTINS_SLICE_INDICES
+    } else if (attr == MP_QSTR_indices) {
+        dest[0] = MP_OBJ_FROM_PTR(&slice_indices_obj);
+        dest[1] = self_in;
+    #endif
     }
 }
 #endif
 
+#if MICROPY_PY_BUILTINS_SLICE_INDICES && !MICROPY_PY_BUILTINS_SLICE_ATTRS
+STATIC const mp_rom_map_elem_t slice_locals_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_indices), MP_ROM_PTR(&slice_indices_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(slice_locals_dict, slice_locals_dict_table);
+#endif
+
 const mp_obj_type_t mp_type_slice = {
     { &mp_type_type },
     .name = MP_QSTR_slice,
     .print = slice_print,
 #if MICROPY_PY_BUILTINS_SLICE_ATTRS
     .attr = slice_attr,
+#elif MICROPY_PY_BUILTINS_SLICE_INDICES
+    .locals_dict = (mp_obj_dict_t*)&slice_locals_dict,
 #endif
 };
 
@@ -96,4 +127,69 @@ void mp_obj_slice_get(mp_obj_t self_in, mp_obj_t *start, mp_obj_t *stop, mp_obj_
     *step = self->step;
 }
 
+// Return the real index and step values for a slice when applied to a sequence of
+// the given length, resolving missing components, negative values and values off
+// the end of the sequence.
+void mp_obj_slice_indices(mp_obj_t self_in, mp_int_t length, mp_bound_slice_t *result) {
+    mp_obj_slice_t *self = MP_OBJ_TO_PTR(self_in);
+    mp_int_t start, stop, step;
+
+    if (self->step == mp_const_none) {
+        step = 1;
+    } else {
+        step = mp_obj_get_int(self->step);
+        if (step == 0) {
+            mp_raise_ValueError("slice step cannot be zero");
+        }
+    }
+
+    if (step > 0) {
+        // Positive step
+        if (self->start == mp_const_none) {
+            start = 0;
+        } else {
+            start = mp_obj_get_int(self->start);
+            if (start < 0) {
+                start += length;
+            }
+            start = MIN(length, MAX(start, 0));
+        }
+
+        if (self->stop == mp_const_none) {
+            stop = length;
+        } else {
+            stop = mp_obj_get_int(self->stop);
+            if (stop < 0) {
+                stop += length;
+            }
+            stop = MIN(length, MAX(stop, 0));
+        }
+    } else {
+        // Negative step
+        if (self->start == mp_const_none) {
+            start = length - 1;
+        } else {
+            start = mp_obj_get_int(self->start);
+            if (start < 0) {
+                start += length;
+            }
+            start = MIN(length - 1, MAX(start, -1));
+        }
+
+        if (self->stop == mp_const_none) {
+            stop = -1;
+        } else {
+            stop = mp_obj_get_int(self->stop);
+            if (stop < 0) {
+                stop += length;
+            }
+            stop = MIN(length - 1, MAX(stop, -1));
+        }
+    }
+
+    result->start = start;
+    result->stop = stop;
+    result->step = step;
+}
+
 #endif
diff --git a/py/sequence.c b/py/sequence.c
index 4c19fc69ea..15e925000a 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -46,78 +46,20 @@ void mp_seq_multiply(const void *items, size_t item_sz, size_t len, size_t times
 #if MICROPY_PY_BUILTINS_SLICE
 
 bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice_t *indexes) {
-    mp_obj_t ostart, ostop, ostep;
-    mp_int_t start, stop;
-    mp_obj_slice_get(slice, &ostart, &ostop, &ostep);
+    mp_obj_slice_indices(slice, len, indexes);
 
-    if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) {
-        indexes->step = mp_obj_get_int(ostep);
-        if (indexes->step == 0) {
-            mp_raise_ValueError("slice step cannot be zero");
-        }
-    } else {
-        indexes->step = 1;
-    }
-
-    if (ostart == mp_const_none) {
-        if (indexes->step > 0) {
-            start = 0;
-        } else {
-            start = len - 1;
-        }
-    } else {
-        start = mp_obj_get_int(ostart);
-    }
-    if (ostop == mp_const_none) {
-        if (indexes->step > 0) {
-            stop = len;
-        } else {
-            stop = 0;
-        }
-    } else {
-        stop = mp_obj_get_int(ostop);
-        if (stop >= 0 && indexes->step < 0) {
-            stop += 1;
-        }
-    }
-
-    // Unlike subscription, out-of-bounds slice indexes are never error
-    if (start < 0) {
-        start = len + start;
-        if (start < 0) {
-            if (indexes->step < 0) {
-                start = -1;
-            } else {
-                start = 0;
-            }
-        }
-    } else if (indexes->step > 0 && (mp_uint_t)start > len) {
-        start = len;
-    } else if (indexes->step < 0 && (mp_uint_t)start >= len) {
-        start = len - 1;
-    }
-    if (stop < 0) {
-        stop = len + stop;
-        if (stop < 0) {
-            stop = -1;
-        }
-        if (indexes->step < 0) {
-            stop += 1;
-        }
-    } else if ((mp_uint_t)stop > len) {
-        stop = len;
+    // If the index is negative then stop points to the last item, not after it
+    if (indexes->step < 0) {
+        indexes->stop++;
     }
 
     // CPython returns empty sequence in such case, or point for assignment is at start
-    if (indexes->step > 0 && start > stop) {
-        stop = start;
-    } else if (indexes->step < 0 && start < stop) {
-        stop = start + 1;
+    if (indexes->step > 0 && indexes->start > indexes->stop) {
+        indexes->stop = indexes->start;
+    } else if (indexes->step < 0 && indexes->start < indexes->stop) {
+        indexes->stop = indexes->start + 1;
     }
 
-    indexes->start = start;
-    indexes->stop = stop;
-
     return indexes->step == 1;
 }
 
diff --git a/tests/basics/slice_indices.py b/tests/basics/slice_indices.py
new file mode 100644
index 0000000000..b7f439ccca
--- /dev/null
+++ b/tests/basics/slice_indices.py
@@ -0,0 +1,27 @@
+# Test builtin slice indices resolution
+
+# A class that returns an item key
+class A:
+    def __getitem__(self, idx):
+        return idx
+
+# Make sure that we have slices and .indices()
+try:
+    A()[2:5].indices(10)
+except:
+    print("SKIP")
+    raise SystemExit
+
+print(A()[:].indices(10))
+print(A()[2:].indices(10))
+print(A()[:7].indices(10))
+print(A()[2:7].indices(10))
+print(A()[2:7:2].indices(10))
+print(A()[2:7:-2].indices(10))
+print(A()[7:2:2].indices(10))
+print(A()[7:2:-2].indices(10))
+
+print(A()[2:7:2].indices(5))
+print(A()[2:7:-2].indices(5))
+print(A()[7:2:2].indices(5))
+print(A()[7:2:-2].indices(5))
diff --git a/tests/unicode/unicode_slice.py b/tests/unicode/unicode_slice.py
new file mode 100644
index 0000000000..d9237088f8
--- /dev/null
+++ b/tests/unicode/unicode_slice.py
@@ -0,0 +1,12 @@
+# Test slicing of Unicode strings
+
+s = "Привет"
+
+print(s[:])
+print(s[2:])
+print(s[:5])
+print(s[2:5])
+print(s[2:5:1])
+print(s[2:10])
+print(s[-3:10])
+print(s[-4:10])