diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 01049f5..3da40dd 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -220,6 +220,38 @@
         return unicode_empty;                           \
     } while (0)
 
+static inline void
+unicode_fill(enum PyUnicode_Kind kind, void *data, Py_UCS4 value,
+             Py_ssize_t start, Py_ssize_t length)
+{
+    assert(0 <= start);
+    assert(kind != PyUnicode_WCHAR_KIND);
+    switch (kind) {
+    case PyUnicode_1BYTE_KIND: {
+        Py_UCS1 ch = (unsigned char)value;
+        Py_UCS1 *to = (Py_UCS1 *)data + start;
+        memset(to, ch, length);
+        break;
+    }
+    case PyUnicode_2BYTE_KIND: {
+        Py_UCS2 ch = (Py_UCS2)value;
+        Py_UCS2 *to = (Py_UCS2 *)data + start;
+        const Py_UCS2 *end = to + length;
+        for (; to < end; ++to) *to = ch;
+        break;
+    }
+    case PyUnicode_4BYTE_KIND: {
+        Py_UCS4 ch = value;
+        Py_UCS4 * to = (Py_UCS4 *)data + start;
+        const Py_UCS4 *end = to + length;
+        for (; to < end; ++to) *to = ch;
+        break;
+    }
+    default: Py_UNREACHABLE();
+    }
+}
+
+
 /* Forward declaration */
 static inline int
 _PyUnicodeWriter_WriteCharInline(_PyUnicodeWriter *writer, Py_UCS4 ch);
@@ -790,7 +822,6 @@
 #include "stringlib/count.h"
 #include "stringlib/find.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs1lib.h"
@@ -801,7 +832,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs2lib.h"
@@ -812,7 +842,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/ucs4lib.h"
@@ -823,7 +852,6 @@
 #include "stringlib/find.h"
 #include "stringlib/replace.h"
 #include "stringlib/find_max_char.h"
-#include "stringlib/localeutil.h"
 #include "stringlib/undef.h"
 
 #include "stringlib/unicodedefs.h"
@@ -9323,86 +9351,149 @@
     return result;
 }
 
+/* _PyUnicode_InsertThousandsGrouping() helper functions */
+#include "stringlib/localeutil.h"
+
+/**
+ * InsertThousandsGrouping:
+ * @writer: Unicode writer.
+ * @n_buffer: Number of characters in @buffer.
+ * @digits: Digits we're reading from. If count is non-NULL, this is unused.
+ * @d_pos: Start of digits string.
+ * @n_digits: The number of digits in the string, in which we want
+ *            to put the grouping chars.
+ * @min_width: The minimum width of the digits in the output string.
+ *             Output will be zero-padded on the left to fill.
+ * @grouping: see definition in localeconv().
+ * @thousands_sep: see definition in localeconv().
+ *
+ * There are 2 modes: counting and filling. If @writer is NULL,
+ *  we are in counting mode, else filling mode.
+ * If counting, the required buffer size is returned.
+ * If filling, we know the buffer will be large enough, so we don't
+ *  need to pass in the buffer size.
+ * Inserts thousand grouping characters (as defined by grouping and
+ *  thousands_sep) into @writer.
+ *
+ * Return value: -1 on error, number of characters otherwise.
+ **/
 Py_ssize_t
 _PyUnicode_InsertThousandsGrouping(
-    PyObject *unicode, Py_ssize_t index,
+    _PyUnicodeWriter *writer,
     Py_ssize_t n_buffer,
-    void *digits, Py_ssize_t n_digits,
+    PyObject *digits,
+    Py_ssize_t d_pos,
+    Py_ssize_t n_digits,
     Py_ssize_t min_width,
-    const char *grouping, PyObject *thousands_sep,
+    const char *grouping,
+    PyObject *thousands_sep,
     Py_UCS4 *maxchar)
 {
-    unsigned int kind, thousands_sep_kind;
-    char *data, *thousands_sep_data;
-    Py_ssize_t thousands_sep_len;
-    Py_ssize_t len;
-
-    if (unicode != NULL) {
-        kind = PyUnicode_KIND(unicode);
-        data = (char *) PyUnicode_DATA(unicode) + index * kind;
+    if (writer) {
+        assert(digits != NULL);
+        assert(maxchar == NULL);
     }
     else {
-        kind = PyUnicode_1BYTE_KIND;
-        data = NULL;
+        assert(digits == NULL);
+        assert(maxchar != NULL);
     }
-    thousands_sep_kind = PyUnicode_KIND(thousands_sep);
-    thousands_sep_data = PyUnicode_DATA(thousands_sep);
-    thousands_sep_len = PyUnicode_GET_LENGTH(thousands_sep);
-    if (unicode != NULL && thousands_sep_kind != kind) {
-        if (thousands_sep_kind < kind) {
-            thousands_sep_data = _PyUnicode_AsKind(thousands_sep, kind);
-            if (!thousands_sep_data)
-                return -1;
+    assert(0 <= d_pos);
+    assert(0 <= n_digits);
+    assert(0 <= min_width);
+    assert(grouping != NULL);
+
+    if (digits != NULL) {
+        if (PyUnicode_READY(digits) == -1) {
+            return -1;
         }
-        else {
-            data = _PyUnicode_AsKind(unicode, thousands_sep_kind);
-            if (!data)
-                return -1;
-        }
+    }
+    if (PyUnicode_READY(thousands_sep) == -1) {
+        return -1;
     }
 
-    switch (kind) {
-    case PyUnicode_1BYTE_KIND:
-        if (unicode != NULL && PyUnicode_IS_ASCII(unicode))
-            len = asciilib_InsertThousandsGrouping(
-                (Py_UCS1 *) data, n_buffer, (Py_UCS1 *) digits, n_digits,
-                min_width, grouping,
-                (Py_UCS1 *) thousands_sep_data, thousands_sep_len);
-        else
-            len = ucs1lib_InsertThousandsGrouping(
-                (Py_UCS1*)data, n_buffer, (Py_UCS1*)digits, n_digits,
-                min_width, grouping,
-                (Py_UCS1 *) thousands_sep_data, thousands_sep_len);
-        break;
-    case PyUnicode_2BYTE_KIND:
-        len = ucs2lib_InsertThousandsGrouping(
-            (Py_UCS2 *) data, n_buffer, (Py_UCS2 *) digits, n_digits,
-            min_width, grouping,
-            (Py_UCS2 *) thousands_sep_data, thousands_sep_len);
-        break;
-    case PyUnicode_4BYTE_KIND:
-        len = ucs4lib_InsertThousandsGrouping(
-            (Py_UCS4 *) data, n_buffer, (Py_UCS4 *) digits, n_digits,
-            min_width, grouping,
-            (Py_UCS4 *) thousands_sep_data, thousands_sep_len);
-        break;
-    default:
-        Py_UNREACHABLE();
+    Py_ssize_t count = 0;
+    Py_ssize_t n_zeros;
+    int loop_broken = 0;
+    int use_separator = 0; /* First time through, don't append the
+                              separator. They only go between
+                              groups. */
+    Py_ssize_t buffer_pos;
+    Py_ssize_t digits_pos;
+    Py_ssize_t len;
+    Py_ssize_t n_chars;
+    Py_ssize_t remaining = n_digits; /* Number of chars remaining to
+                                        be looked at */
+    /* A generator that returns all of the grouping widths, until it
+       returns 0. */
+    GroupGenerator groupgen;
+    GroupGenerator_init(&groupgen, grouping);
+    const Py_ssize_t thousands_sep_len = PyUnicode_GET_LENGTH(thousands_sep);
+
+    /* if digits are not grouped, thousands separator
+       should be an empty string */
+    assert(!(grouping[0] == CHAR_MAX && thousands_sep_len != 0));
+
+    digits_pos = d_pos + n_digits;
+    if (writer) {
+        buffer_pos = writer->pos + n_buffer;
+        assert(buffer_pos <= PyUnicode_GET_LENGTH(writer->buffer));
+        assert(digits_pos <= PyUnicode_GET_LENGTH(digits));
     }
-    if (unicode != NULL && thousands_sep_kind != kind) {
-        if (thousands_sep_kind < kind)
-            PyMem_Free(thousands_sep_data);
-        else
-            PyMem_Free(data);
+    else {
+        buffer_pos = n_buffer;
     }
-    if (unicode == NULL) {
+
+    if (!writer) {
         *maxchar = 127;
-        if (len != n_digits) {
-            *maxchar = Py_MAX(*maxchar,
-                                   PyUnicode_MAX_CHAR_VALUE(thousands_sep));
-        }
     }
-    return len;
+
+    while ((len = GroupGenerator_next(&groupgen)) > 0) {
+        len = Py_MIN(len, Py_MAX(Py_MAX(remaining, min_width), 1));
+        n_zeros = Py_MAX(0, len - remaining);
+        n_chars = Py_MAX(0, Py_MIN(remaining, len));
+
+        /* Use n_zero zero's and n_chars chars */
+
+        /* Count only, don't do anything. */
+        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
+
+        /* Copy into the writer. */
+        InsertThousandsGrouping_fill(writer, &buffer_pos,
+                                     digits, &digits_pos,
+                                     n_chars, n_zeros,
+                                     use_separator ? thousands_sep : NULL,
+                                     thousands_sep_len, maxchar);
+
+        /* Use a separator next time. */
+        use_separator = 1;
+
+        remaining -= n_chars;
+        min_width -= len;
+
+        if (remaining <= 0 && min_width <= 0) {
+            loop_broken = 1;
+            break;
+        }
+        min_width -= thousands_sep_len;
+    }
+    if (!loop_broken) {
+        /* We left the loop without using a break statement. */
+
+        len = Py_MAX(Py_MAX(remaining, min_width), 1);
+        n_zeros = Py_MAX(0, len - remaining);
+        n_chars = Py_MAX(0, Py_MIN(remaining, len));
+
+        /* Use n_zero zero's and n_chars chars */
+        count += (use_separator ? thousands_sep_len : 0) + n_zeros + n_chars;
+
+        /* Copy into the writer. */
+        InsertThousandsGrouping_fill(writer, &buffer_pos,
+                                     digits, &digits_pos,
+                                     n_chars, n_zeros,
+                                     use_separator ? thousands_sep : NULL,
+                                     thousands_sep_len, maxchar);
+    }
+    return count;
 }
 
 
@@ -10021,30 +10112,6 @@
     return NULL;
 }
 
-#define FILL(kind, data, value, start, length) \
-    do { \
-        Py_ssize_t i_ = 0; \
-        assert(kind != PyUnicode_WCHAR_KIND); \
-        switch ((kind)) { \
-        case PyUnicode_1BYTE_KIND: { \
-            unsigned char * to_ = (unsigned char *)((data)) + (start); \
-            memset(to_, (unsigned char)value, (length)); \
-            break; \
-        } \
-        case PyUnicode_2BYTE_KIND: { \
-            Py_UCS2 * to_ = (Py_UCS2 *)((data)) + (start); \
-            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
-            break; \
-        } \
-        case PyUnicode_4BYTE_KIND: { \
-            Py_UCS4 * to_ = (Py_UCS4 *)((data)) + (start); \
-            for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
-            break; \
-        } \
-        default: Py_UNREACHABLE(); \
-        } \
-    } while (0)
-
 void
 _PyUnicode_FastFill(PyObject *unicode, Py_ssize_t start, Py_ssize_t length,
                     Py_UCS4 fill_char)
@@ -10056,7 +10123,7 @@
     assert(fill_char <= PyUnicode_MAX_CHAR_VALUE(unicode));
     assert(start >= 0);
     assert(start + length <= PyUnicode_GET_LENGTH(unicode));
-    FILL(kind, data, fill_char, start, length);
+    unicode_fill(kind, data, fill_char, start, length);
 }
 
 Py_ssize_t
@@ -10127,9 +10194,9 @@
     kind = PyUnicode_KIND(u);
     data = PyUnicode_DATA(u);
     if (left)
-        FILL(kind, data, fill, 0, left);
+        unicode_fill(kind, data, fill, 0, left);
     if (right)
-        FILL(kind, data, fill, left + _PyUnicode_LENGTH(self), right);
+        unicode_fill(kind, data, fill, left + _PyUnicode_LENGTH(self), right);
     _PyUnicode_FastCopyCharacters(u, left, self, 0, _PyUnicode_LENGTH(self));
     assert(_PyUnicode_CheckConsistency(u, 1));
     return u;
@@ -11516,7 +11583,7 @@
             if (tabsize > 0) {
                 incr = tabsize - (line_pos % tabsize);
                 line_pos += incr;
-                FILL(kind, dest_data, ' ', j, incr);
+                unicode_fill(kind, dest_data, ' ', j, incr);
                 j += incr;
             }
         }
@@ -14792,7 +14859,7 @@
     /* Pad left with the fill character if needed */
     if (arg->width > len && !(arg->flags & F_LJUST)) {
         sublen = arg->width - len;
-        FILL(writer->kind, writer->data, fill, writer->pos, sublen);
+        unicode_fill(writer->kind, writer->data, fill, writer->pos, sublen);
         writer->pos += sublen;
         arg->width = len;
     }
@@ -14824,7 +14891,7 @@
     /* Pad right with the fill character if needed */
     if (arg->width > len) {
         sublen = arg->width - len;
-        FILL(writer->kind, writer->data, ' ', writer->pos, sublen);
+        unicode_fill(writer->kind, writer->data, ' ', writer->pos, sublen);
         writer->pos += sublen;
     }
     return 0;