closes bpo-37966: Fully implement the UAX #15 quick-check algorithm. (GH-15558)

The purpose of the `unicodedata.is_normalized` function is to answer
the question `str == unicodedata.normalized(form, str)` more
efficiently than writing just that, by using the "quick check"
optimization described in the Unicode standard in UAX #15.

However, it turns out the code doesn't implement the full algorithm
from the standard, and as a result we often miss the optimization and
end up having to compute the whole normalized string after all.

Implement the standard's algorithm.  This greatly speeds up
`unicodedata.is_normalized` in many cases where our partial variant
of quick-check had been returning MAYBE and the standard algorithm
returns NO.

At a quick test on my desktop, the existing code takes about 4.4 ms/MB
(so 4.4 ns per byte) when the partial quick-check returns MAYBE and it
has to do the slow normalize-and-compare:

  $ build.base/python -m timeit -s 'import unicodedata; s = "\uf900"*500000' \
      -- 'unicodedata.is_normalized("NFD", s)'
  50 loops, best of 5: 4.39 msec per loop

With this patch, it gets the answer instantly (58 ns) on the same 1 MB
string:

  $ build.dev/python -m timeit -s 'import unicodedata; s = "\uf900"*500000' \
      -- 'unicodedata.is_normalized("NFD", s)'
  5000000 loops, best of 5: 58.2 nsec per loop

This restores a small optimization that the original version of this
code had for the `unicodedata.normalize` use case.

With this, that case is actually faster than in master!

$ build.base/python -m timeit -s 'import unicodedata; s = "\u0338"*500000' \
    -- 'unicodedata.normalize("NFD", s)'
500 loops, best of 5: 561 usec per loop

$ build.dev/python -m timeit -s 'import unicodedata; s = "\u0338"*500000' \
    -- 'unicodedata.normalize("NFD", s)'
500 loops, best of 5: 512 usec per loop

1 files changed, 51 insertions, 24 deletions

diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index ae0d4e4..5e8ba60 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -19,6 +19,8 @@
 #include "ucnhash.h"
 #include "structmember.h"
 
+#include <stdbool.h>
+
 _Py_IDENTIFIER(NFC);
 _Py_IDENTIFIER(NFD);
 _Py_IDENTIFIER(NFKC);
@@ -775,25 +777,40 @@ nfc_nfkc(PyObject *self, PyObject *input, int k)
     return result;
 }
 
-typedef enum {YES, NO, MAYBE} NormalMode;
-
-/* Return YES if the input is certainly normalized, NO or MAYBE if it might not be. */
-static NormalMode
-is_normalized(PyObject *self, PyObject *input, int nfc, int k)
+// This needs to match the logic in makeunicodedata.py
+// which constructs the quickcheck data.
+typedef enum {YES = 0, MAYBE = 1, NO = 2} QuickcheckResult;
+
+/* Run the Unicode normalization "quickcheck" algorithm.
+ *
+ * Return YES or NO if quickcheck determines the input is certainly
+ * normalized or certainly not, and MAYBE if quickcheck is unable to
+ * tell.
+ *
+ * If `yes_only` is true, then return MAYBE as soon as we determine
+ * the answer is not YES.
+ *
+ * For background and details on the algorithm, see UAX #15:
+ *   https://www.unicode.org/reports/tr15/#Detecting_Normalization_Forms
+ */
+static QuickcheckResult
+is_normalized_quickcheck(PyObject *self, PyObject *input,
+                         int nfc, int k, bool yes_only)
 {
-    Py_ssize_t i, len;
-    int kind;
-    void *data;
-    unsigned char prev_combining = 0, quickcheck_mask;
-
     /* An older version of the database is requested, quickchecks must be
        disabled. */
     if (self && UCD_Check(self))
         return NO;
 
-    /* The two quickcheck bits at this shift mean 0=Yes, 1=Maybe, 2=No,
-       as described in http://unicode.org/reports/tr15/#Annex8. */
-    quickcheck_mask = 3 << ((nfc ? 4 : 0) + (k ? 2 : 0));
+    Py_ssize_t i, len;
+    int kind;
+    void *data;
+    unsigned char prev_combining = 0;
+
+    /* The two quickcheck bits at this shift have type QuickcheckResult. */
+    int quickcheck_shift = (nfc ? 4 : 0) + (k ? 2 : 0);
+
+    QuickcheckResult result = YES; /* certainly normalized, unless we find something */
 
     i = 0;
     kind = PyUnicode_KIND(input);
@@ -802,16 +819,26 @@ is_normalized(PyObject *self, PyObject *input, int nfc, int k)
     while (i < len) {
         Py_UCS4 ch = PyUnicode_READ(kind, data, i++);
         const _PyUnicode_DatabaseRecord *record = _getrecord_ex(ch);
-        unsigned char combining = record->combining;
-        unsigned char quickcheck = record->normalization_quick_check;
 
-        if (quickcheck & quickcheck_mask)
-            return MAYBE; /* this string might need normalization */
+        unsigned char combining = record->combining;
         if (combining && prev_combining > combining)
             return NO; /* non-canonical sort order, not normalized */
         prev_combining = combining;
+
+        unsigned char quickcheck_whole = record->normalization_quick_check;
+        if (yes_only) {
+            if (quickcheck_whole & (3 << quickcheck_shift))
+                return MAYBE;
+        } else {
+            switch ((quickcheck_whole >> quickcheck_shift) & 3) {
+            case NO:
+              return NO;
+            case MAYBE:
+              result = MAYBE; /* this string might need normalization */
+            }
+        }
     }
-    return YES; /* certainly normalized */
+    return result;
 }
 
 /*[clinic input]
@@ -844,7 +871,7 @@ unicodedata_UCD_is_normalized_impl(PyObject *self, PyObject *form,
     PyObject *result;
     int nfc = 0;
     int k = 0;
-    NormalMode m;
+    QuickcheckResult m;
 
     PyObject *cmp;
     int match = 0;
@@ -867,7 +894,7 @@ unicodedata_UCD_is_normalized_impl(PyObject *self, PyObject *form,
         return NULL;
     }
 
-    m = is_normalized(self, input, nfc, k);
+    m = is_normalized_quickcheck(self, input, nfc, k, false);
 
     if (m == MAYBE) {
         cmp = (nfc ? nfc_nfkc : nfd_nfkd)(self, input, k);
@@ -913,28 +940,28 @@ unicodedata_UCD_normalize_impl(PyObject *self, PyObject *form,
     }
 
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFC)) {
-        if (is_normalized(self, input, 1, 0) == YES) {
+        if (is_normalized_quickcheck(self, input, 1, 0, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfc_nfkc(self, input, 0);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFKC)) {
-        if (is_normalized(self, input, 1, 1) == YES) {
+        if (is_normalized_quickcheck(self, input, 1, 1, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfc_nfkc(self, input, 1);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFD)) {
-        if (is_normalized(self, input, 0, 0) == YES) {
+        if (is_normalized_quickcheck(self, input, 0, 0, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfd_nfkd(self, input, 0);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFKD)) {
-        if (is_normalized(self, input, 0, 1) == YES) {
+        if (is_normalized_quickcheck(self, input, 0, 1, true) == YES) {
             Py_INCREF(input);
             return input;
         }