Tweak conditional phrasing to match loop terminating criteria, add comment

explaining why a guard is not necessary in adaptive_find.

Author: duaneg

Objects/stringlib/fastsearch.h

@@ -595,7 +595,7 @@ STRINGLIB(default_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
                 continue;
             }
             /* miss: check if next character is part of pattern */
-            if (i < w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
+            if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
                 i = i + m;
             }
             else {
@@ -604,7 +604,7 @@ STRINGLIB(default_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
         }
         else {
             /* skip: check if next character is part of pattern */
-            if (i < w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
+            if (i + 1 <= w && !STRINGLIB_BLOOM(mask, ss[i+1])) {
                 i = i + m;
             }
         }
@@ -667,7 +667,16 @@ STRINGLIB(adaptive_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
                     return res + count;
                 }
             }
-            /* miss: check if next character is part of pattern */
+
+            /* Miss: check if next character is part of pattern.
+               Note that in contrast to default_find and default_rfind we do
+               *not* need to prevent the algorithm from reading one character
+               beyond the last character in the input that the pattern could
+               start in. I.e. if i == w it is safe to read ss[i + 1] since the
+               input and pattern length requirements on when this variant
+               algorithm will be called ensure it will always be a valid part
+               of the input. In that case it doesn't matter what the character
+               read is since the loop will terminate regardless. */
             if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
                 i = i + m;
             }
@@ -676,7 +685,9 @@ STRINGLIB(adaptive_find)(const STRINGLIB_CHAR* s, Py_ssize_t n,
             }
         }
         else {
-            /* skip: check if next character is part of pattern */
+            /* Skip: check if next character is part of pattern.
+               See comment above re safety of accessing ss[i+1] when i == w.
+             */
             if (!STRINGLIB_BLOOM(mask, ss[i+1])) {
                 i = i + m;
             }