Add comments about handled code ranges in each branch

mdboom · mdboom · commit f741a9d20244 · 2022-08-31T13:53:09.000-04:00
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
@@ -501,19 +501,35 @@ valid_utf8(const unsigned char* s)
         /* single-byte code */
         return 1;
     } else if (*s < 0xE0) {
+        /* \xC2\x80-\xDF\xBF -- 0080-07FF */
         if (*s < 0xC2) {
+            /* invalid sequence
+               \x80-\xBF -- continuation byte
+               \xC0-\xC1 -- fake 0000-007F */
             return 0;
         }
         expected = 1;
     } else if (*s < 0xF0) {
+        /* \xE0\xA0\x80-\xEF\xBF\xBF -- 0800-FFFF */
         if (*s == 0xE0 && *(s + 1) < 0xA0) {
+            /* invalid sequence
+               \xE0\x80\x80-\xE0\x9F\xBF -- fake 0000-0800 */
             return 0;
         } else if (*s == 0xED && *(s + 1) >= 0xA0) {
+            /* Decoding UTF-8 sequences in range \xED\xA0\x80-\xED\xBF\xBF
+               will result in surrogates in range D800-DFFF. Surrogates are
+               not valid UTF-8 so they are rejected.
+               See https://www.unicode.org/versions/Unicode5.2.0/ch03.pdf
+               (table 3-7) and http://www.rfc-editor.org/rfc/rfc3629.txt */
             return 0;
         }
         expected = 2;
     } else if (*s < 0xF5) {
+        /* \xF0\x90\x80\x80-\xF4\x8F\xBF\xBF -- 10000-10FFFF */
         if (*(s + 1) < 0x90 ? *s == 0xF0 : *s == 0xF4) {
+            /* invalid sequence -- one of:
+               \xF0\x80\x80\x80-\xF0\x8F\xBF\xBF -- fake 0000-FFFF
+               \xF4\x90\x80\x80- -- 110000- overflow */
             return 0;
         }
         expected = 3;
