Handle unescaping directly in n_string2 if space permits

src/parser3.h

@@ -461,6 +461,11 @@ inline PRESERVE_NONE WeaselJsonStatus n_string(Parser3 *self, char *buf,
   MUSTTAIL return Parser3::keepGoing(self, buf, bufEnd);
 }
 
+inline int32_t read4_hex(const char *buf) {
+  return tables.hex[uint8_t(buf[0])] << 12 | tables.hex[uint8_t(buf[1])] << 8 |
+         tables.hex[uint8_t(buf[2])] << 4 | tables.hex[uint8_t(buf[3])] << 0;
+}
+
 inline PRESERVE_NONE WeaselJsonStatus n_string2(Parser3 *self, char *buf,
                                                 char *bufEnd) {
   if (auto s = scan_string(self, buf, bufEnd)) {
@@ -474,10 +479,69 @@ inline PRESERVE_NONE WeaselJsonStatus n_string2(Parser3 *self, char *buf,
     MUSTTAIL return Parser3::keepGoing(self, buf, bufEnd);
   case '\\':
     ++buf;
-    if (auto s = self->push({N_STRING_FOLLOWING_ESCAPE})) {
+    if (bufEnd - buf < /*strlen("u0000\\u0000")*/ 11) {
-      return s;
+      if (auto s = self->push({N_STRING_FOLLOWING_ESCAPE})) {
+        return s;
+      }
+      MUSTTAIL return Parser3::keepGoing(self, buf, bufEnd);
+    } else {
+      if (*buf == 'u') {
+        ++buf;
+        int32_t codepoint = read4_hex(buf);
+        if (codepoint < 0) [[unlikely]] {
+          return WeaselJson_REJECT;
+        }
+        buf += 4;
+        if (0xd800 <= codepoint && codepoint <= 0xdfff) {
+          // utf-16 surrogate
+          int32_t codepoint2 = read4_hex(buf + 2);
+          if (!(buf[0] == '\\' && buf[1] == 'u' && codepoint2 >= 0))
+              [[unlikely]] {
+            return WeaselJson_REJECT;
+          }
+          codepoint =
+              0x10000 + (codepoint - 0xd800) * 0x400 + (codepoint2 - 0xdc00);
+          assert(codepoint >= 0x10000);
+          if (codepoint > 0x10FFFF) [[unlikely]] {
+            return WeaselJson_REJECT;
+          }
+          buf += 6;
+          assert(codepoint < 0x10ffff);
+          self->writeBuf[3] = (0b00111111 & codepoint) | 0b10000000;
+          codepoint >>= 6;
+          self->writeBuf[2] = (0b00111111 & codepoint) | 0b10000000;
+          codepoint >>= 6;
+          self->writeBuf[1] = (0b00111111 & codepoint) | 0b10000000;
+          codepoint >>= 6;
+          self->writeBuf[0] = (0b00000111 & codepoint) | 0b11110000;
+          self->writeBuf += 4;
+        } else {
+          if (codepoint < 0x80) {
+            *self->writeBuf++ = codepoint;
+          } else if (codepoint < 0x800) {
+            self->writeBuf[1] = (0b00111111 & codepoint) | 0b10000000;
+            codepoint >>= 6;
+            self->writeBuf[0] = (0b00011111 & codepoint) | 0b11000000;
+            self->writeBuf += 2;
+          } else {
+            assert(codepoint < 0x10000);
+            self->writeBuf[2] = (0b00111111 & codepoint) | 0b10000000;
+            codepoint >>= 6;
+            self->writeBuf[1] = (0b00111111 & codepoint) | 0b10000000;
+            codepoint >>= 6;
+            self->writeBuf[0] = (0b00001111 & codepoint) | 0b11100000;
+            self->writeBuf += 3;
+          }
+        }
+      } else {
+        auto unescaped = tables.unescape[uint8_t(*buf++)];
+        if (unescaped == 0) [[unlikely]] {
+          return WeaselJson_REJECT;
+        }
+        *self->writeBuf++ = unescaped;
+      }
+      MUSTTAIL return n_string2(self, buf, bufEnd);
     }
-    MUSTTAIL return Parser3::keepGoing(self, buf, bufEnd);
   default:
     [[unlikely]] return WeaselJson_REJECT;
   }
@@ -760,7 +824,8 @@ constexpr inline struct ContinuationTable {
     symbolNames[T_EOF] = "t_eof";
     symbolNames[T_BACKSLASH] = "singleChar<'\\'>";
 
-    // All others can assume that there's at least one byte when they're called
+    // All others can assume that there's at least one byte when they're
+    // called
     acceptsEmptyString[N_NUMBER] = true;
     acceptsEmptyString[N_WHITESPACE] = true;
     acceptsEmptyString[T_EOF] = true;