# HG changeset patch
# User jbe
# Date 1406805755 -7200
# Node ID d476b3c8960df4ff0b72b4d0acc99e8f0e4c373a
# Parent  d5e5e8a9b79a86dab257a013a42d5ccd478ea8db
Speed up JSON library by better approximation of required buffer length when parsing string values

diff -r d5e5e8a9b79a -r d476b3c8960d libraries/json/json.c
--- a/libraries/json/json.c	Thu Jul 31 03:57:11 2014 +0200
+++ b/libraries/json/json.c	Thu Jul 31 13:22:35 2014 +0200
@@ -297,62 +297,80 @@
   case '"':
     // consume quote character:
     pos++;
-    // prepare buffer to decode string (with maximum possible length) and set write position to zero:
-    cbuf = luaL_buffinitsize(L, &luabuf, total-pos);
-    writepos = 0;
-    // loop through the characters until encountering end quote:
-    while ((c = str[pos++]) != '"') {
-      if (c == 0) {
-        // handle unexpected end of JSON document:
-        goto json_import_unexpected_eof;
-      } else if (c < 32 || c == 127) {
-        // do not allow ASCII control characters:
-        // NOTE: illegal UTF-8 sequences and extended control characters are not sanitized
-        //       by this parser to allow different encodings than Unicode
-        lua_pushnil(L);
-        lua_pushliteral(L, "Unexpected control character in JSON string");
-        return 2;
-      } else if (c == '\\') {
-        // read next char after backslash escape:
-        c = str[pos++];
-        switch (c) {
-        // unexpected end-of-string:
-        case 0:
+    // determine buffer length:
+    writepos = pos;
+    while ((c = str[writepos]) != '"') {
+      // consume one character:
+      writepos++;
+      // handle unexpected end of JSON document:
+      if (c == 0) goto json_import_unexpected_eof;
+      // consume one extra character when encountering an escaped quote:
+      else if (c == '\\' && str[writepos] == '"') writepos++;
+    }
+    writepos -= pos;
+    // check if string is non empty:
+    if (writepos) {
+      // prepare buffer to decode string (with maximum possible length) and set write position to zero:
+      cbuf = luaL_buffinitsize(L, &luabuf, writepos);
+      writepos = 0;
+      // loop through the characters until encountering end quote:
+      while ((c = str[pos++]) != '"') {
+        if (c == 0) {
+          // handle unexpected end of JSON document:
           goto json_import_unexpected_eof;
-        // unescaping of quotation mark, slash, and backslash:
-        case '"':
-        case '/':
-        case '\\':
+        } else if (c < 32 || c == 127) {
+          // do not allow ASCII control characters:
+          // NOTE: illegal UTF-8 sequences and extended control characters are not sanitized
+          //       by this parser to allow different encodings than Unicode
+          lua_pushnil(L);
+          lua_pushliteral(L, "Unexpected control character in JSON string");
+          return 2;
+        } else if (c == '\\') {
+          // read next char after backslash escape:
+          c = str[pos++];
+          switch (c) {
+          // unexpected end-of-string:
+          case 0:
+            goto json_import_unexpected_eof;
+          // unescaping of quotation mark, slash, and backslash:
+          case '"':
+          case '/':
+          case '\\':
+            cbuf[writepos++] = c;
+            break;
+          // unescaping of backspace:
+          case 'b': cbuf[writepos++] = '\b'; break;
+          // unescaping of form-feed:
+          case 'f': cbuf[writepos++] = '\f'; break;
+          // unescaping of new-line:
+          case 'n': cbuf[writepos++] = '\n'; break;
+          // unescaping of carriage-return:
+          case 'r': cbuf[writepos++] = '\r'; break;
+          // unescaping of tabulator:
+          case 't': cbuf[writepos++] = '\t'; break;
+          // unescaping of UTF-16 characters
+          case 'u':
+            lua_pushnil(L);
+            lua_pushliteral(L, "JSON unicode escape sequences are not implemented yet");  // TODO
+            return 2;
+          // unexpected escape sequence:
+          default:
+            lua_pushnil(L);
+            lua_pushliteral(L, "Unexpected string escape sequence in JSON document");
+            return 2;
+          }
+        } else {
+          // normal character:
           cbuf[writepos++] = c;
-          break;
-        // unescaping of backspace:
-        case 'b': cbuf[writepos++] = '\b'; break;
-        // unescaping of form-feed:
-        case 'f': cbuf[writepos++] = '\f'; break;
-        // unescaping of new-line:
-        case 'n': cbuf[writepos++] = '\n'; break;
-        // unescaping of carriage-return:
-        case 'r': cbuf[writepos++] = '\r'; break;
-        // unescaping of tabulator:
-        case 't': cbuf[writepos++] = '\t'; break;
-        // unescaping of UTF-16 characters
-        case 'u':
-          lua_pushnil(L);
-          lua_pushliteral(L, "JSON unicode escape sequences are not implemented yet");  // TODO
-          return 2;
-        // unexpected escape sequence:
-        default:
-          lua_pushnil(L);
-          lua_pushliteral(L, "Unexpected string escape sequence in JSON document");
-          return 2;
         }
-      } else {
-        // normal character:
-        cbuf[writepos++] = c;
       }
+      // process buffer to Lua string:
+      luaL_pushresultsize(&luabuf, writepos);
+    } else {
+      // if JSON string is empty,
+      // push empty Lua string:
+      lua_pushliteral(L, "");
     }
-    // process buffer to Lua string:
-    luaL_pushresultsize(&luabuf, writepos);
     // continue with processing of decoded string:
     goto json_import_process_value;
   }