Optimize JsonReader further -- don't make conditional array lookups w…

…here they are not really necessary

It gives solid 5-10% of throughpuut

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonParser.kt

@@ -38,7 +38,7 @@ internal class JsonParser(
  }
 
  private fun readArray(): JsonElement {
- var lastToken = reader.consumeNextToken(TC_BEGIN_LIST)
+ var lastToken = reader.consumeNextToken()
  // Prohibit leading comma
  if (reader.peekNextToken() == TC_COMMA) reader.fail("Unexpected leading comma")
  val result = arrayListOf<JsonElement>()

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonReader.kt

@@ -166,6 +166,7 @@ internal class JsonReader(private val source: String) {
  private var length = 0 // length of string
  private var buf = CharArray(16) // only used for strings with escapes
 
+ // TODO consider replacing usages of this method in JsonParser with char overload
  fun consumeNextToken(expected: Byte): Byte {
  val token = consumeNextToken()
  if (token != expected) {
@@ -174,6 +175,17 @@ internal class JsonReader(private val source: String) {
  return token
  }
 
+ fun consumeNextToken(expected: Char) {
+ val source = source
+ while (currentPosition < source.length) {
+ val c = source[currentPosition++]
+ if (c == ' ' || c == '\n' || c == '\r' || c == '\t') continue
+ if (c == expected) return
+ fail(charToTokenClass(expected))
+ }
+ fail(charToTokenClass(expected)) // EOF
+ }
+
  private fun fail(expectedToken: Byte) {
  // We know that the token was consumed prior to this call
  // Slow path, never called in normal code, can avoid optimizing it
@@ -288,7 +300,6 @@ internal class JsonReader(private val source: String) {
  }
  val startPosition = currentPosition - 1
  var lastPosition = currentPosition
- length = 0
  var char = source[currentPosition] // Avoid two double checks visible in the profiler
  while (char != STRING) {
  if (char == STRING_ESC) {
@@ -308,7 +319,10 @@ internal class JsonReader(private val source: String) {
  } else {
  // some escaped chars were there
  appendRange(source, lastPosition, currentPosition)
- buf.concatToString(0, length)
+ val l = length
+ length = 0
+ buf.concatToString(0, l)
+
  }
  this.currentPosition = currentPosition + 1
  return string
@@ -325,14 +339,17 @@ internal class JsonReader(private val source: String) {
  * `indexOf` for escape symbol. It works almost 20% faster for both large and small JSON strings.
  */
  fun consumeKeyString(): String {
- consumeNextToken(TC_STRING)
+ consumeNextToken(STRING)
  val current = currentPosition
  val closingQuote = source.indexOf('"', current)
  if (closingQuote == -1) fail(TC_STRING) // Better error message?
  // TODO explain
  for (i in current until closingQuote) {
  // Encountered escape sequence, should fallback to "slow" path
- if (source[i] == '\\') TODO()
+ if (source[i] == '\\') {
+ TODO()
+ break
+ }
  }
  this.currentPosition = closingQuote + 1
  return source.substring(current, closingQuote)
@@ -493,8 +510,7 @@ internal class JsonReader(private val source: String) {
  ++current
  hasChars = current != source.length
  val digit = ch - '0'
- if (digit !in 0..9)
- fail("Unexpected symbol '$ch' in numeric literal")
+ if (digit !in 0..9) fail("Unexpected symbol '$ch' in numeric literal")
  accumulator = accumulator * 10 - digit
  if (accumulator > 0) fail("Numeric value overflow")
  }

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt

@@ -34,7 +34,7 @@ internal open class StreamingJsonDecoder(
 
  override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder {
  val newMode = json.switchMode(descriptor)
- reader.consumeNextToken(newMode.beginTc)
+ reader.consumeNextToken(newMode.begin)
  checkLeadingComma()
  return when (newMode) {
  // In fact resets current index that these modes rely on
@@ -52,7 +52,7 @@ internal open class StreamingJsonDecoder(
  }
 
  override fun endStructure(descriptor: SerialDescriptor) {
- reader.consumeNextToken(mode.endTc)
+ reader.consumeNextToken(mode.end)
  }
 
  override fun decodeNotNullMark(): Boolean {
@@ -86,7 +86,7 @@ internal open class StreamingJsonDecoder(
  hasComma = reader.tryConsumeComma()
  }
  } else {
- reader.consumeNextToken(TC_COLON)
+ reader.consumeNextToken(COLON)
  }
 
  return if (reader.canConsumeValue()) {
@@ -126,7 +126,7 @@ internal open class StreamingJsonDecoder(
  while (reader.canConsumeValue()) { // TODO: consider merging comma consumption and this check
  hasComma = false
  val key = decodeStringKey()
- reader.consumeNextToken(TC_COLON)
+ reader.consumeNextToken(COLON)
  val index = descriptor.getElementIndex(key)
  val isUnknown = if (index != UNKNOWN_NAME) {
  if (configuration.coerceInputValues && coerceInputValue(descriptor, index)) {