Minor cleanup and simplification of readString (#376)

* Request Long.MAX_VALUE from the source at once (note: no need for a loop as we are unable to build the string of such size anyway)
* Get rid of redundant checks in commonReadUtf8

Ticks one of the boxes in #342

benchmarks/README.md

@@ -8,4 +8,13 @@ Currently, the suite includes benchmarks on:
 - segment pooling performance.
 
 The suite doesn't include benchmarks for more complex APIs inherited from Okio as these APIs are subject to change.
-Such benchmarks will be added later along with corresponding changes in the library.
+Such benchmarks will be added later along with corresponding changes in the library.
+
+### Quickstart
+
+For JVM:
+```
+./gradlew :kotlinx-io-benchmarks:jvmJar
+
+java -jar benchmarks/build/benchmarks/jvm/jars/kotlinx-io-benchmarks-jvm-jmh-0.6.0-SNAPSHOT-JMH.jar  ReadStringBenchmark -f 1 -wi 5 -i 5 -tu us -w 1 -r 1
+```

benchmarks/src/commonMain/kotlin/ReadStringBenchmark.kt

@@ -0,0 +1,42 @@
+package kotlinx.io.benchmarks
+
+import kotlinx.benchmark.Benchmark
+import kotlinx.benchmark.Param
+import kotlinx.benchmark.Scope
+import kotlinx.benchmark.Setup
+import kotlinx.benchmark.State
+import kotlinx.io.Buffer
+import kotlinx.io.Source
+import kotlinx.io.readCodePointValue
+import kotlinx.io.readString
+import kotlinx.io.writeCodePointValue
+import kotlinx.io.writeString
+import kotlin.random.Random
+
+
+@State(Scope.Benchmark)
+open class ReadStringBenchmark() {
+
+    @Param("16", "64", "512") // Fits into a single segment, so the benchmark does not measure segment boundaries crossing
+    var size: Int = 0
+
+    val buffer: Buffer = Buffer()
+
+    @Setup
+    fun setup() {
+        val string = buildString { repeat(size) { append(('a'..'z').random()) } }
+        buffer.writeString(string)
+    }
+
+
+    @Benchmark
+    fun bufferReadString(): String {
+        return buffer.copy().readString()
+    }
+
+    @Benchmark
+    fun sourceReadString(): String {
+        val source: Source = buffer.copy()
+        return source.readString()
+    }
+}

core/common/src/Utf8.kt

@@ -206,10 +206,7 @@ public fun Sink.writeString(chars: CharSequence, startIndex: Int = 0, endIndex:
  */
 @OptIn(InternalIoApi::class)
 public fun Source.readString(): String {
-    var req: Long = Segment.SIZE.toLong()
-    while (request(req)) {
-        req *= 2
-    }
+    request(Long.MAX_VALUE) // Request all data
     return buffer.commonReadUtf8(buffer.size)
 }
 
@@ -607,10 +604,7 @@ private fun Buffer.commonWriteUtf8CodePoint(codePoint: Int) {
 
 @OptIn(UnsafeIoApi::class)
 private fun Buffer.commonReadUtf8(byteCount: Long): String {
-    require(byteCount >= 0 && byteCount <= Int.MAX_VALUE) {
-        "byteCount ($byteCount) is not within the range [0..${Int.MAX_VALUE})"
-    }
-    require(byteCount)
+    // Invariant: byteCount was request()'ed into this buffer beforehand
     if (byteCount == 0L) return ""
 
     UnsafeBufferOperations.iterate(this) { ctx, head ->
@@ -624,7 +618,6 @@ private fun Buffer.commonReadUtf8(byteCount: Long): String {
             }
         }
     }
-
-    // If the string spans multiple segments, delegate to readBytes().
-    return readByteArray(byteCount.toInt()).commonToUtf8String()
-}
+        // If the string spans multiple segments, delegate to readBytes()
+        return readByteArray(byteCount.toInt()).commonToUtf8String()
+    }