Add buffer relocation in reader

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add buffer relocation in reader
+
 ## [2.4.0] - 2024-03-27
 
 This is a feature release of **picocsv**.   

src/main/java/nbbrd/picocsv/Csv.java

@@ -677,23 +677,29 @@ public boolean readLine() throws IOException {
             // WARNING: default value in JDK21 -XX:MinJumpTableSize=10
             switch (state) {
                 case STATE_0_READY:
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_1_FIRST:
                     skipRemainingFields();
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_2_NOT_LAST:
                     skipRemainingFields();
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_3_LAST:
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_4_SINGLE:
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_5_MISSING:
+                    relocateBuffer();
                     parseNextField(true);
                     return state != STATE_6_DONE;
                 case STATE_6_DONE:
@@ -779,9 +785,21 @@ private void skipRemainingFields() throws IOException {
             } while (state == STATE_2_NOT_LAST);
         }
 
+        private void relocateBuffer() throws IOException {
+            if (bufferIndex > buffer.length / 2) {
+                int remainingChars = buffer.length - bufferIndex;
+                System.arraycopy(buffer, bufferIndex, buffer, 0, remainingChars);
+                int newChars = charReader.read(buffer, remainingChars, bufferIndex);
+                bufferIndex = 0;
+                bufferLength = remainingChars + newChars;
+            }
+        }
+
         // WARNING: main loop; lots of duplication to maximize performances
         // WARNING: comparing ints more performant than comparing chars
         // WARNING: local var access slightly quicker that field access
+        // WARNING: the main trick is to never get out of this method when reading a field
+        // WARNING: and therefore never have to fill the buffer
         private void parseNextField(final boolean firstField) throws IOException {
             int fieldLength = this.fieldLength;
             int i = this.bufferIndex;