feat: add PrettyFormatInlineArrays feature to keep arrays on single line

Fixes #2972

Added new JSONWriter.Feature.PrettyFormatInlineArrays that, when used with
PrettyFormat, keeps array elements on a single line instead of expanding
each element to its own line. This matches the original fastjson behavior.

Changes:
- Added PrettyFormatInlineArrays feature flag (bit 46)
- Added levelArray bitmask to track array vs object context at each level
- Modified startArray/endArray/writeComma in JSONWriterUTF8 and JSONWriterUTF16
- Added comprehensive tests for edge cases

Author: sujaykarve111

core/src/main/java/com/alibaba/fastjson2/JSONWriter.java

@@ -84,12 +84,22 @@ public abstract class JSONWriter
 
     protected boolean startObject;
     protected int level;
+    /**
+     * Bitmask tracking which nesting levels are arrays (vs objects).
+     * Bit N is set if level N is an array context.
+     */
+    protected long levelArray;
     protected int off;
     protected Object rootObject;
     protected IdentityHashMap<Object, Path> refs;
     protected Path path;
     protected String lastReference;
     protected byte pretty;
+    /**
+     * When true and pretty formatting is enabled, arrays are written inline
+     * (on a single line) rather than with line breaks between elements.
+     */
+    protected boolean prettyInlineArrays;
     protected Object attachment;
 
     protected JSONWriter(
@@ -119,6 +129,7 @@ protected JSONWriter(
         } else {
             pretty = PRETTY_NON;
         }
+        prettyInlineArrays = (context.features & PrettyFormatInlineArrays.mask) != 0;
     }
 
     /**
@@ -4430,7 +4441,18 @@ public enum Feature {
          *
          * @since 2.0.61
          */
-        WriteFloatSpecialAsString(1L << 45);
+        WriteFloatSpecialAsString(1L << 45),
+
+        /**
+         * Feature that controls whether arrays are formatted inline (on a single line) when using PrettyFormat.
+         * When enabled along with PrettyFormat, array elements will be written on a single line like [1, 2, 3]
+         * instead of having each element on a separate line.
+         *
+         * <p>This feature requires {@link PrettyFormat} to also be enabled to have any effect.</p>
+         *
+         * @since 2.0.61
+         */
+        PrettyFormatInlineArrays(1L << 46);
 
         public final long mask;
 

core/src/main/java/com/alibaba/fastjson2/JSONWriterUTF16.java

@@ -188,7 +188,11 @@ public final void writeComma() {
 
         chars[off++] = (byte) ',';
         if (pretty != PRETTY_NON) {
-            off = indent(chars, off);
+            // Check if we're in an array context with inline arrays enabled
+            boolean inArrayContext = (levelArray & (1L << level)) != 0;
+            if (!(prettyInlineArrays && inArrayContext)) {
+                off = indent(chars, off);
+            }
         }
         this.off = off;
     }
@@ -198,6 +202,8 @@ public final void startArray() {
         if (++level > context.maxLevel) {
             overflowLevel();
         }
+        // Mark current level as array context
+        levelArray |= (1L << level);
 
         int off = this.off;
         int minCapacity = off + 3 + pretty * level;
@@ -207,14 +213,16 @@ public final void startArray() {
         }
 
         chars[off++] = (byte) '[';
-        if (pretty != PRETTY_NON) {
+        if (pretty != PRETTY_NON && !prettyInlineArrays) {
             off = indent(chars, off);
         }
         this.off = off;
     }
 
     @Override
     public final void endArray() {
+        // Clear array context for current level before decrementing
+        levelArray &= ~(1L << level);
         level--;
         int off = this.off;
         int minCapacity = off + 1 + (pretty == 0 ? 0 : pretty * level + 1);
@@ -223,7 +231,7 @@ public final void endArray() {
             chars = grow(minCapacity);
         }
 
-        if (pretty != PRETTY_NON) {
+        if (pretty != PRETTY_NON && !prettyInlineArrays) {
             off = indent(chars, off);
         }
         chars[off] = (byte) ']';

core/src/main/java/com/alibaba/fastjson2/JSONWriterUTF8.java

@@ -255,7 +255,11 @@ public final void writeComma() {
         }
         bytes[off++] = ',';
         if (pretty != PRETTY_NON) {
-            off = indent(bytes, off);
+            // Check if we're in an array context with inline arrays enabled
+            boolean inArrayContext = (levelArray & (1L << level)) != 0;
+            if (!(prettyInlineArrays && inArrayContext)) {
+                off = indent(bytes, off);
+            }
         }
         this.off = off;
     }
@@ -265,6 +269,8 @@ public final void startArray() {
         if (++level > context.maxLevel) {
             overflowLevel();
         }
+        // Mark current level as array context
+        levelArray |= (1L << level);
 
         int off = this.off;
         int minCapacity = off + 3 + pretty * level;
@@ -273,22 +279,24 @@ public final void startArray() {
             bytes = grow(minCapacity);
         }
         bytes[off++] = '[';
-        if (pretty != PRETTY_NON) {
+        if (pretty != PRETTY_NON && !prettyInlineArrays) {
             off = indent(bytes, off);
         }
         this.off = off;
     }
 
     @Override
     public final void endArray() {
+        // Clear array context for current level before decrementing
+        levelArray &= ~(1L << level);
         level--;
         int off = this.off;
         int minCapacity = off + 1 + (pretty == 0 ? 0 : pretty * level + 1);
         byte[] bytes = this.bytes;
         if (minCapacity > bytes.length) {
             bytes = grow(minCapacity);
         }
-        if (pretty != PRETTY_NON) {
+        if (pretty != PRETTY_NON && !prettyInlineArrays) {
             off = indent(bytes, off);
         }
         bytes[off] = ']';