From 9f96b71b014e7b113dbb35a989b54709342ee69a Mon Sep 17 00:00:00 2001
From: Ziya Kapoor <ziya@research.digiscrypt.com>
Date: Thu, 22 Oct 2026 20:16:22 +0530
Subject: [PATCH] Reject undersized map in
 FrequentLongsSketch.getInstance(String)

---
 .../frequencies/FrequentLongsSketch.java      | 10 ++++++
 .../frequencies/LongsSketchTest.java          | 31 +++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/main/java/org/apache/datasketches/frequencies/FrequentLongsSketch.java b/src/main/java/org/apache/datasketches/frequencies/FrequentLongsSketch.java
index 81fba6fc8..001e80ad2 100644
--- a/src/main/java/org/apache/datasketches/frequencies/FrequentLongsSketch.java
+++ b/src/main/java/org/apache/datasketches/frequencies/FrequentLongsSketch.java
@@ -335,6 +335,16 @@ public static FrequentLongsSketch getInstance(final String string) {
           "Possible Corruption: Incorrect # of tokens: " + numTokens
             + ", numActive: " + numActive);
     }
+    if (lgCur < LG_MIN_MAP_SIZE) {
+      throw new SketchesArgumentException(
+          "Possible Corruption: Map length must be at least " + (1 << LG_MIN_MAP_SIZE) + ": "
+            + tokens[STR_PREAMBLE_TOKENS + 1]);
+    }
+    final long maxActive = (long) ((1L << lgCur) * ReversePurgeLongHashMap.getLoadFactor());
+    if ((numActive < 0) || (numActive > maxActive)) {
+      throw new SketchesArgumentException(
+          "Possible Corruption: numActive exceeds map capacity: " + numActive);
+    }
 
     final FrequentLongsSketch sketch = new FrequentLongsSketch(lgMax, lgCur);
     sketch.streamWeight = streamWt;
diff --git a/src/test/java/org/apache/datasketches/frequencies/LongsSketchTest.java b/src/test/java/org/apache/datasketches/frequencies/LongsSketchTest.java
index 4df537c93..650405751 100644
--- a/src/test/java/org/apache/datasketches/frequencies/LongsSketchTest.java
+++ b/src/test/java/org/apache/datasketches/frequencies/LongsSketchTest.java
@@ -577,6 +577,37 @@ public void checkStringDeserEmptyFlagWithStreamLength() {
     FrequentLongsSketch.getInstance(s);
   }
 
+  @Test(expectedExceptions = SketchesArgumentException.class, timeOut = 5000)
+  public void checkStringDeserMapLengthTooSmall() {
+    final String s = "1,"  //serVer
+             + "10," //FamID
+             + "3,"  //lgMaxMapSz
+             + "5,"  //Empty Flag
+             + "0,"  //stream Len: empty
+             + "0,"  //error offset
+             + "0,"  //numActive
+             + "2,"; //curMapLen ... corrupted, below the minimum of 8
+    final FrequentLongsSketch fls = FrequentLongsSketch.getInstance(s);
+    fls.update(1);
+    fls.update(2);
+    fls.update(3);
+  }
+
+  @Test(expectedExceptions = SketchesArgumentException.class, timeOut = 5000)
+  public void checkStringDeserNumActiveExceedsMapCapacity() {
+    final StringBuilder sb = new StringBuilder();
+    sb.append("1,")  //serVer
+      .append("10,") //FamID
+      .append("3,")  //lgMaxMapSz
+      .append("0,")  //Empty Flag = false
+      .append("9,")  //stream Len
+      .append("0,")  //error offset
+      .append("9,")  //numActive ... corrupted, more than the map can hold
+      .append("8,"); //curMapLen
+    for (int i = 1; i <= 9; i++) { sb.append(i).append(",1,"); }
+    FrequentLongsSketch.getInstance(sb.toString());
+  }
+
   @Test
   public void checkGetEpsilon() {
     assertEquals(FrequentLongsSketch.getEpsilon(1024), 3.5 / 1024, 0.0);