From d43de5dec25ee7dd9df1574e34459a5936507bac Mon Sep 17 00:00:00 2001
From: Eduardo Alonso <edu@error404software.com>
Date: Wed, 17 Mar 2021 21:02:27 +0100
Subject: [PATCH 1/2] Add rolling window result centering

Pandas-like center parameter; allows centering the result values instead of having a bunch of NaNs at the beginning of the column.
---
 .../java/tech/tablesaw/columns/Column.java    |  4 ++++
 .../tech/tablesaw/table/RollingColumn.java    | 17 ++++++++++++++--
 .../tablesaw/table/RollingColumnTest.java     | 20 +++++++++++++++----
 3 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/core/src/main/java/tech/tablesaw/columns/Column.java b/core/src/main/java/tech/tablesaw/columns/Column.java
index 689a3d17c..9004962c3 100644
--- a/core/src/main/java/tech/tablesaw/columns/Column.java
+++ b/core/src/main/java/tech/tablesaw/columns/Column.java
@@ -164,6 +164,10 @@ default RollingColumn rolling(final int windowSize) {
     return new RollingColumn(this, windowSize);
   }
 
+  default RollingColumn rolling(final int windowSize, final boolean center) {
+    return new RollingColumn(this, windowSize, center);
+  }
+
   String getUnformattedString(int r);
 
   boolean isMissing(int rowNumber);
diff --git a/core/src/main/java/tech/tablesaw/table/RollingColumn.java b/core/src/main/java/tech/tablesaw/table/RollingColumn.java
index c59bcde41..0dfb16bde 100644
--- a/core/src/main/java/tech/tablesaw/table/RollingColumn.java
+++ b/core/src/main/java/tech/tablesaw/table/RollingColumn.java
@@ -11,10 +11,16 @@ public class RollingColumn {
 
   protected final Column<?> column;
   protected final int window;
+  protected final boolean center;
 
-  public RollingColumn(Column<?> column, int window) {
+  public RollingColumn(Column<?> column, int window, boolean center) {
     this.column = column;
     this.window = window;
+    this.center = center;
+  }
+
+  public RollingColumn(Column<?> column, int window) {
+    this(column, window, false);
   }
 
   protected String generateNewColumnName(AggregateFunction<?, ?> function) {
@@ -31,7 +37,9 @@ protected String generateNewColumnName(AggregateFunction<?, ?> function) {
   public <INCOL extends Column<?>, OUT> Column<?> calc(AggregateFunction<INCOL, OUT> function) {
     // TODO: the subset operation copies the array. creating a view would likely be more efficient
     Column<?> result = function.returnType().create(generateNewColumnName(function));
-    for (int i = 0; i < window - 1; i++) {
+    int bound = center ? (window - 1) / 2 : window - 1;
+
+    for (int i = 0; i < bound; i++) {
       result.appendMissing();
     }
     for (int origColIndex = 0; origColIndex < column.size() - window + 1; origColIndex++) {
@@ -46,6 +54,11 @@ public <INCOL extends Column<?>, OUT> Column<?> calc(AggregateFunction<INCOL, OU
         result.appendObj(answer);
       }
     }
+    if (center) {
+      for (int i = 0; i < bound; i++) {
+        result.appendMissing();
+      }
+    }
     return result;
   }
 }
diff --git a/core/src/test/java/tech/tablesaw/table/RollingColumnTest.java b/core/src/test/java/tech/tablesaw/table/RollingColumnTest.java
index d39febd88..85e38230f 100644
--- a/core/src/test/java/tech/tablesaw/table/RollingColumnTest.java
+++ b/core/src/test/java/tech/tablesaw/table/RollingColumnTest.java
@@ -1,9 +1,7 @@
 package tech.tablesaw.table;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static tech.tablesaw.aggregate.AggregateFunctions.countTrue;
-import static tech.tablesaw.aggregate.AggregateFunctions.latestDateTime;
+import static org.junit.jupiter.api.Assertions.*;
+import static tech.tablesaw.aggregate.AggregateFunctions.*;
 
 import java.time.LocalDate;
 import java.time.LocalDateTime;
@@ -52,4 +50,18 @@ public void testRollingCountTrue() {
     assertEquals(1, result.getDouble(3), 0.0);
     assertEquals(2, result.getDouble(4), 0.0);
   }
+
+  @Test
+  public void testCenter() {
+    double[] data = new double[] {1, 2, 3, 4, 5};
+    DoubleColumn doubleColumn = DoubleColumn.create("data", data);
+
+    DoubleColumn result = (DoubleColumn) doubleColumn.rolling(3, true).calc(mean);
+
+    assertEquals(Double.NaN, result.getDouble(0), 0.0);
+    assertNotEquals(Double.NaN, result.getDouble(1), 0.0);
+    assertNotEquals(Double.NaN, result.getDouble(2), 0.0);
+    assertNotEquals(Double.NaN, result.getDouble(3), 0.0);
+    assertEquals(Double.NaN, result.getDouble(4), 0.0);
+  }
 }

From 33f3a655625e8ea06efe5795ea5ac6fc58b6bde1 Mon Sep 17 00:00:00 2001
From: Eduardo Alonso <edu@error404software.com>
Date: Thu, 18 Mar 2021 01:56:07 +0100
Subject: [PATCH 2/2] Add javadoc

---
 core/src/main/java/tech/tablesaw/columns/Column.java | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/core/src/main/java/tech/tablesaw/columns/Column.java b/core/src/main/java/tech/tablesaw/columns/Column.java
index 9004962c3..136543626 100644
--- a/core/src/main/java/tech/tablesaw/columns/Column.java
+++ b/core/src/main/java/tech/tablesaw/columns/Column.java
@@ -164,6 +164,10 @@ default RollingColumn rolling(final int windowSize) {
     return new RollingColumn(this, windowSize);
   }
 
+  /**
+   * @param windowSize size of the rolling window to use
+   * @param center set the operation result at the center of the window or at the end of it
+   */
   default RollingColumn rolling(final int windowSize, final boolean center) {
     return new RollingColumn(this, windowSize, center);
   }