Add a class for assist efficient traversal of dimensions in an IndexedTensor.

2 files changed, 42 insertions, 0 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/tensor/DirectIndexedAddress.java b/vespajlib/src/main/java/com/yahoo/tensor/DirectIndexedAddress.java
new file mode 100644
index 00000000000..80cb545238c
--- /dev/null
+++ b/vespajlib/src/main/java/com/yahoo/tensor/DirectIndexedAddress.java
@@ -0,0 +1,38 @@
+package com.yahoo.tensor;
+
+/**
+ * Utility class for efficient access and iteration along dimensions in Indexed tensors.
+ * Usage: Use setIndex to lock the indexes of the dimensions that don't change in this iteration.
+ *        long base = addr.getIndex();
+ *        long stride = addr.getStride(dimension)
+ *        i = 0...size_of_dimension
+ *            double value = tensor.get(base + i * stride);
+ */
+public final class DirectIndexedAddress {
+    private final DimensionSizes sizes;
+    private final int [] indexes;
+    private long directIndex;
+    private DirectIndexedAddress(DimensionSizes sizes) {
+        this.sizes = sizes;
+        indexes = new int[sizes.dimensions()];
+        directIndex = 0;
+    }
+    static DirectIndexedAddress of(DimensionSizes sizes) {
+        return new DirectIndexedAddress(sizes);
+    }
+    /** Sets the current index of a dimension */
+    public void setIndex(int dimension, int index) {
+        if (index < 0 || index >= sizes.size(dimension)) {
+            throw new IndexOutOfBoundsException("Index " + index + " outside of [0," + sizes.size(dimension) + ">");
+        }
+        int diff = index - indexes[dimension];
+        directIndex += getStride(dimension) * diff;
+        indexes[dimension] = index;
+    }
+    /** Retrieve the index that can be used for direct lookup in an indexed tensor. */
+    public long getIndex() { return directIndex; }
+    /** returns the stride to be used for the given dimension */
+    public long getStride(int dimension) {
+        return sizes.productOfDimensionsAfter(dimension);
+    }
+}
diff --git a/vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java b/vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java
index 1319675f5d4..d43b4a03f72 100644
--- a/vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java
+++ b/vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java
@@ -93,6 +93,10 @@ public abstract class IndexedTensor implements Tensor {
         return get(toValueIndex(indexes, dimensionSizes));
     }
 
+    public double get(DirectIndexedAddress address) {
+        return get(address.getIndex());
+    }
+    public DirectIndexedAddress directAddress() { return DirectIndexedAddress.of(dimensionSizes); }
     /**
      * Returns the value at the given indexes as a float
      *