alamb commented on code in PR #13333:
URL: https://github.com/apache/arrow/pull/13333#discussion_r971268286
##########
docs/source/format/Columnar.rst:
##########
@@ -765,6 +765,66 @@ application.
We discuss dictionary encoding as it relates to serialization further
below.
+.. _run-length-encoded-layout:
+
+Run-Length Encoded Layout
+-------------------------
+
+Run-Length is a data representation that represents data as sequences of the
+same value, called runs. Each run is represented as a value, and an integer
+describing how often this value is repeated.
+
+Any array can be run-length encoded. A run-length encoded array has no buffers
+by itself, but has two child arrays. The first one holds a signed 32-bit
integer
Review Comment:
```suggestion
by itself, but has two child arrays. The first one holds a signed 32-bit
integer called a "run end"
```
##########
docs/source/format/Columnar.rst:
##########
@@ -765,6 +765,66 @@ application.
We discuss dictionary encoding as it relates to serialization further
below.
+.. _run-length-encoded-layout:
+
+Run-Length Encoded Layout
+-------------------------
+
+Run-Length is a data representation that represents data as sequences of the
+same value, called runs. Each run is represented as a value, and an integer
+describing how often this value is repeated.
+
+Any array can be run-length encoded. A run-length encoded array has no buffers
+by itself, but has two child arrays. The first one holds a signed 32-bit
integer
+for each run. The actual values of each run are held the second child array.
+
+The values in the first child array represent the length of each run. They do
+not hold the length of the respective run directly, but the accumulated length
+of all runs from the first to the current one, i.e. the logical index where the
+current run ends. This allows relatively efficient random access from a logical
+index using binary search. The length of an individual run can be determined by
+subtracting two adjacent values.
+
+A run has to have a length of at least 1. This means the values in the
+run ends array all positive and in strictly ascending order. A run end cannot
be
+null.
+
+As an example, you could have the following data: ::
+
+ type: Float32
+ [1.0, 1.0, 1.0, 1.0, null, null, 2.0]
+
+In Run-length-encoded form, this could appear as:
+
+::
+
+ * Length: 7, Null count: 2
+ * Children arrays:
+
+ * run ends (Int32):
+ * Length: 3, Null count: 0
+ * Validity bitmap buffer: Not required
Review Comment:
```suggestion
* Validity bitmap buffer: Not present (not allowed)
```
See comment above
##########
docs/source/format/Columnar.rst:
##########
@@ -765,6 +765,66 @@ application.
We discuss dictionary encoding as it relates to serialization further
below.
+.. _run-length-encoded-layout:
+
+Run-Length Encoded Layout
+-------------------------
+
+Run-Length is a data representation that represents data as sequences of the
+same value, called runs. Each run is represented as a value, and an integer
+describing how often this value is repeated.
+
+Any array can be run-length encoded. A run-length encoded array has no buffers
+by itself, but has two child arrays. The first one holds a signed 32-bit
integer
+for each run. The actual values of each run are held the second child array.
+
+The values in the first child array represent the length of each run. They do
+not hold the length of the respective run directly, but the accumulated length
+of all runs from the first to the current one, i.e. the logical index where the
+current run ends. This allows relatively efficient random access from a logical
+index using binary search. The length of an individual run can be determined by
+subtracting two adjacent values.
+
+A run has to have a length of at least 1. This means the values in the
+run ends array all positive and in strictly ascending order. A run end cannot
be
+null.
Review Comment:
I recommend adding something to this section saying "RLE Arrays must not
have have a validity bitmap" which would also imply a run end could not be
null. As worded I think it is unclear if a validity bitmap is allowed but must
be all `valid` or if the bitmap is not allowed
##########
docs/source/format/Columnar.rst:
##########
@@ -765,6 +765,66 @@ application.
We discuss dictionary encoding as it relates to serialization further
below.
+.. _run-length-encoded-layout:
+
+Run-Length Encoded Layout
+-------------------------
+
+Run-Length is a data representation that represents data as sequences of the
+same value, called runs. Each run is represented as a value, and an integer
+describing how often this value is repeated.
+
+Any array can be run-length encoded. A run-length encoded array has no buffers
+by itself, but has two child arrays. The first one holds a signed 32-bit
integer
+for each run. The actual values of each run are held the second child array.
+
+The values in the first child array represent the length of each run. They do
+not hold the length of the respective run directly, but the accumulated length
+of all runs from the first to the current one, i.e. the logical index where the
+current run ends. This allows relatively efficient random access from a logical
+index using binary search. The length of an individual run can be determined by
+subtracting two adjacent values.
+
+A run has to have a length of at least 1. This means the values in the
Review Comment:
```suggestion
A run must have have a length of at least 1. This means the values in the
```
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