alamb commented on code in PR #11710:
URL: https://github.com/apache/arrow-rs/pull/11710#discussion_r4315537186
##########
arrow-array/src/array/run_array.rs:
##########
@@ -73,12 +73,59 @@ macro_rules! ree_map {
/// └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─┘ │ B │
/// ├─────────────────┤
/// RunArray │ B │ run length
of 'B' = run_ends[2] - run_ends[1] = 3
-/// length = 3 └─────────────────┘
+/// offset = 0, len = 6 └─────────────────┘
Review Comment:
this was a bug
##########
arrow-array/src/array/run_array.rs:
##########
@@ -73,12 +73,59 @@ macro_rules! ree_map {
/// └ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─┘ │ B │
/// ├─────────────────┤
/// RunArray │ B │ run length
of 'B' = run_ends[2] - run_ends[1] = 3
-/// length = 3 └─────────────────┘
+/// offset = 0, len = 6 └─────────────────┘
///
/// Logical array
/// Contents
/// ```
///
+/// # `offset` and `len`
+///
+/// The [`Array::len`] and [`Array::offset`] for a `RunArray` refer to the
+/// **logical** length, not the `run_ends` and `values` child arrays. In the
+/// example above the `RunArray` has length `6` even though `run_ends` and
+/// `values` each contain `3` elements.
+///
+/// The values in `run_ends` represent indexes of the *unsliced* logical array.
+/// This allows zero-copy [`RunArray::slice`]: `run_ends` and `values` are left
+/// untouched and only `offset` and `len` are adjusted.
+///
+/// A [`RunArray`] must satisfy the following invariants:
+///
+/// * every run end is greater than zero and `run_ends` is strictly increasing
+/// * `run_ends` and `values` have the same length
+/// * if `len > 0`, the last run end is greater than or equal to `offset +
len`.
+///
+/// The example below shows the same array as above after calling `slice(1, 3)`
+///
+/// ```text
+/// ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─┐
+/// ┌─────────────────┐ ┌─────────┐ ┌ ─ ─ ─ ─ ─ ─ ─ ─ ┐
Review Comment:
I really wanted this PR when reviewing
https://github.com/apache/arrow-rs/pull/11668#pullrequestreview-5573669641
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