Loki: add backend-forward mode to queries, update log-row-context (#47726)

* loki: add helper function to sort dataframe by time

* loki: add direction-attribute to queries

* loki: make log-row-context code backward-compatible

* better comment

Co-authored-by: Ivana Huckova <30407135+ivanahuckova@users.noreply.github.com>

* fixed test

* simplified code

Co-authored-by: Ivana Huckova <30407135+ivanahuckova@users.noreply.github.com>

public/app/plugins/datasource/loki/sortDataFrame.ts

@@ -0,0 +1,70 @@
+import { DataFrame, Field, SortedVector } from '@grafana/data';
+
+type SortDirection = 'ASCENDING' | 'DESCENDING';
+
+// creates the `index` for the sorting.
+// this is needed by the `SortedVector`.
+// the index is an array of numbers, and it defines an order.
+// at every slot in the index the values is the position of
+// the sorted item.
+// for example, an index of [3,1,2] means that
+// in the dataframe, that has 3 rows, after sorting:
+// - the third row will become the first
+// - the first row will become the second
+// - the second row will become the third
+function makeIndex(field: Field<string>, dir: SortDirection): number[] {
+  const fieldValues: string[] = field.values.toArray();
+
+  // we first build an array which is [0,1,2,3....]
+  const index = Array(fieldValues.length);
+  for (let i = 0; i < index.length; i++) {
+    index[i] = i;
+  }
+
+  const isAsc = dir === 'ASCENDING';
+
+  index.sort((a: number, b: number): number => {
+    // we need to answer this question:
+    // in the field-used-for-sorting, how would we compare value-at-index-a to value-at-index-b?
+    const valA = fieldValues[a];
+    const valB = fieldValues[b];
+    if (valA < valB) {
+      return isAsc ? -1 : 1;
+    }
+
+    if (valA > valB) {
+      return isAsc ? 1 : -1;
+    }
+
+    return 0;
+  });
+
+  return index;
+}
+
+// sort a dataframe that is in the Loki format ascending or descending,
+// based on the nanosecond-timestamp
+export function sortDataFrameByTime(frame: DataFrame, dir: SortDirection): DataFrame {
+  const { fields, ...rest } = frame;
+
+  // we use the approach used in @grafana/data/sortDataframe.
+  // we cannot use it directly, because our tsNs field has a type=time,
+  // so we have to build the `index` manually.
+
+  const tsNsField = fields.find((field) => field.name === 'tsNs');
+  if (tsNsField === undefined) {
+    throw new Error('missing nanosecond-timestamp field. should never happen');
+  }
+
+  const index = makeIndex(tsNsField, dir);
+
+  return {
+    ...rest,
+    fields: fields.map((field) => ({
+      ...field,
+      values: new SortedVector(field.values, index),
+    })),
+  };
+
+  return frame;
+}