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diff --git a/devtools/client/memory/actions/diffing.js b/devtools/client/memory/actions/diffing.js
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+++ b/devtools/client/memory/actions/diffing.js
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+const { assert, reportException } = require("devtools/shared/DevToolsUtils");
+const { actions, diffingState, viewState } = require("../constants");
+const telemetry = require("../telemetry");
+const {
+  getSnapshot,
+  censusIsUpToDate,
+  snapshotIsDiffable,
+  findSelectedSnapshot,
+} = require("../utils");
+// This is a circular dependency, so do not destructure the needed properties.
+const snapshotActions = require("./snapshot");
+
+/**
+ * Toggle diffing mode on or off.
+ */
+const toggleDiffing = exports.toggleDiffing = function () {
+  return function (dispatch, getState) {
+    dispatch({
+      type: actions.CHANGE_VIEW,
+      newViewState: getState().diffing ? viewState.CENSUS : viewState.DIFFING,
+      oldDiffing: getState().diffing,
+      oldSelected: findSelectedSnapshot(getState()),
+    });
+  };
+};
+
+/**
+ * Select the given snapshot for diffing.
+ *
+ * @param {snapshotModel} snapshot
+ */
+const selectSnapshotForDiffing = exports.selectSnapshotForDiffing = function (snapshot) {
+  assert(snapshotIsDiffable(snapshot),
+         "To select a snapshot for diffing, it must be diffable");
+  return { type: actions.SELECT_SNAPSHOT_FOR_DIFFING, snapshot };
+};
+
+/**
+ * Compute the difference between the first and second snapshots.
+ *
+ * @param {HeapAnalysesClient} heapWorker
+ * @param {snapshotModel} first
+ * @param {snapshotModel} second
+ */
+const takeCensusDiff = exports.takeCensusDiff = function (heapWorker, first, second) {
+  return function* (dispatch, getState) {
+    assert(snapshotIsDiffable(first),
+           `First snapshot must be in a diffable state, found ${first.state}`);
+    assert(snapshotIsDiffable(second),
+           `Second snapshot must be in a diffable state, found ${second.state}`);
+
+    let report, parentMap;
+    let display = getState().censusDisplay;
+    let filter = getState().filter;
+
+    if (censusIsUpToDate(filter, display, getState().diffing.census)) {
+      return;
+    }
+
+    do {
+      if (!getState().diffing
+          || getState().diffing.firstSnapshotId !== first.id
+          || getState().diffing.secondSnapshotId !== second.id) {
+        // If we stopped diffing or stopped and then started diffing a different
+        // pair of snapshots, then just give up with diffing this pair. In the
+        // latter case, a newly spawned task will handle the diffing for the new
+        // pair.
+        return;
+      }
+
+      display = getState().censusDisplay;
+      filter = getState().filter;
+
+      dispatch({
+        type: actions.TAKE_CENSUS_DIFF_START,
+        first,
+        second,
+        filter,
+        display,
+      });
+
+      let opts = display.inverted
+        ? { asInvertedTreeNode: true }
+        : { asTreeNode: true };
+      opts.filter = filter || null;
+
+      try {
+        ({ delta: report, parentMap } = yield heapWorker.takeCensusDiff(
+          first.path,
+          second.path,
+          { breakdown: display.breakdown },
+          opts));
+      } catch (error) {
+        reportException("actions/diffing/takeCensusDiff", error);
+        dispatch({ type: actions.DIFFING_ERROR, error });
+        return;
+      }
+    }
+    while (filter !== getState().filter
+           || display !== getState().censusDisplay);
+
+    dispatch({
+      type: actions.TAKE_CENSUS_DIFF_END,
+      first,
+      second,
+      report,
+      parentMap,
+      filter,
+      display,
+    });
+
+    telemetry.countDiff({ filter, display });
+  };
+};
+
+/**
+ * Ensure that the current diffing data is up to date with the currently
+ * selected display, filter, etc. If the state is not up-to-date, then a
+ * recompute is triggered.
+ *
+ * @param {HeapAnalysesClient} heapWorker
+ */
+const refreshDiffing = exports.refreshDiffing = function (heapWorker) {
+  return function* (dispatch, getState) {
+    if (getState().diffing.secondSnapshotId === null) {
+      return;
+    }
+
+    assert(getState().diffing.firstSnapshotId,
+           "Should have first snapshot id");
+
+    if (getState().diffing.state === diffingState.TAKING_DIFF) {
+      // There is an existing task that will ensure that the diffing data is
+      // up-to-date.
+      return;
+    }
+
+    const { firstSnapshotId, secondSnapshotId } = getState().diffing;
+
+    const first = getSnapshot(getState(), firstSnapshotId);
+    const second = getSnapshot(getState(), secondSnapshotId);
+    dispatch(takeCensusDiff(heapWorker, first, second));
+  };
+};
+
+/**
+ * Select the given snapshot for diffing and refresh the diffing data if
+ * necessary (for example, if two snapshots are now selected for diffing).
+ *
+ * @param {HeapAnalysesClient} heapWorker
+ * @param {snapshotModel} snapshot
+ */
+const selectSnapshotForDiffingAndRefresh = exports.selectSnapshotForDiffingAndRefresh = function (heapWorker, snapshot) {
+  return function* (dispatch, getState) {
+    assert(getState().diffing,
+           "If we are selecting for diffing, we must be in diffing mode");
+    dispatch(selectSnapshotForDiffing(snapshot));
+    yield dispatch(refreshDiffing(heapWorker));
+  };
+};
+
+/**
+ * Expand the given node in the diffing's census's delta-report.
+ *
+ * @param {CensusTreeNode} node
+ */
+const expandDiffingCensusNode = exports.expandDiffingCensusNode = function (node) {
+  return {
+    type: actions.EXPAND_DIFFING_CENSUS_NODE,
+    node,
+  };
+};
+
+/**
+ * Collapse the given node in the diffing's census's delta-report.
+ *
+ * @param {CensusTreeNode} node
+ */
+const collapseDiffingCensusNode = exports.collapseDiffingCensusNode = function (node) {
+  return {
+    type: actions.COLLAPSE_DIFFING_CENSUS_NODE,
+    node,
+  };
+};
+
+/**
+ * Focus the given node in the snapshot's census's report.
+ *
+ * @param {DominatorTreeNode} node
+ */
+const focusDiffingCensusNode = exports.focusDiffingCensusNode = function (node) {
+  return {
+    type: actions.FOCUS_DIFFING_CENSUS_NODE,
+    node,
+  };
+};