change(web): spin off method, add unit tests to verify handling of keep vs revert suggestions

web/src/engine/predictive-text/worker-thread/src/main/correction/context-transition.ts

@@ -19,6 +19,15 @@ import Reversion = LexicalModelTypes.Reversion;
 import Suggestion = LexicalModelTypes.Suggestion;
 import Transform = LexicalModelTypes.Transform;
 
+export interface TransitionReversionView extends Pick<ContextTransition, 'reversion'> {
+  /**
+   * Gets the context state resulting from the context transition event,
+   * including any generated suggestions and data regarding potential
+   * application thereof.
+   */
+  final: Pick<ContextState, 'suggestions'>
+}
+
 /**
  * Represents the transition between two context states as triggered
  * by input keystrokes or applied suggestions.

web/src/engine/predictive-text/worker-thread/src/main/model-compositor.ts

@@ -2,7 +2,7 @@ import * as models from '@keymanapp/models-templates';
 import { LexicalModelTypes } from '@keymanapp/common-types';
 
 import { TransformUtils } from './transformUtils.js';
-import { applySuggestionCasing, correctAndEnumerate, createDefaultKeep, dedupeSuggestions, finalizeSuggestions, predictionAutoSelect, processSimilarity, toAnnotatedSuggestion, tupleDisplayOrderSort } from './predict-helpers.js';
+import { applySuggestionCasing, correctAndEnumerate, createDefaultKeep, dedupeSuggestions, finalizeSuggestions, predictionAutoSelect, prependReversion, processSimilarity, toAnnotatedSuggestion, tupleDisplayOrderSort } from './predict-helpers.js';
 import { detectCurrentCasing, determineModelTokenizer, determineModelWordbreaker, determinePunctuationFromModel } from './model-helpers.js';
 
 import { ContextTracker } from './correction/context-tracker.js';
@@ -206,18 +206,8 @@ export class ModelCompositor {
       this.SUGGESTION_ID_SEED++;
     });
 
-    if(revertableTransitionId) {
-      const reversion = this.contextTracker.peek(revertableTransitionId)?.reversion;
-      if(reversion) {
-        if(suggestions[0]?.tag == 'keep') {
-          const keep = suggestions.shift();
-          suggestions.unshift(reversion);
-          suggestions.unshift(keep);
-        } else {
-          suggestions.unshift(reversion)
-        }
-      }
-    }
+    const transitionToRevert = this.contextTracker?.peek(revertableTransitionId);
+    prependReversion(suggestions, transitionToRevert);
 
     // Store the suggestions on the final token of the current context state (if it exists).
     // Or, once phrase-level suggestions are possible, on whichever token serves as each prediction's root.
@@ -239,8 +229,8 @@ export class ModelCompositor {
     // Step 1:  re-use the original input Transform as the reversion's Transform.
     // The Web engine will restore the original state of the context before accepting
     // and before reverting; all we need to do is put the original keystroke back in place.
-    let reversionTransform: Transform = originalInput 
-      ? { ...originalInput } 
+    let reversionTransform: Transform = originalInput
+      ? { ...originalInput }
       : { insert: '', deleteLeft: 0, id: suggestion.transform.id };
 
     // Step 2:  building the proper 'displayAs' string for the Reversion

web/src/engine/predictive-text/worker-thread/src/main/predict-helpers.ts

@@ -9,7 +9,7 @@ import { ContextTokenization } from './correction/context-tokenization.js';
 import { ContextTracker } from './correction/context-tracker.js';
 import { ContextToken } from './correction/context-token.js';
 import { ContextState, determineContextSlideTransform } from './correction/context-state.js';
-import { ContextTransition } from './correction/context-transition.js';
+import { ContextTransition, TransitionReversionView } from './correction/context-transition.js';
 import { ExecutionTimer } from './correction/execution-timer.js';
 import { ModelCompositor } from './model-compositor.js';
 import { getBestTokenMatches } from './correction/distance-modeler.js';
@@ -1196,4 +1196,35 @@ export function toAnnotatedSuggestion(
   }
 
   return result;
+}
+
+/**
+ * For applicable scenarios, this mutates the passed-in suggestion array by
+ * prepending a predictive-text reversion that restores the context to a prior
+ * state.  Otherwise, it leaves the suggestion array unaltered.
+ * @param suggestions
+ * @param transitionToRevert
+ * @returns
+ */
+export function prependReversion(suggestions: Suggestion[], transitionToRevert: TransitionReversionView) {
+  if(transitionToRevert) {
+    const reversion = transitionToRevert.reversion;
+    if(reversion) {
+      if(suggestions[0]?.tag == 'keep') {
+        const appliedId = -reversion.id;
+        const appliedSuggestion = transitionToRevert.final.suggestions.find((s) => s.id == appliedId);
+        // If the selected suggestion was itself a `keep`, we don't need a
+        // reversion.  They'd do the same thing.
+        if(appliedSuggestion.tag != 'keep') {
+          const keep = suggestions.shift();
+          suggestions.unshift(reversion);
+          suggestions.unshift(keep);
+        }
+      } else {
+        suggestions.unshift(reversion);
+      }
+    }
+  }
+
+  return suggestions;
 }

web/src/engine/predictive-text/worker-thread/src/main/test-index.ts

@@ -3,7 +3,7 @@ export * from './correction/context-state.js';
 export * from './correction/context-token.js';
 export * from './correction/context-tokenization.js';
 export { ContextTracker } from './correction/context-tracker.js';
-export { ContextTransition } from './correction/context-transition.js';
+export * from './correction/context-transition.js';
 export * from './correction/correction-searchable.js';
 export * from './correction/correction-result-mapping.js';
 export * from './correction/distance-modeler.js';

web/src/test/auto/headless/engine/predictive-text/worker-thread/prediction-helpers/prepend-reversion.tests.ts

@@ -0,0 +1,101 @@
+
+/*
+ * Keyman is copyright (C) SIL Global. MIT License.
+ *
+ * Created by jahorton on 2026-07-27
+ *
+ * This file contains tests designed to ensure predictive text does not
+ * provide matching 'keep' and 'revert' suggestions in any context.
+ */
+
+
+import { assert } from 'chai';
+
+import { LexicalModelTypes } from '@keymanapp/common-types';
+import { prependReversion, type TransitionReversionView } from "@keymanapp/lm-worker/test-index";
+
+import Suggestion = LexicalModelTypes.Suggestion;
+
+describe('prependReversion', () => {
+  it(`prepends reversions when reverting a non-'keep' suggestion`, () => {
+    // context: Original was appl+u, corrected to apply.  Reached via bksp.
+    const suggestions: Suggestion[] = [{
+      tag: 'keep',
+      transform: { insert: 'apply', deleteLeft: 6, id: 3 },
+      displayAs: '"apply"',
+      id: 5,
+      matchesModel: false
+    } as Suggestion];
+
+    const revertable: TransitionReversionView = {
+      reversion: {
+        tag: 'revert',
+        transform: { insert: 'u', deleteLeft: 0, id: 3 },
+        id: -3,
+        displayAs: '"applu"'
+      },
+      final: {
+        suggestions: [{
+          tag: 'keep',
+          transform: { insert: 'applu', deleteLeft: 4, id: 3 },
+          displayAs: '"applu"',
+          id: 2,
+          matchesModel: false
+        } as Suggestion, {
+          transform: { insert: 'apply', deleteLeft: 4, id: 3 },
+          displayAs: 'apply',
+          id: 3
+        }, {
+          transform: { insert: 'applied', deleteLeft: 4, id: 3 },
+          displayAs: 'applied',
+          id: 4
+        }]
+      }
+    };
+
+    prependReversion(suggestions, revertable);
+
+    assert.includeMembers(suggestions, [revertable.reversion]);
+  });
+
+  it(`does not prepend reversions when reverting a 'keep' suggestion`, () => {
+    // context: Original was appl+u, corrected to apply
+    const suggestions: Suggestion[] = [{
+      tag: 'keep',
+      transform: { insert: 'applu', deleteLeft: 5, id: 3 },
+      displayAs: '"applu"',
+      id: 5,
+      matchesModel: false
+    } as Suggestion];
+
+    const revertable: TransitionReversionView = {
+      reversion: {
+        tag: 'revert',
+        transform: { insert: 'u', deleteLeft: 0, id: 3 },
+        id: -2,
+        displayAs: '"applu"'
+      },
+      final: {
+        suggestions: [{
+          tag: 'keep',
+          transform: { insert: 'applu', deleteLeft: 4, id: 3 },
+          displayAs: '"applu"',
+          id: 2,
+          matchesModel: false
+        } as Suggestion, {
+          transform: { insert: 'apply', deleteLeft: 4, id: 3 },
+          displayAs: 'apply',
+          id: 3
+        }, {
+          transform: { insert: 'applied', deleteLeft: 4, id: 3 },
+          displayAs: 'applied',
+          id: 4
+        }]
+      }
+    };
+
+    prependReversion(suggestions, revertable);
+
+    assert.notIncludeMembers(suggestions, [revertable.reversion]);
+  });
+});