From 7a344388346ae7a7c25d156944d2ee8210435f66 Mon Sep 17 00:00:00 2001
From: zepumph <michael.kauzmann@colorado.edu>
Date: Fri, 28 Jan 2022 10:01:59 -0700
Subject: [PATCH] add failing unit test to show bug, update doc,
 https://github.com/phetsims/ratio-and-proportion/issues/421

---
 js/common/model/RAPModel.ts                   |  4 +--
 .../getKeyboardInputSnappingMapperTests.ts    | 26 +++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/js/common/model/RAPModel.ts b/js/common/model/RAPModel.ts
index 58b822ae..6ee16417 100644
--- a/js/common/model/RAPModel.ts
+++ b/js/common/model/RAPModel.ts
@@ -231,7 +231,7 @@ unclampedFitness: ${unclampedFitness}
   }
 
   /**
-   * Given a ratio component (antecedent or consequent), determine what it should be to make the current ratio equal to
+   * Given a ratioTerm, determine how the provided RatioTerm should change to, to make the current ratio equal to
    * the target ratio.
    */
   public getIdealValueForTerm( ratioTerm: RatioTerm ): number {
@@ -241,7 +241,7 @@ unclampedFitness: ${unclampedFitness}
     if ( ratioTerm === RatioTerm.CONSEQUENT ) {
       return this.ratio.tupleProperty.value.antecedent / this.targetRatioProperty.value;
     }
-    assert && assert( false, 'Invalidat ratioTerm' );
+    assert && assert( false, 'Invalid ratioTerm' );
     return -1;
   }
 
diff --git a/js/common/view/getKeyboardInputSnappingMapperTests.ts b/js/common/view/getKeyboardInputSnappingMapperTests.ts
index 898fffaa..5b814a94 100644
--- a/js/common/view/getKeyboardInputSnappingMapperTests.ts
+++ b/js/common/view/getKeyboardInputSnappingMapperTests.ts
@@ -165,3 +165,29 @@ QUnit.test( 'test case of 1/2 "ish"', assert => {
   ratioTupleProperty.value = ratioTupleProperty.value.withAntecedent( newValue );
   assert.ok( ratioTupleProperty.value.antecedent === 0.22, 'step up from ideal past next even step' );
 } );
+
+QUnit.test( 'test case of snapping on the 1/2 tick mark (not snapping to in proportion', assert => {
+  // Buggy case from https://github.com/phetsims/ratio-and-proportion/issues/354#issuecomment-796067400
+
+  const model = new RAPModel( Tandem.OPT_OUT );
+  const ratioTupleProperty = model.ratio.tupleProperty;
+
+  const getIdealValue = () => model.getIdealValueForTerm( RatioTerm.ANTECEDENT );
+  const snapConserveFunction = getKeyboardInputSnappingMapper( getIdealValue, keyboardStep, keyboardStep * rapConstants.SHIFT_KEY_MULTIPLIER );
+  model.targetRatioProperty.value = 1 / 2;
+  ratioTupleProperty.value = new RAPRatioTuple( 0.25, 0.5 );
+
+  let newValue = null;
+
+  newValue = snapConserveFunction( 0.3, ratioTupleProperty.value.antecedent, false, model.inProportionProperty.value );
+  ratioTupleProperty.value = ratioTupleProperty.value.withAntecedent( newValue );
+  assert.ok( ratioTupleProperty.value.antecedent === 0.3, 'normal step' );
+
+  newValue = snapConserveFunction( 0.31, ratioTupleProperty.value.antecedent, true, model.inProportionProperty.value );
+  ratioTupleProperty.value = ratioTupleProperty.value.withAntecedent( newValue );
+  assert.ok( ratioTupleProperty.value.antecedent === 0.31, 'normal shift step up' );
+
+  newValue = snapConserveFunction( 0.26, ratioTupleProperty.value.antecedent, false, model.inProportionProperty.value );
+  ratioTupleProperty.value = ratioTupleProperty.value.withAntecedent( newValue );
+  // assert.ok( ratioTupleProperty.value.antecedent === 0.26, 'full step down' ); // FAILING RIGHT NOW, TODO: see https://github.com/phetsims/ratio-and-proportion/issues/421
+} );