fix: allow single entry in fractional operator (#1935)

<!-- Please use this template for your pull request. -->
<!-- Please use the sections that you need and delete other sections -->

## This PR
<!-- add the description of the PR here -->

- allows single entries in the fractional operator

### Related Issues
<!-- add here the GitHub issue that this PR resolves if applicable -->

#1924

### Notes
<!-- any additional notes for this PR -->

### Follow-up Tasks
<!-- anything that is related to this PR but not done here should be
noted under this section -->
<!-- if there is a need for a new issue, please link it here -->

### How to test
<!-- if applicable, add testing instructions under this section -->

---------

Signed-off-by: Lea Konvalinka <lea.konvalinka@dynatrace.com>
Signed-off-by: Todd Baert <todd.baert@dynatrace.com>
Co-authored-by: Todd Baert <todd.baert@dynatrace.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

core/pkg/evaluator/fractional.go

@@ -57,8 +57,8 @@ func parseFractionalEvaluationData(values, data any, logger *logger.Logger) (str
 	if !ok {
 		return "", nil, errors.New("fractional evaluation data is not an array")
 	}
-	if len(valuesArray) < 2 {
-		return "", nil, errors.New("fractional evaluation data has length under 2")
+if len(valuesArray) < 1 {
+		return "", nil, errors.New("fractional evaluation data must contain at least one distribution")
 	}
 
 	dataMap, ok := data.(map[string]any)

core/pkg/evaluator/fractional_test.go

@@ -442,6 +442,67 @@ func TestFractionalEvaluation(t *testing.T) {
 			expectedValue:   blueHex,
 			expectedReason:  model.TargetingMatchReason,
 		},
+		"single-entry always returns the sole variant": {
+			flags: []model.Flag{{
+				Key:            "headerColor",
+				State:          "ENABLED",
+				DefaultVariant: redVariant,
+				Variants:       colorVariants,
+				Targeting: []byte(`{
+							"fractional": [
+								["blue", 1]
+							]
+						}`),
+			}},
+			flagKey: "headerColor",
+			context: map[string]any{
+				"targetingKey": "any-user",
+			},
+			expectedVariant: blueVariant,
+			expectedValue:   blueHex,
+			expectedReason:  model.TargetingMatchReason,
+		},
+		"single-entry with explicit bucket-by always returns the sole variant": {
+			flags: []model.Flag{{
+				Key:            "headerColor",
+				State:          "ENABLED",
+				DefaultVariant: redVariant,
+				Variants:       colorVariants,
+				Targeting: []byte(`{
+							"fractional": [
+								{"var": "email"},
+								["green", 100]
+							]
+						}`),
+			}},
+			flagKey: "headerColor",
+			context: map[string]any{
+				"email": "any@user.com",
+			},
+			expectedVariant: greenVariant,
+			expectedValue:   greenHex,
+			expectedReason:  model.TargetingMatchReason,
+		},
+		"single-entry shorthand without weight always returns the sole variant": {
+			flags: []model.Flag{{
+				Key:            "headerColor",
+				State:          "ENABLED",
+				DefaultVariant: redVariant,
+				Variants:       colorVariants,
+				Targeting: []byte(`{
+							"fractional": [
+								["yellow"]
+							]
+						}`),
+			}},
+			flagKey: "headerColor",
+			context: map[string]any{
+				"targetingKey": "any-user",
+			},
+			expectedVariant: yellowVariant,
+			expectedValue:   yellowHex,
+			expectedReason:  model.TargetingMatchReason,
+		},
 	}
 	const reqID = "default"
 	for name, tt := range tests {