diff --git a/Sources/geometrize-cli/main.swift b/Sources/geometrize-cli/main.swift
index 3db1028..bc123fc 100644
--- a/Sources/geometrize-cli/main.swift
+++ b/Sources/geometrize-cli/main.swift
@@ -110,25 +110,25 @@ while shapeData.count <= shapeCount {
     if options.verbose {
         print("Step \(counter)", terminator: "")
     }
-    let shapes = runner.step(
+    let shapeResult = runner.step(
         options: runnerOptions,
         shapeCreator: nil,
         energyFunction: defaultEnergyFunction,
         addShapePrecondition: defaultAddShapePrecondition
     )
-    if shapes.isEmpty {
+    if let shapeResult {
+        shapeData.append(shapeResult)
         if options.verbose {
-            print(", no shapes added.", terminator: "")
+            print(", \(shapeResult.shape.description) added.", terminator: "")
         }
     } else {
         if options.verbose {
-            print(", \(shapes.map(\.shape).map(\.description).joined(separator: ", ")) added.", terminator: "")
+            print(", no shapes added.", terminator: "")
         }
     }
     if options.verbose {
         print(" Total count of shapes \(shapeData.count ).")
     }
-    shapeData.append(contentsOf: shapes)
     counter += 1
 }
 
diff --git a/Sources/geometrize/GeometrizeModelHillClimb.swift b/Sources/geometrize/GeometrizeModelHillClimb.swift
index 56a4a9a..16b4998 100644
--- a/Sources/geometrize/GeometrizeModelHillClimb.swift
+++ b/Sources/geometrize/GeometrizeModelHillClimb.swift
@@ -40,7 +40,8 @@ class GeometrizeModelHillClimb: GeometrizeModelBase {
         return [state]
     }
 
-    /// Steps the primitive optimization/fitting algorithm.
+    /// Concurrently runs several optimization sessions tying improving image geometrization by adding a shape to it
+    /// and returns result of the best optimization or nil if improvement of image wasn't found.
     /// - Parameters:
     ///   - shapeCreator: A function that will produce the shapes.
     ///   - alpha: The alpha of the shape.
@@ -49,8 +50,7 @@ class GeometrizeModelHillClimb: GeometrizeModelBase {
     ///   - maxThreads: The maximum number of threads to use during this step.
     ///   - energyFunction: A function to calculate the energy.
     ///   - addShapePrecondition: A function to determine whether to accept a shape.
-    /// - Returns: A vector containing data about the shapes added to the model in this step.
-    ///     This may be empty if no shape that improved the image could be found.
+    /// - Returns: Returns `ShapeResult` representing a shape added to improve image or nil if improvement wasn't found.
     func step( // swiftlint:disable:this function_parameter_count
         shapeCreator: () -> any Shape,
         alpha: UInt8,
@@ -59,7 +59,7 @@ class GeometrizeModelHillClimb: GeometrizeModelBase {
         maxThreads: Int,
         energyFunction: @escaping EnergyFunction,
         addShapePrecondition: @escaping ShapeAcceptancePreconditionFunction = defaultAddShapePrecondition
-    ) -> [ShapeResult] {
+    ) -> ShapeResult? {
 
         let states: [State] = getHillClimbState(
             shapeCreator: shapeCreator,
@@ -90,14 +90,13 @@ class GeometrizeModelHillClimb: GeometrizeModelBase {
         let newScore: Double = differencePartial(target: targetBitmap, before: before, after: currentBitmap, score: lastScore, lines: lines)
         guard addShapePrecondition(lastScore, newScore, shape, lines, color, before, currentBitmap, targetBitmap) else {
             currentBitmap = before
-            return []
+            return nil
         }
 
         // Improvement - set new baseline and return the new shape
         lastScore = newScore
 
-        let result: ShapeResult = ShapeResult(score: lastScore, color: color, shape: shape)
-        return [result]
+        return ShapeResult(score: lastScore, color: color, shape: shape)
     }
 
     /// Sets the seed that the random number generators of this model use.
diff --git a/Sources/geometrize/ImageRunner.swift b/Sources/geometrize/ImageRunner.swift
index 8ad265a..5340be6 100644
--- a/Sources/geometrize/ImageRunner.swift
+++ b/Sources/geometrize/ImageRunner.swift
@@ -82,19 +82,20 @@ public struct ImageRunner {
         model = GeometrizeModelHillClimb(target: targetBitmap, initial: initialBitmap)
     }
 
-    /// Updates the internal model once.
+    /// Makes one step of geometrization trying to add a shape to image improving its geometrization.
     /// - Parameters:
     ///   - options: Various configurable settings for doing the step e.g. the shape types to consider.
     ///   - shapeCreator: An optional function for creating and mutating shapes.
     ///   - energyFunction: A function to calculate the energy.
     ///   - addShapePrecondition: A function to determine whether to accept a shape.
-    /// - Returns: A vector containing data about the shapes just added to the internal model.
+    /// - Returns: a ShapeResult representing a shape added to image or nil if a shape improving image
+    /// geometrization wasn't found.
     public mutating func step(
         options: ImageRunnerOptions,
         shapeCreator: (() -> any Shape)? = nil,
         energyFunction: @escaping EnergyFunction,
         addShapePrecondition: @escaping ShapeAcceptancePreconditionFunction
-    ) -> [ShapeResult] {
+    ) -> ShapeResult? {
         let (xMin, yMin, xMax, yMax) = mapShapeBoundsToImage(options: options.shapeBounds, image: model.getTarget())
         let types = options.shapeTypes
 
@@ -102,7 +103,7 @@ public struct ImageRunner {
 
         model.setSeed(options.seed)
 
-        let result: [ShapeResult] = model.step(
+        return model.step(
             shapeCreator: shapeCreator,
             alpha: options.alpha,
             shapeCount: options.shapeCount,
@@ -111,8 +112,6 @@ public struct ImageRunner {
             energyFunction: energyFunction,
             addShapePrecondition: addShapePrecondition
         )
-
-        return result
     }
 
     public var currentBitmap: Bitmap {
diff --git a/Tests/geometrizeTests/ImageRunnerTests.swift b/Tests/geometrizeTests/ImageRunnerTests.swift
index 67ffd6f..3fdc339 100644
--- a/Tests/geometrizeTests/ImageRunnerTests.swift
+++ b/Tests/geometrizeTests/ImageRunnerTests.swift
@@ -43,9 +43,11 @@ final class ImageRunnerTests: XCTestCase {
         var counter = 0
         while shapeData.count <= 1000 /* Here set count of shapes final image should have. Remember background is the first shape. */ {
             print("Step \(counter)", terminator: "")
-            let shapes = runner.step(options: options, shapeCreator: nil, energyFunction: defaultEnergyFunction, addShapePrecondition: defaultAddShapePrecondition)
-            print(", \(shapes.isEmpty ? "no" : String(shapes.count)) shape(s) was/were added. Total count of shapes \(shapeData.count ).")
-            shapeData.append(contentsOf: shapes)
+            let shapeResult = runner.step(options: options, shapeCreator: nil, energyFunction: defaultEnergyFunction, addShapePrecondition: defaultAddShapePrecondition)
+            if let shapeResult {
+                shapeData.append(shapeResult)
+            }
+            print(", \(shapeResult == nil ? "no" : "1") shape was added. Total count of shapes \(shapeData.count ).")
             counter += 1
         }