InputMutator.swift

// Copyright 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

/// A mutator that changes the input variables of instructions in a program.
public class InputMutator: BaseInstructionMutator {
    public init() {
        super.init(maxSimultaneousMutations: 3)
    }
    
    override public func canMutate(_ instr: Instruction) -> Bool {
        return instr.numInputs > 0 && instr.isMutable
    }
    
    override public func mutate(_ instr: Instruction, _ b: ProgramBuilder) {
        var inouts = b.adopt(instr.inouts)
        
        // Replace one input
        let selectedInput = Int.random(in: 0..<instr.numInputs)
        // TODO get rid of special casing here somehow?
        var newInput: Variable
        if instr.operation is Copy && selectedInput == 0 {
            newInput = b.randVar(ofGuaranteedType: .phi(of: .anything))!
        } else if instr.isBlockEnd {
            // Need to choose from the outer scope
            newInput = b.randVarFromOuterScope()
        } else {
            newInput = b.randVar()
        }
        inouts[selectedInput] = newInput
                
        b.append(Instruction(operation: instr.operation, inouts: inouts))
    }
}