Make divide-by-zero error, handle error gracefully in grow

grow/definition.go

@@ -52,11 +52,8 @@ func (def *Definition) Grow(statusChan chan<- Status) (*vm.Program, error) {
 
 		score, err := def.score(prog)
 		if err != nil {
-			if statusChan != nil {
-				close(statusChan)
-			}
-
-			return nil, err
+			// Can never get best score
+			continue
 		}
 
 		if score > status.BestScore {
@@ -89,7 +86,10 @@ func (def *Definition) score(prog *vm.Program) (uint64, error) {
 
 		sample.SetInputs(state)
 
-		state.Execute() // ignore error
+		err = state.Execute()
+		if err != nil {
+			return 0, err
+		}
 
 		score += sample.matchingOuts(state)
 	}

vm/ophandler.go

@@ -1,5 +1,6 @@
 package vm
 
+import "fmt"
 import "math"
 
 type opHandler func(*State, *Instruction)
@@ -79,8 +80,7 @@ func (state *State) handleDivideUnsigned(instr *Instruction) {
 	in2 := state.readUnsigned(instr.Operands[1])
 
 	if in2 == 0 {
-		// Divide by zero just returns zero
-		state.writeUnsigned(instr.Operands[0], 0)
+		state.setError(fmt.Errorf("Divide by zero"))
 	} else {
 		state.writeUnsigned(instr.Operands[0], in1/in2)
 	}
@@ -91,8 +91,7 @@ func (state *State) handleDivideSigned(instr *Instruction) {
 	in2 := state.readSigned(instr.Operands[1])
 
 	if in2 == 0 {
-		// Divide by zero just returns zero
-		state.writeSigned(instr.Operands[0], 0)
+		state.setError(fmt.Errorf("Divide by zero"))
 	} else {
 		state.writeSigned(instr.Operands[0], in1/in2)
 	}

vm/state.go

@@ -75,8 +75,7 @@ func (state *State) processInstruction() {
 	state.instructionCount += 1
 
 	if state.program.InstructionLimit > 0 && state.instructionCount > state.program.InstructionLimit {
-		state.err = fmt.Errorf("Instruction limit (%d) exceeded", state.program.InstructionLimit)
-		state.running = false
+		state.setError(fmt.Errorf("Instruction limit (%d) exceeded", state.program.InstructionLimit))
 	}
 
 	for state.functionIndex < 0 ||