From bd84a5b969165e80689e380bf8dcba012ae3048a Mon Sep 17 00:00:00 2001
From: Ian Gulliver <git@flamingcow.io>
Date: Sat, 20 Nov 2021 18:12:17 -1000
Subject: [PATCH] Restrict global and function references to memory size

---
 gen/operand.go    |  4 ++--
 gen/rand.go       | 10 ++++++++++
 test/gen_test.go  | 12 +++++++++++-
 test/rand_test.go | 12 ++++++------
 vm/stackframe.go  |  4 +++-
 vm/state.go       |  4 +++-
 6 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/gen/operand.go b/gen/operand.go
index 0fc659b..8545414 100644
--- a/gen/operand.go
+++ b/gen/operand.go
@@ -12,10 +12,10 @@ func randOperand(t vm.OperandNumericType) *vm.Operand {
 
 		switch op.Type {
 		case vm.GlobalMemoryIndex:
-			op.Value = RandBiasedUint64()
+			op.Value = RandBiasedUint64n(vm.GlobalMemorySize)
 
 		case vm.FunctionMemoryIndex:
-			op.Value = RandBiasedUint64()
+			op.Value = RandBiasedUint64n(vm.FunctionMemorySize)
 
 		case vm.Literal:
 			switch t {
diff --git a/gen/rand.go b/gen/rand.go
index 021ee65..fbf1012 100644
--- a/gen/rand.go
+++ b/gen/rand.go
@@ -9,6 +9,16 @@ func RandBiasedUint64() uint64 {
 	return (rand.Uint64() | 0x8000000000000000) >> rand.Intn(65)
 }
 
+// Like RandBiasedUint64() but always returns < n
+func RandBiasedUint64n(n uint64) uint64 {
+	for {
+		ret := RandBiasedUint64()
+		if ret < n {
+			return ret
+		}
+	}
+}
+
 // Generate a random int64 with an even distribution of the tuple
 // {sign, bits.Len64(math.Abs())}
 func RandBiasedInt64() uint64 {
diff --git a/test/gen_test.go b/test/gen_test.go
index 6a100ff..b8ae5ae 100644
--- a/test/gen_test.go
+++ b/test/gen_test.go
@@ -3,7 +3,17 @@ package test
 import "testing"
 
 import "github.com/firestuff/subcoding/gen"
+import "github.com/firestuff/subcoding/asm"
 
 func TestRandProgram(t *testing.T) {
-	gen.RandProgram()
+	for i := 0; i < 100; i++ {
+		prog := gen.RandProgram()
+
+		src, err := asm.Disassemble(prog)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		t.Log(src)
+	}
 }
diff --git a/test/rand_test.go b/test/rand_test.go
index d13c54e..19c7629 100644
--- a/test/rand_test.go
+++ b/test/rand_test.go
@@ -29,8 +29,8 @@ func TestRandBiasedUint64(t *testing.T) {
 		}
 	}
 
-	if max - min > max / 10 {
-		t.Fatalf("Variance greater than allowed: %d > %d (max=%d min=%d)", max - min, max / 10, max, min)
+	if max-min > max/10 {
+		t.Fatalf("Variance greater than allowed: %d > %d (max=%d min=%d)", max-min, max/10, max, min)
 	}
 }
 
@@ -48,9 +48,9 @@ func TestRandBiasedInt64(t *testing.T) {
 		case val == 0:
 			buckets[63]++
 		case val < 0:
-			buckets[63 + bits.Len64(uint64(val * -1))]++
+			buckets[63+bits.Len64(uint64(val*-1))]++
 		case val > 0:
-			buckets[63 - bits.Len64(uint64(val))]++
+			buckets[63-bits.Len64(uint64(val))]++
 		}
 	}
 
@@ -67,7 +67,7 @@ func TestRandBiasedInt64(t *testing.T) {
 		}
 	}
 
-	if max - min > max / 10 {
-		t.Fatalf("Variance greater than allowed: %d > %d (max=%d min=%d)", max - min, max / 10, max, min)
+	if max-min > max/10 {
+		t.Fatalf("Variance greater than allowed: %d > %d (max=%d min=%d)", max-min, max/10, max, min)
 	}
 }
diff --git a/vm/stackframe.go b/vm/stackframe.go
index bd93c2e..2cdba2c 100644
--- a/vm/stackframe.go
+++ b/vm/stackframe.go
@@ -1,5 +1,7 @@
 package vm
 
+const FunctionMemorySize = 16
+
 type stackFrame struct {
 	previousFunctionIndex    int64
 	previousInstructionIndex int64
@@ -10,6 +12,6 @@ func newStackFrame(state *State) *stackFrame {
 	return &stackFrame{
 		previousFunctionIndex:    state.functionIndex,
 		previousInstructionIndex: state.instructionIndex,
-		functionMemory:           NewMemory(16),
+		functionMemory:           NewMemory(FunctionMemorySize),
 	}
 }
diff --git a/vm/state.go b/vm/state.go
index 2728c5f..dc734c5 100644
--- a/vm/state.go
+++ b/vm/state.go
@@ -2,6 +2,8 @@ package vm
 
 import "fmt"
 
+const GlobalMemorySize = 16
+
 type State struct {
 	running bool
 	err     error
@@ -18,7 +20,7 @@ type State struct {
 func NewState(prog *Program) (*State, error) {
 	return &State{
 		program:      prog,
-		globalMemory: NewMemory(16),
+		globalMemory: NewMemory(GlobalMemorySize),
 	}, nil
 }