compiler: implement fallthrough in switch

Closes #628.

pkg/compiler/codegen.go

@@ -407,7 +407,6 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.SwitchStmt:
-		// fallthrough is not supported
 		ast.Walk(c, n.Tag)
 
 		eqOpcode := c.getEqualityOpcode(n.Tag)
@@ -416,9 +415,13 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		lastSwitch := c.currentSwitch
 		c.currentSwitch = label
 
+		startLabels := make([]uint16, len(n.Body.List))
+		for i := range startLabels {
+			startLabels[i] = c.newLabel()
+		}
 		for i := range n.Body.List {
 			lEnd := c.newLabel()
-			lStart := c.newLabel()
+			lStart := startLabels[i]
 			cc := n.Body.List[i].(*ast.CaseClause)
 
 			if l := len(cc.List); l != 0 { // if not `default`
@@ -435,7 +438,12 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			}
 
 			c.setLabel(lStart)
-			for _, stmt := range cc.Body {
+			last := len(cc.Body) - 1
+			for j, stmt := range cc.Body {
+				if j == last && isFallthroughStmt(stmt) {
+					emit.Jmp(c.prog.BinWriter, opcode.JMP, startLabels[i+1])
+					break
+				}
 				ast.Walk(c, stmt)
 			}
 			emit.Jmp(c.prog.BinWriter, opcode.JMP, switchEnd)
@@ -834,6 +842,11 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 	return c
 }
 
+func isFallthroughStmt(c ast.Node) bool {
+	s, ok := c.(*ast.BranchStmt)
+	return ok && s.Tok == token.FALLTHROUGH
+}
+
 // emitReverse reverses top num items of the stack.
 func (c *codegen) emitReverse(num int) {
 	switch num {

pkg/compiler/switch_test.go

@@ -187,6 +187,41 @@ var switchTestCases = []testCase{
 		}`,
 		big.NewInt(2),
 	},
+	{
+		"simple fallthrough",
+		`package main
+		func Main() int {
+			n := 2
+			switch n {
+			case 1: return 5
+			case 2: fallthrough
+			case 3: return 6
+			}
+			return 7
+		}`,
+		big.NewInt(6),
+	},
+	{
+		"double fallthrough",
+		`package main
+		func Main() int {
+			n := 2
+			k := 5
+			switch n {
+			case 0: return k
+			case 1: fallthrough
+			case 2:
+				k++
+				fallthrough
+			case 3:
+			case 4:
+				k++
+				return k
+			}
+			return k
+		}`,
+		big.NewInt(6),
+	},
 }
 
 func TestSwitch(t *testing.T) {