Fix for...downto loops

src/main/scala/millfork/compiler/AbstractStatementCompiler.scala

@@ -229,6 +229,8 @@ abstract class AbstractStatementCompiler[T <: AbstractCode] {
             names)
         ))
       case (ForDirection.DownTo, _, _) =>
+        // TODO: smarter countdown if end is not a constant
+        val endMinusOne = SumExpression(List(true -> f.end, false -> LiteralExpression(1, 1)), decimal = false).pos(p)
         compile(ctx, List(
           Assignment(vex, f.start).pos(p),
           IfStatement(
@@ -236,7 +238,7 @@ abstract class AbstractStatementCompiler[T <: AbstractCode] {
             List(DoWhileStatement(
               f.body,
               List(decrement),
-              FunctionCallExpression("!=", List(vex, f.end)).pos(p),
+              FunctionCallExpression("!=", List(vex, endMinusOne)).pos(p),
               names
             ).pos(p)),
             Nil)

src/test/scala/millfork/test/ForLoopSuite.scala

@@ -77,6 +77,30 @@ class ForLoopSuite extends FunSuite with Matchers {
     }
   }
 
+  test("For-downto 3") {
+    EmuCrossPlatformBenchmarkRun(Cpu.Mos, Cpu.Z80, Cpu.Intel8080, Cpu.Sharp)(
+      """
+        | array output [55] @$c000
+        | void main () {
+        |   byte i
+        |   output[0] = 0
+        |   output[1] = 0
+        |   output[5] = 0
+        |   output[6] = 0
+        |   for i,5,downto,1 {
+        |     stuff()
+        |     output[i] += 1
+        |   }
+        | }
+        | noinline void stuff() {}
+      """.stripMargin){m =>
+      m.readByte(0xc000) should equal(0)
+      m.readByte(0xc001) should equal(1)
+      m.readByte(0xc005) should equal(1)
+      m.readByte(0xc006) should equal(0)
+    }
+  }
+
   test("For-until") {
     EmuCrossPlatformBenchmarkRun(Cpu.Mos, Cpu.Z80, Cpu.Intel8080, Cpu.Sharp)(
       """