Add descending ranges and for-loop support

docs/Range.md

@@ -25,6 +25,23 @@ Exclusive end ranges are adopted from kotlin either:
     assert(4 in r)
     >>> void
 
+Descending finite ranges are explicit too:
+
+    val r = 5 downTo 1
+    assert(r.isDescending)
+    assert(r.toList() == [5,4,3,2,1])
+    >>> void
+
+Use `downUntil` when the lower bound should be excluded:
+
+    val r = 5 downUntil 1
+    assert(r.toList() == [5,4,3,2])
+    assert(1 !in r)
+    >>> void
+
+This is explicit by design: `5..1` is not treated as a reverse range. It is an
+ordinary ascending range with no values in it when iterated.
+
 In any case, we can test an object to belong to using `in` and `!in` and
 access limits:
     
@@ -73,6 +90,23 @@ but
     >>> 2
     >>> void
 
+Descending ranges work in `for` loops exactly the same way:
+
+    for( i in 3 downTo 1 )
+        println(i)
+    >>> 3
+    >>> 2
+    >>> 1
+    >>> void
+
+And with an exclusive lower bound:
+
+    for( i in 3 downUntil 1 )
+        println(i)
+    >>> 3
+    >>> 2
+    >>> void
+
 ### Stepped ranges
 
 Use `step` to change the iteration increment. The range bounds still define membership,
@@ -80,9 +114,18 @@ so iteration ends when the next value is no longer in the range.
 
     assert( [1,3,5] == (1..5 step 2).toList() )
     assert( [1,3] == (1..<5 step 2).toList() )
+    assert( [5,3,1] == (5 downTo 1 step 2).toList() )
     assert( ['a','c','e'] == ('a'..'e' step 2).toList() )
     >>> void
 
+Descending ranges still use a positive `step`; the direction comes from
+`downTo` / `downUntil`:
+
+    assert( ['e','c','a'] == ('e' downTo 'a' step 2).toList() )
+    >>> void
+
+A negative step with `downTo` / `downUntil` is invalid.
+
 Real ranges require an explicit step:
 
     assert( [0,0.25,0.5,0.75,1.0] == (0.0..1.0 step 0.25).toList() )
@@ -119,6 +162,7 @@ Exclusive end char ranges are supported too:
 |-----------------|------------------------------|---------------|
 | contains(other) | used in `in`                 | Range, or Any |
 | isEndInclusive  | true for '..'                | Bool          |
+| isDescending    | true for `downTo`/`downUntil`| Bool          |
 | isOpen          | at any end                   | Bool          |
 | isIntRange      | both start and end are Int   | Bool          |
 | step            | explicit iteration step      | Any?          |

docs/ai_language_reference.md

@@ -50,8 +50,10 @@ Primary sources used: `lynglib/src/commonMain/kotlin/net/sergeych/lyng/{Parser,T
 - Range literals:
   - inclusive: `a..b`
   - exclusive end: `a..<b`
+  - descending inclusive: `a downTo b`
+  - descending exclusive end: `a downUntil b`
   - open-ended forms are supported (`a..`, `..b`, `..`).
-  - optional step: `a..b step 2`
+  - optional step: `a..b step 2`, `a downTo b step 2`
 - Lambda literal:
   - with params: `{ x, y -> x + y }`
   - implicit `it`: `{ it + 1 }`

docs/tutorial.md

@@ -1359,6 +1359,41 @@ size and index access, like lists:
     "total letters: "+letters
     >>> "total letters: 10"
 
+When you need a counting loop that goes backwards, use an explicit descending
+range:
+
+    var sum = 0
+    for( i in 5 downTo 1 ) {
+        sum += i
+    }
+    sum
+    >>> 15
+
+If the lower bound should be excluded, use `downUntil`:
+
+    val xs = []
+    for( i in 5 downUntil 1 ) {
+        xs.add(i)
+    }
+    xs
+    >>> [5,4,3,2]
+
+This is intentionally explicit: `5..1` is an empty ascending range, not an
+implicit reverse loop.
+
+Descending loops also support `step`:
+
+    val xs = []
+    for( i in 10 downTo 1 step 3 ) {
+        xs.add(i)
+    }
+    xs
+    >>> [10,7,4,1]
+
+For descending ranges, `step` stays positive. The direction comes from
+`downTo` / `downUntil`, so `10 downTo 1 step 3` is valid, while
+`10 downTo 1 step -3` is an error.
+
 For loop support breaks the same as while loops above:
 
     fun search(haystack, needle) {    
@@ -1488,6 +1523,14 @@ It could be open and closed:
     assert( 5 !in (1..<5) )
     >>> void
 
+Descending ranges are explicit too:
+
+    (5 downTo 1).toList()
+    >>> [5,4,3,2,1]
+
+    (5 downUntil 1).toList()
+    >>> [5,4,3,2]
+
 Ranges could be inside other ranges:
 
     assert( (2..3) in (1..10) )
@@ -1505,6 +1548,14 @@ and you can use ranges in for-loops:
     >>> b
     >>> void
 
+Descending character ranges work the same way:
+
+    for( ch in 'e' downTo 'a' step 2 ) println(ch)
+    >>> e
+    >>> c
+    >>> a
+    >>> void
+
 See [Ranges](Range.md) for detailed documentation on it.
 
 # Time routines

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt

@@ -3058,9 +3058,10 @@ class Compiler(
                     }
                 }
 
-                Token.Type.DOTDOT, Token.Type.DOTDOTLT -> {
+                Token.Type.DOTDOT, Token.Type.DOTDOTLT, Token.Type.DOWNTO, Token.Type.DOWNUNTIL -> {
                     // range operator
-                    val isEndInclusive = t.type == Token.Type.DOTDOT
+                    val isEndInclusive = t.type == Token.Type.DOTDOT || t.type == Token.Type.DOWNTO
+                    val isDescending = t.type == Token.Type.DOWNTO || t.type == Token.Type.DOWNUNTIL
                     val left = operand
                     // if it is an open end range, then the end of line could be here that we do not want
                     // to skip in parseExpression:
@@ -3078,12 +3079,19 @@ class Compiler(
                         val lConst = constIntValueOrNull(left)
                         val rConst = constIntValueOrNull(rightRef)
                         if (lConst != null && rConst != null) {
-                            operand = ConstRef(ObjRange(ObjInt.of(lConst), ObjInt.of(rConst), isEndInclusive).asReadonly)
+                            operand = ConstRef(
+                                ObjRange(
+                                    ObjInt.of(lConst),
+                                    ObjInt.of(rConst),
+                                    isEndInclusive,
+                                    isDescending = isDescending
+                                ).asReadonly
+                            )
                         } else {
-                            operand = RangeRef(left, rightRef, isEndInclusive)
+                            operand = RangeRef(left, rightRef, isEndInclusive, isDescending = isDescending)
                         }
                     } else {
-                        operand = RangeRef(left, rightRef, isEndInclusive)
+                        operand = RangeRef(left, rightRef, isEndInclusive, isDescending = isDescending)
                     }
                 }
 
@@ -3098,7 +3106,7 @@ class Compiler(
                             }
                             val leftRef = range.start?.takeUnless { it.isNull }?.let { ConstRef(it.asReadonly) }
                             val rightRef = range.end?.takeUnless { it.isNull }?.let { ConstRef(it.asReadonly) }
-                            RangeRef(leftRef, rightRef, range.isEndInclusive)
+                            RangeRef(leftRef, rightRef, range.isEndInclusive, isDescending = range.isDescending)
                         }
                         else -> {
                             cc.previous()
@@ -3108,7 +3116,13 @@ class Compiler(
                     if (rangeRef.step != null) throw ScriptError(t.pos, "step is already specified for this range")
                     val stepExpr = parseExpression() ?: throw ScriptError(t.pos, "Expected step expression")
                     val stepRef = StatementRef(stepExpr)
-                    operand = RangeRef(rangeRef.left, rangeRef.right, rangeRef.isEndInclusive, stepRef)
+                    operand = RangeRef(
+                        rangeRef.left,
+                        rangeRef.right,
+                        rangeRef.isEndInclusive,
+                        isDescending = rangeRef.isDescending,
+                        step = stepRef
+                    )
                 }
 
                 Token.Type.LBRACE, Token.Type.NULL_COALESCE_BLOCKINVOKE -> {
@@ -7828,15 +7842,23 @@ class Compiler(
                 if (range.step != null && !range.step.isNull) return null
                 val start = range.start?.toLong() ?: return null
                 val end = range.end?.toLong() ?: return null
-                val endExclusive = if (range.isEndInclusive) end + 1 else end
+                val stopBoundary = if (range.isDescending) {
-                return ConstIntRange(start, endExclusive)
+                    if (range.isEndInclusive) end - 1 else end
+                } else {
+                    if (range.isEndInclusive) end + 1 else end
+                }
+                return ConstIntRange(start, stopBoundary, range.isDescending)
             }
             is RangeRef -> {
                 if (ref.step != null) return null
                 val start = constIntValueOrNull(ref.left) ?: return null
                 val end = constIntValueOrNull(ref.right) ?: return null
-                val endExclusive = if (ref.isEndInclusive) end + 1 else end
+                val stopBoundary = if (ref.isDescending) {
-                return ConstIntRange(start, endExclusive)
+                    if (ref.isEndInclusive) end - 1 else end
+                } else {
+                    if (ref.isEndInclusive) end + 1 else end
+                }
+                return ConstIntRange(start, stopBoundary, ref.isDescending)
             }
             else -> return null
         }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Parser.kt

@@ -448,6 +448,8 @@ private class Parser(fromPos: Pos, private val interpolationEnabled: Boolean = t
                             "is" -> Token("is", from, Token.Type.IS)
                             "by" -> Token("by", from, Token.Type.BY)
                             "step" -> Token("step", from, Token.Type.STEP)
+                            "downTo" -> Token("downTo", from, Token.Type.DOWNTO)
+                            "downUntil" -> Token("downUntil", from, Token.Type.DOWNUNTIL)
                             "object" -> Token("object", from, Token.Type.OBJECT)
                             "as" -> {
                                 // support both `as` and tight `as?` without spaces

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Token.kt

@@ -36,7 +36,7 @@ data class Token(val value: String, val pos: Pos, val type: Type) {
         PLUS, MINUS, STAR, SLASH, PERCENT,
         ASSIGN, PLUSASSIGN, MINUSASSIGN, STARASSIGN, SLASHASSIGN, PERCENTASSIGN, IFNULLASSIGN,
         PLUS2, MINUS2,
-        IN, NOTIN, IS, NOTIS, BY, STEP,
+        IN, NOTIN, IS, NOTIS, BY, STEP, DOWNTO, DOWNUNTIL,
         EQ, NEQ, LT, LTE, GT, GTE, REF_EQ, REF_NEQ, MATCH, NOTMATCH,
         SHUTTLE,
         AND, BITAND, OR, BITOR, BITXOR, NOT, BITNOT, DOT, ARROW, EQARROW, QUESTION, COLONCOLON,

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt

@@ -3617,6 +3617,9 @@ class BytecodeCompiler(
         val inclusiveSlot = allocSlot()
         val inclusiveId = builder.addConst(BytecodeConst.Bool(ref.isEndInclusive))
         builder.emit(Opcode.CONST_BOOL, inclusiveId, inclusiveSlot)
+        val descendingSlot = allocSlot()
+        val descendingId = builder.addConst(BytecodeConst.Bool(ref.isDescending))
+        builder.emit(Opcode.CONST_BOOL, descendingId, descendingSlot)
         val stepSlot = if (ref.step != null) {
             val step = compileRefWithFallback(ref.step, null, Pos.builtIn) ?: return null
             ensureObjSlot(step).slot
@@ -3627,7 +3630,7 @@ class BytecodeCompiler(
             slot
         }
         val dst = allocSlot()
-        builder.emit(Opcode.MAKE_RANGE, startSlot, endSlot, inclusiveSlot, stepSlot, dst)
+        builder.emit(Opcode.MAKE_RANGE, startSlot, endSlot, inclusiveSlot, descendingSlot, stepSlot, dst)
         updateSlotType(dst, SlotType.OBJ)
         slotObjClass[dst] = ObjRange.type
         return CompiledValue(dst, SlotType.OBJ)
@@ -6244,11 +6247,14 @@ class BytecodeCompiler(
 
         val iSlot = loopSlotId
         val endSlot = allocSlot()
+        val descendingSlot = allocSlot()
         if (range != null) {
             val startId = builder.addConst(BytecodeConst.IntVal(range.start))
-            val endId = builder.addConst(BytecodeConst.IntVal(range.endExclusive))
+            val endId = builder.addConst(BytecodeConst.IntVal(range.stopBoundary))
+            val descendingId = builder.addConst(BytecodeConst.Bool(range.isDescending))
             builder.emit(Opcode.CONST_INT, startId, iSlot)
             builder.emit(Opcode.CONST_INT, endId, endSlot)
+            builder.emit(Opcode.CONST_BOOL, descendingId, descendingSlot)
             updateSlotType(iSlot, SlotType.INT)
             updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
         } else {
@@ -6258,9 +6264,15 @@ class BytecodeCompiler(
                 val startValue = compileRef(left) ?: return null
                 val endValue = compileRef(right) ?: return null
                 if (startValue.type != SlotType.INT || endValue.type != SlotType.INT) return null
+                val descendingId = builder.addConst(BytecodeConst.Bool(rangeRef.isDescending))
                 emitMove(startValue, iSlot)
                 emitMove(endValue, endSlot)
-                if (rangeRef.isEndInclusive) {
+                builder.emit(Opcode.CONST_BOOL, descendingId, descendingSlot)
+                if (rangeRef.isDescending) {
+                    if (rangeRef.isEndInclusive) {
+                        builder.emit(Opcode.DEC_INT, endSlot)
+                    }
+                } else if (rangeRef.isEndInclusive) {
                     builder.emit(Opcode.INC_INT, endSlot)
                 }
                 updateSlotType(iSlot, SlotType.INT)
@@ -6270,7 +6282,7 @@ class BytecodeCompiler(
                 val rangeValue = compileRef(rangeLocal) ?: return null
                 val rangeObj = ensureObjSlot(rangeValue)
                 val okSlot = allocSlot()
-                builder.emit(Opcode.RANGE_INT_BOUNDS, rangeObj.slot, iSlot, endSlot, okSlot)
+                builder.emit(Opcode.RANGE_INT_BOUNDS, rangeObj.slot, iSlot, endSlot, descendingSlot, okSlot)
                 val badRangeLabel = builder.label()
                 builder.emit(
                     Opcode.JMP_IF_FALSE,
@@ -6294,14 +6306,7 @@ class BytecodeCompiler(
                 val endLabel = builder.label()
                 val doneLabel = builder.label()
                 builder.mark(loopLabel)
-                builder.emit(
+                emitIntForLoopCheck(iSlot, endSlot, descendingSlot, endLabel)
-                    Opcode.JMP_IF_GTE_INT,
-                    listOf(
-                        CmdBuilder.Operand.IntVal(iSlot),
-                        CmdBuilder.Operand.IntVal(endSlot),
-                        CmdBuilder.Operand.LabelRef(endLabel)
-                    )
-                )
                 updateSlotType(iSlot, SlotType.INT)
                 updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
                 loopStack.addLast(
@@ -6324,7 +6329,7 @@ class BytecodeCompiler(
                     builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot!!)
                 }
                 builder.mark(continueLabel)
-                builder.emit(Opcode.INC_INT, iSlot)
+                emitIntForLoopStep(iSlot, descendingSlot)
                 if (hasRealWiden) {
                     emitLoopRealCoercions(realWidenSlots)
                 }
@@ -6377,14 +6382,7 @@ class BytecodeCompiler(
         val continueLabel = builder.label()
         val endLabel = builder.label()
         builder.mark(loopLabel)
-        builder.emit(
+        emitIntForLoopCheck(iSlot, endSlot, descendingSlot, endLabel)
-            Opcode.JMP_IF_GTE_INT,
-            listOf(
-                CmdBuilder.Operand.IntVal(iSlot),
-                CmdBuilder.Operand.IntVal(endSlot),
-                CmdBuilder.Operand.LabelRef(endLabel)
-            )
-        )
         updateSlotType(iSlot, SlotType.INT)
         updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
         loopStack.addLast(
@@ -6407,7 +6405,7 @@ class BytecodeCompiler(
             builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot!!)
         }
         builder.mark(continueLabel)
-        builder.emit(Opcode.INC_INT, iSlot)
+        emitIntForLoopStep(iSlot, descendingSlot)
         if (hasRealWiden) {
             emitLoopRealCoercions(realWidenSlots)
         }
@@ -8719,11 +8717,59 @@ class BytecodeCompiler(
             val end = range.end as? ObjInt ?: return null
             val left = ConstRef(start.asReadonly)
             val right = ConstRef(end.asReadonly)
-            return RangeRef(left, right, range.isEndInclusive)
+            return RangeRef(left, right, range.isEndInclusive, isDescending = range.isDescending)
         }
         return null
     }
 
+    private fun emitIntForLoopCheck(iSlot: Int, stopSlot: Int, descendingSlot: Int, endLabel: CmdBuilder.Label) {
+        val descendingLabel = builder.label()
+        val afterCheckLabel = builder.label()
+        builder.emit(
+            Opcode.JMP_IF_TRUE,
+            listOf(
+                CmdBuilder.Operand.IntVal(descendingSlot),
+                CmdBuilder.Operand.LabelRef(descendingLabel)
+            )
+        )
+        builder.emit(
+            Opcode.JMP_IF_GTE_INT,
+            listOf(
+                CmdBuilder.Operand.IntVal(iSlot),
+                CmdBuilder.Operand.IntVal(stopSlot),
+                CmdBuilder.Operand.LabelRef(endLabel)
+            )
+        )
+        builder.emit(Opcode.JMP, listOf(CmdBuilder.Operand.LabelRef(afterCheckLabel)))
+        builder.mark(descendingLabel)
+        builder.emit(
+            Opcode.JMP_IF_LTE_INT,
+            listOf(
+                CmdBuilder.Operand.IntVal(iSlot),
+                CmdBuilder.Operand.IntVal(stopSlot),
+                CmdBuilder.Operand.LabelRef(endLabel)
+            )
+        )
+        builder.mark(afterCheckLabel)
+    }
+
+    private fun emitIntForLoopStep(iSlot: Int, descendingSlot: Int) {
+        val descendingLabel = builder.label()
+        val afterStepLabel = builder.label()
+        builder.emit(
+            Opcode.JMP_IF_TRUE,
+            listOf(
+                CmdBuilder.Operand.IntVal(descendingSlot),
+                CmdBuilder.Operand.LabelRef(descendingLabel)
+            )
+        )
+        builder.emit(Opcode.INC_INT, iSlot)
+        builder.emit(Opcode.JMP, listOf(CmdBuilder.Operand.LabelRef(afterStepLabel)))
+        builder.mark(descendingLabel)
+        builder.emit(Opcode.DEC_INT, iSlot)
+        builder.mark(afterStepLabel)
+    }
+
     private fun extractRangeFromLocal(source: Statement): RangeRef? {
         val target = if (source is BytecodeStatement) source.original else source
         val expr = target as? ExpressionStatement ?: return null

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt

@@ -147,7 +147,7 @@ class CmdBuilder {
             Opcode.CHECK_IS, Opcode.MAKE_QUALIFIED_VIEW ->
                 listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.RANGE_INT_BOUNDS ->
-                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.RET_LABEL, Opcode.THROW ->
                 listOf(OperandKind.CONST, OperandKind.SLOT)
             Opcode.RESOLVE_SCOPE_SLOT ->
@@ -228,7 +228,7 @@ class CmdBuilder {
             Opcode.SET_INDEX ->
                 listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.MAKE_RANGE ->
-                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT, OperandKind.SLOT)
             Opcode.LIST_LITERAL ->
                 listOf(OperandKind.CONST, OperandKind.SLOT, OperandKind.COUNT, OperandKind.SLOT)
             Opcode.GET_MEMBER_SLOT ->
@@ -311,10 +311,10 @@ class CmdBuilder {
             }
             Opcode.OBJ_TO_BOOL -> CmdObjToBool(operands[0], operands[1])
             Opcode.GET_OBJ_CLASS -> CmdGetObjClass(operands[0], operands[1])
-            Opcode.RANGE_INT_BOUNDS -> CmdRangeIntBounds(operands[0], operands[1], operands[2], operands[3])
+            Opcode.RANGE_INT_BOUNDS -> CmdRangeIntBounds(operands[0], operands[1], operands[2], operands[3], operands[4])
             Opcode.LOAD_THIS -> CmdLoadThis(operands[0])
             Opcode.LOAD_THIS_VARIANT -> CmdLoadThisVariant(operands[0], operands[1])
-            Opcode.MAKE_RANGE -> CmdMakeRange(operands[0], operands[1], operands[2], operands[3], operands[4])
+            Opcode.MAKE_RANGE -> CmdMakeRange(operands[0], operands[1], operands[2], operands[3], operands[4], operands[5])
             Opcode.CHECK_IS -> CmdCheckIs(operands[0], operands[1], operands[2])
             Opcode.ASSERT_IS -> CmdAssertIs(operands[0], operands[1])
             Opcode.MAKE_QUALIFIED_VIEW -> CmdMakeQualifiedView(operands[0], operands[1], operands[2])

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt

@@ -96,11 +96,12 @@ object CmdDisassembler {
             is CmdCheckIs -> Opcode.CHECK_IS to intArrayOf(cmd.objSlot, cmd.typeSlot, cmd.dst)
             is CmdAssertIs -> Opcode.ASSERT_IS to intArrayOf(cmd.objSlot, cmd.typeSlot)
             is CmdMakeQualifiedView -> Opcode.MAKE_QUALIFIED_VIEW to intArrayOf(cmd.objSlot, cmd.typeSlot, cmd.dst)
-            is CmdRangeIntBounds -> Opcode.RANGE_INT_BOUNDS to intArrayOf(cmd.src, cmd.startSlot, cmd.endSlot, cmd.okSlot)
+            is CmdRangeIntBounds -> Opcode.RANGE_INT_BOUNDS to intArrayOf(cmd.src, cmd.startSlot, cmd.endSlot, cmd.descendingSlot, cmd.okSlot)
             is CmdMakeRange -> Opcode.MAKE_RANGE to intArrayOf(
                 cmd.startSlot,
                 cmd.endSlot,
                 cmd.inclusiveSlot,
+                cmd.descendingSlot,
                 cmd.stepSlot,
                 cmd.dst
             )

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt

@@ -273,6 +273,7 @@ class CmdMakeRange(
     internal val startSlot: Int,
     internal val endSlot: Int,
     internal val inclusiveSlot: Int,
+    internal val descendingSlot: Int,
     internal val stepSlot: Int,
     internal val dst: Int,
 ) : Cmd() {
@@ -280,9 +281,10 @@ class CmdMakeRange(
         val start = frame.slotToObj(startSlot)
         val end = frame.slotToObj(endSlot)
         val inclusive = frame.slotToObj(inclusiveSlot).toBool()
+        val descending = frame.slotToObj(descendingSlot).toBool()
         val stepObj = frame.slotToObj(stepSlot)
         val step = if (stepObj.isNull) null else stepObj
-        frame.storeObjResult(dst, ObjRange(start, end, isEndInclusive = inclusive, step = step))
+        frame.storeObjResult(dst, ObjRange(start, end, isEndInclusive = inclusive, isDescending = descending, step = step))
         return
     }
 }
@@ -430,6 +432,7 @@ class CmdRangeIntBounds(
     internal val src: Int,
     internal val startSlot: Int,
     internal val endSlot: Int,
+    internal val descendingSlot: Int,
     internal val okSlot: Int,
 ) : Cmd() {
     override suspend fun perform(frame: CmdFrame) {
@@ -439,10 +442,19 @@ class CmdRangeIntBounds(
             frame.setBool(okSlot, false)
             return
         }
+        if (range.isDescending) {
+            frame.setBool(okSlot, false)
+            return
+        }
         val start = (range.start as ObjInt).value
         val end = (range.end as ObjInt).value
         frame.setInt(startSlot, start)
-        frame.setInt(endSlot, if (range.isEndInclusive) end + 1 else end)
+        frame.setInt(endSlot, if (range.isDescending) {
+            if (range.isEndInclusive) end - 1 else end
+        } else {
+            if (range.isEndInclusive) end + 1 else end
+        })
+        frame.setBool(descendingSlot, range.isDescending)
         frame.setBool(okSlot, true)
         return
     }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/highlight/SimpleLyngHighlighter.kt

@@ -43,7 +43,7 @@ private val fallbackKeywordIds = setOf(
     // declarations & modifiers
     "fun", "fn", "class", "interface", "enum", "val", "var", "type", "import", "package",
     "abstract", "closed", "override", "public", "lazy", "dynamic",
-    "private", "protected", "static", "open", "extern", "init", "get", "set", "by", "step",
+    "private", "protected", "static", "open", "extern", "init", "get", "set", "by", "step", "downTo", "downUntil",
     // control flow and misc
     "if", "else", "when", "while", "do", "for", "try", "catch", "finally",
     "throw", "return", "break", "continue", "this", "null", "true", "false", "unset", "void"
@@ -74,7 +74,7 @@ private fun kindOf(type: Type, value: String): HighlightKind? = when (type) {
     Type.COMMA, Type.SEMICOLON, Type.COLON -> HighlightKind.Punctuation
 
     // textual control keywords
-    Type.IN, Type.NOTIN, Type.IS, Type.NOTIS, Type.AS, Type.ASNULL, Type.BY, Type.STEP, Type.OBJECT,
+    Type.IN, Type.NOTIN, Type.IS, Type.NOTIS, Type.AS, Type.ASNULL, Type.BY, Type.STEP, Type.DOWNTO, Type.DOWNUNTIL, Type.OBJECT,
     Type.AND, Type.OR, Type.NOT -> HighlightKind.Keyword
 
     // labels / annotations

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRange.kt

@@ -28,6 +28,7 @@ class ObjRange(
     val start: Obj?,
     val end: Obj?,
     val isEndInclusive: Boolean,
+    val isDescending: Boolean = false,
     val step: Obj? = null
 ) : Obj() {
 
@@ -39,15 +40,38 @@ class ObjRange(
 
     override suspend fun defaultToString(scope: Scope): ObjString {
         val result = StringBuilder()
-        result.append("${start?.inspect(scope) ?: '∞'} ..")
+        result.append(start?.inspect(scope) ?: "∞")
-        if (!isEndInclusive) result.append('<')
+        when {
-        result.append(" ${end?.inspect(scope) ?: '∞'}")
+            isDescending && isEndInclusive -> result.append(" downTo ")
+            isDescending && !isEndInclusive -> result.append(" downUntil ")
+            else -> {
+                result.append(" ..")
+                if (!isEndInclusive) result.append('<')
+                result.append(' ')
+            }
+        }
+        result.append(end?.inspect(scope) ?: "∞")
         if (hasExplicitStep) {
             result.append(" step ${step?.inspect(scope)}")
         }
         return ObjString(result.toString())
     }
 
+    private data class NormalizedLowerBound(val value: Obj, val inclusive: Boolean)
+    private data class NormalizedUpperBound(val value: Obj, val inclusive: Boolean)
+
+    private fun normalizedLowerBound(): NormalizedLowerBound? =
+        when {
+            isDescending -> end?.takeUnless { it.isNull }?.let { NormalizedLowerBound(it, isEndInclusive) }
+            else -> start?.takeUnless { it.isNull }?.let { NormalizedLowerBound(it, true) }
+        }
+
+    private fun normalizedUpperBound(): NormalizedUpperBound? =
+        when {
+            isDescending -> start?.takeUnless { it.isNull }?.let { NormalizedUpperBound(it, true) }
+            else -> end?.takeUnless { it.isNull }?.let { NormalizedUpperBound(it, isEndInclusive) }
+        }
+
     /**
      * IF end is open (null/ObjNull), returns null
      * Otherwise, return correct value for the exclusive end
@@ -74,29 +98,21 @@ class ObjRange(
         }
 
     suspend fun containsRange(scope: Scope, other: ObjRange): Boolean {
-        if (!isOpenStart) {
+        val ourLower = normalizedLowerBound()
-            // our start is not -∞ so other start should be GTE or is not contained:
+        val otherLower = other.normalizedLowerBound()
-            if (!other.isOpenStart && start!!.compareTo(scope, other.start!!) > 0) return false
+        if (ourLower != null) {
+            if (otherLower == null) return false
+            val cmp = ourLower.value.compareTo(scope, otherLower.value)
+            if (cmp == -2 || cmp > 0) return false
+            if (cmp == 0 && otherLower.inclusive && !ourLower.inclusive) return false
         }
-        if (!isOpenEnd) {
+        val ourUpper = normalizedUpperBound()
-            // same with the end: if it is open, it can't be contained in ours:
+        val otherUpper = other.normalizedUpperBound()
-            if (other.isOpenEnd) return false
+        if (ourUpper != null) {
-            // both exists, now there could be 4 cases:
+            if (otherUpper == null) return false
-            return when {
+            val cmp = ourUpper.value.compareTo(scope, otherUpper.value)
-                other.isEndInclusive && isEndInclusive ->
+            if (cmp == -2 || cmp < 0) return false
-                    end!!.compareTo(scope, other.end!!) >= 0
+            if (cmp == 0 && otherUpper.inclusive && !ourUpper.inclusive) return false
-
-                !other.isEndInclusive && !isEndInclusive ->
-                    end!!.compareTo(scope, other.end!!) >= 0
-
-                other.isEndInclusive && !isEndInclusive ->
-                    end!!.compareTo(scope, other.end!!) > 0
-
-                !other.isEndInclusive && isEndInclusive ->
-                    end!!.compareTo(scope, other.end!!) >= 0
-
-                else -> throw IllegalStateException("unknown comparison")
-            }
         }
         return true
     }
@@ -108,35 +124,38 @@ class ObjRange(
 
         if (net.sergeych.lyng.PerfFlags.PRIMITIVE_FASTOPS) {
             if (start is ObjInt && end is ObjInt && other is ObjInt) {
-                val s = start.value
+                val lower = if (isDescending) end.value else start.value
-                val e = end.value
+                val upper = if (isDescending) start.value else end.value
                 val v = other.value
-                if (v < s) return false
+                if (v < lower || v > upper) return false
-                return if (isEndInclusive) v <= e else v < e
+                return if (isDescending) v != lower || isEndInclusive else v != upper || isEndInclusive
             }
             if (start is ObjChar && end is ObjChar && other is ObjChar) {
-                val s = start.value
+                val lower = if (isDescending) end.value else start.value
-                val e = end.value
+                val upper = if (isDescending) start.value else end.value
                 val v = other.value
-                if (v < s) return false
+                if (v < lower || v > upper) return false
-                return if (isEndInclusive) v <= e else v < e
+                return if (isDescending) v != lower || isEndInclusive else v != upper || isEndInclusive
             }
             if (start is ObjString && end is ObjString && other is ObjString) {
-                val s = start.value
+                val lower = if (isDescending) end.value else start.value
-                val e = end.value
+                val upper = if (isDescending) start.value else end.value
                 val v = other.value
-                if (v < s) return false
+                if (v < lower || v > upper) return false
-                return if (isEndInclusive) v <= e else v < e
+                return if (isDescending) v != lower || isEndInclusive else v != upper || isEndInclusive
             }
         }
 
-        if (isOpenStart && isOpenEnd) return true
+        val lower = normalizedLowerBound()
-        if (!isOpenStart) {
+        val upper = normalizedUpperBound()
-            if (start!!.compareTo(scope, other) > 0) return false
+        if (lower == null && upper == null) return true
+        if (lower != null) {
+            val cmp = lower.value.compareTo(scope, other)
+            if (cmp == -2 || cmp > 0 || (!lower.inclusive && cmp == 0)) return false
         }
-        if (!isOpenEnd) {
+        if (upper != null) {
-            val cmp = end!!.compareTo(scope, other)
+            val cmp = upper.value.compareTo(scope, other)
-            if (isEndInclusive && cmp < 0 || !isEndInclusive && cmp <= 0) return false
+            if (cmp == -2 || cmp < 0 || (!upper.inclusive && cmp == 0)) return false
         }
         return true
     }
@@ -153,7 +172,12 @@ class ObjRange(
         if (!hasExplicitStep && start is ObjInt && end is ObjInt) {
             val s = start.value
             val e = end.value
-            if (isEndInclusive) {
+            if (isDescending) {
+                val last = if (isEndInclusive) e else e + 1
+                for (i in s downTo last) {
+                    if (!callback(ObjInt.of(i))) break
+                }
+            } else if (isEndInclusive) {
                 for (i in s..e) {
                     if (!callback(ObjInt.of(i))) break
                 }
@@ -165,7 +189,14 @@ class ObjRange(
         } else if (!hasExplicitStep && start is ObjChar && end is ObjChar) {
             val s = start.value
             val e = end.value
-            if (isEndInclusive) {
+            if (isDescending) {
+                var c = s.code
+                val last = if (isEndInclusive) e.code else e.code + 1
+                while (c >= last) {
+                    if (!callback(ObjChar(c.toChar()))) break
+                    c--
+                }
+            } else if (isEndInclusive) {
                 for (c in s..e) {
                     if (!callback(ObjChar(c))) break
                 }
@@ -184,6 +215,7 @@ class ObjRange(
             if (start == other.start &&
                 end == other.end &&
                 isEndInclusive == other.isEndInclusive &&
+                isDescending == other.isDescending &&
                 step == other.step
             ) 0 else -1
         }
@@ -194,6 +226,7 @@ class ObjRange(
         var result = start?.hashCode() ?: 0
         result = 31 * result + (end?.hashCode() ?: 0)
         result = 31 * result + isEndInclusive.hashCode()
+        result = 31 * result + isDescending.hashCode()
         result = 31 * result + (step?.hashCode() ?: 0)
         return result
     }
@@ -207,6 +240,7 @@ class ObjRange(
         if (start != other.start) return false
         if (end != other.end) return false
         if (isEndInclusive != other.isEndInclusive) return false
+        if (isDescending != other.isDescending) return false
         if (step != other.step) return false
 
         return true
@@ -264,6 +298,13 @@ class ObjRange(
                 moduleName = "lyng.stdlib",
                 getter = { thisAs<ObjRange>().isEndInclusive.toObj() }
             )
+            addPropertyDoc(
+                name = "isDescending",
+                doc = "Whether the range iterates from the start bound down toward the end bound.",
+                type = type("lyng.Bool"),
+                moduleName = "lyng.stdlib",
+                getter = { thisAs<ObjRange>().isDescending.toObj() }
+            )
             addFnDoc(
                 name = "iterator",
                 doc = "Iterator over elements in this range (optimized for Int ranges).",
@@ -290,18 +331,22 @@ class ObjRange(
             if (startObj is Numeric && explicitStep !is Numeric) {
                 scope.raiseIllegalState("Numeric range step must be numeric")
             }
+            if (isDescending) {
+                val sign = when (explicitStep) {
+                    is ObjInt -> explicitStep.value.compareTo(0)
+                    is Numeric -> explicitStep.doubleValue.compareTo(0.0)
+                    else -> 1
+                }
+                if (sign < 0) scope.raiseIllegalState("Descending range step must be positive")
+                return explicitStep.negate(scope)
+            }
             return explicitStep
         }
         if (startObj is ObjInt) {
-            val cmp = if (end == null || end.isNull) 0 else startObj.compareTo(scope, end)
+            return ObjInt.of(if (isDescending) -1 else 1)
-            val dir = if (cmp >= 0) -1 else 1
-            return ObjInt.of(dir.toLong())
         }
         if (startObj is ObjChar) {
-            val endChar = end as? ObjChar
+            return ObjInt.of(if (isDescending) -1 else 1)
-                ?: scope.raiseIllegalState("Char range requires Char end to infer step")
-            val dir = if (startObj.value >= endChar.value) -1 else 1
-            return ObjInt.of(dir.toLong())
         }
         if (startObj is ObjReal) {
             scope.raiseIllegalState("Real range requires explicit step")

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt

@@ -1028,6 +1028,7 @@ class RangeRef(
     internal val left: ObjRef?,
     internal val right: ObjRef?,
     internal val isEndInclusive: Boolean,
+    internal val isDescending: Boolean = false,
     internal val step: ObjRef? = null
 ) : ObjRef {
     override suspend fun get(scope: Scope): ObjRecord = scope.raiseObjRefEvalDisabled()

lynglib/src/commonMain/kotlin/net/sergeych/lyng/statements.kt

@@ -77,7 +77,11 @@ class IfStatement(
     }
 }
 
-data class ConstIntRange(val start: Long, val endExclusive: Long)
+data class ConstIntRange(
+    val start: Long,
+    val stopBoundary: Long,
+    val isDescending: Boolean,
+)
 
 class ForInStatement(
     val loopVarName: String,

lynglib/src/commonTest/kotlin/ScriptTest.kt

@@ -294,6 +294,18 @@ class ScriptTest {
         assertEquals(Token.Type.INT, tt[0].type)
         assertEquals(Token.Type.DOTDOTLT, tt[1].type)
         assertEquals(Token.Type.INT, tt[2].type)
+
+        tt = parseLyng("5 downTo 4".toSource())
+
+        assertEquals(Token.Type.INT, tt[0].type)
+        assertEquals(Token.Type.DOWNTO, tt[1].type)
+        assertEquals(Token.Type.INT, tt[2].type)
+
+        tt = parseLyng("5 downUntil 4".toSource())
+
+        assertEquals(Token.Type.INT, tt[0].type)
+        assertEquals(Token.Type.DOWNUNTIL, tt[1].type)
+        assertEquals(Token.Type.INT, tt[2].type)
     }
 
     @Test
@@ -1280,6 +1292,36 @@ class ScriptTest {
         assertTrue(convIndex > incIndex, "INT_TO_REAL should appear after INC_INT")
     }
 
+    @Test
+    fun testDescendingForLoopDisasm() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            fun countDown() {
+                var acc = 0
+                for (i in 5 downTo 1) {
+                    acc += i
+                }
+            }
+            fun countDownVar() {
+                var acc = 0
+                val r = 5 downTo 1
+                for (i in r) {
+                    acc += i
+                }
+            }
+            """.trimIndent()
+        )
+        val constDisasm = scope.disassembleSymbol("countDown")
+        val varDisasm = scope.disassembleSymbol("countDownVar")
+        assertTrue("DEC_INT" in constDisasm, "expected DEC_INT in descending for-loop disasm")
+        assertTrue("JMP_IF_LTE_INT" in constDisasm, "expected JMP_IF_LTE_INT in descending for-loop disasm")
+        assertTrue("CALL_MEMBER_SLOT" !in constDisasm, "descending literal range should avoid iterator fallback")
+        assertTrue("DEC_INT" in varDisasm, "expected DEC_INT in descending range-variable for-loop disasm")
+        assertTrue("JMP_IF_LTE_INT" in varDisasm, "expected descending comparison in range-variable for-loop disasm")
+        assertTrue("CALL_MEMBER_SLOT" !in varDisasm, "descending range-variable loop should avoid iterator fallback")
+    }
+
     @Test
     fun testIntClosedRangeInclusive() = runTest {
         eval(
@@ -3485,17 +3527,58 @@ class ScriptTest {
     fun testRangeStepIteration() = runTest {
         val ints = eval("""(1..5 step 2).toList()""") as ObjList
         assertEquals(listOf(1, 3, 5), ints.list.map { it.toInt() })
+        val descending = eval("""(5 downTo 1).toList()""") as ObjList
+        assertEquals(listOf(5, 4, 3, 2, 1), descending.list.map { it.toInt() })
+        val descendingExclusive = eval("""(5 downUntil 1).toList()""") as ObjList
+        assertEquals(listOf(5, 4, 3, 2), descendingExclusive.list.map { it.toInt() })
+        val descendingStep = eval("""(10 downTo 1 step 3).toList()""") as ObjList
+        assertEquals(listOf(10, 7, 4, 1), descendingStep.list.map { it.toInt() })
+        val descendingChars = eval("""('e' downTo 'a' step 2).toList()""") as ObjList
+        assertEquals(listOf('e', 'c', 'a'), descendingChars.list.map { it.toString().single() })
         val chars = eval("""('a'..'e' step 2).toList()""") as ObjList
         assertEquals(listOf('a', 'c', 'e'), chars.list.map { it.toString().single() })
         val reals = eval("""(0.0..1.0 step 0.25).toList()""") as ObjList
         assertEquals(listOf(0.0, 0.25, 0.5, 0.75, 1.0), reals.list.map { it.toDouble() })
         val empty = eval("""(5..1 step 1).toList()""") as ObjList
         assertEquals(0, empty.list.size)
+        val plainDescending = eval("""(5..1).toList()""") as ObjList
+        assertEquals(0, plainDescending.list.size)
         val openEnd = eval("""(0.. step 1).take(3).toList()""") as ObjList
         assertEquals(listOf(0, 1, 2), openEnd.list.map { it.toInt() })
+        assertEquals(
+            true,
+            eval(
+                """
+                val r = 10 downTo 1
+                r.isDescending && r.isEndInclusive && (10 in r) && (1 in r) && (0 !in r)
+                """.trimIndent()
+            ).toBool()
+        )
+        assertEquals(
+            true,
+            eval(
+                """
+                val r = 10 downUntil 1
+                r.isDescending && !r.isEndInclusive && (10 in r) && (1 !in r) && ((8 downTo 3) in r)
+                """.trimIndent()
+            ).toBool()
+        )
+        assertEquals(
+            15,
+            (eval(
+                """
+                var s = 0
+                for (i in 5 downTo 1) s += i
+                s
+                """.trimIndent()
+            ) as ObjInt).toInt()
+        )
         assertFailsWith<ExecutionError> {
             eval("""(0.0..1.0).toList()""")
         }
+        assertFailsWith<ExecutionError> {
+            eval("""(5 downTo 1 step -1).toList()""")
+        }
         assertFailsWith<ExecutionError> {
             eval("""(0..).toList()""")
         }