five/hbrt/thread.go

// Copyright (c) 2026 Charles KWON OhJun (charleskwonohjun@gmail.com)
// All rights reserved.

package hbrt

import (
	"fmt"
	"os"
)

// Default stack/frame sizes
const (
	DefaultStackSize    = 2048 // initial eval stack capacity
	MaxStackSize        = 65536
	MaxCallDepth        = 256
	InitialCallDepth    = 32  // start small, grow if needed
)

// CallFrame saves the state of a function call.
// Harbour equivalent: HB_STACK_STATE
type CallFrame struct {
	symbol     *Symbol // function symbol (for debugging/profiling)
	base       int     // stack base (start of this frame's args)
	localBase  int     // where locals start in the locals slice
	localCount int     // number of locals in this frame
	paramCount int     // number of parameters passed
	retVal     Value   // return value
	module     string  // current PRG source file (updated by DebugLine)
	line       int     // current PRG source line
	localNames []string // PRG-source names of params+locals (nil = none registered)
}

// CurFrame returns the current call frame (for closure capture).
func (t *Thread) CurFrame() *CallFrame { return t.curFrame }

// CurBlock returns the *HbBlock for the codeblock currently executing
// (or nil outside a block body). Used by pcode dispatch to resolve
// detached-local opcodes against the running block's capture slice.
func (t *Thread) CurBlock() *HbBlock { return t.curBlock }

// SetCurBlock installs the executing block. The pcode block wrapper
// pairs Set(block) with a deferred Set(prev) to nest correctly across
// blocks that call other blocks (`{|| eval(b1) + eval(b2) }`).
func (t *Thread) SetCurBlock(b *HbBlock) { t.curBlock = b }

// LocalsSlice returns the underlying locals array (for closure capture).
func (t *Thread) LocalsSlice() []Value { return t.locals }

// GetLocal reads a local variable from a captured frame (1-based index).
func (f *CallFrame) GetLocal(n int, locals []Value) Value {
	idx := f.localBase + n - 1
	return locals[idx]
}

// SetLocal writes a local variable in a captured frame (1-based index).
func (f *CallFrame) SetLocal(n int, v Value, locals []Value) {
	idx := f.localBase + n - 1
	locals[idx] = v
}

// Thread is the per-goroutine execution context.
// Harbour equivalent: HB_STACK (thread-local stack)
//
// Each goroutine that runs Harbour code gets its own Thread.
// No locking needed for stack/locals/calls — they are goroutine-local.
// The TID is VM-unique and assigned at construction time for debugger
// thread listing.
// TraceEntry captures one step of execution history — module+line where
// DebugLine fired. Populated only when the debugger is attached so
// regular runs don't pay the ring-buffer cost.
type TraceEntry struct {
	Module string
	Line   int
}

// Size of the per-thread execution trace ring buffer. 256 entries gives
// enough runway to answer "how did we get here?" across most loops
// without meaningfully bloating per-thread memory.
const TraceRingSize = 256

type Thread struct {
	tid uint32

	// traceRing is a ring buffer of recent DebugLine hits. traceHead
	// points at the slot for the next write. Total recorded entries
	// across the program's lifetime for this thread is tracked via
	// traceCount so the debugger can render "N lines ago".
	traceRing  []TraceEntry
	traceHead  int
	traceCount uint64
	// Eval stack (goroutine-local, no lock needed)
	stack []Value
	sp    int // stack pointer (next free slot)

	// Local variables: flat array, each frame gets a slice via localBase+localCount
	locals []Value

	// Call stack
	calls    []CallFrame
	callSP   int // call stack pointer
	curFrame *CallFrame

	// Return value (passed between caller/callee)
	retVal Value

	// Pending function call stack (PushSymbol pushes, Function pops).
	// Depth=1 is the common case (non-nested call) and gets a scalar
	// fast slot to skip slice append/trim; nested calls fall back to
	// the heap slice. Balanced push/pop keeps the invariant:
	//   pendingSymFast set → slice empty
	//   slice non-empty → pendingSymFast may or may not be set.
	pendingSymFast  *Symbol
	pendingSyms     []*Symbol
	pendingParams   int     // number of params for next Frame call
	pendingCallSym  *Symbol // symbol for next Frame (for PROCNAME)

	// STATIC variables (per-module, shared but rarely written)
	// Accessed via PushStatic/PopStatic with module reference
	statics map[string][]Value

	// OOP: current Self object (set during method dispatch)
	self Value

	// Current code block (set while a block body is executing).
	// Pcode opcodes PcOpPushDetached / PcOpPopDetached read/write
	// Detached[i] through this pointer. The block's wrapper Fn
	// sets it before ExecPcode and restores on exit. Stays nil
	// outside block bodies; nil-checking opcodes fall back to NIL.
	curBlock *HbBlock

	// Error handling: last error from BEGIN SEQUENCE
	lastError *HbError

	// MEMVAR: PUBLIC/PRIVATE variables (shared across call stack)
	Memvars *MemvarTable

	// WorkArea manager (goroutine-local, no locks needed)
	WA      interface{} // *hbrdd.WorkAreaManager — set by caller to avoid import cycle

	// FastFieldGetter is a hot-path closure set by SqlScan (or any other
	// scan loop) to short-circuit PcOpFieldGet. When non-nil, the pcode
	// interpreter calls this instead of going through PushSymbol +
	// Function dispatch + FieldGet RTL's own Frame/EndProc. Caller is
	// responsible for setting and clearing it around a scan.
	FastFieldGetter func(int) Value
	waStack []uint16    // saved workarea numbers for (expr)->(expr) context switching

	// VM reference (shared, read-mostly)
	vm *VM
}

// NewThread creates a new execution thread.
func NewThread(vm *VM) *Thread {
	t := &Thread{
		stack:   make([]Value, DefaultStackSize),
		sp:      0,
		locals:  make([]Value, 256), // will grow as needed
		calls:   make([]CallFrame, InitialCallDepth),
		callSP:  0,
		statics: make(map[string][]Value),
		Memvars: NewMemvarTable(),
		vm:      vm,
	}
	return t
}

// --- Stack operations ---

func (t *Thread) push(v Value) {
	if t.sp >= len(t.stack) {
		if t.sp >= MaxStackSize {
			panic(t.runtimeError("stack overflow"))
		}
		newStack := make([]Value, len(t.stack)*2)
		copy(newStack, t.stack[:t.sp])
		t.stack = newStack
	}
	t.stack[t.sp] = v
	t.sp++
}

func (t *Thread) pop() Value {
	if t.sp <= 0 {
		panic(t.runtimeError("stack underflow"))
	}
	t.sp--
	v := t.stack[t.sp]
	t.stack[t.sp] = cachedNil
	return v
}

func (t *Thread) peek() Value {
	if t.sp <= 0 {
		panic(t.runtimeError("stack underflow (peek)"))
	}
	return t.stack[t.sp-1]
}

func (t *Thread) peekPtr() *Value {
	if t.sp <= 0 {
		panic(t.runtimeError("stack underflow (peekPtr)"))
	}
	return &t.stack[t.sp-1]
}

func (t *Thread) setTop(v Value) {
	if t.sp <= 0 {
		panic(t.runtimeError("stack underflow (setTop)"))
	}
	t.stack[t.sp-1] = v
}

// stackAt returns a pointer to stack item at offset from top.
// 0 = top, -1 = second from top, etc.
func (t *Thread) stackAt(offset int) *Value {
	idx := t.sp - 1 + offset
	if idx < 0 || idx >= t.sp {
		panic(t.runtimeError("stack access out of range"))
	}
	return &t.stack[idx]
}

// --- Push convenience methods (used by generated code) ---

func (t *Thread) PushNil()            { t.push(MakeNil()) }
func (t *Thread) PushBool(b bool)     { t.push(MakeBool(b)) }
func (t *Thread) PushInt(n int)       { t.push(MakeInt(n)) }
func (t *Thread) PushLong(n int64)    { t.push(MakeLong(n)) }
func (t *Thread) PushDouble(v float64, length, decimal uint16) {
	t.push(MakeDouble(v, length, decimal))
}
func (t *Thread) PushString(s string) { t.push(MakeString(s)) }
func (t *Thread) PushValue(v Value)   { t.push(v) }

func (t *Thread) Pop()       { t.pop() }
func (t *Thread) Pop2() Value { return t.pop() } // pop and return
func (t *Thread) Dup()       { t.push(t.peek()) }

// SP returns the current data-stack depth. Paired with SetSP for
// callers that need to clamp the stack across an inner function
// dispatch — used by FrbDo to neutralise pcode-body stack leaks.
func (t *Thread) SP() int { return t.sp }

// SetSP forcibly resets the data-stack depth. Truncates if newSP < sp;
// extends with NIL if newSP > sp (defensive — should never grow here
// in practice). Bounds-checked against the underlying slice so a
// negative or out-of-range value can't corrupt the runtime.
func (t *Thread) SetSP(newSP int) {
	if newSP < 0 {
		newSP = 0
	}
	if newSP > len(t.stack) {
		newSP = len(t.stack)
	}
	// Clear any slots being abandoned so stale values can't surface
	// later through Dup/peek paths.
	for i := newSP; i < t.sp; i++ {
		t.stack[i] = cachedNil
	}
	t.sp = newSP
}

// --- Frame management ---
// Harbour: hb_xvmFrame(params, locals)
// Called at the start of every function.

func (t *Thread) Frame(params, locals int) {
	if t.callSP >= MaxCallDepth {
		panic(t.runtimeError("call stack overflow"))
	}
	// Grow call stack dynamically if needed
	if t.callSP >= len(t.calls) {
		newSize := len(t.calls) * 2
		if newSize > MaxCallDepth {
			newSize = MaxCallDepth
		}
		newCalls := make([]CallFrame, newSize)
		copy(newCalls, t.calls)
		t.calls = newCalls
	}

	// Ensure locals slice has enough space
	localBase := 0
	if t.curFrame != nil {
		localBase = t.curFrame.localBase + t.curFrame.localCount
	}
	needed := localBase + params + locals
	if needed > len(t.locals) {
		newLocals := make([]Value, needed*2)
		copy(newLocals, t.locals)
		t.locals = newLocals
	}

	// Save frame
	// Handle case where fewer args were pushed than declared params
	actual := t.pendingParams
	if actual > params {
		actual = params
	}
	if actual > t.sp {
		actual = t.sp
	}

	frame := &t.calls[t.callSP]
	frame.base = t.sp - actual // only actual args on stack
	frame.localBase = localBase
	frame.localCount = params + locals
	frame.paramCount = t.pendingParams // actual args passed by caller (not declared count)
	frame.retVal = MakeNil()
	frame.symbol = t.pendingCallSym
	t.pendingCallSym = nil

	// Copy actual parameters from stack to locals
	for i := 0; i < actual; i++ {
		t.locals[localBase+i] = t.stack[frame.base+i]
	}

	// Initialize missing params and locals to NIL
	for i := actual; i < params+locals; i++ {
		t.locals[localBase+i] = MakeNil()
	}

	// Pop args from stack (they're now in locals)
	t.sp = frame.base

	t.curFrame = frame
	t.callSP++
}

// EndProc is called via defer at the end of every function.
// Handles recover for BEGIN SEQUENCE and restores frame.
// All panics are re-panicked so the generated SEQUENCE/RECOVER handler
// can catch them. HbError + BreakValue (from Break() in hbrtl) are
// re-panicked silently; unknown panics also re-panic but with a
// diagnostic message on stderr.
func (t *Thread) EndProc() {
	if r := recover(); r != nil {
		t.endFrame()
		if _, ok := r.(*HbError); ok {
			panic(r) // HbError — re-panic silently
		}
		// Check for BreakValue from hbrtl.Break() via duck typing.
		// We can't import hbrtl (cycle), so we check the type name.
		rType := fmt.Sprintf("%T", r)
		if rType == "hbrtl.BreakValue" {
			panic(r) // BreakValue — re-panic silently for RECOVER USING
		}
		fmt.Fprintf(os.Stderr, "Five runtime error: %v [recovered, repanicked]\n", r)
		panic(r)
	}
	t.endFrame()
}

// EndProcFast is called by RTL functions that don't need recover().
// ~3x faster than EndProc (no defer recover overhead).
func (t *Thread) EndProcFast() {
	t.endFrame()
}

// endFrame restores the previous call frame.
func (t *Thread) endFrame() {
	if t.callSP > 0 {
		t.callSP--
		if t.callSP > 0 {
			t.curFrame = &t.calls[t.callSP-1]
		} else {
			t.curFrame = nil
		}
	}
}

// EndProcNoRecover cleans up the frame without recover (used by Break).
func (t *Thread) EndProcNoRecover() {
	if t.callSP > 0 {
		t.callSP--
		if t.callSP > 0 {
			t.curFrame = &t.calls[t.callSP-1]
		} else {
			t.curFrame = nil
		}
	}
}

// --- Local variable access ---
// Harbour convention: local index 1-based (1 = first param or local)

func (t *Thread) PushLocal(n int) {
	idx := t.localIndex(n)
	v := t.locals[idx]
	if v.Type() == tByref {
		t.push((*HbRefCell)(v.ptr).V)
	} else {
		t.push(v)
	}
}

func (t *Thread) PopLocal(n int) {
	idx := t.localIndex(n)
	val := t.pop()
	if e := t.locals[idx]; e.Type() == tByref {
		(*HbRefCell)(e.ptr).V = val
	} else {
		t.locals[idx] = val
	}
}

func (t *Thread) Local(n int) Value {
	v := t.locals[t.localIndex(n)]
	if v.Type() == tByref {
		return (*HbRefCell)(v.ptr).V
	}
	return v
}

func (t *Thread) SetLocal(n int, v Value) {
	idx := t.localIndex(n)
	if e := t.locals[idx]; e.Type() == tByref {
		(*HbRefCell)(e.ptr).V = v
	} else {
		t.locals[idx] = v
	}
}

// Fast variants — no bounds checking (gengo guarantees valid indices).
// Byref-aware: transparently dereference/write-through RefCell.

func (t *Thread) PushLocalFast(n int) {
	v := t.locals[t.curFrame.localBase+n-1]
	if v.Type() == tByref {
		t.push((*HbRefCell)(v.ptr).V)
	} else {
		t.push(v)
	}
}

func (t *Thread) PopLocalFast(n int) {
	idx := t.curFrame.localBase + n - 1
	val := t.pop()
	if e := t.locals[idx]; e.Type() == tByref {
		(*HbRefCell)(e.ptr).V = val
	} else {
		t.locals[idx] = val
	}
}

func (t *Thread) LocalFast(n int) Value {
	v := t.locals[t.curFrame.localBase+n-1]
	if v.Type() == tByref {
		return (*HbRefCell)(v.ptr).V
	}
	return v
}

func (t *Thread) SetLocalFast(n int, v Value) {
	idx := t.curFrame.localBase + n - 1
	if e := t.locals[idx]; e.Type() == tByref {
		(*HbRefCell)(e.ptr).V = v
	} else {
		t.locals[idx] = v
	}
}

// PushLocalRef creates a shared RefCell and pushes it for @param.
// Both caller's local and callee's param point to the same cell.
func (t *Thread) PushLocalRef(n int) {
	idx := t.localIndex(n)
	v := t.locals[idx]
	if v.Type() == tByref {
		t.push(v) // already a RefCell — share it
		return
	}
	cell := &HbRefCell{V: v}
	ref := MakeByref(cell)
	t.locals[idx] = ref // caller's local becomes RefCell
	t.push(ref)         // callee gets same RefCell
}

func (t *Thread) LocalAsString(n int) string {
	return t.Local(n).AsString()
}

// EnsureLocalRef converts a local to a RefCell if it isn't one already.
// Used by closure capture to enable shared mutable access.
func (t *Thread) EnsureLocalRef(n int) {
	idx := t.curFrame.localBase + n - 1
	v := t.locals[idx]
	if v.Type() != tByref {
		cell := &HbRefCell{V: v}
		t.locals[idx] = MakeByref(cell)
	}
}

// LocalRaw returns the raw Value at local slot (including RefCell wrapper).
// Used by closure capture to grab the RefCell itself, not the dereferenced value.
func (t *Thread) LocalRaw(n int) Value {
	return t.locals[t.curFrame.localBase+n-1]
}

// SetLocalRaw sets a local slot to the raw Value (including RefCell wrapper).
// Used by closure to inject shared RefCell into block locals.
func (t *Thread) SetLocalRaw(n int, v Value) {
	t.locals[t.curFrame.localBase+n-1] = v
}

// LocalSetInt is an optimization: set local directly without stack. Byref-aware.
func (t *Thread) LocalSetInt(n int, val int) {
	idx := t.localIndex(n)
	if e := t.locals[idx]; e.Type() == tByref {
		(*HbRefCell)(e.ptr).V = MakeInt(val)
	} else {
		t.locals[idx] = MakeInt(val)
	}
}

func (t *Thread) localIndex(n int) int {
	if t.curFrame == nil {
		panic(t.runtimeError("no active frame"))
	}
	idx := t.curFrame.localBase + n - 1 // 1-based to 0-based
	if idx < t.curFrame.localBase || idx >= t.curFrame.localBase+t.curFrame.localCount {
		panic(t.runtimeError(fmt.Sprintf("local variable index out of range: %d", n)))
	}
	return idx
}

// --- Memvar access ---

// PushMemvar pushes a memvar value onto the stack. Harbour: M->varname
func (t *Thread) PushMemvar(name string) {
	if v, ok := t.Memvars.Get(name); ok {
		t.push(v)
	} else {
		t.push(MakeNil())
	}
}

// PopMemvar pops stack and stores into a memvar. Harbour: M->varname := expr
func (t *Thread) PopMemvar(name string) {
	val := t.pop()
	if !t.Memvars.Set(name, val) {
		// Auto-create as PRIVATE if not exists
		t.Memvars.SetPrivate(name, val, t.callSP)
	}
}

// DeclarePublic creates a PUBLIC memvar with NIL value.
func (t *Thread) DeclarePublic(name string) {
	if !t.Memvars.Exists(name) {
		t.Memvars.SetPublic(name, MakeNil())
	}
}

// DeclarePrivate creates a PRIVATE memvar with NIL value.
func (t *Thread) DeclarePrivate(name string) {
	t.Memvars.SetPrivate(name, MakeNil(), t.callSP)
}

// --- Return value ---

func (t *Thread) RetValue() {
	t.retVal = t.pop()
}

func (t *Thread) RetInt(n int64) {
	t.retVal = MakeNumInt(n)
}

func (t *Thread) RetNil() {
	t.retVal = MakeNil()
}

func (t *Thread) RetString(s string) {
	t.retVal = MakeString(s)
}

func (t *Thread) RetBool(b bool) {
	t.retVal = MakeBool(b)
}

func (t *Thread) RetLong(n int64) {
	t.retVal = MakeLong(n)
}

func (t *Thread) RetDouble(v float64, length, decimal uint16) {
	t.retVal = MakeDouble(v, length, decimal)
}

func (t *Thread) RetPointer(val interface{}) {
	t.retVal = MakePointer(val)
}

func (t *Thread) RetVal(v Value) {
	t.retVal = v
}

// PushRetValue pushes the return value from the last call onto the stack.
func (t *Thread) PushRetValue() {
	t.push(t.retVal)
}

// GetRetValue returns the current return value.
func (t *Thread) GetRetValue() Value {
	return t.retVal
}

// --- Error handling ---

// HbError represents a Harbour runtime error.
type HbError struct {
	Description string
	Operation   string
	Args        []Value
	SubSystem   string
	GenCode     int
	Stack       []DebugStackFrame // snapshot at panic site (pre-unwind)
}

func (e *HbError) Error() string {
	return fmt.Sprintf("Five runtime error: %s (op: %s)", e.Description, e.Operation)
}

func (t *Thread) runtimeError(msg string) *HbError {
	return &HbError{
		Description: msg,
		SubSystem:   "BASE",
		Stack:       t.DebugCallStack(),
	}
}

func (t *Thread) argError(op string, args ...Value) *HbError {
	return &HbError{
		Description: "argument error",
		Operation:   op,
		Args:        args,
		SubSystem:   "BASE",
		GenCode:     1,
		Stack:       t.DebugCallStack(),
	}
}

func (t *Thread) handleSequenceError(err *HbError) {
	// BEGIN SEQUENCE / RECOVER: store error for RECOVER USING
	t.lastError = err
	// The recover block is handled by the generated code's defer/recover pattern.
	// EndProc catches the panic and this function stores the error value.
}

// VM returns the VM this thread belongs to.
func (t *Thread) VM() *VM {
	return t.vm
}

// ParamCount returns the number of parameters passed to the current call.
// Used by RTL functions that call ParamCount() BEFORE Frame() — returns
// pendingParams set by Function(nArgs). This is the original behavior
// that all existing RTL functions depend on.
//
// For PRG-level PCount(), use CallerParamCount() instead (via PCount RTL).
func (t *Thread) ParamCount() int {
	return t.pendingParams
}

// CallerParamCount returns the param count of the calling PRG function
// (one frame below the current). Used by PCount() RTL which needs the
// caller's count, not its own.
func (t *Thread) CallerParamCount() int {
	if t.callSP >= 2 {
		return t.calls[t.callSP-2].paramCount
	}
	return 0
}

// PendingParams2 sets pending param count for direct block calls (AEval, ASort etc.)
func (t *Thread) PendingParams2(n int) {
	t.pendingParams = n
}

func (t *Thread) pushPendingSym(sym *Symbol) {
	// Fast path for depth=1 nesting — store in scalar slot without
	// touching the slice. A nil sym (unresolved symbol, caught later
	// in Function() with a descriptive error) must not use the fast
	// path because `pendingSymFast == nil` already means "empty";
	// falling back to the slice preserves distinguishability.
	if sym != nil && t.pendingSymFast == nil && len(t.pendingSyms) == 0 {
		t.pendingSymFast = sym
		return
	}
	if t.pendingSymFast != nil {
		t.pendingSyms = append(t.pendingSyms, t.pendingSymFast)
		t.pendingSymFast = nil
	}
	t.pendingSyms = append(t.pendingSyms, sym)
}

func (t *Thread) popPendingSym() *Symbol {
	if n := len(t.pendingSyms); n > 0 {
		sym := t.pendingSyms[n-1]
		t.pendingSyms = t.pendingSyms[:n-1]
		return sym
	}
	if sym := t.pendingSymFast; sym != nil {
		t.pendingSymFast = nil
		return sym
	}
	return nil
}

// PushAliasField pushes a field value from a named alias workarea.
// Harbour: alias->field
func (t *Thread) PushAliasField(alias, field string) {
	// Delegate to WorkAreaManager via WA interface
	if t.WA != nil {
		// Use reflection-free interface assertion
		type aliasGetter interface {
			GetAliasField(alias, field string) Value
		}
		if ag, ok := t.WA.(aliasGetter); ok {
			t.push(ag.GetAliasField(alias, field))
			return
		}
	}
	t.push(MakeNil())
}

// PushDynAliasField pushes a field from dynamic alias: (expr)->field
func (t *Thread) PushDynAliasField(alias, field string) {
	t.PushAliasField(alias, field)
}

// GetLastError returns the last error from BEGIN SEQUENCE.
func (t *Thread) GetLastError() *HbError {
	return t.lastError
}

// --- STATIC variable access ---

func (t *Thread) RegisterStatics(module string, statics []Value) {
	t.statics[module] = statics
}

func (t *Thread) PushStatic(module string, n int) {
	statics := t.statics[module]
	if n < 1 || n > len(statics) {
		panic(t.runtimeError(fmt.Sprintf("static index out of range: %s[%d]", module, n)))
	}
	t.push(statics[n-1])
}

func (t *Thread) PopStatic(module string, n int) {
	statics := t.statics[module]
	if n < 1 || n > len(statics) {
		panic(t.runtimeError(fmt.Sprintf("static index out of range: %s[%d]", module, n)))
	}
	statics[n-1] = t.pop()
}

// --- Workarea context switching for (alias)->(expr) ---
//
// The waSel interfaces below use CurrentNum() (uint16 area index), NOT
// Current() (which returns the Area interface on WorkAreaManager). An
// earlier version required `Current() uint16` which silently failed the
// type assertion on the real hbrdd.WorkAreaManager implementation —
// `alias->(expr)` expressions appeared to "work" on the first area but
// collapsed to no-op as soon as a sibling area was opened, because the
// switch/save/restore block was skipped entirely. See repro in
// /tmp/repro_xarea.prg.

func (t *Thread) WASaveAndSelect(areaNum int) {
	type waSel interface {
		SelectByNum(uint16)
		CurrentNum() uint16
	}
	if wam, ok := t.WA.(waSel); ok {
		t.waStack = append(t.waStack, wam.CurrentNum())
		wam.SelectByNum(uint16(areaNum))
	}
}

func (t *Thread) WASaveAndSelectAlias(alias string) {
	type waSel interface {
		SelectByAlias(string) bool
		SelectByNum(uint16)
		CurrentNum() uint16
	}
	if wam, ok := t.WA.(waSel); ok {
		t.waStack = append(t.waStack, wam.CurrentNum())
		if !wam.SelectByAlias(alias) {
			// Alias not open: switch to the no-area sentinel so the
			// inner expression's Current() returns nil and ops like
			// DbCloseArea/FieldGet/RecCount short-circuit. Without
			// this, the inner expression silently runs against the
			// originally-selected WA — which led to `CLOSE bad_alias`
			// closing the *current* area.
			wam.SelectByNum(0)
		}
	}
}

func (t *Thread) WARestore() {
	if n := len(t.waStack); n > 0 {
		saved := t.waStack[n-1]
		t.waStack = t.waStack[:n-1]
		type waSel interface{ SelectByNum(uint16) }
		if wam, ok := t.WA.(waSel); ok {
			wam.SelectByNum(saved)
		}
	}
}