// 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
}

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

// 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.
type Thread struct {
	// 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)
	// Stack needed for nested calls: Double(Add(3,4))
	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

	// 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

	// 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 {
	t.sp--
	v := t.stack[t.sp]
	t.stack[t.sp] = cachedNil // help GC (no alloc)
	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()) }

// --- 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 = params
	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.
func (t *Thread) EndProc() {
	if r := recover(); r != nil {
		if hbErr, ok := r.(*HbError); ok {
			t.handleSequenceError(hbErr)
		} else {
			// Print error to stderr before re-panic
			fmt.Fprintf(os.Stderr, "Five runtime error: %v\n", r)
			panic(r)
		}
	}

	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)
	t.push(t.locals[idx])
}

func (t *Thread) PopLocal(n int) {
	idx := t.localIndex(n)
	t.locals[idx] = t.pop()
}

func (t *Thread) Local(n int) Value {
	return t.locals[t.localIndex(n)]
}

func (t *Thread) SetLocal(n int, v Value) {
	t.locals[t.localIndex(n)] = v
}

// PushLocalRef pushes a reference to a local variable (for @param).
// Harbour: hb_vmPushLocalByRef
// Simplified: pushes the value (true BYREF needs refcell pattern).
// TODO: implement proper ByRef with shared mutation.
func (t *Thread) PushLocalRef(n int) {
	t.push(t.Local(n)) // simplified: pass by value for now
}

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

// LocalSetInt is an optimization: set local directly without stack.
// Harbour: hb_xvmLocalSetInt(n, val)
func (t *Thread) LocalSetInt(n int, val int) {
	t.locals[t.localIndex(n)] = 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
}

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",
	}
}

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

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 variadic RTL functions (QOut, etc.).
func (t *Thread) ParamCount() int {
	return t.pendingParams
}

// 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) {
	t.pendingSyms = append(t.pendingSyms, sym)
}

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

// 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()
}