package jsonrpc2

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"reflect"
)

var (
	ctxT = reflect.TypeOf((*context.Context)(nil)).Elem()
	errT = reflect.TypeOf((*error)(nil)).Elem()
)

type methodDispatchFunc func(context.Context, Request) (any, error)

func newMethodDispatchFunc(
	method reflect.Value,
) methodDispatchFunc {
	paramTs := make([]reflect.Type, method.Type().NumIn()-1)
	for i := range paramTs {
		paramTs[i] = method.Type().In(i + 1)
	}

	return func(ctx context.Context, req Request) (any, error) {
		callVals := make([]reflect.Value, 0, len(paramTs)+1)
		callVals = append(callVals, reflect.ValueOf(ctx))

		for i, paramT := range paramTs {
			paramPtrV := reflect.New(paramT)

			err := json.Unmarshal(req.Params[i], paramPtrV.Interface())
			if err != nil {
				// The JSON has already been validated, so this is not an
				// errCodeParse situation. We assume it's an invalid param then,
				// unless the error says otherwise via an UnmarshalJSON method
				// returning an Error of its own.
				if !errors.As(err, new(Error)) {
					err = NewInvalidParamsError(
						"JSON unmarshaling param %d into %T: %v", i, paramT, err,
					)
				}
				return nil, err
			}

			callVals = append(callVals, paramPtrV.Elem())
		}

		var (
			callResV   = method.Call(callVals)
			resV, errV reflect.Value
		)

		if len(callResV) == 1 {
			errV = callResV[0]
		} else {
			resV = callResV[0]
			errV = callResV[1]
		}

		if !errV.IsNil() {
			return nil, errV.Interface().(error)
		}

		if resV.IsValid() {
			return resV.Interface(), nil
		}

		return nil, nil
	}
}

type dispatcher struct {
	methods map[string]methodDispatchFunc
}

// NewDispatchHandler returns a Handler which will use methods on the passed in
// value to dispatch RPC calls. The passed in value must be a pointer. All
// exported methods which look like:
//
//	MethodName(context.Context, ParamType) (ResponseType, error)
//
// will be available via RPC calls.
func NewDispatchHandler(i any) Handler {
	v := reflect.ValueOf(i)
	if v.Kind() != reflect.Pointer {
		panic(fmt.Sprintf("expected pointer but got type %T", i))
	}

	v = v.Elem()

	var (
		t          = v.Type()
		numMethods = t.NumMethod()
		methods    = make(map[string]methodDispatchFunc, numMethods)
	)

	for i := range numMethods {
		var (
			method  = t.Method(i)
			methodV = v.Method(i)
			methodT = methodV.Type()
		)

		if !method.IsExported() ||
			methodT.NumIn() < 1 ||
			methodT.In(0) != ctxT ||
			(methodT.NumOut() == 1 && methodT.Out(0) != errT) ||
			(methodT.NumOut() == 2 && methodT.Out(1) != errT) ||
			methodT.NumOut() > 2 {
			continue
		}

		methods[method.Name] = newMethodDispatchFunc(methodV)
	}

	return &dispatcher{methods}
}

func (d *dispatcher) ServeRPC(ctx context.Context, req Request) (any, error) {
	fn, ok := d.methods[req.Method]
	if !ok {
		return nil, NewError(
			errCodeMethodNotFound, "unknown method %q", req.Method,
		)
	}

	return fn(ctx, req)
}