[HELP] Resolution Changer w/ aspect ratio

#include "ReShade.fxh";

uniform bool bGFXA_doResolution <
	ui_type = "combo";
	ui_label = "Custom Screen Resolution [GFX Adapter]";
> = false;

uniform bool bGFXA_screenStretch <
	ui_type = "combo";
	ui_label = "Stretch Screen [GFX Adapter]";
> = false;

uniform float screenPAR <
	ui_type = "drag";
	ui_min = -5.0; ui_max = 5.0;
	ui_label = "Pixel Aspect Ratio [GFX Adapter]";
> = 1.0;

uniform float iGFXA_resolutionX <
	ui_type = "drag";
	ui_min = 1.0;
	ui_max = BUFFER_WIDTH;
	ui_label = "Screen Resolution Width [GFX Adapter]";
> = 320;

uniform float iGFXA_resolutionY <
	ui_type = "drag";
	ui_min = 1.0;
	ui_max = BUFFER_HEIGHT;
	ui_label = "Screen Resolution Height [GFX Adapter]";
> = 240;

uniform float3 f3GFXA_backgroundColor <
	ui_type = "color";
	ui_label = "Background Color [GFX Adapter]";
> = float3(1,1,1);


float2 p_sh(float2 p_){

	float2 xx = float2(320.0, 240.0); //output screen res				
	float2 ar = float2(1.0,1.0); //final aspect ratio (calculated)
	
	xx = float2(iGFXA_resolutionX,iGFXA_resolutionY);
	ar = ar.xx;
	
	return 0.5 * float2(
		step(0.0, 1.0 - ar.y) * (1.0 - 1.0 / ar.y), 
		step(1.0, ar.y) * (1.0 - ar.y));
}

float3 PS_Resolutionizer(float4 pos : SV_Position, float2 uv : TEXCOORD0) : SV_Target {
        float2 p = uv; //this is Texcoord
	float2 p_ = p*float2((float)BUFFER_WIDTH,(float)BUFFER_HEIGHT);
	float2 xx = float2(320.0,240.0);
	float2 ar = float2(1.0,1.0); //final aspect ratio (calculated)
	
	ar = ar.xx;
	xx = ReShade::ScreenSize.xy;
	
	if (bGFXA_doResolution){
	
	xx = float2(iGFXA_resolutionX, iGFXA_resolutionY); //Resolution, in screenshots, it was 320x240
	
	if(bGFXA_screenStretch==0){
		float sar = ReShade::ScreenSize.x/ReShade::ScreenSize.y;
		float nar = xx.x/xx.y;
		ar = float2(1.0, nar/sar*screenPAR);						
	}	
		p_ = p;
		p_ *= ar;
		
	if(ar.y<1.0) p_ = p_*(1.0/ar.y);
			p_ = floor(p_*xx)/xx;
			p_ +=  p_sh(p_);
	} else {
		p_ = uv;
	}

        float3 col = tex2D(ReShade::BackBuffer, p_).rgb;
        return col;
}

technique Resolutionizer {
	pass Resolutionizer_Pass1 {
		VertexShader=PostProcessVS;
		PixelShader=PS_Resolutionizer;
	}
}

uv -= 0.5;
uv *= scale;
uv += 0.5;