shader_type canvas_item;
render_mode blend_premul_alpha;
const float TEX_SIZE = 4096.0;
const float UV_QUANTIZE = TEX_SIZE;
// I feel like these magic numbers are a bit more intuitive in hex
const float x00FF  = float(0x00FF);   //   255.0
const float x0100  = float(0x0100);   //   256.0
const float x7FFF  = float(0x7FFF);   // 32767.0
const float x8000  = float(0x8000);   // 32768.0
const float xFF00  = float(0xFF00);   // 65280.0
const float xFFFF  = float(0xFFFF);   // 65535.0
const float x10000 = float(0x10000);  // 65536.0

const vec2 INT16_DOT_BE = vec2(xFF00, x00FF);
const vec2 INT16_DOT_LE = vec2(x00FF, xFF00);
uniform sampler2D tex : hint_normal;

float unpack_int16(vec2 int16) {
	// Convert packed 2byte integer, sampled as two [0.0, 1.0] range floats,
	// to the original int value [-32768, 32767] or [0, 65535] but in float32
	float unsigned = dot(int16, INT16_DOT_LE);
	return unsigned - (unsigned < x7FFF ? 0.0 : x10000);
}

float rescale_int16(float int16) {
	// Rescale from [-32768, 32767] to [-1.0, 1.0)
	return int16 / x8000;
}

vec2 pack_float_to_int16(float value) {
	// Convert a float in range [-1.0, 1.0) to a signed 2byte integer [-32768, 32767] packed into two [0.0, 1.0] floats
	float scaled = value * x8000;
	float unsigned = scaled + (scaled < 0.0 ? x10000 : 0.0);
	float unsigned_div_256 = unsigned / x0100;
	float MSB = trunc(unsigned_div_256)         / x00FF;
	float LSB = fract(unsigned_div_256) * x0100 / x00FF;
	return vec2(LSB, MSB);
}

vec4 test_writeback(vec2 uv) {
	// Test importing and exporting the samples,
	// and exporting a value derived from the UV
	vec4 output;
	float sample_1 = rescale_int16(unpack_int16(texture(tex, uv).xw));
	float sample_2 = rescale_int16(dot(trunc(uv*TEX_SIZE), vec2(1.0, TEX_SIZE)));
	output.xy = pack_float_to_int16(sample_1);
	output.zw = pack_float_to_int16(sample_2);
	return output;
}

void fragment() {
	// GLES2
	vec2 uv = vec2(UV.x, 1.0-UV.y);
	uv = (trunc(uv*UV_QUANTIZE)+0.5)/UV_QUANTIZE;

	COLOR.xyzw = test_writeback(uv);
}