use bytes;
use io;
use memio;

// Filter modes supported by PNG files.
export type filter = enum u8 {
	NONE = 0,
	SUB = 1,
	UP = 2,
	AVERAGE = 3,
	PAETH = 4,
};

fn applyfilter(dec: *decoder) void = {
	const mode = dec.filter as filter;
	const bpp = dec.bpp;
	switch (mode) {
	case filter::NONE =>
		yield; // no-op
	case filter::SUB =>
		for (let i = bpp; i < len(dec.cr); i += 1) {
			dec.cr[i] += dec.cr[i - bpp];
		};
	case filter::UP =>
		for (let i = 0z; i < len(dec.cr); i += 1) {
			dec.cr[i] += dec.pr[i];
		};
	case filter::AVERAGE =>
		for (let i = 0z; i < bpp; i += 1) {
			dec.cr[i] += dec.pr[i] / 2;
		};
		for (let i = bpp; i < len(dec.cr); i += 1) {
			dec.cr[i] += ((dec.cr[i - bpp]: int + dec.pr[i]: int) / 2): u8;
		};
	case filter::PAETH =>
		applypaeth(dec);
	};
	dec.filter = void;
	dec.pr[..] = dec.cr[..];
};

@test fn filter_none() void = {
	const src = memio::fixed(no_filtering);
	const image = load(&src)!;
	defer image_finish(&image);
	assert(bytes::equal(image.pixels, no_filtering_data));
};

@test fn filter_sub() void = {
	const src = memio::fixed(filter_sub_png);
	const image = load(&src)!;
	defer image_finish(&image);
	assert(bytes::equal(image.pixels, filter_sub_data));
};

@test fn filter_up() void = {
	const src = memio::fixed(filter_up_png);
	const image = load(&src)!;
	defer image_finish(&image);
	assert(bytes::equal(image.pixels, filter_up_data));
};

@test fn filter_average() void = {
	const src = memio::fixed(filter_avg_png);
	const image = load(&src)!;
	defer image_finish(&image);
	assert(bytes::equal(image.pixels, filter_avg_data));
};

@test fn filter_peath() void = {
	const src = memio::fixed(filter_peath_png);
	const image = load(&src)!;
	defer image_finish(&image);
	assert(bytes::equal(image.pixels, filter_peath_data));
};