const indexTable = [
	-1, -1, -1, -1, 2, 4, 6, 8,
	-1, -1, -1, -1, 2, 4, 6, 8
]

const stepTable = [
	7, 8, 9, 10, 11, 12, 13, 14,
	16, 17, 19, 21, 23, 25, 28, 31,
	34, 37, 41, 45, 50, 55, 60, 66,
	73, 80, 88, 97, 107, 118, 130, 143,
	157, 173, 190, 209, 230, 253, 279, 307,
	337, 371, 408, 449, 494, 544, 598, 658,
	724, 796, 876, 963, 1060, 1166, 1282, 1411,
	1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
	3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
	7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
	15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794,
	32767
]

const byteDepth = 2, // We are working with 16-bit samples. TODO(Trek): make configurable.
	initSamps = 2, // Number of samples used to initialise the encoder.
	initBytes = initSamps * byteDepth,
	headBytes = 4, // Number of bytes in the header of ADPCM.
	samplesPerEnc = 2, // Number of sample encoded at a time eg. 2 16-bit samples get encoded into 1 byte.
	bytesPerEnc = samplesPerEnc * byteDepth,
	compFact = 4; // In general ADPCM compresses by a factor of 4.

let est = 0, // Estimation of sample based on quantised ADPCM nibble.
	idx = 0, // Index to step used for estimation.
	step = 0;

function decodeSample(nibble) {

	let diff = 0;
	if ((nibble & 4) != 0) {
		diff += step;
	}
	if ((nibble & 2) != 0) {
		diff += step >> 1;
	}
	if ((nibble & 1) != 0) {
		diff += step >> 2;
	}
	diff += step >> 3;

	if ((nibble & 8) != 0) {
		diff = -diff;
	}
	est += diff;
	idx += indexTable[nibble];

	if (idx < 0) {
		idx = 0;
	} else if (idx > stepTable.length - 1) {
		idx = stepTable.length - 1;
	}

	step = stepTable[idx];

	result = new Uint8Array(new Uint16Array([est]).buffer);
	return result;
}

function decode(b) {
	// b should be a Uint8Array
	if (!(b instanceof Uint8Array)) {
		console.log("Error: data is not a Uint8Array");
		return;
	}

	// Initialize Decoder with first 4 bytes of b.
	est = (new Uint16Array(b.slice(0, 2).buffer))[0];
	idx = b[byteDepth];
	step = stepTable[idx];

	var result = new Uint8Array(b.slice(0, 2));

	for (var i = headBytes; i < b.length - b[3]; i++) {
		var twoNibs = b[i];
		var nib2 = twoNibs >> 4;
		var nib1 = (nib2 << 4) ^ twoNibs
		var firstBytes = decodeSample(nib1)
		result = concat(result, firstBytes)

		var secondBytes = decodeSample(nib2)
		result = concat(result, secondBytes)
	}
	if (b[3] == 1) {
		var padNib = b[b.length - 1]
		var samp = decodeSample(padNib)
		result = concat(result, samp)
	}
	return result;
}

// concat concatenates TypedArrays a and b of same type.
function concat(a, b) {
	var c = new(a.constructor)(a.length + b.length);
	c.set(a, 0);
	c.set(b, a.length);
	return c;
}