Add readSigned3BytesInDataView() and writeSigned3BytesInDataView() utils

src/tests/utils/byteOps.test.ts

@@ -1,6 +1,8 @@
 import {
 	read3BytesInDataView,
-	write3BytesInDataView
+	write3BytesInDataView,
+	readSigned3BytesInDataView,
+	writeSigned3BytesInDataView
 } from '../../utils/byteOps';
 
 let view: DataView;
@@ -10,7 +12,10 @@ beforeEach(() =>
 	const array = new Uint8Array(
 		[
 			0b00000000, 0b00000001, 0b00000010, 0b00000011,
-			0b10000000, 0b01000000, 0b00100000, 0b00010000
+			0b10000000, 0b01000000, 0b00100000, 0b00010000,
+			0b01111111, 0b11111111, 0b11111111, 0b00000000,
+			0b11111111, 0b11111111, 0b11111111, 0b00000000,
+			0b10000000, 0b00000000, 0b00000000, 0b00000000
 		]
 	);
 
@@ -32,3 +37,31 @@ test('write3BytesInDataView()', () =>
 	write3BytesInDataView({ view, pos: 1, value: 8405024 });
 	expect(read3BytesInDataView({ view, pos: 1 })).toBe(8405024);
 });
+
+test('readSigned3BytesInDataView()', () =>
+{
+	// Bytes 8, 9 and 10 in the array are number 8388607 since first bit is 0 and
+	// all other bits are 1, so it must be maximum positive 24 bits signed integer,
+	// which is Math.pow(2, 23) - 1 = 8388607.
+	expect(readSigned3BytesInDataView({ view, pos: 8 })).toBe(8388607);
+
+	// Bytes 12, 13 and 14 in the array are number -1.
+	expect(readSigned3BytesInDataView({ view, pos: 12 })).toBe(-1);
+
+	// Bytes 16, 17 and 18 in the array are number -8388608 since first bit is 1
+	// and all other bits are 0, so it must be minimum negative 24 bits signed
+	// integer, which is -1 * Math.pow(2, 23) = -8388608.
+	expect(readSigned3BytesInDataView({ view, pos: 16 })).toBe(-8388608);
+});
+
+test('writeSigned3BytesInDataView()', () =>
+{
+	writeSigned3BytesInDataView({ view, pos: 0, value: 8388607 });
+	expect(readSigned3BytesInDataView({ view, pos: 0 })).toBe(8388607);
+
+	writeSigned3BytesInDataView({ view, pos: 0, value: -1 });
+	expect(readSigned3BytesInDataView({ view, pos: 0 })).toBe(-1);
+
+	writeSigned3BytesInDataView({ view, pos: 0, value: -8388608 });
+	expect(readSigned3BytesInDataView({ view, pos: 0 })).toBe(-8388608);
+});

src/utils/byteOps.ts

@@ -13,7 +13,7 @@ export function read3BytesInDataView(
 }
 
 /**
- * Write a unsigned value in 3 bytes starting from byte `pos` in `view`.
+ * Write an unsigned value in 3 bytes starting from byte `pos` in `view`.
  *
  * @category Utils
  * @hidden
@@ -26,3 +26,39 @@ export function write3BytesInDataView(
 	view.setUint8(pos, value >> 16);
 	view.setUint16(pos + 1, value);
 }
+
+/**
+ * Read 3 bytes starting from byte `pos` in `view` as signed value.
+ *
+ * @category Utils
+ * @hidden
+ */
+export function readSigned3BytesInDataView(
+	{ view, pos }:
+	{ view: DataView; pos: number }
+): number
+{
+	const byte2 = view.getUint8(pos);
+	const byte1 = view.getUint8(pos + 1);
+	const byte0 = view.getUint8(pos + 2);
+
+	// Check bit 7 (sign).
+	const extension = byte2 & 0b10000000 ? 0b11111111 : 0b00000000;
+
+	return byte0 | (byte1 << 8) | (byte2 << 16) | (extension << 24);
+}
+
+/**
+ * Write a signed value in 3 bytes starting from byte `pos` in `view`.
+ *
+ * @category Utils
+ * @hidden
+ */
+export function writeSigned3BytesInDataView(
+	{ view, pos, value }:
+	{ view: DataView; pos: number; value: number }
+): void
+{
+	view.setInt8(pos, value >> 16);
+	view.setUint16(pos + 1, value);
+}