feat: parse framerate from fraction

Author: connerdouglass

src/rate.test.ts

@@ -0,0 +1,47 @@
+import {
+	Rate,
+	RateFromFraction,
+	Rate_23_976,
+	Rate_24,
+	Rate_29_97,
+	Rate_30,
+	Rate_59_94,
+	Rate_60,
+} from './rate';
+
+describe('testing create framerate from fraction', () => {
+	test('frame rate from fraction', () => {
+		const cases: [number, number, Rate][] = [
+			[24000, 1001, Rate_23_976],
+			[24, 1, Rate_24],
+			[30, 1, Rate_30],
+			[30000, 1001, Rate_29_97],
+			[60, 1, Rate_60],
+			[60000, 1001, Rate_59_94],
+		];
+		for (const [num, den, expected] of cases) {
+			const rate = RateFromFraction(num, den);
+			expect(rate.num).toBe(expected.num);
+			expect(rate.den).toBe(expected.den);
+			expect(rate.nominal).toBe(expected.nominal);
+			expect(rate.drop).toBe(expected.drop);
+		}
+	});
+	test('frame rate from fraction for built-in rates', () => {
+		const cases: Rate[] = [
+			Rate_23_976,
+			Rate_24,
+			Rate_30,
+			Rate_29_97,
+			Rate_60,
+			Rate_59_94,
+		];
+		for (const rate of cases) {
+			const newRate = RateFromFraction(rate.num, rate.den);
+			expect(newRate.num).toBe(rate.num);
+			expect(newRate.den).toBe(rate.den);
+			expect(newRate.nominal).toBe(rate.nominal);
+			expect(newRate.drop).toBe(rate.drop);
+		}
+	});
+});

src/rate.ts

@@ -1,3 +1,6 @@
+const SECONDS_PER_HOUR = 3600;
+const DROP_OCCURRENCES_PER_HOUR = 54;
+
 // Rate represents a frame rate for a timecode
 export interface Rate {
 	nominal: number;
@@ -26,6 +29,35 @@ export function ParseRate(str: string): Rate | null {
 	}
 }
 
+export function RateFromFraction(num: number, den: number): Rate {
+	// Calculate the nominal frame rate (number of frames in a second without drops)
+	const nominal = num % den === 0 ? num / den : den - (num % den);
+
+	// format it as a string (ie. 23.976)
+	let str = (num / den).toFixed(3);
+	while (str.endsWith('0')) {
+		str = str.slice(0, -1);
+	}
+	if (str.endsWith('.')) {
+		str = str.slice(0, -1);
+	}
+
+	// Calculate the number of frames to skip per drop occurrence
+	const actualFramesPerHour = Math.floor((num * SECONDS_PER_HOUR) / den);
+	const nominalFramesPerHour = nominal * SECONDS_PER_HOUR;
+	const totalFramesDropped = nominalFramesPerHour - actualFramesPerHour;
+	const framesPerDrop = Math.round(
+		totalFramesDropped / DROP_OCCURRENCES_PER_HOUR
+	);
+
+	return {
+		nominal,
+		drop: framesPerDrop,
+		num,
+		den,
+	};
+}
+
 // 23.976 has exactly 24 frames per second. However, the textual representation of timecodes using this rate
 // skip two frames every minute, except when the minute is a multiple of 10. This is because 23.976 footage
 // actually does display at a rate of 23.976 frames each second on televisions. To ensure that the first