Start implementation

src/codec.ts

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+import { AudioCodec, VideoCodec } from "./source";
+
+export const buildVideoCodecString = (codec: VideoCodec, width: number, height: number) => {
+	if (codec === 'avc') {
+		let profileIndication = 0x64; // Default to High Profile
+
+		if (width <= 768 && height <= 432) {
+			profileIndication = 0x42; // Baseline for smaller videos
+		} else if (width <= 1920 && height <= 1080) {
+			profileIndication = 0x4D; // Main for HD
+		}
+
+		const profileCompatibility = 0x00;
+
+		// Default to Level 4.1 (0x29) for most content, only bump to Level 5.0 (0x32) for 4K content
+		const levelIndication = (width > 1920 || height > 1080) ? 0x32 : 0x29;
+
+		const hexProfileIndication = profileIndication.toString(16).padStart(2, '0');
+		const hexProfileCompatibility = profileCompatibility.toString(16).padStart(2, '0');
+		const hexLevelIndication = levelIndication.toString(16).padStart(2, '0');
+
+		return `avc1.${hexProfileIndication}${hexProfileCompatibility}${hexLevelIndication}`;
+	} else if (codec === 'hevc') {
+		// Start with general_profile_space and general_profile_idc
+		let profileSpace = 0; // Assuming general_profile_space == 0 (most common)
+		let profileIdc = 1;   // Assuming Main Profile (1)
+		
+		// Generate compatibility flags (32 bits in reverse order)
+		// For basic compatibility, we'll set the main profile bit
+		const compatibilityFlags = Array(32).fill(0);
+		compatibilityFlags[profileIdc] = 1;  // Set bit for current profile
+		const compatibilityHex = parseInt(compatibilityFlags.reverse().join(''), 2)
+			.toString(16)
+			.replace(/^0+/, ''); // Remove leading zeroes
+			
+		// Determine tier and level based on resolution
+		let tier = 'L';  // L for Main Tier, H for High Tier
+		let level = 120; // Default level 4.0 (120)
+		
+		// Adjust level based on resolution (simplified)
+		if (width <= 1280 && height <= 720) {
+			level = 93;  // Level 3.1
+		} else if (width <= 1920 && height <= 1080) {
+			level = 120; // Level 4.0
+		} else if (width <= 3840 && height <= 2160) {
+			level = 150; // Level 5.0
+		} else {
+			tier = 'H';  // Use High Tier for very high resolutions
+			level = 180; // Level 6.0
+		}
+		
+		// Generate constraint flags (6 bytes)
+		// Using B0 as a simple default (progressive source flag)
+		const constraintFlags = 'B0';
+		
+		// Construct the final string following the format
+		// If profile_space is 0, start with the profile_idc directly
+		const profilePrefix = profileSpace === 0 ? '' : 
+			String.fromCharCode(65 + profileSpace - 1);
+		
+		return `hev1.${profilePrefix}${profileIdc}.${compatibilityHex}.${tier}${level}.${constraintFlags}`;
+	} else if (codec === 'vp8') {
+		return 'vp8'; // Easy, this one
+	} else if (codec === 'vp9') {
+		// Default to Profile 0 (most common)
+        const profile = "00";
+        
+        // Determine level based on resolution
+        // VP9 levels are specified as two digits: major.minor
+        let level;
+        if (width <= 854 && height <= 480) {
+            level = "21"; // Level 2.1
+        } else if (width <= 1280 && height <= 720) {
+            level = "31"; // Level 3.1
+        } else if (width <= 1920 && height <= 1080) {
+            level = "41"; // Level 4.1
+        } else if (width <= 3840 && height <= 2160) {
+            level = "51"; // Level 5.1
+        } else {
+            level = "61"; // Level 6.1
+        }
+        
+        // Default to 8-bit depth
+        const bitDepth = "08";
+        
+        return `vp09.${profile}.${level}.${bitDepth}`;
+	} else if (codec === 'av1') {
+		// Default to Main Profile (0)
+        const profile = 0;
+        
+        // Determine level based on resolution
+        // Using a simplified level selection based on common resolutions
+        let level;
+        if (width <= 854 && height <= 480) {
+            level = "01"; // Level 2.1
+        } else if (width <= 1280 && height <= 720) {
+            level = "03"; // Level 2.3
+        } else if (width <= 1920 && height <= 1080) {
+            level = "04"; // Level 3.0
+        } else if (width <= 3840 && height <= 2160) {
+            level = "07"; // Level 4.0
+        } else {
+            level = "09"; // Level 4.2
+        }
+        
+        // Default to Main tier
+        const tier = "M";
+        
+        // Default to 8-bit depth
+        const bitDepth = "08";
+	
+        return `av01.${profile}.${level}${tier}.${bitDepth}`;
+	}
+
+	throw new Error(`Unhandled codec '${codec}'.`);
+};
+
+export const buildAudioCodecString = (codec: AudioCodec, numberOfChannels: number, sampleRate: number) => {
+	if (codec === 'aac') {
+		// If stereo or higher channels and lower sample rate, likely using HE-AAC v2 with PS
+		if (numberOfChannels >= 2 && sampleRate <= 24000) {
+			return 'mp4a.40.29'; // HE-AAC v2 (AAC LC + SBR + PS)
+		}
+
+		// If sample rate is low, likely using HE-AAC v1 with SBR
+		if (sampleRate <= 24000) {
+			return 'mp4a.40.5'; // HE-AAC v1 (AAC LC + SBR)
+		}
+
+		// Default to standard AAC-LC for higher sample rates
+		return 'mp4a.40.2'; // AAC-LC
+	} else if (codec === 'opus') {
+		return 'opus'; // Easy, this one
+	} else if (codec === 'vorbis') {
+		return 'vorbis'; // Also easy, this one
+	}
+
+	throw new Error(`Unhandled codec '${codec}'.`);
+};