<?php
/** Structural integrity of the MP3 format requested by the narration pipeline. */
namespace Accessibility_Audio_TTS\Core;

if ( ! defined( 'ABSPATH' ) ) { exit; }

class Audio_Validation {
	/**
	 * Walk every complete Layer III frame, not merely an ID3/sync prefix.
	 * ID3v2/ID3v1 tags between complete segments are allowed for merged responses.
	 * This checks transport/container integrity; it does not recognize spoken words.
	 */
	public static function inspect_mp3( string $content ): array {
		$size = strlen( $content ); $offset = 0; $frames = 0; $duration = 0.0;
		$bad = array( 'valid' => false, 'error' => 'Invalid or incomplete MP3 audio received.' );
		while ( $offset < $size ) {
			if ( 'ID3' === substr( $content, $offset, 3 ) ) {
				if ( $size - $offset < 10 ) { return $bad; }
				$version = ord( $content[ $offset + 3 ] ); $flags = ord( $content[ $offset + 5 ] );
				if ( ! in_array( $version, array( 2, 3, 4 ), true ) || 0xFF === ord( $content[ $offset + 4 ] ) ) { return $bad; }
				$length = 0;
				for ( $i = 6; $i < 10; ++$i ) { $part = ord( $content[ $offset + $i ] ); if ( $part & 0x80 ) { return $bad; } $length = ( $length << 7 ) | $part; }
				$footer = 4 === $version && ( $flags & 0x10 ) ? 10 : 0;
				if ( $offset + 10 + $length + $footer > $size ) { return $bad; }
				if ( $footer && '3DI' !== substr( $content, $offset + 10 + $length, 3 ) ) { return $bad; }
				$offset += 10 + $length + $footer;
				continue;
			}
			if ( 'TAG' === substr( $content, $offset, 3 ) ) {
				if ( $size - $offset < 128 || 0 === $frames ) { return $bad; }
				$offset += 128; continue;
			}
			if ( $size - $offset < 4 ) { return $bad; }
			$a = ord( $content[ $offset ] ); $b = ord( $content[ $offset + 1 ] ); $c = ord( $content[ $offset + 2 ] );
			$version = ( $b >> 3 ) & 3; $layer = ( $b >> 1 ) & 3; $rate_index = ( $c >> 4 ) & 15; $sample_index = ( $c >> 2 ) & 3;
			if ( 0xFF !== $a || 0xE0 !== ( $b & 0xE0 ) || 1 === $version || 1 !== $layer || 0 === $rate_index || 15 === $rate_index || 3 === $sample_index ) { return $bad; }
			$rates = 3 === $version ? array( 0,32,40,48,56,64,80,96,112,128,160,192,224,256,320 ) : array( 0,8,16,24,32,40,48,56,64,80,96,112,128,144,160 );
			$sample_rates = array( 44100, 48000, 32000 );
			$sample_rate = $sample_rates[ $sample_index ] / ( 3 === $version ? 1 : ( 2 === $version ? 2 : 4 ) );
			$length = (int) floor( ( 3 === $version ? 144000 : 72000 ) * $rates[ $rate_index ] / $sample_rate ) + ( ( $c >> 1 ) & 1 );
			if ( $length < 4 || $offset + $length > $size ) { return $bad; }
			$offset += $length; ++$frames; $duration += ( 3 === $version ? 1152 : 576 ) / $sample_rate;
		}
		return $frames > 0 ? array( 'valid' => true, 'frames' => $frames, 'duration' => $duration ) : $bad;
	}

	public static function valid_mp3( string $content ): bool { return self::inspect_mp3( $content )['valid']; }

	/** Require actual frames even when an upstream server labels arbitrary bytes audio. */
	public static function valid_mp3_response( array $response ): bool {
		$type = strtolower( trim( explode( ';', (string) ( $response['content_type'] ?? '' ) )[0] ) );
		return ! empty( $response['success'] ) && in_array( $type, array( 'audio/mpeg', 'audio/mp3' ), true ) && isset( $response['body'] ) && is_string( $response['body'] ) && self::valid_mp3( $response['body'] );
	}
}