package audio

import (
	"encoding/binary"
	"errors"
	"fmt"
)

// ParsePCM memvalidasi file WAV (RIFF) dan mengembalikan isi chunk "data" (PCM).
// Standar aset tia (PRD Appendix C): 44.1 kHz, mono, 16-bit — selain itu ditolak
// agar kesalahan aset ketahuan saat dev, bukan saat runtime.
func ParsePCM(b []byte) ([]byte, error) {
	if len(b) < 44 || string(b[0:4]) != "RIFF" || string(b[8:12]) != "WAVE" {
		return nil, errors.New("bukan file RIFF/WAVE")
	}
	var (
		format, channels, bits, rate int
		data                         []byte
	)
	for off := 12; off+8 <= len(b); {
		id := string(b[off : off+4])
		size := int(binary.LittleEndian.Uint32(b[off+4 : off+8]))
		off += 8
		if size < 0 || off+size > len(b) {
			return nil, fmt.Errorf("chunk %q terpotong", id)
		}
		switch id {
		case "fmt ":
			if size < 16 {
				return nil, errors.New("chunk fmt terlalu pendek")
			}
			format = int(binary.LittleEndian.Uint16(b[off : off+2]))
			channels = int(binary.LittleEndian.Uint16(b[off+2 : off+4]))
			rate = int(binary.LittleEndian.Uint32(b[off+4 : off+8]))
			bits = int(binary.LittleEndian.Uint16(b[off+14 : off+16]))
		case "data":
			data = b[off : off+size]
		}
		off += size + size%2 // chunk berukuran ganjil dipad 1 byte
	}
	if format != 1 {
		return nil, fmt.Errorf("format %d bukan PCM", format)
	}
	if channels != 1 || bits != 16 || rate != 44100 {
		return nil, fmt.Errorf("hanya mono/16-bit/44100Hz didukung (dapat %dch/%dbit/%dHz)", channels, bits, rate)
	}
	if len(data) == 0 {
		return nil, errors.New("chunk data kosong")
	}
	return data, nil
}