package capture

import (
	"fmt"
	"os"

	"github.com/gopacket/gopacket"
	"github.com/gopacket/gopacket/layers"
	"github.com/gopacket/gopacket/pcapgo"
	"golang.org/x/net/bpf"
)

// ReadPcapFile opens a pcap file and returns a channel of packets.
// If filter is non-empty, applies software BPF filtering (kernel BPF unavailable for files).
// Per D-03: replaces -i as packet source. Per D-04: --filter works with --read.
// The returned channel is closed when all packets have been emitted.
func ReadPcapFile(path string, filter string) (<-chan gopacket.Packet, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("cannot open pcap file %q: %w", path, err)
	}
	r, err := pcapgo.NewReader(f)
	if err != nil {
		f.Close()
		return nil, fmt.Errorf("not a valid pcap file %q: %w", path, err)
	}

	// Compile software BPF filter if provided.
	var vm *bpf.VM
	if filter != "" {
		vm, err = CompileSoftwareBPF(filter)
		if err != nil {
			f.Close()
			return nil, err
		}
	}

	lt := layers.LinkType(r.LinkType())
	packetSource := gopacket.NewPacketSource(r, lt)
	packetSource.NoCopy = false // CRITICAL: do NOT use NoCopy=true with pcapgo (Pitfall 3)

	packets := make(chan gopacket.Packet, 512)
	go func() {
		defer close(packets)
		defer f.Close()
		for pkt := range packetSource.Packets() {
			// Apply software BPF filter if active.
			if vm != nil {
				result, err := vm.Run(pkt.Data())
				if err != nil || result == 0 {
					continue // packet does not match filter
				}
			}
			packets <- pkt
		}
	}()
	return packets, nil
}