package batch

import (
	"encoding/binary"
	"testing"

	"github.com/slackhq/nebula/overlay/tio"
)

// nopTunWriter is a zero-alloc tio.GSOWriter for benchmarks. Discards
// everything but satisfies the interface the coalescer detects.
type nopTunWriter struct{}

func (nopTunWriter) Write(p []byte) (int, error) { return len(p), nil }
func (nopTunWriter) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, _ tio.GSOProto) error {
	return nil
}
func (nopTunWriter) Capabilities() tio.Capabilities {
	return tio.Capabilities{TSO: true, USO: true}
}

// buildTCPv4BulkFlow returns a slice of N adjacent ACK-only TCP segments
// on a single 5-tuple, each carrying payloadLen bytes. Seq numbers are
// contiguous so every packet is coalesceable onto the previous one.
func buildTCPv4BulkFlow(n, payloadLen int) [][]byte {
	pkts := make([][]byte, n)
	pay := make([]byte, payloadLen)
	seq := uint32(1000)
	for i := range n {
		pkts[i] = buildTCPv4(seq, tcpAck, pay)
		seq += uint32(payloadLen)
	}
	return pkts
}

// buildTCPv4Interleaved returns nFlows * perFlow packets with per-flow
// seq continuity but round-robin across flows — worst case for any
// "last-slot" cache.
func buildTCPv4Interleaved(nFlows, perFlow, payloadLen int) [][]byte {
	pay := make([]byte, payloadLen)
	seqs := make([]uint32, nFlows)
	for i := range seqs {
		seqs[i] = uint32(1000 + i*1000000)
	}
	pkts := make([][]byte, 0, nFlows*perFlow)
	for range perFlow {
		for f := range nFlows {
			sport := uint16(10000 + f)
			pkts = append(pkts, buildTCPv4Ports(sport, 2000, seqs[f], tcpAck, pay))
			seqs[f] += uint32(payloadLen)
		}
	}
	return pkts
}

// buildICMPv4 returns a minimal non-TCP packet that takes the passthrough
// branch in Commit.
func buildICMPv4() []byte {
	pkt := make([]byte, 28)
	pkt[0] = 0x45
	binary.BigEndian.PutUint16(pkt[2:4], 28)
	pkt[9] = 1 // ICMP
	copy(pkt[12:16], []byte{10, 0, 0, 1})
	copy(pkt[16:20], []byte{10, 0, 0, 2})
	return pkt
}

// runCommitBench drives Commit over pkts batchSize at a time, flushing
// between batches, and reports per-packet cost.
func runCommitBench(b *testing.B, pkts [][]byte, batchSize int) {
	b.Helper()
	c := NewTCPCoalescer(nopTunWriter{})
	b.ReportAllocs()
	b.SetBytes(int64(len(pkts[0])))
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		pkt := pkts[i%len(pkts)]
		if err := c.Commit(pkt); err != nil {
			b.Fatal(err)
		}
		if (i+1)%batchSize == 0 {
			if err := c.Flush(); err != nil {
				b.Fatal(err)
			}
		}
	}
	// Drain any trailing partial batch so slot state doesn't leak across runs.
	_ = c.Flush()
}

// BenchmarkCommitSingleFlow is the bulk-TCP steady state: one flow,
// contiguous seq, 1200-byte payloads. Every packet past the seed should
// append onto the open slot. This is the case we most care about.
func BenchmarkCommitSingleFlow(b *testing.B) {
	pkts := buildTCPv4BulkFlow(tcpCoalesceMaxSegs, 1200)
	runCommitBench(b, pkts, tcpCoalesceMaxSegs)
}

// BenchmarkCommitInterleaved4 has 4 concurrent bulk flows round-robined.
// A single-entry fast-path cache will miss on every packet; an N-way
// cache or map lookup carries the weight.
func BenchmarkCommitInterleaved4(b *testing.B) {
	pkts := buildTCPv4Interleaved(4, tcpCoalesceMaxSegs, 1200)
	runCommitBench(b, pkts, len(pkts))
}

// BenchmarkCommitInterleaved16 stresses the map at higher flow counts.
func BenchmarkCommitInterleaved16(b *testing.B) {
	pkts := buildTCPv4Interleaved(16, tcpCoalesceMaxSegs, 1200)
	runCommitBench(b, pkts, len(pkts))
}

// BenchmarkCommitPassthrough exercises the non-TCP branch: parseTCPBase
// bails early and addPassthrough is the only work.
func BenchmarkCommitPassthrough(b *testing.B) {
	pkt := buildICMPv4()
	pkts := make([][]byte, 64)
	for i := range pkts {
		pkts[i] = pkt
	}
	runCommitBench(b, pkts, 64)
}

// BenchmarkCommitNonCoalesceableTCP sends SYN|ACK packets on one flow.
// Each packet takes the "TCP but not admissible" branch which does a
// map delete + passthrough. Measures the seal-without-slot cost.
func BenchmarkCommitNonCoalesceableTCP(b *testing.B) {
	pay := make([]byte, 0)
	pkts := make([][]byte, 64)
	for i := range pkts {
		pkts[i] = buildTCPv4(uint32(1000+i), tcpSyn|tcpAck, pay)
	}
	runCommitBench(b, pkts, 64)
}

// runMultiCommitBench drives MultiCoalescer.Commit. The dispatcher does
// the IP/L4 parse once and passes the parsed struct to the lane, so this
// is the bench that shows the savings of skipping the lane's re-parse.
func runMultiCommitBench(b *testing.B, pkts [][]byte, batchSize int) {
	b.Helper()
	m := NewMultiCoalescer(nopTunWriter{}, true, true)
	b.ReportAllocs()
	b.SetBytes(int64(len(pkts[0])))
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		pkt := pkts[i%len(pkts)]
		if err := m.Commit(pkt); err != nil {
			b.Fatal(err)
		}
		if (i+1)%batchSize == 0 {
			if err := m.Flush(); err != nil {
				b.Fatal(err)
			}
		}
	}
	_ = m.Flush()
}

// BenchmarkMultiCommitSingleFlow is the multi-lane analogue of
// BenchmarkCommitSingleFlow — same workload but routed through the
// dispatcher. The delta vs the single-lane bench measures dispatcher
// overhead.
func BenchmarkMultiCommitSingleFlow(b *testing.B) {
	pkts := buildTCPv4BulkFlow(tcpCoalesceMaxSegs, 1200)
	runMultiCommitBench(b, pkts, tcpCoalesceMaxSegs)
}

// BenchmarkMultiCommitInterleaved4 mirrors BenchmarkCommitInterleaved4
// through the dispatcher.
func BenchmarkMultiCommitInterleaved4(b *testing.B) {
	pkts := buildTCPv4Interleaved(4, tcpCoalesceMaxSegs, 1200)
	runMultiCommitBench(b, pkts, len(pkts))
}