2 files changed, 75 insertions, 1 deletions

diff --git a/pkg/api/api0/client.go b/pkg/api/api0/client.go
index d3097b1..5a8aaf6 100644
--- a/pkg/api/api0/client.go
+++ b/pkg/api/api0/client.go
@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/netip"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/pg9182/atlas/pkg/origin"
@@ -455,7 +456,25 @@ func (h *Handler) handleClientServers(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
-	respMaybeCompress(w, r, http.StatusOK, h.ServerList.csGetJSON())
+
+	buf := h.ServerList.csGetJSON()
+	for _, e := range strings.Split(r.Header.Get("Accept-Encoding"), ",") {
+		if t, _, _ := strings.Cut(e, ";"); strings.TrimSpace(t) == "gzip" {
+			if zbuf, ok := h.ServerList.csGetJSONGzip(); ok {
+				buf = zbuf
+				w.Header().Set("Content-Encoding", "gzip")
+			} else {
+				hlog.FromRequest(r).Error().Msg("failed to gzip server list")
+			}
+			break
+		}
+	}
+
+	w.Header().Set("Content-Length", strconv.Itoa(len(buf)))
+	w.WriteHeader(http.StatusOK)
+	if r.Method != http.MethodHead {
+		w.Write(buf)
+	}
 }
 
 /*
diff --git a/pkg/api/api0/serverlist.go b/pkg/api/api0/serverlist.go
index de48ae0..9c9d04e 100644
--- a/pkg/api/api0/serverlist.go
+++ b/pkg/api/api0/serverlist.go
@@ -2,6 +2,7 @@ package api0
 
 import (
 	"bytes"
+	"compress/gzip"
 	"errors"
 	"fmt"
 	"net/netip"
@@ -35,6 +36,12 @@ type ServerList struct {
 	csUpdateWg sync.WaitGroup            // allows other goroutines to wait for that update to complete
 	csBytes    atomic.Pointer[[]byte]    // contents of buffer must not be modified; only swapped
 
+	// /client/servers gzipped json
+	csgzUpdate   atomic.Pointer[*byte]  // pointer to the first byte of the last known json (works because it must be swapped, not modified)
+	csgzUpdateMu sync.Mutex             // ensures only one update runs at a time
+	csgzUpdateWg sync.WaitGroup         // allows other goroutines to wait for that update to complete
+	csgzBytes    atomic.Pointer[[]byte] // gzipped
+
 	// for unit tests
 	__clock func() time.Time
 }
@@ -239,6 +246,54 @@ func (s *ServerList) csGetJSON() []byte {
 	return b.Bytes()
 }
 
+// csGetJSONGzip is like csGetJSON, but returns it gzipped with true, or false
+// if an error occurs.
+func (s *ServerList) csGetJSONGzip() ([]byte, bool) {
+	buf := s.csGetJSON()
+	if len(buf) == 0 {
+		return nil, false
+	}
+	cur := &buf[0]
+
+	last := s.csgzUpdate.Load()
+	if last != nil && *last == cur {
+		if zbuf := s.csgzBytes.Load(); zbuf != nil && *zbuf != nil {
+			return *zbuf, true
+		}
+	}
+
+	if !s.csgzUpdateMu.TryLock() {
+		s.csgzUpdateWg.Wait()
+		if zbuf := s.csgzBytes.Load(); zbuf != nil && *zbuf != nil {
+			return *zbuf, true
+		}
+		return nil, false
+	} else {
+		defer s.csgzUpdateMu.Unlock()
+		s.csgzUpdateWg.Add(1)
+		defer s.csgzUpdateWg.Done()
+	}
+
+	var b bytes.Buffer
+	zw := gzip.NewWriter(&b)
+	if _, err := zw.Write(buf); err != nil {
+		s.csgzBytes.Store(nil)
+		s.csgzUpdate.Store(&cur)
+		return nil, false
+	}
+	if err := zw.Close(); err != nil {
+		s.csgzBytes.Store(nil)
+		s.csgzUpdate.Store(&cur)
+		return nil, false
+	}
+
+	zbuf := b.Bytes()
+	s.csgzBytes.Store(&zbuf)
+	s.csgzUpdate.Store(&cur)
+
+	return zbuf, true
+}
+
 // csUpdateNextUpdateTime updates the next update time for the cached
 // /client/servers response. It must be called after any time updates while
 // holding a write lock on s.mu.