struct.go: Cache structIndexes

This should help save some CPU time and avoid some memory churn.

3 files changed, 19 insertions, 2 deletions

diff --git a/go.mod b/go.mod
index 74386a3..452d2ff 100644
--- a/go.mod
+++ b/go.mod
@@ -2,7 +2,10 @@ module git.lukeshu.com/go/lowmemjson
 
 go 1.18
 
-require github.com/stretchr/testify v1.8.0
+require (
+	git.lukeshu.com/go/typedsync v0.0.0-20230126205501-1e8afc0ceb1e
+	github.com/stretchr/testify v1.8.0
+)
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
diff --git a/go.sum b/go.sum
index 5164829..76cf271 100644
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,5 @@
+git.lukeshu.com/go/typedsync v0.0.0-20230126205501-1e8afc0ceb1e h1:ZAzzElMx7aMgJXC9QXOxIPyoZrWxX00eP2sR4UHYP+4=
+git.lukeshu.com/go/typedsync v0.0.0-20230126205501-1e8afc0ceb1e/go.mod h1:EAn7NcfoGeGMv3DWxKQnifcT/rYPAIEqp9Rsz//oYqY=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
diff --git a/struct.go b/struct.go
index b7fc287..8a664c6 100644
--- a/struct.go
+++ b/struct.go
@@ -7,6 +7,8 @@ package lowmemjson
 import (
 	"reflect"
 
+	"git.lukeshu.com/go/typedsync"
+
 	"git.lukeshu.com/go/lowmemjson/internal"
 )
 
@@ -25,9 +27,19 @@ type structIndex struct {
 	byName map[string]int
 }
 
+var structIndexCache typedsync.CacheMap[reflect.Type, structIndex]
+
 // indexStruct takes a struct Type, and indexes its fields for use by
-// Decoder.Decode() and Encoder.Encode().
+// Decoder.Decode() and Encoder.Encode().  indexStruct caches its
+// results.
 func indexStruct(typ reflect.Type) structIndex {
+	ret, _ := structIndexCache.LoadOrCompute(typ, indexStructReal)
+	return ret
+}
+
+// indexStructReal is like indexStruct, but is the real indexer,
+// bypassing the cache.
+func indexStructReal(typ reflect.Type) structIndex {
 	var byPos []structField
 	byName := make(map[string][]int)