feat: add OrderedMap, the table that keeps its key order

Assisted-by: GLM 5.3 Flash

interpres.go

@@ -26,6 +26,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"reflect"
 	"slices"
 	"strings"
 )
@@ -237,12 +238,15 @@ func Unmarshal(data []byte, v any) error {
 
 // UnmarshalContext is the cancellable variant of Unmarshal.
 func UnmarshalContext(ctx context.Context, data []byte, v any) error {
-	tree, err := ParseMapContext(ctx, data)
+	// Only a destination that can reach an OrderedMap needs the node tree the
+	// written key order is read from; every other decode skips building it.
+	tree, doc, err := parseWithOptions(ctx, data, parseOptions{}, typeWantsOrder(reflect.TypeOf(v)))
 	if err != nil {
 		return err
 	}
 	dec := newDecoder()
 	dec.ctx = ctx
+	dec.nodes = indexNodes(doc.Root())
 	return dec.decode(tree, v)
 }
 
@@ -307,17 +311,19 @@ func (d *Decoder) Decode(data []byte, v any) error {
 
 // DecodeContext is the cancellable variant of Decode.
 func (d *Decoder) DecodeContext(ctx context.Context, data []byte, v any) error {
-	tree, _, err := parseWithOptions(ctx, data, parseOptions{
+	opts := parseOptions{
 		maxDepth:     d.maxDepth,
 		maxInputSize: d.maxInputSize,
 		useNumber:    d.useNumber,
-	}, false)
+	}
+	tree, doc, err := parseWithOptions(ctx, data, opts, typeWantsOrder(reflect.TypeOf(v)))
 	if err != nil {
 		return err
 	}
 	dec := newDecoder()
 	dec.disallowUnknown = d.disallowUnknown
 	dec.ctx = ctx
+	dec.nodes = indexNodes(doc.Root())
 	return dec.decode(tree, v)
 }