From c2c311a859b50449d9fe2a147c740eb94a835a21 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Balv=C3=ADn?= <opensource@petrbalvin.org>
Date: Sun, 4 Oct 2026 22:09:21 +0200
Subject: [PATCH] fix: catch a cyclic map by its identity on the document walk

Assisted-by: DeepSeek V4.1 Flash
---
 document.go | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/document.go b/document.go
index fa9e077..f0db02a 100644
--- a/document.go
+++ b/document.go
@@ -5,6 +5,7 @@ package interpres
 
 import (
 	"maps"
+	"reflect"
 	"slices"
 )
 
@@ -402,13 +403,24 @@ func (t *Table) Set(key string, value any) {
 // newOrderedTable builds a table node for a value the caller set, its keys
 // entered in sorted order, the order Marshal writes maps in.
 func newOrderedTable(m map[string]any) *Table {
-	return orderedTable(m, 0)
+	return orderedTable(m, 0, map[uintptr]bool{})
 }
 
 // orderedTable is newOrderedTable's recursion. The depth bound is the value
 // encoder's: a cyclic map stopped here is written by the value writer, which
-// reports it instead of running the stack out.
-func orderedTable(m map[string]any, depth int) *Table {
+// reports it instead of running the stack out. seen carries the maps the walk
+// is inside of, so a map that repeats one of them is caught by its identity
+// at once and takes the depth limit's shape: its entries are recorded and the
+// walk stops, instead of the walk and the emission materialising a chain the
+// input does not hold.
+func orderedTable(m map[string]any, depth int, seen map[uintptr]bool) *Table {
+	id := reflect.ValueOf(m).Pointer()
+	if seen[id] {
+		depth = maxEncodeDepth
+	} else {
+		seen[id] = true
+		defer delete(seen, id)
+	}
 	t := newTable(m)
 	for _, k := range slices.Sorted(maps.Keys(m)) {
 		v := m[k]
@@ -418,11 +430,11 @@ func orderedTable(m map[string]any, depth int) *Table {
 		}
 		switch val := v.(type) {
 		case map[string]any:
-			e.child = orderedTable(val, depth+1)
+			e.child = orderedTable(val, depth+1, seen)
 		case []map[string]any:
 			e.elements = make([]*Table, len(val))
 			for i, item := range val {
-				e.elements[i] = orderedTable(item, depth+1)
+				e.elements[i] = orderedTable(item, depth+1, seen)
 			}
 		}
 	}