From 4f04f7245854ccbfadb86ef7774f53477f71da87 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 16:19:52 +0300
Subject: [PATCH] json: output of json encode/decode of structs containing f64
 arrays is not equivalent to the input (fixes #22860)

---
 vlib/json/json_primitives.c.v                 | 22 +++++++++++++++++--
 .../tests/json_f64_array_roundtrip_test.v     | 14 ++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 vlib/json/tests/json_f64_array_roundtrip_test.v

diff --git a/vlib/json/json_primitives.c.v b/vlib/json/json_primitives.c.v
index 081fd37be..e1e718d11 100644
--- a/vlib/json/json_primitives.c.v
+++ b/vlib/json/json_primitives.c.v
@@ -3,6 +3,8 @@
 // that can be found in the LICENSE file.
 module json
 
+import math
+
 #flag -I @VEXEROOT/thirdparty/cJSON
 #flag @VEXEROOT/thirdparty/cJSON/cJSON.o
 #include "cJSON.h"
@@ -28,6 +30,7 @@ fn C.cJSON_IsObject(&C.cJSON) bool
 fn C.cJSON_IsArray(&C.cJSON) bool
 
 fn C.cJSON_CreateNumber(f64) &C.cJSON
+fn C.cJSON_CreateRaw(&char) &C.cJSON
 
 fn C.cJSON_CreateBool(bool) &C.cJSON
 
@@ -233,12 +236,12 @@ fn encode_u64(val u64) &C.cJSON {
 
 @[markused]
 fn encode_f32(val f32) &C.cJSON {
-	return C.cJSON_CreateNumber(val)
+	return C.cJSON_CreateRaw(&char(json_float_to_raw_string(val).str))
 }
 
 @[markused]
 fn encode_f64(val f64) &C.cJSON {
-	return C.cJSON_CreateNumber(val)
+	return C.cJSON_CreateRaw(&char(json_float_to_raw_string(val).str))
 }
 
 @[markused]
@@ -256,6 +259,21 @@ fn encode_string(val string) &C.cJSON {
 	return C.cJSON_CreateString(&char(val.str))
 }
 
+// json_float_to_raw_string uses V's float formatter so json.encode keeps exact float round-trips.
+fn json_float_to_raw_string[T](val T) string {
+	if val == 0 {
+		return '0'
+	}
+	if math.is_nan(f64(val)) || math.is_inf(f64(val), 0) {
+		return 'null'
+	}
+	mut raw := val.str()
+	if raw.len > 2 && raw[raw.len - 2] == `.` && raw[raw.len - 1] == `0` {
+		raw = raw[..raw.len - 2]
+	}
+	return raw
+}
+
 // ///////////////////////
 // user := decode_User(json_parse(js_string_var))
 @[markused]
diff --git a/vlib/json/tests/json_f64_array_roundtrip_test.v b/vlib/json/tests/json_f64_array_roundtrip_test.v
new file mode 100644
index 000000000..c85989962
--- /dev/null
+++ b/vlib/json/tests/json_f64_array_roundtrip_test.v
@@ -0,0 +1,14 @@
+import json
+
+struct F64ArrayPayload {
+	arr []f64
+}
+
+fn test_encode_decode_struct_with_f64_array_roundtrips() ! {
+	original := F64ArrayPayload{
+		arr: [0.9716157205240175, 0.9336099585062241]
+	}
+	encoded := json.encode(original)
+	assert encoded == '{"arr":[0.9716157205240175,0.9336099585062241]}'
+	assert json.decode(F64ArrayPayload, encoded)! == original
+}
-- 
2.39.5