sys/bcd: add bcd_buf_from_u32()

sys/bcd/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/bcd/bcd.c

@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2025 ML!PA Consulting GmbH
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser General
+ * Public License v2.1. See the file LICENSE in the top level directory for more
+ * details.
+ */
+
+/**
+ * @ingroup     sys_bcd
+ * @{
+ *
+ * @file
+ * @brief       Library to de- and encode binary coded decimals
+ *
+ * @author      Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ *
+ * @}
+ */
+
+#include "bcd.h"
+#include <errno.h>
+#include <string.h>
+
+static inline uint8_t swap_nibbles(uint8_t b)
+{
+    return (b << 4) | (b >> 4);
+}
+
+int bcd_buf_from_u32(uint32_t val, void *dst, size_t len)
+{
+    uint8_t *tgt = dst;
+    uint8_t hex = 0;
+    uint8_t idx = 0;
+
+    memset(dst, 0, len);
+    len *= 2;
+
+    do {
+        hex <<= 4;
+        hex += val % 10;
+        val /= 10;
+        if (++idx % 2 == 0) {
+            *tgt++ = swap_nibbles(hex);
+            hex = 0;
+        }
+    } while (val && idx <= len);
+
+    if (idx > len) {
+        return -ENOBUFS;
+    }
+
+    if (idx % 2) {
+        *tgt++ = hex;
+    }
+
+    return (uintptr_t)tgt - (uintptr_t)dst;
+}

sys/include/bcd.h

@@ -20,6 +20,7 @@
 #ifndef BCD_H
 #define BCD_H
 
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus
@@ -52,6 +53,22 @@ static inline uint8_t bcd_to_byte(uint8_t bcd)
     return bcd - (6 * (bcd >> 4));
 }
 
+/**
+ * @brief   Convert a decimal value into a BCD buffer.
+ *          (This looks like the decimal integer value when printed as hex)
+ *
+ * This will e.g. turn the value 123 -> 0x123 (decimal: 291)
+ *
+ * @param[in]  val  Decimal value to print
+ * @param[out] dst  Destination buffer
+ * @param[in]  len  Size of the destination buffer
+ *
+ * @return number of bytes written
+ * @return -ENOBUFS if @p dst is not large enough
+ *         In that case the state of @p dst is undefined.
+ */
+int bcd_buf_from_u32(uint32_t val, void *dst, size_t len);
+
 #ifdef __cplusplus
 }
 #endif