sys: fmt: add fmt_lpad()

sys/fmt/fmt.c

@@ -243,6 +243,33 @@ size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits)
     return pos;
 }
 
+size_t fmt_lpad(char *out, size_t in_len, size_t pad_len, char pad_char)
+{
+    if (in_len >= pad_len) {
+        return in_len;
+    }
+
+    size_t n = pad_len - in_len;
+
+    if (FMT_USE_MEMMOVE) {
+        memmove(out + n, out, in_len);
+        memset(out, pad_char, n);
+    }
+    else {
+        char *pos = out + pad_len - 1;
+        out += in_len -1;
+
+        while(in_len--) {
+            *pos-- = *out--;
+        }
+
+        while (n--) {
+            *pos-- = pad_char;
+        }
+    }
+    return pad_len;
+}
+
 uint32_t scn_u32_dec(const char *str, size_t n)
 {
     uint32_t res = 0;

sys/include/fmt.h

@@ -41,6 +41,10 @@
 extern "C" {
 #endif
 
+#ifndef FMT_USE_MEMMOVE
+#define FMT_USE_MEMMOVE (1) /**< use memmove() or internal implementation */
+#endif
+
 /**
  * @brief Format a byte value as hex
  *
@@ -294,6 +298,28 @@ void print_u64_dec(uint64_t val);
  */
 void print_str(const char* str);
 
+/**
+ * @brief Pad string to the left
+ *
+ * This function left-pads a given string @p out with @p pad_char
+ *
+ * For example, calling
+ *
+ *     fmt_lpad("abcd", 4, 7, ' ');
+ *
+ * would result in "   abcd".
+ *
+ * @note Caller must ensure @p str can take pad_len characters!
+ *
+ * @param[inout]    str         string to pad
+ * @param[in]       in_len      length of str
+ * @param[in]       pad_len     total length after padding
+ * @param[in]       pad_char    char to use as pad char
+ *
+ * @returns pad_len
+ */
+size_t fmt_lpad(char *str, size_t in_len, size_t pad_len, char pad_char);
+
 #ifdef __cplusplus
 }
 #endif