fmt: add signed 32 bit float point formating

sys/fmt/fmt.c

@@ -35,6 +35,17 @@ ssize_t write(int fildes, const void *buf, size_t nbyte);
 
 static const char _hex_chars[16] = "0123456789ABCDEF";
 
+static const uint32_t _tenmap[] = {
+    0,
+    10LU,
+    100LU,
+    1000LU,
+    10000LU,
+    100000LU,
+    1000000LU,
+    10000000LU,
+};
+
 static inline int _is_digit(char c)
 {
     return (c >= '0' && c <= '9');
@@ -243,16 +254,48 @@ size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits)
     return pos;
 }
 
-static const uint32_t _tenmap[] = {
-    0,
-    10LU,
-    100LU,
-    1000LU,
-    10000LU,
-    100000LU,
-    1000000LU,
-    10000000LU,
-};
+size_t fmt_s32_dfp(char *out, int32_t val, unsigned fp_digits)
+{
+    int32_t absolute, divider;
+    unsigned div_len, len, pos = 0;
+    char tmp[9];
+
+    if (fp_digits > 9) {
+        return 0;
+    }
+    if (fp_digits == 0) {
+        return fmt_s32_dec(out, val);
+    }
+    if (val < 0) {
+        if (out) {
+            out[pos++] = '-';
+        }
+        val = -val;
+    }
+
+    uint32_t e = _tenmap[fp_digits];
+    absolute = (val / e);
+    divider = val - (absolute * e);
+
+    pos += fmt_s32_dec(&out[pos], absolute);
+
+    if (!out) {
+        return pos + 1 + fp_digits;     /* abs len + decimal point + divider */
+    }
+
+    out[pos++] = '.';
+    len = pos + fp_digits;
+    div_len = fmt_s32_dec(tmp, divider);
+
+    while (pos < (len - div_len)) {
+        out[pos++] = '0';
+    }
+    for (size_t i = 0; i < div_len; i++) {
+        out[pos++] = tmp[i];
+    }
+
+    return pos;
+}
 
 /* this is very probably not the most efficient implementation, as it at least
  * pulls in floating point math.  But it works, and it's always nice to have

sys/include/fmt.h

@@ -203,6 +203,34 @@ size_t fmt_s16_dec(char *out, int16_t val);
  */
 size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits);
 
+/**
+ * @brief Convert 32-bit fixed point number to a decimal string
+ *
+ * The input for this function is a signed 32-bit integer holding the fixed
+ * point value as well as an unsigned integer defining the position of the
+ * decimal point, so this value defines the number of decimal digits after the
+ * decimal point.
+ *
+ * Will add a leading "-" if @p val is negative.
+ *
+ * The resulting string will always be patted with zeros after the decimal point.
+ *
+ * For example: if @p val is -314159 and @p fp_digits is 5, the resulting string
+ * will be "-3.14159". For @p val := 16777215 and @p fp_digits := 6 the result
+ * will be "16.777215".
+ *
+ * If @p out is NULL, will only return the number of bytes that would have
+ * been written.
+ *
+ * @param[out] out          Pointer to the output buffer, or NULL
+ * @param[in]  val          Fixed point value
+ * @param[in]  fp_digits    Number of digits after the decimal point, MUST be <= 9
+ *
+ * @return      Length of the resulting string
+ * @return      0 if @p fp_digits is > 9
+ */
+size_t fmt_s32_dfp(char *out, int32_t val, unsigned fp_digits);
+
 /**
  * @brief Format float to string
  *