sys/fmt: added format for fixed floating points

sys/fmt/fmt.c

@@ -1,15 +1,20 @@
-/**
+/*
  * Copyright (C) 2015 Kaspar Schleiser <kaspar@schleiser.de>
  *
  * 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_fmt
  * @{
+ *
  * @file
  * @brief       String formatting library implementation
+ *
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ *
  * @}
  */
 
@@ -34,6 +39,17 @@ static inline int _is_digit(char c)
     return (c >= '0' && c <= '9');
 }
 
+static inline unsigned pwr(unsigned val, unsigned exp)
+{
+    unsigned res = 1;
+
+    for (unsigned i = 0; i < exp; i++) {
+        res *= val;
+    }
+
+    return res;
+}
+
 size_t fmt_byte_hex(char *out, uint8_t byte)
 {
     if (out) {
@@ -122,6 +138,56 @@ size_t fmt_s32_dec(char *out, int32_t val)
     return fmt_u32_dec(out, val) + negative;
 }
 
+size_t fmt_s16_dec(char *out, int16_t val)
+{
+    return fmt_s32_dec(out, val);
+}
+
+size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits)
+{
+    int16_t absolute, divider;
+    size_t pos = 0;
+    size_t div_len, len;
+    unsigned e;
+    char tmp[4];
+
+    if (fp_digits > 4) {
+        return 0;
+    }
+    if (fp_digits == 0) {
+        return fmt_s16_dec(out, val);
+    }
+    if (val < 0) {
+        if (out) {
+            out[pos++] = '-';
+        }
+        val *= -1;
+    }
+
+    e = pwr(10, fp_digits);
+    absolute = (val / (int)e);
+    divider = val - (absolute * e);
+
+    pos += fmt_s16_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_s16_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;
+}
+
 uint32_t scn_u32_dec(const char *str, size_t n)
 {
     uint32_t res = 0;

sys/include/fmt.h

@@ -12,8 +12,10 @@
  * @brief       Provides simple string formatting functions
  *
  * @{
+ *
  * @file
- * @brief   string formatting API
+ * @brief       String formatting API
+ *
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
  */
 
@@ -127,6 +129,49 @@ size_t fmt_u16_dec(char *out, uint16_t val);
  */
 size_t fmt_s32_dec(char *out, int32_t val);
 
+/**
+ * @brief Convert a int16 value to decimal string.
+ *
+ * Will add a leading "-" if @p val is negative.
+ *
+ * If @p out is NULL, will only return the number of bytes that would have
+ * been written.
+ *
+ * @param[out]  out  Pointer to output buffer, or NULL
+ * @param[in]   val  Value to convert
+ *
+ * @return      nr of characters written to (or needed in) @p out
+ */
+size_t fmt_s16_dec(char *out, int16_t val);
+
+/**
+ * @brief Convert 16-bit fixed point number to a decimal string
+ *
+ * The input for this function is a signed 16-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.
+ *
+ * The resulting string will always be patted with zeros after the decimal point.
+ *
+ * For example: if @p val is -3548 and @p fp_digits is 2, the resulting string
+ * will be "-35.48". For @p val := 12010 and @p fp_digits := 3 the result will
+ * be "12.010".
+ *
+ * Will add a leading "-" if @p val is negative.
+ *
+ * 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, MUST be <= 4
+ * @param[in]  fp_digits    Number of digits after the decimal point
+ *
+ * @return      Length of the resulting string
+ * @return      0 if @p fp_digits is > 4
+ */
+size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits);
+
 /**
  * @brief Count characters until '\0' (exclusive) in @p str
  *