sys/phydat: added phydat_to_json converter

sys/include/phydat.h

@@ -240,6 +240,51 @@ char phydat_prefix_from_scale(int8_t scale);
  */
 void phydat_fit(phydat_t *dat, const int32_t *values, unsigned int dim);
 
+/**
+ * @brief   Convert the given phydat_t structure into a JSON string
+ *
+ * The output string written to @p buf will be `\0` terminated. You must make
+ * sure, that the given @p buf is large enough to hold the resulting string. You
+ * can call the function with `@p buf := NULL` to simply calculate the size of
+ * the JSON string without writing anything.
+ *
+ * The formatted JSON string will have the following format:
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ {.json}
+ * // case (dim == 1):
+ * {
+ *   "d": 21.45,
+ *   "u": "°C"
+ * }
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ {.json}
+ * // case (dim > 1), dim := 3 in this case:
+ * {
+ *   "d": [1.02, 0.23, -0.81],
+ *   "u": "g"
+ * }
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * The data will be encoded as fixed point number based on the given scale
+ * factor.
+ *
+ * For encoding the unit, this function uses the extended
+ * phydat_unit_to_str_verbose() function to also print units for non-SI types,
+ * e.g. it will produce `..."u":"date"}` for @ref UNIT_DATE or `..."u":"none"}`
+ * for @ref UNIT_NONE.
+ *
+ * @param[in]  data     data to encode
+ * @param[in]  dim      dimensions used in @p data, MUST be > 0 and < PHYDAT_DIM
+ * @param[out] buf      target buffer for the JSON string, or NULL
+ *
+ * @pre     @p dim > 0
+ * @pre     @p dim < PHYDAT_DIM
+ *
+ * @return  number of bytes (potentially) written to @p buf, including `\0`
+ *          terminator
+ */
+size_t phydat_to_json(const phydat_t *data, size_t dim, char *buf);
+
 #ifdef __cplusplus
 }
 #endif

sys/phydat/phydat_json.c

@@ -0,0 +1,99 @@
+/*
+ * Copyright (C) 2017 Freie Universität Berlin
+ *
+ * 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_phydat
+ * @{
+ *
+ * @file
+ * @brief       Convert phydat_t structs to human readable JSON strings
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include <string.h>
+
+#include "fmt.h"
+#include "assert.h"
+#include "phydat.h"
+
+#define STATIC_LEN      (14U)
+
+/**
+ * @note    @p buf must be at least 5 bytes of size
+ */
+static size_t _bool_to_str(int16_t val, char *buf)
+{
+    if (val) {
+        memcpy(buf, "true", 4);
+        return 4;
+    }
+    else {
+        memcpy(buf, "false", 5);
+        return 5;
+    }
+}
+
+size_t phydat_to_json(const phydat_t *data, size_t dim, char *buf)
+{
+    assert((dim > 0) && (dim < PHYDAT_DIM));
+
+    size_t pos = 0;
+
+    if (buf == NULL) {
+        pos = STATIC_LEN;
+        if (dim > 1) {
+            pos += (2 + (dim - 1));       /* array parens + separating commas */
+        }
+        for (size_t i = 0; i < dim; i++) {
+            if (data->unit != UNIT_BOOL) {
+                pos += fmt_s16_dfp(NULL, data->val[i], (int)data->scale);
+            }
+            else {
+                pos += (data->val[i]) ? 4 : 5;  /* true: 4, false: 5 */
+            }
+        }
+        pos += strlen(phydat_unit_to_str_verbose(data->unit));
+    }
+    else {
+        memcpy(buf, "{\"d\":", 5);
+        pos += 5;
+        /* write data */
+        if (dim > 1) {
+            buf[pos++] = '[';
+        }
+        for (size_t i = 0; i < dim; i++) {
+            if (data->unit != UNIT_BOOL) {
+                pos += fmt_s16_dfp(&buf[pos], data->val[i], (int)data->scale);
+            }
+            else {
+                pos += _bool_to_str(data->val[i], &buf[pos]);
+            }
+            buf[pos++] = ',';
+        }
+        /* override last comma if needed */
+        if (dim > 1) {
+            buf[pos - 1] = ']';
+            buf[pos++] = ',';
+        }
+        /* add unit */
+        memcpy(&buf[pos], "\"u\":\"", 5);
+        pos += 5;
+        const char *u = phydat_unit_to_str_verbose(data->unit);
+        strcpy(&buf[pos], u);
+        pos += strlen(u);
+        /* terminate the JSON string */
+        memcpy(&buf[pos], "\"}", 2);
+        pos += 2;
+        buf[pos++] = '\0';
+    }
+
+    return pos;
+}