Document init_ibm_float() and init_ieee_float()

src/grib_ibmfloat.cc

@@ -51,6 +51,11 @@ struct ibm_table_t
 
 static ibm_table_t ibm_table = { 0, {0,}, {0,}, 0, 0 };
 
+
+/**
+ * @brief (see the documentation for init_ieee_float() in grib_ieeefloat.cc)
+ */
+
 static void init_ibm_table()
 {
     if (!ibm_table.inited) {

src/grib_ieeefloat.cc

@@ -56,12 +56,34 @@ struct ieee_table_t
 
 static ieee_table_t ieee_table = { 0, {0,}, {0,}, 0, 0 };
 
+
+/**
+ * @brief Initializes the ieee_table with IEE754 single precision (32-bit) values.
+ *
+ * Nearest smaller values (e.g., reference values for grid_simple and grid_ccsds)
+ * are taken from this table.
+ *
+ * @details The table layout is as follows:
+ * 
+ * | idx (i) | multiplier (e) | value (v = mmin * e) |
+ * |---------|----------------|----------------------|
+ * | 1       | 2^(-149)       | 0x800000 * 2^(-149)  |
+ * | 2       | 2^(-148)       | 0x800000 * 2^(-148)  |
+ * | ...     | ...            | ...                  |
+ * | 253     | 2^103          | 0x800000 * 2^103     |
+ * | 254     | 2^104          | 0x800000 * 2^104     |
+ *
+ * The vmin and vmax boundaries are defined as:
+ * - vmin =  0x800000 * 2^(-149)
+ * - vmax =  0xffffff * 2^104
+ */
+
 static void init_ieee_table()
 {
     if (!ieee_table.inited) {
         unsigned long i;
-        unsigned long mmin = 0x800000;
+        unsigned long mmin = 0x800000;  // minimum mantissa
-        unsigned long mmax = 0xffffff;
+        unsigned long mmax = 0xffffff;  // maximum mantissa
         double e           = 1;
         for (i = 1; i <= 104; i++) {
             e *= 2;