new option encoding function

ChangeLog

@@ -1,3 +1,8 @@
+2012-10-25  Olaf Bergmann  <bergmann@tzi.org>
+
+	* option.c (coap_opt_encode): new function for option encoding
+	with option jumps
+
 2012-03-23  Olaf Bergmann  <bergmann@tzi.org>
 
 	* examples/client.c (clear_obs): clear observation relationship after

option.c

@@ -15,6 +15,7 @@
 #endif
 
 #include <stdio.h>
+#include <string.h>
 
 #include "option.h"
 
@@ -91,3 +92,119 @@ coap_check_option(coap_pdu_t *pdu, unsigned char type,
 
   return oi->option && oi->type == type ? oi->option : NULL;
 }
+
+size_t
+coap_opt_encode(coap_opt_t *opt, size_t n, unsigned short delta,
+		unsigned char *val, unsigned short len) {
+  size_t l = 1;
+  size_t length = len; 
+  unsigned int D = delta;
+  coap_opt_t *O = opt;
+  unsigned short N;
+
+  /* option length must not exceed 1034 bytes */
+  if (length > 1034)
+    return 0;
+
+  /* check for option jumps */
+
+  if (delta < 15) { 
+    /* no option jump required, proceed with normal encoding if
+       sufficient space is available */
+    if (n < length + 1)
+      return 0;
+
+    goto encode;
+  } 
+
+  if (delta < 30) {		/* encode with short option jump */
+    /* check if sufficient storage space is available */
+    if (n < length + 2)
+      return 0;
+    
+    delta -= 15;
+    l++;		       /* need one additional byte for 0xf1 */
+    *opt++ = 0xf1;
+
+    goto encode;
+  } 
+
+  /* Values up to 2070 can be encoded with a single jump value
+   * (((0xff + 2) << 3) + 14) == 257 * 8 + 14 = 2070). For values 
+   * below 2064, we use the last three bits from delta as the 
+   * delta value and always encode N as (delta >> 3) - 2. Above
+   * 2064, N will be (delta >> 3) - 3, and 
+   * delta = delta - ((N + 2) << 3).
+   */ 
+  if (delta < 2071) {		/* longer option jump */
+    /* check if sufficient storage space is available */
+    if (n < length + 2)
+      return 0;
+
+    N = (delta >> 3) - 2;
+
+    if (N > 255) {
+      N--;
+      delta = (delta + 8) & 0x0f;
+    } else
+      delta = (delta & 0x07);
+
+    l += 2;		   /* need two additional bytes for 0xf2 nn */
+    *opt++ = 0xf2;
+    *opt++ = N & 0xff;
+
+    goto encode;
+  }
+
+  /* longest option jump (delta <= 65535) */
+
+  /* check if sufficient storage space is available */
+  if (n < length + 3)
+    return 0;
+
+  N = (delta >> 3) - 258;
+  delta = (delta & 0x07);
+
+  l += 3;	      /* need three additional bytes for 0xf3 nn nn */
+  *opt++ = 0xf3;
+  *opt++ = (N >> 8) & 0xff;
+  *opt++ = N & 0xff;
+
+ encode:
+  {
+    size_t k;
+    
+    printf("delta(%d) = ", D);
+
+    for (k = 0; k < l-1; ++k) {
+      printf("%02x ", O[k]);
+    }
+    printf("%xx\n", delta & 0x0f);
+  }
+  return 0;
+  if (length < 15) {
+    *opt++ = (delta << 4) | (length & 0x0f);
+    n--;
+  } else {
+    *opt = ((delta & 0x0f) << 4) | 0x0f;
+    opt++; n--;
+    length -= 15;
+
+    while (255 <= length && len < n) {
+      ++l;
+      *opt++ = 0xff;
+      length -= 255;
+      n--;
+    }
+
+    if (len < n) {
+      ++l;
+      *opt++ = length & 0xff;
+      n--;
+    } else
+      return 0;
+  }
+    
+  memcpy(opt, val, len);
+  return l + len;
+}

option.h

@@ -38,6 +38,20 @@ typedef unsigned char coap_opt_t;
 		  ? (*(PCHAR(opt) + 1) + 15)			\
 		  : (*PCHAR(opt) & 0x0f)))
 
+/**
+ * Encodes the option delta, length and value.
+ *
+ * @param opt start of the given option
+ * @param n   maximum length of the option
+ * @param delta the option delta
+ * @param val the value to write
+ * @param length option length
+ * @return the number of bytes that are used to encode the option length
+ *         or @c 0 in case of error.
+ */
+size_t coap_opt_encode(coap_opt_t *opt, size_t n, unsigned char delta,
+		       unsigned char *val, unsigned short len);
+
 #define COAP_OPT_SETLENGTH(opt,val)					\
   if ((val) < 15)							\
     *PCHAR(opt) = ((*PCHAR(opt) & 0xf0) | ((val) & 0x0f));		\
@@ -217,6 +231,23 @@ coap_opt_t *coap_check_option(coap_pdu_t *pdu,
 			      unsigned char type, 
 			      coap_opt_iterator_t *oi);
 
+/**
+ * Encodes option with given @p delta into @p opt. This function returns
+ * the number of bytes written to @p opt or @c 0 on error. This happens
+ * especially when @p opt does not provide sufficient space to store
+ * the option value, delta, and option jumps when required.
+ *
+ * @param opt   The option buffer space where @p val is written
+ * @param n     Maximum length of @p opt.
+ * @param delta The option delta.
+ * @param val   The option value to copy into @p opt.
+ * @param len   The actual length of @p val.
+ * @return The number of bytes that have been written to @p opt or
+ *         @c 0 on error. The return value will always be less than @p n.
+ */
+size_t coap_opt_encode(coap_opt_t *opt, size_t n, unsigned short delta,
+		       unsigned char *val, unsigned short len);
+
 /** @} */
 
 #endif /* _OPTION_H_ */