Byte header

byte_header.c

@@ -0,0 +1,94 @@
+#include <assert.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+ * Pack header data into a byte.
+ * 3 bits are used for type (2^3 = 8 different types)
+ * 5 bits are used for length (2^5 = 32 max length, unit is up to you)
+ */
+
+typedef enum {
+    Ok = 0,
+    OversizedType,
+    OversizedLen,
+} Result;
+
+#define TYPE_LEN (4)
+#define LEN      (4)
+
+Result msg_create_header(uint8_t type, uint8_t len, uint8_t *out);
+void   msg_parse_header(uint8_t header, uint8_t *type_out, uint8_t *len_out);
+
+int main(void) {
+    assert(TYPE_LEN + LEN <= 8);
+    uint8_t maxlen = (1 << LEN) - 1;
+    uint8_t maxtype = (1 << TYPE_LEN) - 1;
+
+    {
+        uint8_t hdr;
+        Result  res = msg_create_header(3, 8, &hdr);
+
+        if (res == OversizedType)
+            printf("The type is out of range");
+        if (res == OversizedLen)
+            printf("The length is out of range");
+
+        printf("Header: 0x%02X\n", hdr);
+        printf("Error: %d\n", res);
+
+        uint8_t plen, ptype;
+        msg_parse_header(hdr, &ptype, &plen);
+        printf("Type: %d\n", ptype);
+        printf("Len: %d\n", plen);
+    }
+    {
+        printf("Max len: %d\n", maxlen);
+        printf("Max type: %d\n", maxtype);
+
+        uint8_t hdr;
+        Result  res = msg_create_header(maxtype, maxlen, &hdr);
+        assert(res == Ok);
+
+        uint8_t plen, ptype;
+        msg_parse_header(hdr, &ptype, &plen);
+        printf("plen: %d\n", plen);
+        printf("ptype: %d\n", ptype);
+        assert(ptype == maxtype);
+        assert(plen == maxlen);
+    }
+    {
+        for (int i = 0; i < 1000; i++) {
+            uint8_t len = arc4random() % maxlen;
+            uint8_t type = arc4random() % maxtype;
+            uint8_t hdr;
+            Result  res = msg_create_header(type, len, &hdr);
+            assert(res == Ok);
+
+            uint8_t plen, ptype;
+            msg_parse_header(hdr, &ptype, &plen);
+            assert(ptype == type);
+            assert(plen == len);
+        }
+    }
+
+    return 0;
+}
+
+Result msg_create_header(uint8_t type, uint8_t len, uint8_t *out) {
+    if (type > (1 << TYPE_LEN) - 1)
+        return OversizedType;
+
+    if (len > (1 << LEN) - 1)
+        return OversizedLen;
+
+    *out = (type << LEN) | (len & (0xFF >> TYPE_LEN));
+
+    return Ok;
+}
+
+void msg_parse_header(uint8_t header, uint8_t *type_out, uint8_t *len_out) {
+    *type_out = (header >> LEN) & ((1 << LEN) - 1);
+    *len_out = header & (0xFF >> TYPE_LEN);
+}