Let's analyze the code step by step to understand the CWE-787 (Out-of-bounds Write) vulnerability:

1. Input analysis:
   - The function takes three parameters: 'type' (int), 'r' (unsigned char pointer), and 'size' (size_t).
   - 'r' is the destination buffer, and 'size' is its intended size.

2. Initial operations (lines 5-10):
   - Some bit manipulations are performed on 'type'.
   - The first three bytes of 'r' are set based on these manipulations.

3. Main loop (lines 11-14):
   - A loop starts with i = 3 and doubles i in each iteration (i *= 2).
   - The loop continues while i < size / 2.

4. Vulnerable memcpy (line 12):
   - Inside the loop, memcpy(r + i, r, i) is called.
   - This copies i bytes from the start of 'r' to the position 'r + i'.

5. Vulnerability analysis:
   - The vulnerability occurs because there's no check to ensure that 'i + i' doesn't exceed 'size'.
   - In the last iteration of the loop, 'i' could be just below size/2.
   - This means 'i + i' could be larger than 'size', causing an out-of-bounds write.

6. Example scenario:
   - If size = 1000 and i = 512 in the last iteration:
   - memcpy(r + 512, r, 512) would attempt to write up to index 1023, which is beyond the allocated size.

7. Consequences:
   - This out-of-bounds write can corrupt memory adjacent to the 'r' buffer.
   - It may lead to crashes, data corruption, or potential security vulnerabilities.

Conclusion:
The CWE-787 vulnerability arises because the memcpy operation in the loop doesn't properly check if the destination range (r + i) plus the copy size (i) exceeds the total allocated size of the buffer. This can lead to writing beyond the bounds of the allocated memory for 'r', resulting in an out-of-bounds write vulnerability.