Hyperdimensional computing (HDC) is a human brain-inspired computing paradigm that processes neural activity patterns with high dimensional vectors. Existing HDC accelerators usually utilize different hardware architectures to process encoding phases and comparison phases of HDC applications separately. They are unable to adapt to dynamic workloads for various datasets, resulting in resource underutilization. In this article, we propose a resistive random access memory (ReRAM)-based HDC accelerator called ReHDC for general HDC. We abstract the computing paradigms in encoding and comparison phases, and provide uniform primitive operators to efficiently process these two phases with the same hardware architecture. In the unified processing engine, ReHDC utilizes analog crossbar arrays to accelerate accumulation operations, and digital crossbar arrays to speed up high-dimensional element-wise operations (xor). Experimental results show that ReHDC can accelerate the HDC training by$69.4\times $and$1.93\times $, and can also improve the energy efficiency by$51.5\times $and$2.2\times $, compared with NVIDIA Tesla P100 GPU and the ReRAM-based HDC accelerator DUAL, respectively. Moreover, the performance speedup and energy efficiency for HDC inference are similar to that of HDC training.