IO：字节、字符、缓冲与资源关闭

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
public class Main {
    static long count(InputStream input, byte[] pattern, int block) throws IOException {
        if (input == null || pattern == null || pattern.length == 0 || block < 1 || block > 65536)
            throw new IllegalArgumentException("input");
        int[] prefix = new int[pattern.length];
        for (int i = 1, j = 0; i < pattern.length; i++) {
            while (j > 0 && pattern[i] != pattern[j]) j = prefix[j-1];
            if (pattern[i] == pattern[j]) j++;
            prefix[i] = j;
        }
        byte[] buffer = new byte[block];
        int matched = 0; // 跨读取块保留状态，不能放进 read 循环内部。
        long hits = 0;
        for (int n; (n = input.read(buffer)) != -1;) {
            if (n == 0) throw new IOException("non-progressing source");
            for (int i = 0; i < n; i++) { // 只处理本次有效字节。
                while (matched > 0 && buffer[i] != pattern[matched])
                    matched = prefix[matched-1];
                if (buffer[i] == pattern[matched]) matched++;
                if (matched == pattern.length) {
                    hits = Math.addExact(hits,1L);
                    matched = prefix[matched-1]; // 保留重叠匹配的可能。
                }
            }
        }
        return hits;
    }
    static void check(long wanted, String text, String pattern) throws IOException {
        byte[] bytes = text.getBytes(StandardCharsets.UTF_8);
        try (InputStream input = new ByteArrayInputStream(bytes) {
            @Override public synchronized int read(byte[] b, int off, int len) {
                return super.read(b,off,Math.min(len,1));
            }
        }) {
            if (count(input,pattern.getBytes(StandardCharsets.UTF_8),3) != wanted)
                throw new AssertionError(text);
        }
    }
    public static void main(String[] args) throws Exception {
        // 单字节短读强制覆盖块边界，另验空流和重叠。
        check(3,"aaaa","aa");
        check(2,"ababa","aba");
        check(0,"","a");
        check(1,"清欢清欢","欢清");
        try (InputStream input = new ByteArrayInputStream(new byte[]{0,-1,0})) {
            if (count(input,new byte[]{-1,0},2) != 1) throw new AssertionError("binary");
        }
        boolean rejected = false;
        try { count(new ByteArrayInputStream(new byte[0]),new byte[0],1); }
        catch (IllegalArgumentException e) { rejected = true; }
        if (!rejected) throw new AssertionError("empty pattern");
        System.out.println("PASS 6");
    }
}