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grep.c

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+// Simple grep.  Only supports ^ . * $ operators.
+
+#include "types.h"
+#include "stat.h"
+#include "user.h"
+
+char buf[1024];
+int match(char*, char*);
+
+void grep(char *pattern, int fd) {
+    int n, m;
+    char *p, *q;
+
+    m = 0;
+    while ((n = read(fd, buf + m, sizeof(buf) - m - 1)) > 0) {
+        m += n;
+        buf[m] = '\0';
+        p = buf;
+        while ((q = strchr(p, '\n')) != 0) {
+            *q = 0;
+            if (match(pattern, p)) {
+                *q = '\n';
+                write(1, p, q + 1 - p);
+            }
+            p = q + 1;
+        }
+        if (p == buf) {
+            m = 0;
+        }
+        if (m > 0) {
+            m -= p - buf;
+            memmove(buf, p, m);
+        }
+    }
+}
+
+int main(int argc, char *argv[])     {
+    int fd, i;
+    char *pattern;
+
+    if (argc <= 1) {
+        printf(2, "usage: grep pattern [file ...]\n");
+        exit();
+    }
+    pattern = argv[1];
+
+    if (argc <= 2) {
+        grep(pattern, 0);
+        exit();
+    }
+
+    for (i = 2; i < argc; i++) {
+        if ((fd = open(argv[i], 0)) < 0) {
+            printf(1, "grep: cannot open %s\n", argv[i]);
+            exit();
+        }
+        grep(pattern, fd);
+        close(fd);
+    }
+    exit();
+}
+
+// Regexp matcher from Kernighan & Pike,
+// The Practice of Programming, Chapter 9.
+
+int matchhere(char*, char*);
+int matchstar(int, char*, char*);
+
+int match(char *re, char *text) {
+    if (re[0] == '^') {
+        return matchhere(re + 1, text);
+    }
+    do { // must look at empty string
+        if (matchhere(re, text)) {
+            return 1;
+        }
+    }
+    while (*text++ != '\0');
+    return 0;
+}
+
+// matchhere: search for re at beginning of text
+int matchhere(char *re, char *text){
+    if (re[0] == '\0') {
+        return 1;
+    }
+    if (re[1] == '*') {
+        return matchstar(re[0], re + 2, text);
+    }
+    if (re[0] == '$' && re[1] == '\0') {
+        return *text == '\0';
+    }
+    if (*text != '\0' && (re[0] == '.' || re[0] == *text)) {
+        return matchhere(re + 1, text + 1);
+    }
+    return 0;
+}
+
+// matchstar: search for c*re at beginning of text
+int matchstar(int c, char *re, char *text) {
+    do { // a * matches zero or more instances
+        if (matchhere(re, text)) {
+            return 1;
+        }
+    }
+    while (*text != '\0' && (*text++ == c || c == '.'));
+    return 0;
+}
+