// 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)) > 0){
        m += n;
        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;
}